Merge #9306 'vim-patch: integrate xdiff library'

closes #1466
closes #1007
closes #1391

30 files changed, 6015 insertions, 306 deletions

diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index 4702656c41..c34dbaaf60 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -628,6 +628,11 @@ CursorMoved			After the cursor was moved in Normal or Visual
 CursorMovedI			After the cursor was moved in Insert mode.
 				Not triggered when the popup menu is visible.
 				Otherwise the same as CursorMoved.
+							*DiffUpdated*
+DiffUpdated			After diffs have been updated.  Depending on
+				what kind of diff is being used (internal or
+				external) this can be triggered on every
+				change or when doing |:diffupdate|.
 							*DirChanged*
 DirChanged			After the |current-directory| was changed.
 				Sets these |v:event| keys:
diff --git a/runtime/doc/diff.txt b/runtime/doc/diff.txt
index 766240dfb0..ab002b4dcb 100644
--- a/runtime/doc/diff.txt
+++ b/runtime/doc/diff.txt
@@ -27,7 +27,9 @@ The second and following arguments may also be a directory name.  Vim will
 then append the file name of the first argument to the directory name to find
 the file.
 
-This only works when a standard "diff" command is available.  See 'diffexpr'.
+By default an internal diff library will be used.  When 'diffopt' or
+'diffexpr' has been set an external "diff" command will be used.  This only
+works when such a diff program is available.
 
 Diffs are local to the current tab page |tab-page|.  You can't see diffs with
 a window in another tab page.  This does make it possible to have several
@@ -332,8 +334,9 @@ between file1 and file2: >
 
 The ">" is replaced with the value of 'shellredir'.
 
-The output of "diff" must be a normal "ed" style diff.  Do NOT use a context
-diff.  This example explains the format that Vim expects: >
+The output of "diff" must be a normal "ed" style diff or a unified diff.  Do
+NOT use a context diff.  This example explains the format that Vim expects for
+the "ed" style diff: >
 
 	1a2
 	> bbb
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 29d9a60aa2..9aaae6a527 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1859,13 +1859,13 @@ A jump table for the options with a short description can be found at |Q_op|.
 						*'dex'* *'diffexpr'*
 'diffexpr' 'dex'	string	(default "")
 			global
-	Expression which is evaluated to obtain an ed-style diff file from two
-	versions of a file.  See |diff-diffexpr|.
+	Expression which is evaluated to obtain a diff file (either ed-style
+	or unified-style) from two versions of a file.  See |diff-diffexpr|.
 	This option cannot be set from a |modeline| or in the |sandbox|, for
 	security reasons.
 
 						*'dip'* *'diffopt'*
-'diffopt' 'dip'		string	(default "filler")
+'diffopt' 'dip'		string	(default "internal,filler")
 			global
 	Option settings for diff mode.  It can consist of the following items.
 	All are optional.  Items must be separated by a comma.
@@ -1925,11 +1925,31 @@ A jump table for the options with a short description can be found at |Q_op|.
 		foldcolumn:{n}	Set the 'foldcolumn' option to {n} when
 				starting diff mode.  Without this 2 is used.
 
-	Examples: >
+		internal	Use the internal diff library.  This is
+				ignored when 'diffexpr' is set.  *E960*
+				When running out of memory when writing a
+				buffer this item will be ignored for diffs
+				involving that buffer.  Set the 'verbose'
+				option to see when this happens.
+
+		indent-heuristic
+				Use the indent heuristic for the internal
+				diff library.
+
+                algorithm:{text} Use the specified diff algorithm with the
+				internal diff engine. Currently supported 
+				algorithms are:
+				myers      the default algorithm
+				minimal    spend extra time to generate the
+					   smallest possible diff
+				patience   patience diff algorithm
+				histogram  histogram diff algorithm
 
-		:set diffopt=filler,context:4
+	Examples: >
+		:set diffopt=internal,filler,context:4
 		:set diffopt=
-		:set diffopt=filler,foldcolumn:3
+		:set diffopt=internal,filler,foldcolumn:3
+		:set diffopt-=internal  " do NOT use the internal diff parser
 <
 				     *'digraph'* *'dg'* *'nodigraph'* *'nodg'*
 'digraph' 'dg'		boolean	(default off)
diff --git a/src/nvim/CMakeLists.txt b/src/nvim/CMakeLists.txt
index 29a4e1e163..a2c4e677d4 100644
--- a/src/nvim/CMakeLists.txt
+++ b/src/nvim/CMakeLists.txt
@@ -79,6 +79,8 @@ file(MAKE_DIRECTORY ${LINT_SUPPRESSES_ROOT}/src)
 
 file(GLOB NVIM_SOURCES *.c)
 file(GLOB NVIM_HEADERS *.h)
+file(GLOB XDIFF_SOURCES xdiff/*.c)
+file(GLOB XDIFF_HEADERS xdiff/*.h)
 
 foreach(subdir
         os
@@ -148,12 +150,13 @@ set(CONV_SOURCES
   syntax.c
   tag.c
   window.c)
-
 foreach(sfile ${CONV_SOURCES})
   if(NOT EXISTS "${CMAKE_CURRENT_LIST_DIR}/${sfile}")
     message(FATAL_ERROR "${sfile} doesn't exist (it was added to CONV_SOURCES)")
   endif()
 endforeach()
+# xdiff: inlined external project, we don't maintain it. #9306
+list(APPEND CONV_SOURCES ${XDIFF_SOURCES})
 
 if(NOT MSVC)
   set_source_files_properties(
@@ -404,7 +407,8 @@ if(POLICY CMP0069)
 endif()
 
 add_executable(nvim ${NVIM_GENERATED_FOR_SOURCES} ${NVIM_GENERATED_FOR_HEADERS}
-  ${NVIM_GENERATED_SOURCES} ${NVIM_SOURCES} ${NVIM_HEADERS})
+  ${NVIM_GENERATED_SOURCES} ${NVIM_SOURCES} ${NVIM_HEADERS}
+  ${XDIFF_SOURCES} ${XDIFF_HEADERS})
 target_link_libraries(nvim ${NVIM_EXEC_LINK_LIBRARIES})
 install_helper(TARGETS nvim)
 
@@ -490,6 +494,7 @@ add_library(
   EXCLUDE_FROM_ALL
   ${NVIM_SOURCES} ${NVIM_GENERATED_SOURCES}
   ${NVIM_HEADERS} ${NVIM_GENERATED_FOR_SOURCES} ${NVIM_GENERATED_FOR_HEADERS}
+  ${XDIFF_SOURCES} ${XDIFF_HEADERS}
 )
 set_property(TARGET libnvim APPEND PROPERTY
              INCLUDE_DIRECTORIES ${LUA_PREFERRED_INCLUDE_DIRS})
@@ -514,6 +519,7 @@ else()
     EXCLUDE_FROM_ALL
     ${NVIM_SOURCES} ${NVIM_GENERATED_SOURCES}
     ${NVIM_HEADERS} ${NVIM_GENERATED_FOR_SOURCES} ${NVIM_GENERATED_FOR_HEADERS}
+    ${XDIFF_SOURCES} ${XDIFF_HEADERS}
     ${UNIT_TEST_FIXTURES}
   )
   target_link_libraries(nvim-test ${NVIM_TEST_LINK_LIBRARIES})
diff --git a/src/nvim/auevents.lua b/src/nvim/auevents.lua
index a6290aaac1..ab92e84799 100644
--- a/src/nvim/auevents.lua
+++ b/src/nvim/auevents.lua
@@ -33,6 +33,7 @@ return {
     'CursorHoldI',            -- idem, in Insert mode
     'CursorMoved',            -- cursor was moved
     'CursorMovedI',           -- cursor was moved in Insert mode
+    'DiffUpdated',            -- diffs have been updated
     'DirChanged',             -- directory changed
     'EncodingChanged',        -- after changing the 'encoding' option
     'ExitPre',                -- before exiting
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index e86eaf010f..d270714611 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -58,10 +58,10 @@ typedef struct {
  * functions that set or reset the flags.
  *
  * VALID_BOTLINE    VALID_BOTLINE_AP
- *     on		on		w_botline valid
- *     off		on		w_botline approximated
- *     off		off		w_botline not valid
- *     on		off		not possible
+ *     on       on      w_botline valid
+ *     off      on      w_botline approximated
+ *     off      off     w_botline not valid
+ *     on       off     not possible
  */
 #define VALID_WROW      0x01    /* w_wrow (window row) is valid */
 #define VALID_WCOL      0x02    /* w_wcol (window col) is valid */
@@ -159,7 +159,7 @@ typedef struct {
   int wo_arab;
 # define w_p_arab w_onebuf_opt.wo_arab  /* 'arabic' */
   int wo_bri;
-# define w_p_bri w_onebuf_opt.wo_bri	/* 'breakindent' */
+# define w_p_bri w_onebuf_opt.wo_bri    // 'breakindent'
   char_u *wo_briopt;
 # define w_p_briopt w_onebuf_opt.wo_briopt /* 'breakindentopt' */
   int wo_diff;
@@ -412,13 +412,13 @@ typedef struct {
    * syntax state too often.
    * b_sst_array[] is allocated to hold the state for all displayed lines,
    * and states for 1 out of about 20 other lines.
-   * b_sst_array	pointer to an array of synstate_T
-   * b_sst_len	number of entries in b_sst_array[]
-   * b_sst_first	pointer to first used entry in b_sst_array[] or NULL
-   * b_sst_firstfree	pointer to first free entry in b_sst_array[] or NULL
-   * b_sst_freecount	number of free entries in b_sst_array[]
-   * b_sst_check_lnum	entries after this lnum need to be checked for
-   *			validity (MAXLNUM means no check needed)
+   * b_sst_array        pointer to an array of synstate_T
+   * b_sst_len          number of entries in b_sst_array[]
+   * b_sst_first        pointer to first used entry in b_sst_array[] or NULL
+   * b_sst_firstfree    pointer to first free entry in b_sst_array[] or NULL
+   * b_sst_freecount    number of free entries in b_sst_array[]
+   * b_sst_check_lnum   entries after this lnum need to be checked for
+   *                    validity (MAXLNUM means no check needed)
    */
   synstate_T  *b_sst_array;
   int b_sst_len;
@@ -475,15 +475,15 @@ struct file_buffer {
   int b_locked;                 // Buffer is being closed or referenced, don't
                                 // let autocommands wipe it out.
 
-  /*
-   * b_ffname has the full path of the file (NULL for no name).
-   * b_sfname is the name as the user typed it (or NULL).
-   * b_fname is the same as b_sfname, unless ":cd" has been done,
-   *		then it is the same as b_ffname (NULL for no name).
-   */
-  char_u      *b_ffname;        /* full path file name */
-  char_u      *b_sfname;        /* short file name */
-  char_u      *b_fname;         /* current file name */
+  //
+  // b_ffname   has the full path of the file (NULL for no name).
+  // b_sfname   is the name as the user typed it (or NULL).
+  // b_fname    is the same as b_sfname, unless ":cd" has been done,
+  //            then it is the same as b_ffname (NULL for no name).
+  //
+  char_u      *b_ffname;        // full path file name
+  char_u      *b_sfname;        // short file name
+  char_u      *b_fname;         // current file name
 
   bool file_id_valid;
   FileID file_id;
@@ -587,12 +587,12 @@ struct file_buffer {
 
   bool b_scanned;               /* ^N/^P have scanned this buffer */
 
-  /* flags for use of ":lmap" and IM control */
-  long b_p_iminsert;            /* input mode for insert */
-  long b_p_imsearch;            /* input mode for search */
-#define B_IMODE_USE_INSERT -1   /*	Use b_p_iminsert value for search */
-#define B_IMODE_NONE 0          /*	Input via none */
-#define B_IMODE_LMAP 1          /*	Input via langmap */
+  // flags for use of ":lmap" and IM control
+  long b_p_iminsert;            // input mode for insert
+  long b_p_imsearch;            // input mode for search
+#define B_IMODE_USE_INSERT -1   //  Use b_p_iminsert value for search
+#define B_IMODE_NONE 0          //  Input via none
+#define B_IMODE_LMAP 1          //  Input via langmap
 # define B_IMODE_LAST 1
 
   short b_kmap_state;           /* using "lmap" mappings */
@@ -760,7 +760,7 @@ struct file_buffer {
   /* Two special kinds of buffers:
    * help buffer  - used for help files, won't use a swap file.
    * spell buffer - used for spell info, never displayed and doesn't have a
-   *		      file name.
+   *                file name.
    */
   bool b_help;                  /* TRUE for help file buffer (when set b_p_bt
                                    is "help") */
@@ -787,6 +787,8 @@ struct file_buffer {
   // array of channelids which have asked to receive updates for this
   // buffer.
   kvec_t(uint64_t) update_channels;
+
+  int b_diff_failed;    // internal diff failed for this buffer
 };
 
 /*
@@ -840,6 +842,7 @@ struct tabpage_S {
   diff_T          *tp_first_diff;
   buf_T           *(tp_diffbuf[DB_COUNT]);
   int tp_diff_invalid;              ///< list of diffs is outdated
+  int tp_diff_update;               ///< update diffs before redrawing
   frame_T         *(tp_snapshot[SNAP_COUNT]);    ///< window layout snapshots
   ScopeDictDictItem tp_winvar;      ///< Variable for "t:" Dictionary.
   dict_T          *tp_vars;         ///< Internal variables, local to tab page.
@@ -916,9 +919,9 @@ typedef struct {
 /// Same as lpos_T, but with additional field len.
 typedef struct
 {
-    linenr_T    lnum;	///< line number
-    colnr_T     col;	///< column number
-    int         len;	///< length: 0 - to the end of line
+    linenr_T    lnum;   ///< line number
+    colnr_T     col;    ///< column number
+    int         len;    ///< length: 0 - to the end of line
 } llpos_T;
 
 /// posmatch_T provides an array for storing match items for matchaddpos()
@@ -926,10 +929,10 @@ typedef struct
 typedef struct posmatch posmatch_T;
 struct posmatch
 {
-    llpos_T     pos[MAXPOSMATCH];	///< array of positions
-    int         cur;			///< internal position counter 
-    linenr_T    toplnum;		///< top buffer line
-    linenr_T    botlnum;		///< bottom buffer line
+    llpos_T     pos[MAXPOSMATCH];   ///< array of positions
+    int         cur;                ///< internal position counter
+    linenr_T    toplnum;            ///< top buffer line
+    linenr_T    botlnum;            ///< bottom buffer line
 };
 
 /*
@@ -1036,7 +1039,7 @@ struct window_S {
    * Recomputing is minimized by storing the result of computations.
    * Use functions in screen.c to check if they are valid and to update.
    * w_valid is a bitfield of flags, which indicate if specific values are
-   * valid or need to be recomputed.	
+   * valid or need to be recomputed.
    */
   int w_valid;
   pos_T w_valid_cursor;             /* last known position of w_cursor, used
diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index 4bfe67c9d3..2e0b198c13 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -4,11 +4,17 @@
 /// @file diff.c
 ///
 /// Code for diff'ing two, three or four buffers.
+///
+/// There are three ways to diff:
+/// - Shell out to an external diff program, using files.
+/// - Use the compiled-in xdiff library.
+/// - Let 'diffexpr' do the work, using files.
 
 #include <inttypes.h>
 #include <stdbool.h>
 
 #include "nvim/vim.h"
+#include "xdiff/xdiff.h"
 #include "nvim/ascii.h"
 #include "nvim/diff.h"
 #include "nvim/buffer.h"
@@ -36,16 +42,24 @@
 #include "nvim/os/os.h"
 #include "nvim/os/shell.h"
 
-static int diff_busy = FALSE;    // ex_diffgetput() is busy
+static int diff_busy = false;         // using diff structs, don't change them
+static int diff_need_update = false;  // ex_diffupdate needs to be called
 
 // Flags obtained from the 'diffopt' option
-#define DIFF_FILLER     1        // display filler lines
-#define DIFF_ICASE      2        // ignore case
-#define DIFF_IWHITE     4        // ignore change in white space
-#define DIFF_HORIZONTAL 8        // horizontal splits
-#define DIFF_VERTICAL   16       // vertical splits
-#define DIFF_HIDDEN_OFF 32       // diffoff when hidden
-static int diff_flags = DIFF_FILLER;
+#define DIFF_FILLER     0x001   // display filler lines
+#define DIFF_IBLANK     0x002   // ignore empty lines
+#define DIFF_ICASE      0x004   // ignore case
+#define DIFF_IWHITE     0x008   // ignore change in white space
+#define DIFF_IWHITEALL  0x010   // ignore all white space changes
+#define DIFF_IWHITEEOL  0x020   // ignore change in white space at EOL
+#define DIFF_HORIZONTAL 0x040   // horizontal splits
+#define DIFF_VERTICAL   0x080   // vertical splits
+#define DIFF_HIDDEN_OFF 0x100   // diffoff when hidden
+#define DIFF_INTERNAL   0x200   // use internal xdiff algorithm
+#define ALL_WHITE_DIFF (DIFF_IWHITE | DIFF_IWHITEALL | DIFF_IWHITEEOL)
+static int diff_flags = DIFF_INTERNAL | DIFF_FILLER;
+
+static long diff_algorithm = 0;
 
 #define LBUFLEN 50               // length of line in diff file
 
@@ -53,6 +67,25 @@ static int diff_flags = DIFF_FILLER;
 // kNone when not checked yet
 static TriState diff_a_works = kNone;
 
+// used for diff input
+typedef struct {
+    char_u   *din_fname;   // used for external diff
+    mmfile_t  din_mmfile;  // used for internal diff
+} diffin_T;
+
+// used for diff result
+typedef struct {
+    char_u   *dout_fname;  // used for external diff
+    garray_T  dout_ga;     // used for internal diff
+} diffout_T;
+
+// two diff inputs and one result
+typedef struct {
+    diffin_T    dio_orig;      // original file input
+    diffin_T    dio_new;       // new file input
+    diffout_T   dio_diff;      // diff result
+    int         dio_internal;  // using internal diff
+} diffio_T;
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "diff.c.generated.h"
@@ -68,10 +101,10 @@ void diff_buf_delete(buf_T *buf)
 
     if (i != DB_COUNT) {
       tp->tp_diffbuf[i] = NULL;
-      tp->tp_diff_invalid = TRUE;
+      tp->tp_diff_invalid = true;
 
       if (tp == curtab) {
-        diff_redraw(TRUE);
+        diff_redraw(true);
       }
     }
   }
@@ -98,8 +131,8 @@ void diff_buf_adjust(win_T *win)
       int i = diff_buf_idx(win->w_buffer);
       if (i != DB_COUNT) {
         curtab->tp_diffbuf[i] = NULL;
-        curtab->tp_diff_invalid = TRUE;
-        diff_redraw(TRUE);
+        curtab->tp_diff_invalid = true;
+        diff_redraw(true);
       }
     }
   } else {
@@ -127,8 +160,8 @@ void diff_buf_add(buf_T *buf)
   for (i = 0; i < DB_COUNT; ++i) {
     if (curtab->tp_diffbuf[i] == NULL) {
       curtab->tp_diffbuf[i] = buf;
-      curtab->tp_diff_invalid = TRUE;
-      diff_redraw(TRUE);
+      curtab->tp_diff_invalid = true;
+      diff_redraw(true);
       return;
     }
   }
@@ -192,9 +225,9 @@ void diff_invalidate(buf_T *buf)
   FOR_ALL_TABS(tp) {
     int i = diff_buf_idx_tp(buf, tp);
     if (i != DB_COUNT) {
-      tp->tp_diff_invalid = TRUE;
+      tp->tp_diff_invalid = true;
       if (tp == curtab) {
-        diff_redraw(TRUE);
+        diff_redraw(true);
       }
     }
   }
@@ -234,6 +267,15 @@ void diff_mark_adjust(linenr_T line1, linenr_T line2, long amount,
 static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
                                 linenr_T line2, long amount, long amount_after)
 {
+  if (diff_internal()) {
+    // Will update diffs before redrawing.  Set _invalid to update the
+    // diffs themselves, set _update to also update folds properly just
+    // before redrawing.
+    // Do update marks here, it is needed for :%diffput.
+    tp->tp_diff_invalid = true;
+    tp->tp_diff_update = true;
+  }
+
   int inserted;
   int deleted;
   if (line2 == MAXLNUM) {
@@ -293,14 +335,14 @@ static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
 
     //
     // Check for these situations:
-    //	  1  2	3
-    //	  1  2	3
-    // line1     2	3  4  5
-    //	     2	3  4  5
-    //	     2	3  4  5
-    // line2     2	3  4  5
-    //		3     5  6
-    //		3     5  6
+    //    1  2  3
+    //    1  2  3
+    // line1     2  3  4  5
+    //       2  3  4  5
+    //       2  3  4  5
+    // line2     2  3  4  5
+    //      3     5  6
+    //      3     5  6
 
     // compute last line of this change
     last = dp->df_lnum[idx] + dp->df_count[idx] - 1;
@@ -316,7 +358,7 @@ static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
         }
         dp->df_lnum[idx] += amount_after;
       } else {
-        int check_unchanged = FALSE;
+        int check_unchanged = false;
 
         // 2. 3. 4. 5.: inserted/deleted lines touching this diff.
         if (deleted > 0) {
@@ -341,7 +383,7 @@ static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
               // 5. delete lines at or just before top of diff
               n = off;
               dp->df_count[idx] -= line2 - dp->df_lnum[idx] + 1;
-              check_unchanged = TRUE;
+              check_unchanged = true;
             }
             dp->df_lnum[idx] = line1;
           } else {
@@ -361,7 +403,7 @@ static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
               } else {
                 n = line2 - last;
               }
-              check_unchanged = TRUE;
+              check_unchanged = true;
             } else {
               // 3. delete lines inside the diff
               n = 0;
@@ -380,7 +422,7 @@ static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
           if (dp->df_lnum[idx] <= line1) {
             // inserted lines somewhere in this diff
             dp->df_count[idx] += inserted;
-            check_unchanged = TRUE;
+            check_unchanged = true;
           } else {
             // inserted lines somewhere above this diff
             dp->df_lnum[idx] += inserted;
@@ -447,12 +489,12 @@ static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
   }
 
   if (tp == curtab) {
-    diff_redraw(TRUE);
+    diff_redraw(true);
 
     // Need to recompute the scroll binding, may remove or add filler
     // lines (e.g., when adding lines above w_topline). But it's slow when
     // making many changes, postpone until redrawing.
-    diff_need_scrollbind = TRUE;
+    diff_need_scrollbind = true;
   }
 }
 
@@ -519,7 +561,7 @@ static void diff_check_unchanged(tabpage_T *tp, diff_T *dp)
       }
       char_u *line_org = vim_strsave(ml_get_buf(tp->tp_diffbuf[i_org],
                                                 dp->df_lnum[i_org] + off_org,
-                                                FALSE));
+                                                false));
 
       int i_new;
       for (i_new = i_org + 1; i_new < DB_COUNT; ++i_new) {
@@ -538,7 +580,7 @@ static void diff_check_unchanged(tabpage_T *tp, diff_T *dp)
 
         if (diff_cmp(line_org, ml_get_buf(tp->tp_diffbuf[i_new],
                                           dp->df_lnum[i_new] + off_new,
-                                          FALSE)) != 0) {
+                                          false)) != 0) {
           break;
         }
       }
@@ -612,42 +654,243 @@ static void diff_redraw(int dofold)
       } else if ((n > 0) && (n > wp->w_topfill)) {
         wp->w_topfill = n;
       }
-      check_topfill(wp, FALSE);
+      check_topfill(wp, false);
+    }
+  }
+}
+
+static void clear_diffin(diffin_T *din)
+{
+  if (din->din_fname == NULL) {
+    xfree(din->din_mmfile.ptr);
+    din->din_mmfile.ptr = NULL;
+  } else {
+    os_remove((char *)din->din_fname);
+  }
+}
+
+static void clear_diffout(diffout_T *dout)
+{
+  if (dout->dout_fname == NULL) {
+    ga_clear_strings(&dout->dout_ga);
+  } else {
+    os_remove((char *)dout->dout_fname);
+  }
+}
+
+/// Write buffer "buf" to a memory buffer.
+///
+/// @param buf
+/// @param din
+///
+/// @return FAIL for failure.
+static int diff_write_buffer(buf_T *buf, diffin_T *din)
+{
+  linenr_T   lnum;
+  char_u    *s;
+  long       len = 0;
+  char_u    *ptr;
+
+  // xdiff requires one big block of memory with all the text.
+  for (lnum = 1; lnum <= buf->b_ml.ml_line_count; lnum++) {
+    len += (long)STRLEN(ml_get_buf(buf, lnum, false)) + 1;
+  }
+  ptr = xmalloc(len);
+  if (ptr == NULL) {
+    // Allocating memory failed.  This can happen, because we try to read
+    // the whole buffer text into memory.  Set the failed flag, the diff
+    // will be retried with external diff.  The flag is never reset.
+    buf->b_diff_failed = true;
+    if (p_verbose > 0) {
+      verbose_enter();
+      smsg(_("Not enough memory to use internal diff for buffer \"%s\""),
+           buf->b_fname);
+      verbose_leave();
+    }
+    return FAIL;
+  }
+  din->din_mmfile.ptr = (char *)ptr;
+  din->din_mmfile.size = len;
+
+  len = 0;
+  for (lnum = 1; lnum <= buf->b_ml.ml_line_count; lnum++) {
+    for (s = ml_get_buf(buf, lnum, false); *s != NUL; ) {
+      if (diff_flags & DIFF_ICASE) {
+        int c;
+
+        // xdiff doesn't support ignoring case, fold-case the text.
+        int     orig_len;
+        char_u  cbuf[MB_MAXBYTES + 1];
+
+        c = PTR2CHAR(s);
+        c = enc_utf8 ? utf_fold(c) : TOLOWER_LOC(c);
+        orig_len = MB_PTR2LEN(s);
+        if (utf_char2bytes(c, cbuf) != orig_len) {
+          // TODO(Bram): handle byte length difference
+          memmove(ptr + len, s, orig_len);
+        } else {
+          memmove(ptr + len, cbuf, orig_len);
+        }
+
+        s += orig_len;
+        len += orig_len;
+      } else {
+        ptr[len++] = *s++;
+      }
     }
+    ptr[len++] = NL;
   }
+  return OK;
 }
 
-/// Write buffer "buf" to file "name".
+/// Write buffer "buf" to file or memory buffer.
 ///
 /// Always use 'fileformat' set to "unix".
 ///
 /// @param buf
-/// @param fname
+/// @param din
 ///
 /// @return FAIL for failure
-static int diff_write(buf_T *buf, char_u *fname)
+static int diff_write(buf_T *buf, diffin_T *din)
 {
+  if (din->din_fname == NULL) {
+    return diff_write_buffer(buf, din);
+  }
+
+  // Always use 'fileformat' set to "unix".
   char_u *save_ff = buf->b_p_ff;
   buf->b_p_ff = vim_strsave((char_u *)FF_UNIX);
-  int r = buf_write(buf, fname, NULL, (linenr_T)1, buf->b_ml.ml_line_count,
-                    NULL, FALSE, FALSE, FALSE, TRUE);
+  int r = buf_write(buf, din->din_fname, NULL,
+                    (linenr_T)1, buf->b_ml.ml_line_count,
+                    NULL, false, false, false, true);
   free_string_option(buf->b_p_ff);
   buf->b_p_ff = save_ff;
   return r;
 }
 
+///
+/// Update the diffs for all buffers involved.
+///
+/// @param dio
+/// @param idx_orig
+/// @param eap   can be NULL
+static void diff_try_update(diffio_T    *dio,
+                            int     idx_orig,
+                            exarg_T     *eap)
+{
+  buf_T *buf;
+  int    idx_new;
+
+  if (dio->dio_internal) {
+    ga_init(&dio->dio_diff.dout_ga, sizeof(char *), 1000);
+  } else {
+    // We need three temp file names.
+    dio->dio_orig.din_fname = vim_tempname();
+    dio->dio_new.din_fname = vim_tempname();
+    dio->dio_diff.dout_fname = vim_tempname();
+    if (dio->dio_orig.din_fname == NULL
+        || dio->dio_new.din_fname == NULL
+        || dio->dio_diff.dout_fname == NULL) {
+      goto theend;
+    }
+  }
+
+  // Check external diff is actually working.
+  if (!dio->dio_internal && check_external_diff(dio) == FAIL) {
+    goto theend;
+  }
+
+  // :diffupdate!
+  if (eap != NULL && eap->forceit) {
+    for (idx_new = idx_orig; idx_new < DB_COUNT; idx_new++) {
+      buf = curtab->tp_diffbuf[idx_new];
+      if (buf_valid(buf)) {
+        buf_check_timestamp(buf, false);
+      }
+    }
+  }
+
+  // Write the first buffer to a tempfile or mmfile_t.
+  buf = curtab->tp_diffbuf[idx_orig];
+  if (diff_write(buf, &dio->dio_orig) == FAIL) {
+    goto theend;
+  }
+
+  // Make a difference between the first buffer and every other.
+  for (idx_new = idx_orig + 1; idx_new < DB_COUNT; idx_new++) {
+    buf = curtab->tp_diffbuf[idx_new];
+    if (buf == NULL || buf->b_ml.ml_mfp == NULL) {
+      continue;  // skip buffer that isn't loaded
+    }
+
+    // Write the other buffer and diff with the first one.
+    if (diff_write(buf, &dio->dio_new) == FAIL) {
+      continue;
+    }
+    if (diff_file(dio) == FAIL) {
+      continue;
+    }
+
+    // Read the diff output and add each entry to the diff list.
+    diff_read(idx_orig, idx_new, &dio->dio_diff);
+
+    clear_diffin(&dio->dio_new);
+    clear_diffout(&dio->dio_diff);
+  }
+  clear_diffin(&dio->dio_orig);
+
+theend:
+  xfree(dio->dio_orig.din_fname);
+  xfree(dio->dio_new.din_fname);
+  xfree(dio->dio_diff.dout_fname);
+}
+
+///
+/// Return true if the options are set to use the internal diff library.
+/// Note that if the internal diff failed for one of the buffers, the external
+/// diff will be used anyway.
+///
+int diff_internal(void)
+{
+  return (diff_flags & DIFF_INTERNAL) != 0 && *p_dex == NUL;
+}
+
+///
+/// Return true if the internal diff failed for one of the diff buffers.
+///
+static int diff_internal_failed(void)
+{
+  int idx;
+
+  // Only need to do something when there is another buffer.
+  for (idx = 0; idx < DB_COUNT; idx++) {
+    if (curtab->tp_diffbuf[idx] != NULL
+        && curtab->tp_diffbuf[idx]->b_diff_failed) {
+      return true;
+    }
+  }
+  return false;
+}
+
 /// Completely update the diffs for the buffers involved.
 ///
-/// This uses the ordinary "diff" command.
-/// The buffers are written to a file, also for unmodified buffers (the file
-/// could have been produced by autocommands, e.g. the netrw plugin).
+/// When using the external "diff" command the buffers are written to a file,
+/// also for unmodified buffers (the file could have been produced by
+/// autocommands, e.g. the netrw plugin).
 ///
 /// @param eap can be NULL
 void ex_diffupdate(exarg_T *eap)
 {
+  if (diff_busy) {
+    diff_need_update = true;
+    return;
+  }
+
+  int had_diffs = curtab->tp_first_diff != NULL;
+
   // Delete all diffblocks.
   diff_clear(curtab);
-  curtab->tp_diff_invalid = FALSE;
+  curtab->tp_diff_invalid = false;
 
   // Use the first buffer as the original text.
   int idx_orig;
@@ -658,7 +901,7 @@ void ex_diffupdate(exarg_T *eap)
   }
 
   if (idx_orig == DB_COUNT) {
-    return;
+    goto theend;
   }
 
   // Only need to do something when there is another buffer.
@@ -670,49 +913,70 @@ void ex_diffupdate(exarg_T *eap)
   }
 
   if (idx_new == DB_COUNT) {
-    return;
+    goto theend;
   }
 
-  // We need three temp file names.
-  char *tmp_orig = (char *) vim_tempname();
-  char *tmp_new = (char *) vim_tempname();
-  char *tmp_diff = (char *) vim_tempname();
+  // Only use the internal method if it did not fail for one of the buffers.
+  diffio_T  diffio;
+  memset(&diffio, 0, sizeof(diffio));
+  diffio.dio_internal = diff_internal() && !diff_internal_failed();
 
-  if ((tmp_orig == NULL) || (tmp_new == NULL) || (tmp_diff == NULL)) {
-    goto theend;
+  diff_try_update(&diffio, idx_orig, eap);
+  if (diffio.dio_internal && diff_internal_failed()) {
+    // Internal diff failed, use external diff instead.
+    memset(&diffio, 0, sizeof(diffio));
+    diff_try_update(&diffio, idx_orig, eap);
+  }
+
+  // force updating cursor position on screen
+  curwin->w_valid_cursor.lnum = 0;
+
+theend:
+  // A redraw is needed if there were diffs and they were cleared, or there
+  // are diffs now, which means they got updated.
+  if (had_diffs || curtab->tp_first_diff != NULL) {
+    diff_redraw(true);
+    apply_autocmds(EVENT_DIFFUPDATED, NULL, NULL, false, curbuf);
   }
+}
 
-  // Do a quick test if "diff" really works.  Otherwise it looks like there
-  // are no differences.  Can't use the return value, it's non-zero when
-  // there are differences.
+///
+/// Do a quick test if "diff" really works.  Otherwise it looks like there
+/// are no differences.  Can't use the return value, it's non-zero when
+/// there are differences.
+///
+static int check_external_diff(diffio_T *diffio)
+{
   // May try twice, first with "-a" and then without.
   int io_error = false;
   TriState ok = kFalse;
   for (;;) {
     ok = kFalse;
-    FILE *fd = mch_fopen(tmp_orig, "w");
+    FILE *fd = mch_fopen((char *)diffio->dio_orig.din_fname, "w");
 
     if (fd == NULL) {
-      io_error = TRUE;
+      io_error = true;
     } else {
       if (fwrite("line1\n", (size_t)6, (size_t)1, fd) != 1) {
-        io_error = TRUE;
+        io_error = true;
       }
       fclose(fd);
-      fd = mch_fopen(tmp_new, "w");
+      fd = mch_fopen((char *)diffio->dio_new.din_fname, "w");
 
       if (fd == NULL) {
-        io_error = TRUE;
+        io_error = true;
       } else {
         if (fwrite("line2\n", (size_t)6, (size_t)1, fd) != 1) {
-          io_error = TRUE;
+          io_error = true;
         }
         fclose(fd);
-        diff_file(tmp_orig, tmp_new, tmp_diff);
-        fd = mch_fopen(tmp_diff, "r");
+        fd = NULL;
+        if (diff_file(diffio) == OK) {
+          fd = mch_fopen((char *)diffio->dio_diff.dout_fname, "r");
+        }
 
         if (fd == NULL) {
-          io_error = TRUE;
+          io_error = true;
         } else {
           char_u linebuf[LBUFLEN];
 
@@ -728,10 +992,10 @@ void ex_diffupdate(exarg_T *eap)
           }
           fclose(fd);
         }
-        os_remove(tmp_diff);
-        os_remove(tmp_new);
+        os_remove((char *)diffio->dio_diff.dout_fname);
+        os_remove((char *)diffio->dio_new.din_fname);
       }
-      os_remove(tmp_orig);
+      os_remove((char *)diffio->dio_orig.din_fname);
     }
 
     // When using 'diffexpr' break here.
@@ -757,83 +1021,92 @@ void ex_diffupdate(exarg_T *eap)
     }
     EMSG(_("E97: Cannot create diffs"));
     diff_a_works = kNone;
-    goto theend;
-  }
-
-  // :diffupdate!
-  if ((eap != NULL) && eap->forceit) {
-    for (idx_new = idx_orig; idx_new < DB_COUNT; ++idx_new) {
-      buf_T *buf = curtab->tp_diffbuf[idx_new];
-      if (buf_valid(buf)) {
-        buf_check_timestamp(buf, FALSE);
-      }
-    }
+    return FAIL;
   }
+  return OK;
+}
 
-  // Write the first buffer to a tempfile.
-  buf_T *buf = curtab->tp_diffbuf[idx_orig];
-  if (diff_write(buf, (char_u *) tmp_orig) == FAIL) {
-    goto theend;
-  }
+///
+/// Invoke the xdiff function.
+///
+static int diff_file_internal(diffio_T *diffio)
+{
+  xpparam_t     param;
+  xdemitconf_t    emit_cfg;
+  xdemitcb_t        emit_cb;
 
-  // Make a difference between the first buffer and every other.
-  for (idx_new = idx_orig + 1; idx_new < DB_COUNT; ++idx_new) {
-    buf_T *buf = curtab->tp_diffbuf[idx_new];
-    if (buf == NULL || buf->b_ml.ml_mfp == NULL) {
-      continue;  // skip buffer that isn't loaded
-    }
+  memset(&param, 0, sizeof(param));
+  memset(&emit_cfg, 0, sizeof(emit_cfg));
+  memset(&emit_cb, 0, sizeof(emit_cb));
 
-    if (diff_write(buf, (char_u *) tmp_new) == FAIL) {
-      continue;
-    }
-    diff_file(tmp_orig, tmp_new, tmp_diff);
+  param.flags = diff_algorithm;
 
-    // Read the diff output and add each entry to the diff list.
-    diff_read(idx_orig, idx_new, (char_u *) tmp_diff);
-    os_remove(tmp_diff);
-    os_remove(tmp_new);
+  if (diff_flags & DIFF_IWHITE) {
+    param.flags |= XDF_IGNORE_WHITESPACE_CHANGE;
+  }
+  if (diff_flags & DIFF_IWHITEALL) {
+    param.flags |= XDF_IGNORE_WHITESPACE;
+  }
+  if (diff_flags & DIFF_IWHITEEOL) {
+    param.flags |= XDF_IGNORE_WHITESPACE_AT_EOL;
+  }
+  if (diff_flags & DIFF_IBLANK) {
+    param.flags |= XDF_IGNORE_BLANK_LINES;
   }
-  os_remove(tmp_orig);
-
-  // force updating cursor position on screen
-  curwin->w_valid_cursor.lnum = 0;
-
-  diff_redraw(TRUE);
 
-theend:
-  xfree(tmp_orig);
-  xfree(tmp_new);
-  xfree(tmp_diff);
+  emit_cfg.ctxlen = 0;  // don't need any diff_context here
+  emit_cb.priv = &diffio->dio_diff;
+  emit_cb.outf = xdiff_out;
+  if (xdl_diff(&diffio->dio_orig.din_mmfile,
+               &diffio->dio_new.din_mmfile,
+               &param, &emit_cfg, &emit_cb) < 0) {
+    EMSG(_("E960: Problem creating the internal diff"));
+    return FAIL;
+  }
+  return OK;
 }
 
 /// Make a diff between files "tmp_orig" and "tmp_new", results in "tmp_diff".
 ///
-/// @param tmp_orig
-/// @param tmp_new
-/// @param tmp_diff
-static void diff_file(const char *const tmp_orig, const char *const tmp_new,
-                      const char *const tmp_diff)
+/// @param dio
+///
+/// @return OK or FAIL
+static int diff_file(diffio_T *dio)
 {
+  char  *tmp_orig = (char *)dio->dio_orig.din_fname;
+  char  *tmp_new = (char *)dio->dio_new.din_fname;
+  char  *tmp_diff = (char *)dio->dio_diff.dout_fname;
   if (*p_dex != NUL) {
     // Use 'diffexpr' to generate the diff file.
     eval_diff(tmp_orig, tmp_new, tmp_diff);
+    return OK;
+  }
+  // Use xdiff for generating the diff.
+  if (dio->dio_internal) {
+    return diff_file_internal(dio);
   } else {
     const size_t len = (strlen(tmp_orig) + strlen(tmp_new) + strlen(tmp_diff)
                         + STRLEN(p_srr) + 27);
     char *const cmd = xmalloc(len);
+    if (cmd == NULL) {
+      return FAIL;
+    }
 
-    /* We don't want $DIFF_OPTIONS to get in the way. */
+    // We don't want $DIFF_OPTIONS to get in the way.
     if (os_getenv("DIFF_OPTIONS")) {
       os_unsetenv("DIFF_OPTIONS");
     }
 
-    /* Build the diff command and execute it.  Always use -a, binary
-     * differences are of no use.  Ignore errors, diff returns
-     * non-zero when differences have been found. */
-    vim_snprintf(cmd, len, "diff %s%s%s%s%s %s",
+    // Build the diff command and execute it.  Always use -a, binary
+    // differences are of no use.  Ignore errors, diff returns
+    // non-zero when differences have been found.
+    vim_snprintf((char *)cmd, len, "diff %s%s%s%s%s%s%s%s %s",
                  diff_a_works == kFalse ? "" : "-a ",
                  "",
                  (diff_flags & DIFF_IWHITE) ? "-b " : "",
+                 (diff_flags & DIFF_IWHITEALL) ? "-w " : "",
+                 (diff_flags & DIFF_IWHITEEOL) ? "-Z " : "",
+                 (diff_flags & DIFF_IBLANK) ? "-B " : "",
                  (diff_flags & DIFF_ICASE) ? "-i " : "",
                  tmp_orig, tmp_new);
     append_redir(cmd, len, (char *) p_srr, tmp_diff);
@@ -843,6 +1116,7 @@ static void diff_file(const char *const tmp_orig, const char *const tmp_new,
                      NULL);
     unblock_autocmds();
     xfree(cmd);
+    return OK;
   }
 }
 
@@ -876,7 +1150,7 @@ void ex_diffpatch(exarg_T *eap)
   // Write the current buffer to "tmp_orig".
   if (buf_write(curbuf, tmp_orig, NULL,
                 (linenr_T)1, curbuf->b_ml.ml_line_count,
-                NULL, FALSE, FALSE, FALSE, TRUE) == FAIL) {
+                NULL, false, false, false, true) == FAIL) {
     goto theend;
   }
 
@@ -907,7 +1181,7 @@ void ex_diffpatch(exarg_T *eap)
       tempdir = "/tmp";
     }
     os_chdir(tempdir);
-    shorten_fnames(TRUE);
+    shorten_fnames(true);
   }
 #endif
 
@@ -934,7 +1208,7 @@ void ex_diffpatch(exarg_T *eap)
     if (os_chdir((char *)dirbuf) != 0) {
       EMSG(_(e_prev_dir));
     }
-    shorten_fnames(TRUE);
+    shorten_fnames(true);
   }
 #endif
 
@@ -971,8 +1245,8 @@ void ex_diffpatch(exarg_T *eap)
       // check that split worked and editing tmp_new
       if ((curwin != old_curwin) && win_valid(old_curwin)) {
         // Set 'diff', 'scrollbind' on and 'wrap' off.
-        diff_win_options(curwin, TRUE);
-        diff_win_options(old_curwin, TRUE);
+        diff_win_options(curwin, true);
+        diff_win_options(old_curwin, true);
 
         if (newname != NULL) {
           // do a ":file filename.new" on the patched buffer
@@ -1025,7 +1299,7 @@ void ex_diffsplit(exarg_T *eap)
   if (win_split(0, (diff_flags & DIFF_VERTICAL) ? WSP_VERT : 0) != FAIL) {
     // Pretend it was a ":split fname" command
     eap->cmdidx = CMD_split;
-    curwin->w_p_diff = TRUE;
+    curwin->w_p_diff = true;
     do_exedit(eap, old_curwin);
 
     // split must have worked
@@ -1052,7 +1326,7 @@ void ex_diffsplit(exarg_T *eap)
 void ex_diffthis(exarg_T *eap)
 {
   // Set 'diff', 'scrollbind' on and 'wrap' off.
-  diff_win_options(curwin, TRUE);
+  diff_win_options(curwin, true);
 }
 
 static void set_diff_option(win_T *wp, int value)
@@ -1085,12 +1359,12 @@ void diff_win_options(win_T *wp, int addbuf)
   if (!wp->w_p_diff) {
     wp->w_p_scb_save = wp->w_p_scb;
   }
-  wp->w_p_scb = TRUE;
+  wp->w_p_scb = true;
 
   if (!wp->w_p_diff) {
     wp->w_p_crb_save = wp->w_p_crb;
   }
-  wp->w_p_crb = TRUE;
+  wp->w_p_crb = true;
 
   if (!wp->w_p_diff) {
     wp->w_p_wrap_save = wp->w_p_wrap;
@@ -1116,7 +1390,7 @@ void diff_win_options(win_T *wp, int addbuf)
     wp->w_p_fdl_save = wp->w_p_fdl;
   }
   wp->w_p_fdc = diff_foldcolumn;
-  wp->w_p_fen = TRUE;
+  wp->w_p_fen = true;
   wp->w_p_fdl = 0;
   foldUpdateAll(wp);
 
@@ -1211,88 +1485,97 @@ void ex_diffoff(exarg_T *eap)
 ///
 /// @param idx_orig idx of original file
 /// @param idx_new idx of new file
-/// @param fname name of diff output file
-static void diff_read(int idx_orig, int idx_new, char_u *fname)
+/// @dout diff output
+static void diff_read(int idx_orig, int idx_new, diffout_T *dout)
 {
-  FILE *fd;
+  FILE *fd = NULL;
+  int line_idx = 0;
   diff_T *dprev = NULL;
   diff_T *dp = curtab->tp_first_diff;
   diff_T *dn, *dpl;
-  long f1, l1, f2, l2;
-  char_u linebuf[LBUFLEN]; // only need to hold the diff line
-  int difftype;
-  char_u *p;
+  char_u linebuf[LBUFLEN];  // only need to hold the diff line
+  char_u *line;
   long off;
   int i;
   linenr_T lnum_orig, lnum_new;
   long count_orig, count_new;
-  int notset = TRUE; // block "*dp" not set yet
-
-  fd = mch_fopen((char *)fname, "r");
-
-  if (fd == NULL) {
-    EMSG(_("E98: Cannot read diff output"));
-    return;
+  int notset = true;  // block "*dp" not set yet
+  enum {
+    DIFF_ED,
+    DIFF_UNIFIED,
+    DIFF_NONE
+  } diffstyle = DIFF_NONE;
+
+  if (dout->dout_fname == NULL) {
+    diffstyle = DIFF_UNIFIED;
+  } else {
+    fd = mch_fopen((char *)dout->dout_fname, "r");
+    if (fd == NULL) {
+      EMSG(_("E98: Cannot read diff output"));
+      return;
+    }
   }
 
   for (;;) {
-    if (vim_fgets(linebuf, LBUFLEN, fd)) {
-      // end of file
-      break;
-    }
-
-    if (!isdigit(*linebuf)) {
-      // not the start of a diff block
-      continue;
-    }
-
-    // This line must be one of three formats:
-    // {first}[,{last}]c{first}[,{last}]
-    // {first}a{first}[,{last}]
-    // {first}[,{last}]d{first}
-    p = linebuf;
-    f1 = getdigits_long(&p);
-
-    if (*p == ',') {
-      ++p;
-      l1 = getdigits_long(&p);
-    } else {
-      l1 = f1;
-    }
-
-    if ((*p != 'a') && (*p != 'c') && (*p != 'd')) {
-      // invalid diff format
-      continue;
-    }
-    difftype = *p++;
-    f2 = getdigits_long(&p);
-
-    if (*p == ',') {
-      ++p;
-      l2 = getdigits_long(&p);
-    } else {
-      l2 = f2;
-    }
-
-    if ((l1 < f1) || (l2 < f2)) {
-      // invalid line range
-      continue;
-    }
-
-    if (difftype == 'a') {
-      lnum_orig = f1 + 1;
-      count_orig = 0;
+    if (fd == NULL) {
+      if (line_idx >= dout->dout_ga.ga_len) {
+        break;      // did last line
+      }
+      line = ((char_u **)dout->dout_ga.ga_data)[line_idx++];
     } else {
-      lnum_orig = f1;
-      count_orig = l1 - f1 + 1;
+      if (vim_fgets(linebuf, LBUFLEN, fd)) {
+        break;      // end of file
+      }
+      line = linebuf;
+    }
+
+    if (diffstyle == DIFF_NONE) {
+      // Determine diff style.
+      // ed like diff looks like this:
+      // {first}[,{last}]c{first}[,{last}]
+      // {first}a{first}[,{last}]
+      // {first}[,{last}]d{first}
+      //
+      // unified diff looks like this:
+      // --- file1       2018-03-20 13:23:35.783153140 +0100
+      // +++ file2       2018-03-20 13:23:41.183156066 +0100
+      // @@ -1,3 +1,5 @@
+      if (isdigit(*line)) {
+        diffstyle = DIFF_ED;
+      } else if ((STRNCMP(line, "@@ ", 3) == 0)) {
+        diffstyle = DIFF_UNIFIED;
+      } else if ((STRNCMP(line, "--- ", 4) == 0)
+                 && (vim_fgets(linebuf, LBUFLEN, fd) == 0)
+                 && (STRNCMP(line, "+++ ", 4) == 0)
+                 && (vim_fgets(linebuf, LBUFLEN, fd) == 0)
+                 && (STRNCMP(line, "@@ ", 3) == 0)) {
+        diffstyle = DIFF_UNIFIED;
+      } else {
+        // Format not recognized yet, skip over this line.  Cygwin diff
+        // may put a warning at the start of the file.
+        continue;
+      }
     }
 
-    if (difftype == 'd') {
-      lnum_new = f2 + 1;
-      count_new = 0;
+    if (diffstyle == DIFF_ED) {
+      if (!isdigit(*line)) {
+        continue;   // not the start of a diff block
+      }
+      if (parse_diff_ed(line, &lnum_orig, &count_orig,
+                        &lnum_new, &count_new) == FAIL) {
+        continue;
+      }
+    } else if (diffstyle == DIFF_UNIFIED) {
+      if (STRNCMP(line, "@@ ", 3)  != 0) {
+        continue;   // not the start of a diff block
+      }
+      if (parse_diff_unified(line, &lnum_orig, &count_orig,
+                             &lnum_new, &count_new) == FAIL) {
+        continue;
+      }
     } else {
-      lnum_new = f2;
-      count_new = l2 - f2 + 1;
+      EMSG(_("E959: Invalid diff format."));
+      break;
     }
 
     // Go over blocks before the change, for which orig and new are equal.
@@ -1304,7 +1587,7 @@ static void diff_read(int idx_orig, int idx_new, char_u *fname)
       }
       dprev = dp;
       dp = dp->df_next;
-      notset = TRUE;
+      notset = true;
     }
 
     if ((dp != NULL)
@@ -1391,7 +1674,7 @@ static void diff_read(int idx_orig, int idx_new, char_u *fname)
         }
       }
     }
-    notset = FALSE; // "*dp" has been set
+    notset = false;  // "*dp" has been set
   }
 
   // for remaining diff blocks orig and new are equal
@@ -1401,10 +1684,12 @@ static void diff_read(int idx_orig, int idx_new, char_u *fname)
     }
     dprev = dp;
     dp = dp->df_next;
-    notset = TRUE;
+    notset = true;
   }
 
-  fclose(fd);
+  if (fd != NULL) {
+    fclose(fd);
+  }
 }
 
 /// Copy an entry at "dp" from "idx_orig" to "idx_new".
@@ -1503,23 +1788,23 @@ int diff_check(win_T *wp, linenr_T lnum)
   }
 
   if (lnum < dp->df_lnum[idx] + dp->df_count[idx]) {
-    int zero = FALSE;
+    int zero = false;
 
     // Changed or inserted line.  If the other buffers have a count of
     // zero, the lines were inserted.  If the other buffers have the same
     // count, check if the lines are identical.
-    cmp = FALSE;
+    cmp = false;
 
     for (i = 0; i < DB_COUNT; ++i) {
       if ((i != idx) && (curtab->tp_diffbuf[i] != NULL)) {
         if (dp->df_count[i] == 0) {
-          zero = TRUE;
+          zero = true;
         } else {
           if (dp->df_count[i] != dp->df_count[idx]) {
             // nr of lines changed.
             return -1;
           }
-          cmp = TRUE;
+          cmp = true;
         }
       }
     }
@@ -1543,7 +1828,7 @@ int diff_check(win_T *wp, linenr_T lnum)
     // the difference.  Can't remove the entry here, we might be halfway
     // through updating the window.  Just report the text as unchanged.
     // Other windows might still show the change though.
-    if (zero == FALSE) {
+    if (zero == false) {
       return 0;
     }
     return -2;
@@ -1635,20 +1920,28 @@ static bool diff_equal_char(const char_u *const p1, const char_u *const p2,
 /// @return on-zero if the two strings are different.
 static int diff_cmp(char_u *s1, char_u *s2)
 {
-  if ((diff_flags & (DIFF_ICASE | DIFF_IWHITE)) == 0) {
+  if ((diff_flags & DIFF_IBLANK)
+      && (*skipwhite(s1) == NUL || *skipwhite(s2) == NUL)) {
+    return 0;
+  }
+
+  if ((diff_flags & (DIFF_ICASE | ALL_WHITE_DIFF)) == 0) {
     return STRCMP(s1, s2);
   }
 
-  if ((diff_flags & DIFF_ICASE) && !(diff_flags & DIFF_IWHITE)) {
+  if ((diff_flags & DIFF_ICASE) && !(diff_flags & ALL_WHITE_DIFF)) {
     return mb_stricmp((const char *)s1, (const char *)s2);
   }
 
-  // Ignore white space changes and possibly ignore case.
   char_u *p1 = s1;
   char_u *p2 = s2;
 
+  // Ignore white space changes and possibly ignore case.
   while (*p1 != NUL && *p2 != NUL) {
-    if (ascii_iswhite(*p1) && ascii_iswhite(*p2)) {
+    if (((diff_flags & DIFF_IWHITE)
+         && ascii_iswhite(*p1) && ascii_iswhite(*p2))
+        || ((diff_flags & DIFF_IWHITEALL)
+            && (ascii_iswhite(*p1) || ascii_iswhite(*p2)))) {
       p1 = skipwhite(p1);
       p2 = skipwhite(p2);
     } else {
@@ -1815,6 +2108,8 @@ int diffopt_changed(void)
   int diff_context_new = 6;
   int diff_flags_new = 0;
   int diff_foldcolumn_new = 2;
+  long diff_algorithm_new = 0;
+  long diff_indent_heuristic = 0;
 
   char_u *p = p_dip;
   while (*p != NUL) {
@@ -1824,9 +2119,18 @@ int diffopt_changed(void)
     } else if ((STRNCMP(p, "context:", 8) == 0) && ascii_isdigit(p[8])) {
       p += 8;
       diff_context_new = getdigits_int(&p);
+    } else if (STRNCMP(p, "iblank", 6) == 0) {
+      p += 6;
+      diff_flags_new |= DIFF_IBLANK;
     } else if (STRNCMP(p, "icase", 5) == 0) {
       p += 5;
       diff_flags_new |= DIFF_ICASE;
+    } else if (STRNCMP(p, "iwhiteall", 9) == 0) {
+      p += 9;
+      diff_flags_new |= DIFF_IWHITEALL;
+    } else if (STRNCMP(p, "iwhiteeol", 9) == 0) {
+      p += 9;
+      diff_flags_new |= DIFF_IWHITEEOL;
     } else if (STRNCMP(p, "iwhite", 6) == 0) {
       p += 6;
       diff_flags_new |= DIFF_IWHITE;
@@ -1842,6 +2146,29 @@ int diffopt_changed(void)
     } else if (STRNCMP(p, "hiddenoff", 9) == 0) {
       p += 9;
       diff_flags_new |= DIFF_HIDDEN_OFF;
+    } else if (STRNCMP(p, "indent-heuristic", 16) == 0) {
+      p += 16;
+      diff_indent_heuristic = XDF_INDENT_HEURISTIC;
+    } else if (STRNCMP(p, "internal", 8) == 0) {
+      p += 8;
+      diff_flags_new |= DIFF_INTERNAL;
+    } else if (STRNCMP(p, "algorithm:", 10) == 0) {
+      p += 10;
+      if (STRNCMP(p, "myers", 5) == 0) {
+        p += 5;
+        diff_algorithm_new = 0;
+      } else if (STRNCMP(p, "minimal", 7) == 0) {
+        p += 7;
+        diff_algorithm_new = XDF_NEED_MINIMAL;
+      } else if (STRNCMP(p, "patience", 8) == 0) {
+        p += 8;
+        diff_algorithm_new = XDF_PATIENCE_DIFF;
+      } else if (STRNCMP(p, "histogram", 9) == 0) {
+        p += 9;
+        diff_algorithm_new = XDF_HISTOGRAM_DIFF;
+      } else {
+        return FAIL;
+      }
     }
 
     if ((*p != ',') && (*p != NUL)) {
@@ -1853,23 +2180,27 @@ int diffopt_changed(void)
     }
   }
 
+  diff_algorithm_new |= diff_indent_heuristic;
+
   // Can't have both "horizontal" and "vertical".
   if ((diff_flags_new & DIFF_HORIZONTAL) && (diff_flags_new & DIFF_VERTICAL)) {
     return FAIL;
   }
 
-  // If "icase" or "iwhite" was added or removed, need to update the diff.
-  if (diff_flags != diff_flags_new) {
+  // If flags were added or removed, or the algorithm was changed, need to
+  // update the diff.
+  if (diff_flags != diff_flags_new || diff_algorithm != diff_algorithm_new) {
     FOR_ALL_TABS(tp) {
-      tp->tp_diff_invalid = TRUE;
+      tp->tp_diff_invalid = true;
     }
   }
 
   diff_flags = diff_flags_new;
   diff_context = diff_context_new;
   diff_foldcolumn = diff_foldcolumn_new;
+  diff_algorithm = diff_algorithm_new;
 
-  diff_redraw(TRUE);
+  diff_redraw(true);
 
   // recompute the scroll binding with the new option value, may
   // remove or add filler lines
@@ -1910,7 +2241,7 @@ bool diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp)
   int l;
 
   // Make a copy of the line, the next ml_get() will invalidate it.
-  char_u *line_org = vim_strsave(ml_get_buf(wp->w_buffer, lnum, FALSE));
+  char_u *line_org = vim_strsave(ml_get_buf(wp->w_buffer, lnum, false));
 
   int idx = diff_buf_idx(wp->w_buffer);
   if (idx == DB_COUNT) {
@@ -1942,15 +2273,18 @@ bool diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp)
       }
       added = false;
       line_new = ml_get_buf(curtab->tp_diffbuf[i],
-                            dp->df_lnum[i] + off, FALSE);
+                            dp->df_lnum[i] + off, false);
 
       // Search for start of difference
       si_org = si_new = 0;
 
       while (line_org[si_org] != NUL) {
-        if ((diff_flags & DIFF_IWHITE)
-            && ascii_iswhite(line_org[si_org])
-            && ascii_iswhite(line_new[si_new])) {
+        if (((diff_flags & DIFF_IWHITE)
+             && ascii_iswhite(line_org[si_org])
+             && ascii_iswhite(line_new[si_new]))
+            || ((diff_flags & DIFF_IWHITEALL)
+                && (ascii_iswhite(line_org[si_org])
+                    || ascii_iswhite(line_new[si_new])))) {
           si_org = (int)(skipwhite(line_org + si_org) - line_org);
           si_new = (int)(skipwhite(line_new + si_new) - line_new);
         } else {
@@ -1980,9 +2314,12 @@ bool diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp)
                && ei_new >= si_new
                && ei_org >= 0
                && ei_new >= 0) {
-          if ((diff_flags & DIFF_IWHITE)
-              && ascii_iswhite(line_org[ei_org])
-              && ascii_iswhite(line_new[ei_new])) {
+          if (((diff_flags & DIFF_IWHITE)
+               && ascii_iswhite(line_org[ei_org])
+               && ascii_iswhite(line_new[ei_new]))
+              || ((diff_flags & DIFF_IWHITEALL)
+                  && (ascii_iswhite(line_org[ei_org])
+                      || ascii_iswhite(line_new[ei_new])))) {
             while (ei_org >= *startp && ascii_iswhite(line_org[ei_org])) {
               ei_org--;
             }
@@ -2117,7 +2454,7 @@ void ex_diffgetput(exarg_T *eap)
   int start_skip, end_skip;
   int new_count;
   int buf_empty;
-  int found_not_ma = FALSE;
+  int found_not_ma = false;
   int idx_other;
   int idx_from;
   int idx_to;
@@ -2138,7 +2475,7 @@ void ex_diffgetput(exarg_T *eap)
             || MODIFIABLE(curtab->tp_diffbuf[idx_other])) {
           break;
         }
-        found_not_ma = TRUE;
+        found_not_ma = true;
       }
     }
 
@@ -2176,7 +2513,7 @@ void ex_diffgetput(exarg_T *eap)
       // digits only
       i = atol((char *)eap->arg);
     } else {
-      i = buflist_findpat(eap->arg, p, FALSE, TRUE, FALSE);
+      i = buflist_findpat(eap->arg, p, false, true, false);
 
       if (i < 0) {
         // error message already given
@@ -2202,7 +2539,7 @@ void ex_diffgetput(exarg_T *eap)
     }
   }
 
-  diff_busy = TRUE;
+  diff_busy = true;
 
   // When no range given include the line above or below the cursor.
   if (eap->addr_count == 0) {
@@ -2239,7 +2576,7 @@ void ex_diffgetput(exarg_T *eap)
     change_warning(0);
     if (diff_buf_idx(curbuf) != idx_to) {
       EMSG(_("E787: Buffer changed unexpectedly"));
-      return;
+      goto theend;
     }
   }
 
@@ -2308,7 +2645,7 @@ void ex_diffgetput(exarg_T *eap)
       for (i = 0; i < count; ++i) {
         // remember deleting the last line of the buffer
         buf_empty = curbuf->b_ml.ml_line_count == 1;
-        ml_delete(lnum, FALSE);
+        ml_delete(lnum, false);
         added--;
       }
 
@@ -2317,15 +2654,15 @@ void ex_diffgetput(exarg_T *eap)
         if (nr > curtab->tp_diffbuf[idx_from]->b_ml.ml_line_count) {
           break;
         }
-        p = vim_strsave(ml_get_buf(curtab->tp_diffbuf[idx_from], nr, FALSE));
-        ml_append(lnum + i - 1, p, 0, FALSE);
+        p = vim_strsave(ml_get_buf(curtab->tp_diffbuf[idx_from], nr, false));
+        ml_append(lnum + i - 1, p, 0, false);
         xfree(p);
         added++;
         if (buf_empty && (curbuf->b_ml.ml_line_count == 2)) {
           // Added the first line into an empty buffer, need to
           // delete the dummy empty line.
-          buf_empty = FALSE;
-          ml_delete((linenr_T)2, FALSE);
+          buf_empty = false;
+          ml_delete((linenr_T)2, false);
         }
       }
       new_count = dp->df_count[idx_to] + added;
@@ -2399,20 +2736,31 @@ void ex_diffgetput(exarg_T *eap)
     // another buffer.  Sync undo if the command was typed.  This isn't
     // 100% right when ":diffput" is used in a function or mapping.
     if (KeyTyped) {
-      u_sync(FALSE);
+      u_sync(false);
     }
     aucmd_restbuf(&aco);
   }
 
-  diff_busy = FALSE;
+theend:
+  diff_busy = false;
+  if (diff_need_update) {
+    ex_diffupdate(NULL);
+  }
 
-  // Check that the cursor is on a valid character and update it's position.
-  // When there were filler lines the topline has become invalid.
+  // Check that the cursor is on a valid character and update its
+  // position.  When there were filler lines the topline has become
+  // invalid.
   check_cursor();
   changed_line_abv_curs();
 
-  // Also need to redraw the other buffers.
-  diff_redraw(FALSE);
+  if (diff_need_update) {
+    // redraw already done by ex_diffupdate()
+    diff_need_update = false;
+  } else {
+    // Also need to redraw the other buffers.
+    diff_redraw(false);
+    apply_autocmds(EVENT_DIFFUPDATED, NULL, NULL, false, curbuf);
+  }
 }
 
 /// Update folds for all diff buffers for entry "dp".
@@ -2637,3 +2985,145 @@ linenr_T diff_lnum_win(linenr_T lnum, win_T *wp)
   }
   return n;
 }
+
+///
+/// Handle an ED style diff line.
+/// Return FAIL if the line does not contain diff info.
+///
+static int parse_diff_ed(char_u     *line,
+                         linenr_T    *lnum_orig,
+                         long       *count_orig,
+                         linenr_T    *lnum_new,
+                         long       *count_new)
+{
+  char_u *p;
+  long    f1, l1, f2, l2;
+  int       difftype;
+
+  // The line must be one of three formats:
+  // change: {first}[,{last}]c{first}[,{last}]
+  // append: {first}a{first}[,{last}]
+  // delete: {first}[,{last}]d{first}
+  p = line;
+  f1 = getdigits(&p);
+  if (*p == ',') {
+    p++;
+    l1 = getdigits(&p);
+  } else {
+    l1 = f1;
+  }
+  if (*p != 'a' && *p != 'c' && *p != 'd') {
+    return FAIL;        // invalid diff format
+  }
+  difftype = *p++;
+  f2 = getdigits(&p);
+  if (*p == ',') {
+    p++;
+    l2 = getdigits(&p);
+  } else {
+    l2 = f2;
+  }
+  if (l1 < f1 || l2 < f2) {
+    return FAIL;
+  }
+
+  if (difftype == 'a') {
+    *lnum_orig = f1 + 1;
+    *count_orig = 0;
+  } else {
+    *lnum_orig = f1;
+    *count_orig = l1 - f1 + 1;
+  }
+  if (difftype == 'd') {
+    *lnum_new = f2 + 1;
+    *count_new = 0;
+  } else {
+    *lnum_new = f2;
+    *count_new = l2 - f2 + 1;
+  }
+  return OK;
+}
+
+///
+/// Parses unified diff with zero(!) context lines.
+/// Return FAIL if there is no diff information in "line".
+///
+static int parse_diff_unified(char_u        *line,
+                              linenr_T    *lnum_orig,
+                              long      *count_orig,
+                              linenr_T    *lnum_new,
+                              long      *count_new)
+{
+  char_u *p;
+  long    oldline, oldcount, newline, newcount;
+
+  // Parse unified diff hunk header:
+  // @@ -oldline,oldcount +newline,newcount @@
+  p = line;
+  if (*p++ == '@' && *p++ == '@' && *p++ == ' ' && *p++ == '-') {
+    oldline = getdigits(&p);
+    if (*p == ',') {
+      p++;
+      oldcount = getdigits(&p);
+    } else {
+      oldcount = 1;
+    }
+    if (*p++ == ' ' && *p++ == '+') {
+      newline = getdigits(&p);
+      if (*p == ',') {
+        p++;
+        newcount = getdigits(&p);
+      } else {
+        newcount = 1;
+      }
+    } else {
+      return FAIL;  // invalid diff format
+    }
+
+    if (oldcount == 0) {
+      oldline += 1;
+    }
+    if (newcount == 0) {
+      newline += 1;
+    }
+    if (newline == 0) {
+      newline = 1;
+    }
+
+    *lnum_orig = oldline;
+    *count_orig = oldcount;
+    *lnum_new = newline;
+    *count_new = newcount;
+
+    return OK;
+  }
+
+  return FAIL;
+}
+
+///
+/// Callback function for the xdl_diff() function.
+/// Stores the diff output in a grow array.
+///
+static int xdiff_out(void *priv, mmbuffer_t *mb, int nbuf)
+{
+  diffout_T *dout = (diffout_T *)priv;
+  char_u    *p;
+
+  // The header line always comes by itself, text lines in at least two
+  // parts.  We drop the text part.
+  if (nbuf > 1) {
+    return 0;
+  }
+
+  // sanity check
+  if (STRNCMP(mb[0].ptr, "@@ ", 3)  != 0) {
+    return 0;
+  }
+
+  ga_grow(&dout->dout_ga, 1);
+
+  p = vim_strnsave((char_u *)mb[0].ptr, mb[0].size);
+  ((char_u **)dout->dout_ga.ga_data)[dout->dout_ga.ga_len++] = p;
+  return 0;
+}
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 7e8a7d1a35..8e5455bc67 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -1739,13 +1739,13 @@ failed:
     close(0);
 #ifndef WIN32
     // On Unix, use stderr for stdin, makes shell commands work.
-    ignored = dup(2);
+    vim_ignored = dup(2);
 #else
     // On Windows, use the console input handle for stdin.
     HANDLE conin = CreateFile("CONIN$", GENERIC_READ | GENERIC_WRITE,
                               FILE_SHARE_READ, (LPSECURITY_ATTRIBUTES)NULL,
                               OPEN_EXISTING, 0, (HANDLE)NULL);
-    ignored = _open_osfhandle(conin, _O_RDONLY);
+    vim_ignored = _open_osfhandle(conin, _O_RDONLY);
 #endif
   }
 
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index df74e8b2ff..04ff1320ce 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -1125,8 +1125,7 @@ EXTERN FILE *time_fd INIT(= NULL);  /* where to write startup timing */
  * can't do anything useful with the value.  Assign to this variable to avoid
  * the warning.
  */
-EXTERN int ignored;
-EXTERN char *ignoredp;
+EXTERN int vim_ignored;
 
 // Start a msgpack-rpc channel over stdin/stdout.
 EXTERN bool embedded_mode INIT(= false);
diff --git a/src/nvim/misc1.c b/src/nvim/misc1.c
index d4a406cdba..d77c196a3a 100644
--- a/src/nvim/misc1.c
+++ b/src/nvim/misc1.c
@@ -1953,10 +1953,10 @@ changed_lines(
 {
   changed_lines_buf(curbuf, lnum, lnume, xtra);
 
-  if (xtra == 0 && curwin->w_p_diff) {
-    /* When the number of lines doesn't change then mark_adjust() isn't
-     * called and other diff buffers still need to be marked for
-     * displaying. */
+  if (xtra == 0 && curwin->w_p_diff && !diff_internal()) {
+    // When the number of lines doesn't change then mark_adjust() isn't
+    // called and other diff buffers still need to be marked for
+    // displaying.
     linenr_T wlnum;
 
     FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
@@ -2025,6 +2025,10 @@ static void changed_common(linenr_T lnum, colnr_T col, linenr_T lnume, long xtra
   /* mark the buffer as modified */
   changed();
 
+  if (curwin->w_p_diff && diff_internal()) {
+    curtab->tp_diff_update = true;
+  }
+
   /* set the '. mark */
   if (!cmdmod.keepjumps) {
     RESET_FMARK(&curbuf->b_last_change, ((pos_T) {lnum, col, 0}), 0);
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index f87de52a82..38ee0936aa 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -1325,6 +1325,14 @@ static int normal_check(VimState *state)
     normal_check_cursor_moved(s);
     normal_check_text_changed(s);
 
+    // Updating diffs from changed() does not always work properly,
+    // esp. updating folds.  Do an update just before redrawing if
+    // needed.
+    if (curtab->tp_diff_update || curtab->tp_diff_invalid) {
+      ex_diffupdate(NULL);
+      curtab->tp_diff_update = false;
+    }
+
     // Scroll-binding for diff mode may have been postponed until
     // here.  Avoids doing it for every change.
     if (diff_need_scrollbind) {
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index b0575df7ec..0cc6f58c5f 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -615,7 +615,7 @@ return {
       alloced=true,
       redraw={'current_window'},
       varname='p_dip',
-      defaults={if_true={vi="filler"}}
+      defaults={if_true={vi="internal,filler"}}
     },
     {
       full_name='digraph', abbreviation='dg',
diff --git a/src/nvim/testdir/test_diffmode.vim b/src/nvim/testdir/test_diffmode.vim
index afe289b262..ad3eec3274 100644
--- a/src/nvim/testdir/test_diffmode.vim
+++ b/src/nvim/testdir/test_diffmode.vim
@@ -2,6 +2,9 @@
 
 func Test_diff_fold_sync()
   enew!
+  let g:update_count = 0
+  au DiffUpdated * let g:update_count += 1
+
   let l = range(50)
   call setline(1, l)
   diffthis
@@ -27,12 +30,27 @@ func Test_diff_fold_sync()
   call win_gotoid(winone)
   call assert_equal(23, getcurpos()[1])
 
+  call assert_equal(1, g:update_count)
+  au! DiffUpdated
+
   windo diffoff
   close!
   set nomodified
 endfunc
 
 func Test_vert_split()
+  set diffopt=filler
+  call Common_vert_split()
+  set diffopt&
+endfunc
+
+func Test_vert_split_internal()
+  set diffopt=internal,filler
+  call Common_vert_split()
+  set diffopt&
+endfunc
+
+func Common_vert_split()
   " Disable the title to avoid xterm keeping the wrong one.
   set notitle noicon
   new
@@ -201,6 +219,26 @@ func Test_diffget_diffput()
   %bwipe!
 endfunc
 
+" Test putting two changes from one buffer to another
+func Test_diffput_two()
+  new a
+  let win_a = win_getid()
+  call setline(1, range(1, 10))
+  diffthis
+  new b
+  let win_b = win_getid()
+  call setline(1, range(1, 10))
+  8del
+  5del
+  diffthis
+  call win_gotoid(win_a)
+  %diffput
+  call win_gotoid(win_b)
+  call assert_equal(map(range(1, 10), 'string(v:val)'), getline(1, '$'))
+  bwipe! a
+  bwipe! b
+endfunc
+
 func Test_dp_do_buffer()
   e! one
   let bn1=bufnr('%')
@@ -257,6 +295,28 @@ func Test_dp_do_buffer()
   %bwipe!
 endfunc
 
+func Test_do_lastline()
+  e! one
+  call setline(1, ['1','2','3','4','5','6'])
+  diffthis
+
+  new two
+  call setline(1, ['2','4','5'])
+  diffthis
+
+  1
+  norm dp]c
+  norm dp]c
+  wincmd w
+  call assert_equal(4, line('$'))
+  norm G
+  norm do
+  call assert_equal(3, line('$'))
+
+  windo diffoff
+  %bwipe!
+endfunc
+
 func Test_diffoff()
   enew!
   call setline(1, ['Two', 'Three'])
@@ -275,10 +335,8 @@ func Test_diffoff()
   bwipe!
 endfunc
 
-func Test_diffopt_icase()
-  set diffopt=icase,foldcolumn:0
-
-  e one
+func Common_icase_test()
+  edit one
   call setline(1, ['One', 'Two', 'Three', 'Four', 'Fi#ve'])
   redraw
   let normattr = screenattr(1, 1)
@@ -300,32 +358,54 @@ func Test_diffopt_icase()
 
   diffoff!
   %bwipe!
+endfunc
+
+func Test_diffopt_icase()
+  set diffopt=icase,foldcolumn:0
+  call Common_icase_test()
   set diffopt&
 endfunc
 
-func Test_diffopt_iwhite()
-  set diffopt=iwhite,foldcolumn:0
+func Test_diffopt_icase_internal()
+  set diffopt=icase,foldcolumn:0,internal
+  call Common_icase_test()
+  set diffopt&
+endfunc
 
-  e one
-  " Difference in trailing spaces should be ignored,
+func Common_iwhite_test()
+  edit one
+  " Difference in trailing spaces and amount of spaces should be ignored,
   " but not other space differences.
-  call setline(1, ["One \t", 'Two', 'Three', 'Four'])
+  call setline(1, ["One \t", 'Two', 'Three', 'one two', 'one two', 'Four'])
   redraw
   let normattr = screenattr(1, 1)
   diffthis
 
   botright vert new two
-  call setline(1, ["One\t ", "Two\t ", 'Three', ' Four'])
+  call setline(1, ["One\t ", "Two\t ", 'Three', 'one   two', 'onetwo', ' Four'])
   diffthis
 
   redraw
   call assert_equal(normattr, screenattr(1, 1))
   call assert_equal(normattr, screenattr(2, 1))
   call assert_equal(normattr, screenattr(3, 1))
-  call assert_notequal(normattr, screenattr(4, 1))
+  call assert_equal(normattr, screenattr(4, 1))
+  call assert_notequal(normattr, screenattr(5, 1))
+  call assert_notequal(normattr, screenattr(6, 1))
 
   diffoff!
   %bwipe!
+endfunc
+
+func Test_diffopt_iwhite()
+  set diffopt=iwhite,foldcolumn:0
+  call Common_iwhite_test()
+  set diffopt&
+endfunc
+
+func Test_diffopt_iwhite_internal()
+  set diffopt=internal,iwhite,foldcolumn:0
+  call Common_iwhite_test()
   set diffopt&
 endfunc
 
@@ -339,8 +419,13 @@ func Test_diffopt_context()
 
   set diffopt=context:2
   call assert_equal('+--  2 lines: 1', foldtextresult(1))
+  set diffopt=internal,context:2
+  call assert_equal('+--  2 lines: 1', foldtextresult(1))
+
   set diffopt=context:1
   call assert_equal('+--  3 lines: 1', foldtextresult(1))
+  set diffopt=internal,context:1
+  call assert_equal('+--  3 lines: 1', foldtextresult(1))
 
   diffoff!
   %bwipe!
@@ -348,7 +433,7 @@ func Test_diffopt_context()
 endfunc
 
 func Test_diffopt_horizontal()
-  set diffopt=horizontal
+  set diffopt=internal,horizontal
   diffsplit
 
   call assert_equal(&columns, winwidth(1))
@@ -362,7 +447,7 @@ func Test_diffopt_horizontal()
 endfunc
 
 func Test_diffopt_vertical()
-  set diffopt=vertical
+  set diffopt=internal,vertical
   diffsplit
 
   call assert_equal(&lines - 2, winheight(1))
@@ -376,7 +461,7 @@ func Test_diffopt_vertical()
 endfunc
 
 func Test_diffopt_hiddenoff()
-  set diffopt=filler,foldcolumn:0,hiddenoff
+  set diffopt=internal,filler,foldcolumn:0,hiddenoff
   e! one
   call setline(1, ['Two', 'Three'])
   redraw
@@ -399,7 +484,7 @@ func Test_diffopt_hiddenoff()
 endfunc
 
 func Test_diffoff_hidden()
-  set diffopt=filler,foldcolumn:0
+  set diffopt=internal,filler,foldcolumn:0
   e! one
   call setline(1, ['Two', 'Three'])
   redraw
@@ -480,7 +565,9 @@ func Test_diffexpr()
   endif
 
   func DiffExpr()
-    silent exe '!diff ' . v:fname_in . ' ' . v:fname_new . '>' . v:fname_out
+    " Prepend some text to check diff type detection
+    call writefile(['warning', '  message'], v:fname_out)
+    silent exe '!diff ' . v:fname_in . ' ' . v:fname_new . '>>' . v:fname_out
   endfunc
   set diffexpr=DiffExpr()
   set diffopt=foldcolumn:0
@@ -523,7 +610,7 @@ func Test_diffpatch()
   3
 + 4
 .
-  saveas Xpatch
+  saveas! Xpatch
   bwipe!
   new
   call assert_fails('diffpatch Xpatch', 'E816:')
@@ -569,6 +656,22 @@ func Test_diff_nomodifiable()
   %bwipe!
 endfunc
 
+func Test_diff_filler()
+  new
+  call setline(1, [1, 2, 3, 'x', 4])
+  diffthis
+  vnew
+  call setline(1, [1, 2, 'y', 'y', 3, 4])
+  diffthis
+  redraw
+
+  call assert_equal([0, 0, 0, 0, 0, 0, 0, 1, 0], map(range(-1, 7), 'diff_filler(v:val)'))
+  wincmd w
+  call assert_equal([0, 0, 0, 0, 2, 0, 0, 0], map(range(-1, 6), 'diff_filler(v:val)'))
+
+  %bwipe!
+endfunc
+
 func Test_diff_lastline()
   enew!
   only!
@@ -615,3 +718,23 @@ func Test_diff_with_cursorline()
   call StopVimInTerminal(buf)
   call delete('Xtest_diff_cursorline')
 endfunc
+
+func Test_diff_of_diff()
+  if !CanRunVimInTerminal()
+    return
+  endif
+
+  call writefile([
+	\ 'call setline(1, ["aa","bb","cc","@@ -3,2 +5,7 @@","dd","ee","ff"])',
+	\ 'vnew',
+	\ 'call setline(1, ["aa","bb","cc"])',
+	\ 'windo diffthis',
+	\ ], 'Xtest_diff_diff')
+  let buf = RunVimInTerminal('-S Xtest_diff_diff', {})
+
+  call VerifyScreenDump(buf, 'Test_diff_of_diff_01', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('Xtest_diff_diff')
+endfunc
diff --git a/src/nvim/xdiff/COPYING b/src/nvim/xdiff/COPYING
new file mode 100644
index 0000000000..f3f1b3b65e
--- /dev/null
+++ b/src/nvim/xdiff/COPYING
@@ -0,0 +1,504 @@
+		  GNU LESSER GENERAL PUBLIC LICENSE
+		       Version 2.1, February 1999
+
+ Copyright (C) 1991, 1999 Free Software Foundation, Inc.
+     59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+[This is the first released version of the Lesser GPL.  It also counts
+ as the successor of the GNU Library Public License, version 2, hence
+ the version number 2.1.]
+
+			    Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+Licenses are intended to guarantee your freedom to share and change
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+
+  This license, the Lesser General Public License, applies to some
+specially designated software packages--typically libraries--of the
+Free Software Foundation and other authors who decide to use it.  You
+can use it too, but we suggest you first think carefully about whether
+this license or the ordinary General Public License is the better
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+
+  When we speak of free software, we are referring to freedom of use,
+not price.  Our General Public Licenses are designed to make sure that
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+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
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diff --git a/src/nvim/xdiff/README.txt b/src/nvim/xdiff/README.txt
new file mode 100644
index 0000000000..1afe74095b
--- /dev/null
+++ b/src/nvim/xdiff/README.txt
@@ -0,0 +1,16 @@
+The files in this directory come from the xdiff implementation in git.
+You can find it here: https://github.com/git/git/tree/master/xdiff
+The files were last updated 2018 September 10.
+
+This is originally based on libxdiff, which can be found here:
+http://www.xmailserver.org/xdiff-lib.html
+
+The git version was used because it has been maintained and improved.
+And since it's part of git it is expected to be reliable.
+
+The code is distributed under the GNU LGPL license.  It is included in the
+COPYING file.
+
+Changes in these files were made to avoid compiler warnings.
+
+The first work for including xdiff in Vim was done by Christian Brabandt.
diff --git a/src/nvim/xdiff/xdiff.h b/src/nvim/xdiff/xdiff.h
new file mode 100644
index 0000000000..bc26fb64fd
--- /dev/null
+++ b/src/nvim/xdiff/xdiff.h
@@ -0,0 +1,143 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#if !defined(XDIFF_H)
+#define XDIFF_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif /* #ifdef __cplusplus */
+
+/* xpparm_t.flags */
+#define XDF_NEED_MINIMAL (1 << 0)
+
+#define XDF_IGNORE_WHITESPACE (1 << 1)
+#define XDF_IGNORE_WHITESPACE_CHANGE (1 << 2)
+#define XDF_IGNORE_WHITESPACE_AT_EOL (1 << 3)
+#define XDF_IGNORE_CR_AT_EOL (1 << 4)
+#define XDF_WHITESPACE_FLAGS (XDF_IGNORE_WHITESPACE | \
+			      XDF_IGNORE_WHITESPACE_CHANGE | \
+			      XDF_IGNORE_WHITESPACE_AT_EOL | \
+			      XDF_IGNORE_CR_AT_EOL)
+
+#define XDF_IGNORE_BLANK_LINES (1 << 7)
+
+#define XDF_PATIENCE_DIFF (1 << 14)
+#define XDF_HISTOGRAM_DIFF (1 << 15)
+#define XDF_DIFF_ALGORITHM_MASK (XDF_PATIENCE_DIFF | XDF_HISTOGRAM_DIFF)
+#define XDF_DIFF_ALG(x) ((x) & XDF_DIFF_ALGORITHM_MASK)
+
+#define XDF_INDENT_HEURISTIC (1 << 23)
+
+/* xdemitconf_t.flags */
+#define XDL_EMIT_FUNCNAMES (1 << 0)
+#define XDL_EMIT_FUNCCONTEXT (1 << 2)
+
+/* merge simplification levels */
+#define XDL_MERGE_MINIMAL 0
+#define XDL_MERGE_EAGER 1
+#define XDL_MERGE_ZEALOUS 2
+#define XDL_MERGE_ZEALOUS_ALNUM 3
+
+/* merge favor modes */
+#define XDL_MERGE_FAVOR_OURS 1
+#define XDL_MERGE_FAVOR_THEIRS 2
+#define XDL_MERGE_FAVOR_UNION 3
+
+/* merge output styles */
+#define XDL_MERGE_DIFF3 1
+
+typedef struct s_mmfile {
+	char *ptr;
+	long size;
+} mmfile_t;
+
+typedef struct s_mmbuffer {
+	char *ptr;
+	long size;
+} mmbuffer_t;
+
+typedef struct s_xpparam {
+	unsigned long flags;
+
+	/* See Documentation/diff-options.txt. */
+	char **anchors;
+	size_t anchors_nr;
+} xpparam_t;
+
+typedef struct s_xdemitcb {
+	void *priv;
+	int (*outf)(void *, mmbuffer_t *, int);
+} xdemitcb_t;
+
+typedef long (*find_func_t)(const char *line, long line_len, char *buffer, long buffer_size, void *priv);
+
+typedef int (*xdl_emit_hunk_consume_func_t)(long start_a, long count_a,
+					    long start_b, long count_b,
+					    void *cb_data);
+
+typedef struct s_xdemitconf {
+	long ctxlen;
+	long interhunkctxlen;
+	unsigned long flags;
+	find_func_t find_func;
+	void *find_func_priv;
+	xdl_emit_hunk_consume_func_t hunk_func;
+} xdemitconf_t;
+
+typedef struct s_bdiffparam {
+	long bsize;
+} bdiffparam_t;
+
+#include "../memory.h"
+
+#define xdl_malloc(x) xmalloc((x))
+#define xdl_free(ptr) xfree(ptr)
+#define xdl_realloc(ptr,x) xrealloc((ptr),(x))
+
+void *xdl_mmfile_first(mmfile_t *mmf, long *size);
+long xdl_mmfile_size(mmfile_t *mmf);
+
+int xdl_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+	     xdemitconf_t const *xecfg, xdemitcb_t *ecb);
+
+typedef struct s_xmparam {
+	xpparam_t xpp;
+	int marker_size;
+	int level;
+	int favor;
+	int style;
+	const char *ancestor;	/* label for orig */
+	const char *file1;	/* label for mf1 */
+	const char *file2;	/* label for mf2 */
+} xmparam_t;
+
+#define DEFAULT_CONFLICT_MARKER_SIZE 7
+
+int xdl_merge(mmfile_t *orig, mmfile_t *mf1, mmfile_t *mf2,
+		xmparam_t const *xmp, mmbuffer_t *result);
+
+#ifdef __cplusplus
+}
+#endif /* #ifdef __cplusplus */
+
+#endif /* #if !defined(XDIFF_H) */
diff --git a/src/nvim/xdiff/xdiffi.c b/src/nvim/xdiff/xdiffi.c
new file mode 100644
index 0000000000..96d5277027
--- /dev/null
+++ b/src/nvim/xdiff/xdiffi.c
@@ -0,0 +1,1043 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003	Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#include "xinclude.h"
+
+#define XDL_MAX_COST_MIN 256
+#define XDL_HEUR_MIN_COST 256
+#define XDL_LINE_MAX (long)((1UL << (CHAR_BIT * sizeof(long) - 1)) - 1)
+#define XDL_SNAKE_CNT 20
+#define XDL_K_HEUR 4
+
+typedef struct s_xdpsplit {
+	long i1, i2;
+	int min_lo, min_hi;
+} xdpsplit_t;
+
+/*
+ * See "An O(ND) Difference Algorithm and its Variations", by Eugene Myers.
+ * Basically considers a "box" (off1, off2, lim1, lim2) and scan from both
+ * the forward diagonal starting from (off1, off2) and the backward diagonal
+ * starting from (lim1, lim2). If the K values on the same diagonal crosses
+ * returns the furthest point of reach. We might end up having to expensive
+ * cases using this algorithm is full, so a little bit of heuristic is needed
+ * to cut the search and to return a suboptimal point.
+ */
+static long xdl_split(unsigned long const *ha1, long off1, long lim1,
+		      unsigned long const *ha2, long off2, long lim2,
+		      long *kvdf, long *kvdb, int need_min, xdpsplit_t *spl,
+		      xdalgoenv_t *xenv) {
+	long dmin = off1 - lim2, dmax = lim1 - off2;
+	long fmid = off1 - off2, bmid = lim1 - lim2;
+	long odd = (fmid - bmid) & 1;
+	long fmin = fmid, fmax = fmid;
+	long bmin = bmid, bmax = bmid;
+	long ec, d, i1, i2, prev1, best, dd, v, k;
+
+	/*
+	 * Set initial diagonal values for both forward and backward path.
+	 */
+	kvdf[fmid] = off1;
+	kvdb[bmid] = lim1;
+
+	for (ec = 1;; ec++) {
+		int got_snake = 0;
+
+		/*
+		 * We need to extent the diagonal "domain" by one. If the next
+		 * values exits the box boundaries we need to change it in the
+		 * opposite direction because (max - min) must be a power of two.
+		 * Also we initialize the external K value to -1 so that we can
+		 * avoid extra conditions check inside the core loop.
+		 */
+		if (fmin > dmin)
+			kvdf[--fmin - 1] = -1;
+		else
+			++fmin;
+		if (fmax < dmax)
+			kvdf[++fmax + 1] = -1;
+		else
+			--fmax;
+
+		for (d = fmax; d >= fmin; d -= 2) {
+			if (kvdf[d - 1] >= kvdf[d + 1])
+				i1 = kvdf[d - 1] + 1;
+			else
+				i1 = kvdf[d + 1];
+			prev1 = i1;
+			i2 = i1 - d;
+			for (; i1 < lim1 && i2 < lim2 && ha1[i1] == ha2[i2]; i1++, i2++);
+			if (i1 - prev1 > xenv->snake_cnt)
+				got_snake = 1;
+			kvdf[d] = i1;
+			if (odd && bmin <= d && d <= bmax && kvdb[d] <= i1) {
+				spl->i1 = i1;
+				spl->i2 = i2;
+				spl->min_lo = spl->min_hi = 1;
+				return ec;
+			}
+		}
+
+		/*
+		 * We need to extent the diagonal "domain" by one. If the next
+		 * values exits the box boundaries we need to change it in the
+		 * opposite direction because (max - min) must be a power of two.
+		 * Also we initialize the external K value to -1 so that we can
+		 * avoid extra conditions check inside the core loop.
+		 */
+		if (bmin > dmin)
+			kvdb[--bmin - 1] = XDL_LINE_MAX;
+		else
+			++bmin;
+		if (bmax < dmax)
+			kvdb[++bmax + 1] = XDL_LINE_MAX;
+		else
+			--bmax;
+
+		for (d = bmax; d >= bmin; d -= 2) {
+			if (kvdb[d - 1] < kvdb[d + 1])
+				i1 = kvdb[d - 1];
+			else
+				i1 = kvdb[d + 1] - 1;
+			prev1 = i1;
+			i2 = i1 - d;
+			for (; i1 > off1 && i2 > off2 && ha1[i1 - 1] == ha2[i2 - 1]; i1--, i2--);
+			if (prev1 - i1 > xenv->snake_cnt)
+				got_snake = 1;
+			kvdb[d] = i1;
+			if (!odd && fmin <= d && d <= fmax && i1 <= kvdf[d]) {
+				spl->i1 = i1;
+				spl->i2 = i2;
+				spl->min_lo = spl->min_hi = 1;
+				return ec;
+			}
+		}
+
+		if (need_min)
+			continue;
+
+		/*
+		 * If the edit cost is above the heuristic trigger and if
+		 * we got a good snake, we sample current diagonals to see
+		 * if some of the, have reached an "interesting" path. Our
+		 * measure is a function of the distance from the diagonal
+		 * corner (i1 + i2) penalized with the distance from the
+		 * mid diagonal itself. If this value is above the current
+		 * edit cost times a magic factor (XDL_K_HEUR) we consider
+		 * it interesting.
+		 */
+		if (got_snake && ec > xenv->heur_min) {
+			for (best = 0, d = fmax; d >= fmin; d -= 2) {
+				dd = d > fmid ? d - fmid: fmid - d;
+				i1 = kvdf[d];
+				i2 = i1 - d;
+				v = (i1 - off1) + (i2 - off2) - dd;
+
+				if (v > XDL_K_HEUR * ec && v > best &&
+				    off1 + xenv->snake_cnt <= i1 && i1 < lim1 &&
+				    off2 + xenv->snake_cnt <= i2 && i2 < lim2) {
+					for (k = 1; ha1[i1 - k] == ha2[i2 - k]; k++)
+						if (k == xenv->snake_cnt) {
+							best = v;
+							spl->i1 = i1;
+							spl->i2 = i2;
+							break;
+						}
+				}
+			}
+			if (best > 0) {
+				spl->min_lo = 1;
+				spl->min_hi = 0;
+				return ec;
+			}
+
+			for (best = 0, d = bmax; d >= bmin; d -= 2) {
+				dd = d > bmid ? d - bmid: bmid - d;
+				i1 = kvdb[d];
+				i2 = i1 - d;
+				v = (lim1 - i1) + (lim2 - i2) - dd;
+
+				if (v > XDL_K_HEUR * ec && v > best &&
+				    off1 < i1 && i1 <= lim1 - xenv->snake_cnt &&
+				    off2 < i2 && i2 <= lim2 - xenv->snake_cnt) {
+					for (k = 0; ha1[i1 + k] == ha2[i2 + k]; k++)
+						if (k == xenv->snake_cnt - 1) {
+							best = v;
+							spl->i1 = i1;
+							spl->i2 = i2;
+							break;
+						}
+				}
+			}
+			if (best > 0) {
+				spl->min_lo = 0;
+				spl->min_hi = 1;
+				return ec;
+			}
+		}
+
+		/*
+		 * Enough is enough. We spent too much time here and now we collect
+		 * the furthest reaching path using the (i1 + i2) measure.
+		 */
+		if (ec >= xenv->mxcost) {
+			long fbest, fbest1, bbest, bbest1;
+
+			fbest = fbest1 = -1;
+			for (d = fmax; d >= fmin; d -= 2) {
+				i1 = XDL_MIN(kvdf[d], lim1);
+				i2 = i1 - d;
+				if (lim2 < i2)
+					i1 = lim2 + d, i2 = lim2;
+				if (fbest < i1 + i2) {
+					fbest = i1 + i2;
+					fbest1 = i1;
+				}
+			}
+
+			bbest = bbest1 = XDL_LINE_MAX;
+			for (d = bmax; d >= bmin; d -= 2) {
+				i1 = XDL_MAX(off1, kvdb[d]);
+				i2 = i1 - d;
+				if (i2 < off2)
+					i1 = off2 + d, i2 = off2;
+				if (i1 + i2 < bbest) {
+					bbest = i1 + i2;
+					bbest1 = i1;
+				}
+			}
+
+			if ((lim1 + lim2) - bbest < fbest - (off1 + off2)) {
+				spl->i1 = fbest1;
+				spl->i2 = fbest - fbest1;
+				spl->min_lo = 1;
+				spl->min_hi = 0;
+			} else {
+				spl->i1 = bbest1;
+				spl->i2 = bbest - bbest1;
+				spl->min_lo = 0;
+				spl->min_hi = 1;
+			}
+			return ec;
+		}
+	}
+}
+
+
+/*
+ * Rule: "Divide et Impera". Recursively split the box in sub-boxes by calling
+ * the box splitting function. Note that the real job (marking changed lines)
+ * is done in the two boundary reaching checks.
+ */
+int xdl_recs_cmp(diffdata_t *dd1, long off1, long lim1,
+		 diffdata_t *dd2, long off2, long lim2,
+		 long *kvdf, long *kvdb, int need_min, xdalgoenv_t *xenv) {
+	unsigned long const *ha1 = dd1->ha, *ha2 = dd2->ha;
+
+	/*
+	 * Shrink the box by walking through each diagonal snake (SW and NE).
+	 */
+	for (; off1 < lim1 && off2 < lim2 && ha1[off1] == ha2[off2]; off1++, off2++);
+	for (; off1 < lim1 && off2 < lim2 && ha1[lim1 - 1] == ha2[lim2 - 1]; lim1--, lim2--);
+
+	/*
+	 * If one dimension is empty, then all records on the other one must
+	 * be obviously changed.
+	 */
+	if (off1 == lim1) {
+		char *rchg2 = dd2->rchg;
+		long *rindex2 = dd2->rindex;
+
+		for (; off2 < lim2; off2++)
+			rchg2[rindex2[off2]] = 1;
+	} else if (off2 == lim2) {
+		char *rchg1 = dd1->rchg;
+		long *rindex1 = dd1->rindex;
+
+		for (; off1 < lim1; off1++)
+			rchg1[rindex1[off1]] = 1;
+	} else {
+		xdpsplit_t spl;
+		spl.i1 = spl.i2 = 0;
+
+		/*
+		 * Divide ...
+		 */
+		if (xdl_split(ha1, off1, lim1, ha2, off2, lim2, kvdf, kvdb,
+			      need_min, &spl, xenv) < 0) {
+
+			return -1;
+		}
+
+		/*
+		 * ... et Impera.
+		 */
+		if (xdl_recs_cmp(dd1, off1, spl.i1, dd2, off2, spl.i2,
+				 kvdf, kvdb, spl.min_lo, xenv) < 0 ||
+		    xdl_recs_cmp(dd1, spl.i1, lim1, dd2, spl.i2, lim2,
+				 kvdf, kvdb, spl.min_hi, xenv) < 0) {
+
+			return -1;
+		}
+	}
+
+	return 0;
+}
+
+
+int xdl_do_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+		xdfenv_t *xe) {
+	long ndiags;
+	long *kvd, *kvdf, *kvdb;
+	xdalgoenv_t xenv;
+	diffdata_t dd1, dd2;
+
+	if (XDF_DIFF_ALG(xpp->flags) == XDF_PATIENCE_DIFF)
+		return xdl_do_patience_diff(mf1, mf2, xpp, xe);
+
+	if (XDF_DIFF_ALG(xpp->flags) == XDF_HISTOGRAM_DIFF)
+		return xdl_do_histogram_diff(mf1, mf2, xpp, xe);
+
+	if (xdl_prepare_env(mf1, mf2, xpp, xe) < 0) {
+
+		return -1;
+	}
+
+	/*
+	 * Allocate and setup K vectors to be used by the differential algorithm.
+	 * One is to store the forward path and one to store the backward path.
+	 */
+	ndiags = xe->xdf1.nreff + xe->xdf2.nreff + 3;
+	if (!(kvd = (long *) xdl_malloc((2 * ndiags + 2) * sizeof(long)))) {
+
+		xdl_free_env(xe);
+		return -1;
+	}
+	kvdf = kvd;
+	kvdb = kvdf + ndiags;
+	kvdf += xe->xdf2.nreff + 1;
+	kvdb += xe->xdf2.nreff + 1;
+
+	xenv.mxcost = xdl_bogosqrt(ndiags);
+	if (xenv.mxcost < XDL_MAX_COST_MIN)
+		xenv.mxcost = XDL_MAX_COST_MIN;
+	xenv.snake_cnt = XDL_SNAKE_CNT;
+	xenv.heur_min = XDL_HEUR_MIN_COST;
+
+	dd1.nrec = xe->xdf1.nreff;
+	dd1.ha = xe->xdf1.ha;
+	dd1.rchg = xe->xdf1.rchg;
+	dd1.rindex = xe->xdf1.rindex;
+	dd2.nrec = xe->xdf2.nreff;
+	dd2.ha = xe->xdf2.ha;
+	dd2.rchg = xe->xdf2.rchg;
+	dd2.rindex = xe->xdf2.rindex;
+
+	if (xdl_recs_cmp(&dd1, 0, dd1.nrec, &dd2, 0, dd2.nrec,
+			 kvdf, kvdb, (xpp->flags & XDF_NEED_MINIMAL) != 0, &xenv) < 0) {
+
+		xdl_free(kvd);
+		xdl_free_env(xe);
+		return -1;
+	}
+
+	xdl_free(kvd);
+
+	return 0;
+}
+
+
+static xdchange_t *xdl_add_change(xdchange_t *xscr, long i1, long i2, long chg1, long chg2) {
+	xdchange_t *xch;
+
+	if (!(xch = (xdchange_t *) xdl_malloc(sizeof(xdchange_t))))
+		return NULL;
+
+	xch->next = xscr;
+	xch->i1 = i1;
+	xch->i2 = i2;
+	xch->chg1 = chg1;
+	xch->chg2 = chg2;
+	xch->ignore = 0;
+
+	return xch;
+}
+
+
+static int recs_match(xrecord_t *rec1, xrecord_t *rec2, long flags)
+{
+	return (rec1->ha == rec2->ha &&
+		xdl_recmatch(rec1->ptr, rec1->size,
+			     rec2->ptr, rec2->size,
+			     flags));
+}
+
+/*
+ * If a line is indented more than this, xget_indent() just returns this value.
+ * This avoids having to do absurd amounts of work for data that are not
+ * human-readable text, and also ensures that the output of xget_indent fits within
+ * an int.
+ */
+#define MAX_INDENT 200
+
+/*
+ * Return the amount of indentation of the specified line, treating TAB as 8
+ * columns. Return -1 if line is empty or contains only whitespace. Clamp the
+ * output value at MAX_INDENT.
+ */
+static int xget_indent(xrecord_t *rec)
+{
+	long i;
+	int ret = 0;
+
+	for (i = 0; i < rec->size; i++) {
+		char c = rec->ptr[i];
+
+		if (!XDL_ISSPACE(c))
+			return ret;
+		else if (c == ' ')
+			ret += 1;
+		else if (c == '\t')
+			ret += 8 - ret % 8;
+		/* ignore other whitespace characters */
+
+		if (ret >= MAX_INDENT)
+			return MAX_INDENT;
+	}
+
+	/* The line contains only whitespace. */
+	return -1;
+}
+
+/*
+ * If more than this number of consecutive blank rows are found, just return this
+ * value. This avoids requiring O(N^2) work for pathological cases, and also
+ * ensures that the output of score_split fits in an int.
+ */
+#define MAX_BLANKS 20
+
+/* Characteristics measured about a hypothetical split position. */
+struct split_measurement {
+	/*
+	 * Is the split at the end of the file (aside from any blank lines)?
+	 */
+	int end_of_file;
+
+	/*
+	 * How much is the line immediately following the split indented (or -1 if
+	 * the line is blank):
+	 */
+	int indent;
+
+	/*
+	 * How many consecutive lines above the split are blank?
+	 */
+	int pre_blank;
+
+	/*
+	 * How much is the nearest non-blank line above the split indented (or -1
+	 * if there is no such line)?
+	 */
+	int pre_indent;
+
+	/*
+	 * How many lines after the line following the split are blank?
+	 */
+	int post_blank;
+
+	/*
+	 * How much is the nearest non-blank line after the line following the
+	 * split indented (or -1 if there is no such line)?
+	 */
+	int post_indent;
+};
+
+struct split_score {
+	/* The effective indent of this split (smaller is preferred). */
+	int effective_indent;
+
+	/* Penalty for this split (smaller is preferred). */
+	int penalty;
+};
+
+/*
+ * Fill m with information about a hypothetical split of xdf above line split.
+ */
+static void measure_split(const xdfile_t *xdf, long split,
+			  struct split_measurement *m)
+{
+	long i;
+
+	if (split >= xdf->nrec) {
+		m->end_of_file = 1;
+		m->indent = -1;
+	} else {
+		m->end_of_file = 0;
+		m->indent = xget_indent(xdf->recs[split]);
+	}
+
+	m->pre_blank = 0;
+	m->pre_indent = -1;
+	for (i = split - 1; i >= 0; i--) {
+		m->pre_indent = xget_indent(xdf->recs[i]);
+		if (m->pre_indent != -1)
+			break;
+		m->pre_blank += 1;
+		if (m->pre_blank == MAX_BLANKS) {
+			m->pre_indent = 0;
+			break;
+		}
+	}
+
+	m->post_blank = 0;
+	m->post_indent = -1;
+	for (i = split + 1; i < xdf->nrec; i++) {
+		m->post_indent = xget_indent(xdf->recs[i]);
+		if (m->post_indent != -1)
+			break;
+		m->post_blank += 1;
+		if (m->post_blank == MAX_BLANKS) {
+			m->post_indent = 0;
+			break;
+		}
+	}
+}
+
+/*
+ * The empirically-determined weight factors used by score_split() below.
+ * Larger values means that the position is a less favorable place to split.
+ *
+ * Note that scores are only ever compared against each other, so multiplying
+ * all of these weight/penalty values by the same factor wouldn't change the
+ * heuristic's behavior. Still, we need to set that arbitrary scale *somehow*.
+ * In practice, these numbers are chosen to be large enough that they can be
+ * adjusted relative to each other with sufficient precision despite using
+ * integer math.
+ */
+
+/* Penalty if there are no non-blank lines before the split */
+#define START_OF_FILE_PENALTY 1
+
+/* Penalty if there are no non-blank lines after the split */
+#define END_OF_FILE_PENALTY 21
+
+/* Multiplier for the number of blank lines around the split */
+#define TOTAL_BLANK_WEIGHT (-30)
+
+/* Multiplier for the number of blank lines after the split */
+#define POST_BLANK_WEIGHT 6
+
+/*
+ * Penalties applied if the line is indented more than its predecessor
+ */
+#define RELATIVE_INDENT_PENALTY (-4)
+#define RELATIVE_INDENT_WITH_BLANK_PENALTY 10
+
+/*
+ * Penalties applied if the line is indented less than both its predecessor and
+ * its successor
+ */
+#define RELATIVE_OUTDENT_PENALTY 24
+#define RELATIVE_OUTDENT_WITH_BLANK_PENALTY 17
+
+/*
+ * Penalties applied if the line is indented less than its predecessor but not
+ * less than its successor
+ */
+#define RELATIVE_DEDENT_PENALTY 23
+#define RELATIVE_DEDENT_WITH_BLANK_PENALTY 17
+
+/*
+ * We only consider whether the sum of the effective indents for splits are
+ * less than (-1), equal to (0), or greater than (+1) each other. The resulting
+ * value is multiplied by the following weight and combined with the penalty to
+ * determine the better of two scores.
+ */
+#define INDENT_WEIGHT 60
+
+/*
+ * How far do we slide a hunk at most?
+ */
+#define INDENT_HEURISTIC_MAX_SLIDING 100
+
+/*
+ * Compute a badness score for the hypothetical split whose measurements are
+ * stored in m. The weight factors were determined empirically using the tools and
+ * corpus described in
+ *
+ *     https://github.com/mhagger/diff-slider-tools
+ *
+ * Also see that project if you want to improve the weights based on, for example,
+ * a larger or more diverse corpus.
+ */
+static void score_add_split(const struct split_measurement *m, struct split_score *s)
+{
+	/*
+	 * A place to accumulate penalty factors (positive makes this index more
+	 * favored):
+	 */
+	int post_blank, total_blank, indent, any_blanks;
+
+	if (m->pre_indent == -1 && m->pre_blank == 0)
+		s->penalty += START_OF_FILE_PENALTY;
+
+	if (m->end_of_file)
+		s->penalty += END_OF_FILE_PENALTY;
+
+	/*
+	 * Set post_blank to the number of blank lines following the split,
+	 * including the line immediately after the split:
+	 */
+	post_blank = (m->indent == -1) ? 1 + m->post_blank : 0;
+	total_blank = m->pre_blank + post_blank;
+
+	/* Penalties based on nearby blank lines: */
+	s->penalty += TOTAL_BLANK_WEIGHT * total_blank;
+	s->penalty += POST_BLANK_WEIGHT * post_blank;
+
+	if (m->indent != -1)
+		indent = m->indent;
+	else
+		indent = m->post_indent;
+
+	any_blanks = (total_blank != 0);
+
+	/* Note that the effective indent is -1 at the end of the file: */
+	s->effective_indent += indent;
+
+	if (indent == -1) {
+		/* No additional adjustments needed. */
+	} else if (m->pre_indent == -1) {
+		/* No additional adjustments needed. */
+	} else if (indent > m->pre_indent) {
+		/*
+		 * The line is indented more than its predecessor.
+		 */
+		s->penalty += any_blanks ?
+			RELATIVE_INDENT_WITH_BLANK_PENALTY :
+			RELATIVE_INDENT_PENALTY;
+	} else if (indent == m->pre_indent) {
+		/*
+		 * The line has the same indentation level as its predecessor.
+		 * No additional adjustments needed.
+		 */
+	} else {
+		/*
+		 * The line is indented less than its predecessor. It could be
+		 * the block terminator of the previous block, but it could
+		 * also be the start of a new block (e.g., an "else" block, or
+		 * maybe the previous block didn't have a block terminator).
+		 * Try to distinguish those cases based on what comes next:
+		 */
+		if (m->post_indent != -1 && m->post_indent > indent) {
+			/*
+			 * The following line is indented more. So it is likely
+			 * that this line is the start of a block.
+			 */
+			s->penalty += any_blanks ?
+				RELATIVE_OUTDENT_WITH_BLANK_PENALTY :
+				RELATIVE_OUTDENT_PENALTY;
+		} else {
+			/*
+			 * That was probably the end of a block.
+			 */
+			s->penalty += any_blanks ?
+				RELATIVE_DEDENT_WITH_BLANK_PENALTY :
+				RELATIVE_DEDENT_PENALTY;
+		}
+	}
+}
+
+static int score_cmp(struct split_score *s1, struct split_score *s2)
+{
+	/* -1 if s1.effective_indent < s2->effective_indent, etc. */
+	int cmp_indents = ((s1->effective_indent > s2->effective_indent) -
+			   (s1->effective_indent < s2->effective_indent));
+
+	return INDENT_WEIGHT * cmp_indents + (s1->penalty - s2->penalty);
+}
+
+/*
+ * Represent a group of changed lines in an xdfile_t (i.e., a contiguous group
+ * of lines that was inserted or deleted from the corresponding version of the
+ * file). We consider there to be such a group at the beginning of the file, at
+ * the end of the file, and between any two unchanged lines, though most such
+ * groups will usually be empty.
+ *
+ * If the first line in a group is equal to the line following the group, then
+ * the group can be slid down. Similarly, if the last line in a group is equal
+ * to the line preceding the group, then the group can be slid up. See
+ * group_slide_down() and group_slide_up().
+ *
+ * Note that loops that are testing for changed lines in xdf->rchg do not need
+ * index bounding since the array is prepared with a zero at position -1 and N.
+ */
+struct xdlgroup {
+	/*
+	 * The index of the first changed line in the group, or the index of
+	 * the unchanged line above which the (empty) group is located.
+	 */
+	long start;
+
+	/*
+	 * The index of the first unchanged line after the group. For an empty
+	 * group, end is equal to start.
+	 */
+	long end;
+};
+
+/*
+ * Initialize g to point at the first group in xdf.
+ */
+static void group_init(xdfile_t *xdf, struct xdlgroup *g)
+{
+	g->start = g->end = 0;
+	while (xdf->rchg[g->end])
+		g->end++;
+}
+
+/*
+ * Move g to describe the next (possibly empty) group in xdf and return 0. If g
+ * is already at the end of the file, do nothing and return -1.
+ */
+static inline int group_next(xdfile_t *xdf, struct xdlgroup *g)
+{
+	if (g->end == xdf->nrec)
+		return -1;
+
+	g->start = g->end + 1;
+	for (g->end = g->start; xdf->rchg[g->end]; g->end++)
+		;
+
+	return 0;
+}
+
+/*
+ * Move g to describe the previous (possibly empty) group in xdf and return 0.
+ * If g is already at the beginning of the file, do nothing and return -1.
+ */
+static inline int group_previous(xdfile_t *xdf, struct xdlgroup *g)
+{
+	if (g->start == 0)
+		return -1;
+
+	g->end = g->start - 1;
+	for (g->start = g->end; xdf->rchg[g->start - 1]; g->start--)
+		;
+
+	return 0;
+}
+
+/*
+ * If g can be slid toward the end of the file, do so, and if it bumps into a
+ * following group, expand this group to include it. Return 0 on success or -1
+ * if g cannot be slid down.
+ */
+static int group_slide_down(xdfile_t *xdf, struct xdlgroup *g, long flags)
+{
+	if (g->end < xdf->nrec &&
+	    recs_match(xdf->recs[g->start], xdf->recs[g->end], flags)) {
+		xdf->rchg[g->start++] = 0;
+		xdf->rchg[g->end++] = 1;
+
+		while (xdf->rchg[g->end])
+			g->end++;
+
+		return 0;
+	} else {
+		return -1;
+	}
+}
+
+/*
+ * If g can be slid toward the beginning of the file, do so, and if it bumps
+ * into a previous group, expand this group to include it. Return 0 on success
+ * or -1 if g cannot be slid up.
+ */
+static int group_slide_up(xdfile_t *xdf, struct xdlgroup *g, long flags)
+{
+	if (g->start > 0 &&
+	    recs_match(xdf->recs[g->start - 1], xdf->recs[g->end - 1], flags)) {
+		xdf->rchg[--g->start] = 1;
+		xdf->rchg[--g->end] = 0;
+
+		while (xdf->rchg[g->start - 1])
+			g->start--;
+
+		return 0;
+	} else {
+		return -1;
+	}
+}
+
+static void xdl_bug(const char *msg)
+{
+	fprintf(stderr, "BUG: %s\n", msg);
+	exit(1);
+}
+
+/*
+ * Move back and forward change groups for a consistent and pretty diff output.
+ * This also helps in finding joinable change groups and reducing the diff
+ * size.
+ */
+int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
+	struct xdlgroup g, go;
+	long earliest_end, end_matching_other;
+	long groupsize;
+
+	group_init(xdf, &g);
+	group_init(xdfo, &go);
+
+	while (1) {
+		/* If the group is empty in the to-be-compacted file, skip it: */
+		if (g.end == g.start)
+			goto next;
+
+		/*
+		 * Now shift the change up and then down as far as possible in
+		 * each direction. If it bumps into any other changes, merge them.
+		 */
+		do {
+			groupsize = g.end - g.start;
+
+			/*
+			 * Keep track of the last "end" index that causes this
+			 * group to align with a group of changed lines in the
+			 * other file. -1 indicates that we haven't found such
+			 * a match yet:
+			 */
+			end_matching_other = -1;
+
+			/* Shift the group backward as much as possible: */
+			while (!group_slide_up(xdf, &g, flags))
+				if (group_previous(xdfo, &go))
+					xdl_bug("group sync broken sliding up");
+
+			/*
+			 * This is this highest that this group can be shifted.
+			 * Record its end index:
+			 */
+			earliest_end = g.end;
+
+			if (go.end > go.start)
+				end_matching_other = g.end;
+
+			/* Now shift the group forward as far as possible: */
+			while (1) {
+				if (group_slide_down(xdf, &g, flags))
+					break;
+				if (group_next(xdfo, &go))
+					xdl_bug("group sync broken sliding down");
+
+				if (go.end > go.start)
+					end_matching_other = g.end;
+			}
+		} while (groupsize != g.end - g.start);
+
+		/*
+		 * If the group can be shifted, then we can possibly use this
+		 * freedom to produce a more intuitive diff.
+		 *
+		 * The group is currently shifted as far down as possible, so the
+		 * heuristics below only have to handle upwards shifts.
+		 */
+
+		if (g.end == earliest_end) {
+			/* no shifting was possible */
+		} else if (end_matching_other != -1) {
+			/*
+			 * Move the possibly merged group of changes back to line
+			 * up with the last group of changes from the other file
+			 * that it can align with.
+			 */
+			while (go.end == go.start) {
+				if (group_slide_up(xdf, &g, flags))
+					xdl_bug("match disappeared");
+				if (group_previous(xdfo, &go))
+					xdl_bug("group sync broken sliding to match");
+			}
+		} else if (flags & XDF_INDENT_HEURISTIC) {
+			/*
+			 * Indent heuristic: a group of pure add/delete lines
+			 * implies two splits, one between the end of the "before"
+			 * context and the start of the group, and another between
+			 * the end of the group and the beginning of the "after"
+			 * context. Some splits are aesthetically better and some
+			 * are worse. We compute a badness "score" for each split,
+			 * and add the scores for the two splits to define a
+			 * "score" for each position that the group can be shifted
+			 * to. Then we pick the shift with the lowest score.
+			 */
+			long shift, best_shift = -1;
+			struct split_score best_score;
+
+			shift = earliest_end;
+			if (g.end - groupsize - 1 > shift)
+				shift = g.end - groupsize - 1;
+			if (g.end - INDENT_HEURISTIC_MAX_SLIDING > shift)
+				shift = g.end - INDENT_HEURISTIC_MAX_SLIDING;
+			for (; shift <= g.end; shift++) {
+				struct split_measurement m;
+				struct split_score score = {0, 0};
+
+				measure_split(xdf, shift, &m);
+				score_add_split(&m, &score);
+				measure_split(xdf, shift - groupsize, &m);
+				score_add_split(&m, &score);
+				if (best_shift == -1 ||
+				    score_cmp(&score, &best_score) <= 0) {
+					best_score.effective_indent = score.effective_indent;
+					best_score.penalty = score.penalty;
+					best_shift = shift;
+				}
+			}
+
+			while (g.end > best_shift) {
+				if (group_slide_up(xdf, &g, flags))
+					xdl_bug("best shift unreached");
+				if (group_previous(xdfo, &go))
+					xdl_bug("group sync broken sliding to blank line");
+			}
+		}
+
+	next:
+		/* Move past the just-processed group: */
+		if (group_next(xdf, &g))
+			break;
+		if (group_next(xdfo, &go))
+			xdl_bug("group sync broken moving to next group");
+	}
+
+	if (!group_next(xdfo, &go))
+		xdl_bug("group sync broken at end of file");
+
+	return 0;
+}
+
+
+int xdl_build_script(xdfenv_t *xe, xdchange_t **xscr) {
+	xdchange_t *cscr = NULL, *xch;
+	char *rchg1 = xe->xdf1.rchg, *rchg2 = xe->xdf2.rchg;
+	long i1, i2, l1, l2;
+
+	/*
+	 * Trivial. Collects "groups" of changes and creates an edit script.
+	 */
+	for (i1 = xe->xdf1.nrec, i2 = xe->xdf2.nrec; i1 >= 0 || i2 >= 0; i1--, i2--)
+		if (rchg1[i1 - 1] || rchg2[i2 - 1]) {
+			for (l1 = i1; rchg1[i1 - 1]; i1--);
+			for (l2 = i2; rchg2[i2 - 1]; i2--);
+
+			if (!(xch = xdl_add_change(cscr, i1, i2, l1 - i1, l2 - i2))) {
+				xdl_free_script(cscr);
+				return -1;
+			}
+			cscr = xch;
+		}
+
+	*xscr = cscr;
+
+	return 0;
+}
+
+
+void xdl_free_script(xdchange_t *xscr) {
+	xdchange_t *xch;
+
+	while ((xch = xscr) != NULL) {
+		xscr = xscr->next;
+		xdl_free(xch);
+	}
+}
+
+static int xdl_call_hunk_func(xdfenv_t *xe UNUSED, xdchange_t *xscr, xdemitcb_t *ecb,
+			      xdemitconf_t const *xecfg)
+{
+	xdchange_t *xch, *xche;
+
+	for (xch = xscr; xch; xch = xche->next) {
+		xche = xdl_get_hunk(&xch, xecfg);
+		if (!xch)
+			break;
+		if (xecfg->hunk_func(xch->i1, xche->i1 + xche->chg1 - xch->i1,
+				     xch->i2, xche->i2 + xche->chg2 - xch->i2,
+				     ecb->priv) < 0)
+			return -1;
+	}
+	return 0;
+}
+
+static void xdl_mark_ignorable(xdchange_t *xscr, xdfenv_t *xe, long flags)
+{
+	xdchange_t *xch;
+
+	for (xch = xscr; xch; xch = xch->next) {
+		int ignore = 1;
+		xrecord_t **rec;
+		long i;
+
+		rec = &xe->xdf1.recs[xch->i1];
+		for (i = 0; i < xch->chg1 && ignore; i++)
+			ignore = xdl_blankline(rec[i]->ptr, rec[i]->size, flags);
+
+		rec = &xe->xdf2.recs[xch->i2];
+		for (i = 0; i < xch->chg2 && ignore; i++)
+			ignore = xdl_blankline(rec[i]->ptr, rec[i]->size, flags);
+
+		xch->ignore = ignore;
+	}
+}
+
+int xdl_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+	     xdemitconf_t const *xecfg, xdemitcb_t *ecb) {
+	xdchange_t *xscr;
+	xdfenv_t xe;
+	emit_func_t ef = xecfg->hunk_func ? xdl_call_hunk_func : xdl_emit_diff;
+
+	if (xdl_do_diff(mf1, mf2, xpp, &xe) < 0) {
+
+		return -1;
+	}
+	if (xdl_change_compact(&xe.xdf1, &xe.xdf2, xpp->flags) < 0 ||
+	    xdl_change_compact(&xe.xdf2, &xe.xdf1, xpp->flags) < 0 ||
+	    xdl_build_script(&xe, &xscr) < 0) {
+
+		xdl_free_env(&xe);
+		return -1;
+	}
+	if (xscr) {
+		if (xpp->flags & XDF_IGNORE_BLANK_LINES)
+			xdl_mark_ignorable(xscr, &xe, xpp->flags);
+
+		if (ef(&xe, xscr, ecb, xecfg) < 0) {
+
+			xdl_free_script(xscr);
+			xdl_free_env(&xe);
+			return -1;
+		}
+		xdl_free_script(xscr);
+	}
+	xdl_free_env(&xe);
+
+	return 0;
+}
diff --git a/src/nvim/xdiff/xdiffi.h b/src/nvim/xdiff/xdiffi.h
new file mode 100644
index 0000000000..8f1c7c8b04
--- /dev/null
+++ b/src/nvim/xdiff/xdiffi.h
@@ -0,0 +1,64 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#if !defined(XDIFFI_H)
+#define XDIFFI_H
+
+
+typedef struct s_diffdata {
+	long nrec;
+	unsigned long const *ha;
+	long *rindex;
+	char *rchg;
+} diffdata_t;
+
+typedef struct s_xdalgoenv {
+	long mxcost;
+	long snake_cnt;
+	long heur_min;
+} xdalgoenv_t;
+
+typedef struct s_xdchange {
+	struct s_xdchange *next;
+	long i1, i2;
+	long chg1, chg2;
+	int ignore;
+} xdchange_t;
+
+
+
+int xdl_recs_cmp(diffdata_t *dd1, long off1, long lim1,
+		 diffdata_t *dd2, long off2, long lim2,
+		 long *kvdf, long *kvdb, int need_min, xdalgoenv_t *xenv);
+int xdl_do_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+		xdfenv_t *xe);
+int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags);
+int xdl_build_script(xdfenv_t *xe, xdchange_t **xscr);
+void xdl_free_script(xdchange_t *xscr);
+int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
+		  xdemitconf_t const *xecfg);
+int xdl_do_patience_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+		xdfenv_t *env);
+int xdl_do_histogram_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+		xdfenv_t *env);
+
+#endif /* #if !defined(XDIFFI_H) */
diff --git a/src/nvim/xdiff/xemit.c b/src/nvim/xdiff/xemit.c
new file mode 100644
index 0000000000..d8a6f1ed38
--- /dev/null
+++ b/src/nvim/xdiff/xemit.c
@@ -0,0 +1,332 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003	Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#include "xinclude.h"
+
+static long xdl_get_rec(xdfile_t *xdf, long ri, char const **rec) {
+
+	*rec = xdf->recs[ri]->ptr;
+
+	return xdf->recs[ri]->size;
+}
+
+
+static int xdl_emit_record(xdfile_t *xdf, long ri, char const *pre, xdemitcb_t *ecb) {
+	long size, psize = (long)strlen(pre);
+	char const *rec;
+
+	size = xdl_get_rec(xdf, ri, &rec);
+	if (xdl_emit_diffrec(rec, size, pre, psize, ecb) < 0) {
+
+		return -1;
+	}
+
+	return 0;
+}
+
+
+/*
+ * Starting at the passed change atom, find the latest change atom to be included
+ * inside the differential hunk according to the specified configuration.
+ * Also advance xscr if the first changes must be discarded.
+ */
+xdchange_t *xdl_get_hunk(xdchange_t **xscr, xdemitconf_t const *xecfg)
+{
+	xdchange_t *xch, *xchp, *lxch;
+	long max_common = 2 * xecfg->ctxlen + xecfg->interhunkctxlen;
+	long max_ignorable = xecfg->ctxlen;
+	unsigned long ignored = 0; /* number of ignored blank lines */
+
+	/* remove ignorable changes that are too far before other changes */
+	for (xchp = *xscr; xchp && xchp->ignore; xchp = xchp->next) {
+		xch = xchp->next;
+
+		if (xch == NULL ||
+		    xch->i1 - (xchp->i1 + xchp->chg1) >= max_ignorable)
+			*xscr = xch;
+	}
+
+	if (*xscr == NULL)
+		return NULL;
+
+	lxch = *xscr;
+
+	for (xchp = *xscr, xch = xchp->next; xch; xchp = xch, xch = xch->next) {
+		long distance = xch->i1 - (xchp->i1 + xchp->chg1);
+		if (distance > max_common)
+			break;
+
+		if (distance < max_ignorable && (!xch->ignore || lxch == xchp)) {
+			lxch = xch;
+			ignored = 0;
+		} else if (distance < max_ignorable && xch->ignore) {
+			ignored += xch->chg2;
+		} else if (lxch != xchp &&
+			   xch->i1 + (long)ignored - (lxch->i1 + lxch->chg1) > max_common) {
+			break;
+		} else if (!xch->ignore) {
+			lxch = xch;
+			ignored = 0;
+		} else {
+			ignored += xch->chg2;
+		}
+	}
+
+	return lxch;
+}
+
+
+#if 0
+static long def_ff(const char *rec, long len, char *buf, long sz, void *priv UNUSED)
+{
+	if (len > 0 &&
+			(isalpha((unsigned char)*rec) || /* identifier? */
+			 *rec == '_' || /* also identifier? */
+			 *rec == '$')) { /* identifiers from VMS and other esoterico */
+		if (len > sz)
+			len = sz;
+		while (0 < len && isspace((unsigned char)rec[len - 1]))
+			len--;
+		memcpy(buf, rec, len);
+		return len;
+	}
+	return -1;
+}
+#endif
+
+#if 0
+static long match_func_rec(xdfile_t *xdf, xdemitconf_t const *xecfg, long ri,
+			   char *buf, long sz)
+{
+	const char *rec;
+	long len = xdl_get_rec(xdf, ri, &rec);
+	if (!xecfg->find_func)
+		return def_ff(rec, len, buf, sz, xecfg->find_func_priv);
+	return xecfg->find_func(rec, len, buf, sz, xecfg->find_func_priv);
+}
+#endif
+
+#if 0
+static int is_func_rec(xdfile_t *xdf, xdemitconf_t const *xecfg, long ri)
+{
+	char dummy[1];
+	return match_func_rec(xdf, xecfg, ri, dummy, sizeof(dummy)) >= 0;
+}
+#endif
+
+struct func_line {
+	long len;
+	char buf[80];
+};
+
+#if 0
+static long get_func_line(xdfenv_t *xe, xdemitconf_t const *xecfg,
+			  struct func_line *func_line, long start, long limit)
+{
+	long l, size, step = (start > limit) ? -1 : 1;
+	char *buf, dummy[1];
+
+	buf = func_line ? func_line->buf : dummy;
+	size = func_line ? sizeof(func_line->buf) : sizeof(dummy);
+
+	for (l = start; l != limit && 0 <= l && l < xe->xdf1.nrec; l += step) {
+		long len = match_func_rec(&xe->xdf1, xecfg, l, buf, size);
+		if (len >= 0) {
+			if (func_line)
+				func_line->len = len;
+			return l;
+		}
+	}
+	return -1;
+}
+#endif
+
+#if 0
+static int is_empty_rec(xdfile_t *xdf, long ri)
+{
+	const char *rec;
+	long len = xdl_get_rec(xdf, ri, &rec);
+
+	while (len > 0 && XDL_ISSPACE(*rec)) {
+		rec++;
+		len--;
+	}
+	return !len;
+}
+#endif
+
+int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
+		  xdemitconf_t const *xecfg) {
+	long s1, s2, e1, e2, lctx;
+	xdchange_t *xch, *xche;
+#if 0
+	long funclineprev = -1;
+#endif
+	struct func_line func_line;
+
+	func_line.len = 0;
+
+	for (xch = xscr; xch; xch = xche->next) {
+		xche = xdl_get_hunk(&xch, xecfg);
+		if (!xch)
+			break;
+
+		s1 = XDL_MAX(xch->i1 - xecfg->ctxlen, 0);
+		s2 = XDL_MAX(xch->i2 - xecfg->ctxlen, 0);
+
+#if 0
+		if (xecfg->flags & XDL_EMIT_FUNCCONTEXT) {
+			long fs1, i1 = xch->i1;
+
+			/* Appended chunk? */
+			if (i1 >= xe->xdf1.nrec) {
+				long i2 = xch->i2;
+
+				/*
+				 * We don't need additional context if
+				 * a whole function was added.
+				 */
+				while (i2 < xe->xdf2.nrec) {
+					if (is_func_rec(&xe->xdf2, xecfg, i2))
+						goto post_context_calculation;
+					i2++;
+				}
+
+				/*
+				 * Otherwise get more context from the
+				 * pre-image.
+				 */
+				i1 = xe->xdf1.nrec - 1;
+			}
+
+			fs1 = get_func_line(xe, xecfg, NULL, i1, -1);
+			while (fs1 > 0 && !is_empty_rec(&xe->xdf1, fs1 - 1) &&
+			       !is_func_rec(&xe->xdf1, xecfg, fs1 - 1))
+				fs1--;
+			if (fs1 < 0)
+				fs1 = 0;
+			if (fs1 < s1) {
+				s2 -= s1 - fs1;
+				s1 = fs1;
+			}
+		}
+
+ post_context_calculation:
+#endif
+		lctx = xecfg->ctxlen;
+		lctx = XDL_MIN(lctx, xe->xdf1.nrec - (xche->i1 + xche->chg1));
+		lctx = XDL_MIN(lctx, xe->xdf2.nrec - (xche->i2 + xche->chg2));
+
+		e1 = xche->i1 + xche->chg1 + lctx;
+		e2 = xche->i2 + xche->chg2 + lctx;
+
+#if 0
+		if (xecfg->flags & XDL_EMIT_FUNCCONTEXT) {
+			long fe1 = get_func_line(xe, xecfg, NULL,
+						 xche->i1 + xche->chg1,
+						 xe->xdf1.nrec);
+			while (fe1 > 0 && is_empty_rec(&xe->xdf1, fe1 - 1))
+				fe1--;
+			if (fe1 < 0)
+				fe1 = xe->xdf1.nrec;
+			if (fe1 > e1) {
+				e2 += fe1 - e1;
+				e1 = fe1;
+			}
+
+			/*
+			 * Overlap with next change?  Then include it
+			 * in the current hunk and start over to find
+			 * its new end.
+			 */
+			if (xche->next) {
+				long l = XDL_MIN(xche->next->i1,
+						 xe->xdf1.nrec - 1);
+				if (l - xecfg->ctxlen <= e1 ||
+				    get_func_line(xe, xecfg, NULL, l, e1) < 0) {
+					xche = xche->next;
+					goto post_context_calculation;
+				}
+			}
+		}
+#endif
+
+		/*
+		 * Emit current hunk header.
+		 */
+
+#if 0
+		if (xecfg->flags & XDL_EMIT_FUNCNAMES) {
+			get_func_line(xe, xecfg, &func_line,
+				      s1 - 1, funclineprev);
+			funclineprev = s1 - 1;
+		}
+#endif
+		if (xdl_emit_hunk_hdr(s1 + 1, e1 - s1, s2 + 1, e2 - s2,
+				      func_line.buf, func_line.len, ecb) < 0)
+			return -1;
+
+		/*
+		 * Emit pre-context.
+		 */
+		for (; s2 < xch->i2; s2++)
+			if (xdl_emit_record(&xe->xdf2, s2, " ", ecb) < 0)
+				return -1;
+
+		for (s1 = xch->i1, s2 = xch->i2;; xch = xch->next) {
+			/*
+			 * Merge previous with current change atom.
+			 */
+			for (; s1 < xch->i1 && s2 < xch->i2; s1++, s2++)
+				if (xdl_emit_record(&xe->xdf2, s2, " ", ecb) < 0)
+					return -1;
+
+			/*
+			 * Removes lines from the first file.
+			 */
+			for (s1 = xch->i1; s1 < xch->i1 + xch->chg1; s1++)
+				if (xdl_emit_record(&xe->xdf1, s1, "-", ecb) < 0)
+					return -1;
+
+			/*
+			 * Adds lines from the second file.
+			 */
+			for (s2 = xch->i2; s2 < xch->i2 + xch->chg2; s2++)
+				if (xdl_emit_record(&xe->xdf2, s2, "+", ecb) < 0)
+					return -1;
+
+			if (xch == xche)
+				break;
+			s1 = xch->i1 + xch->chg1;
+			s2 = xch->i2 + xch->chg2;
+		}
+
+		/*
+		 * Emit post-context.
+		 */
+		for (s2 = xche->i2 + xche->chg2; s2 < e2; s2++)
+			if (xdl_emit_record(&xe->xdf2, s2, " ", ecb) < 0)
+				return -1;
+	}
+
+	return 0;
+}
diff --git a/src/nvim/xdiff/xemit.h b/src/nvim/xdiff/xemit.h
new file mode 100644
index 0000000000..1b9887e670
--- /dev/null
+++ b/src/nvim/xdiff/xemit.h
@@ -0,0 +1,36 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#if !defined(XEMIT_H)
+#define XEMIT_H
+
+
+typedef int (*emit_func_t)(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
+			   xdemitconf_t const *xecfg);
+
+xdchange_t *xdl_get_hunk(xdchange_t **xscr, xdemitconf_t const *xecfg);
+int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
+		  xdemitconf_t const *xecfg);
+
+
+
+#endif /* #if !defined(XEMIT_H) */
diff --git a/src/nvim/xdiff/xhistogram.c b/src/nvim/xdiff/xhistogram.c
new file mode 100644
index 0000000000..3fb8974dd4
--- /dev/null
+++ b/src/nvim/xdiff/xhistogram.c
@@ -0,0 +1,386 @@
+/*
+ * Copyright (C) 2010, Google Inc.
+ * and other copyright owners as documented in JGit's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "xinclude.h"
+#include "xtypes.h"
+#include "xdiff.h"
+
+#define MAX_PTR	INT_MAX
+#define MAX_CNT	INT_MAX
+
+#define LINE_END(n) (line##n + count##n - 1)
+#define LINE_END_PTR(n) (*line##n + *count##n - 1)
+
+struct histindex {
+	struct record {
+		unsigned int ptr, cnt;
+		struct record *next;
+	} **records, /* an occurrence */
+	  **line_map; /* map of line to record chain */
+	chastore_t rcha;
+	unsigned int *next_ptrs;
+	unsigned int table_bits,
+		     records_size,
+		     line_map_size;
+
+	unsigned int max_chain_length,
+		     key_shift,
+		     ptr_shift;
+
+	unsigned int cnt,
+		     has_common;
+
+	xdfenv_t *env;
+	xpparam_t const *xpp;
+};
+
+struct region {
+	unsigned int begin1, end1;
+	unsigned int begin2, end2;
+};
+
+#define LINE_MAP(i, a) (i->line_map[(a) - i->ptr_shift])
+
+#define NEXT_PTR(index, ptr) \
+	(index->next_ptrs[(ptr) - index->ptr_shift])
+
+#define CNT(index, ptr) \
+	((LINE_MAP(index, ptr))->cnt)
+
+#define REC(env, s, l) \
+	(env->xdf##s.recs[l - 1])
+
+static int cmp_recs(xpparam_t const *xpp,
+	xrecord_t *r1, xrecord_t *r2)
+{
+	return r1->ha == r2->ha &&
+		xdl_recmatch(r1->ptr, r1->size, r2->ptr, r2->size,
+			    xpp->flags);
+}
+
+#define CMP_ENV(xpp, env, s1, l1, s2, l2) \
+	(cmp_recs(xpp, REC(env, s1, l1), REC(env, s2, l2)))
+
+#define CMP(i, s1, l1, s2, l2) \
+	(cmp_recs(i->xpp, REC(i->env, s1, l1), REC(i->env, s2, l2)))
+
+#define TABLE_HASH(index, side, line) \
+	XDL_HASHLONG((REC(index->env, side, line))->ha, index->table_bits)
+
+static int scanA(struct histindex *index, int line1, int count1)
+{
+	int ptr, tbl_idx;
+	unsigned int chain_len;
+	struct record **rec_chain, *rec;
+
+	for (ptr = LINE_END(1); line1 <= ptr; ptr--) {
+		tbl_idx = TABLE_HASH(index, 1, ptr);
+		rec_chain = index->records + tbl_idx;
+		rec = *rec_chain;
+
+		chain_len = 0;
+		while (rec) {
+			if (CMP(index, 1, rec->ptr, 1, ptr)) {
+				/*
+				 * ptr is identical to another element. Insert
+				 * it onto the front of the existing element
+				 * chain.
+				 */
+				NEXT_PTR(index, ptr) = rec->ptr;
+				rec->ptr = ptr;
+				/* cap rec->cnt at MAX_CNT */
+				rec->cnt = XDL_MIN(MAX_CNT, rec->cnt + 1);
+				LINE_MAP(index, ptr) = rec;
+				goto continue_scan;
+			}
+
+			rec = rec->next;
+			chain_len++;
+		}
+
+		if (chain_len == index->max_chain_length)
+			return -1;
+
+		/*
+		 * This is the first time we have ever seen this particular
+		 * element in the sequence. Construct a new chain for it.
+		 */
+		if (!(rec = xdl_cha_alloc(&index->rcha)))
+			return -1;
+		rec->ptr = ptr;
+		rec->cnt = 1;
+		rec->next = *rec_chain;
+		*rec_chain = rec;
+		LINE_MAP(index, ptr) = rec;
+
+continue_scan:
+		; /* no op */
+	}
+
+	return 0;
+}
+
+static int try_lcs(struct histindex *index, struct region *lcs, int b_ptr,
+	int line1, int count1, int line2, int count2)
+{
+	unsigned int b_next = b_ptr + 1;
+	struct record *rec = index->records[TABLE_HASH(index, 2, b_ptr)];
+	unsigned int as, ae, bs, be, np, rc;
+	int should_break;
+
+	for (; rec; rec = rec->next) {
+		if (rec->cnt > index->cnt) {
+			if (!index->has_common)
+				index->has_common = CMP(index, 1, rec->ptr, 2, b_ptr);
+			continue;
+		}
+
+		as = rec->ptr;
+		if (!CMP(index, 1, as, 2, b_ptr))
+			continue;
+
+		index->has_common = 1;
+		for (;;) {
+			should_break = 0;
+			np = NEXT_PTR(index, as);
+			bs = b_ptr;
+			ae = as;
+			be = bs;
+			rc = rec->cnt;
+
+			while (line1 < (int)as && line2 < (int)bs
+				&& CMP(index, 1, as - 1, 2, bs - 1)) {
+				as--;
+				bs--;
+				if (1 < rc)
+					rc = XDL_MIN(rc, CNT(index, as));
+			}
+			while ((int)ae < LINE_END(1) && (int)be < LINE_END(2)
+				&& CMP(index, 1, ae + 1, 2, be + 1)) {
+				ae++;
+				be++;
+				if (1 < rc)
+					rc = XDL_MIN(rc, CNT(index, ae));
+			}
+
+			if (b_next <= be)
+				b_next = be + 1;
+			if (lcs->end1 - lcs->begin1 < ae - as || rc < index->cnt) {
+				lcs->begin1 = as;
+				lcs->begin2 = bs;
+				lcs->end1 = ae;
+				lcs->end2 = be;
+				index->cnt = rc;
+			}
+
+			if (np == 0)
+				break;
+
+			while (np <= ae) {
+				np = NEXT_PTR(index, np);
+				if (np == 0) {
+					should_break = 1;
+					break;
+				}
+			}
+
+			if (should_break)
+				break;
+
+			as = np;
+		}
+	}
+	return b_next;
+}
+
+static int fall_back_to_classic_diff(xpparam_t const *xpp, xdfenv_t *env,
+		int line1, int count1, int line2, int count2)
+{
+	xpparam_t xpparam;
+	xpparam.flags = xpp->flags & ~XDF_DIFF_ALGORITHM_MASK;
+
+	return xdl_fall_back_diff(env, &xpparam,
+				  line1, count1, line2, count2);
+}
+
+static inline void free_index(struct histindex *index)
+{
+	xdl_free(index->records);
+	xdl_free(index->line_map);
+	xdl_free(index->next_ptrs);
+	xdl_cha_free(&index->rcha);
+}
+
+static int find_lcs(xpparam_t const *xpp, xdfenv_t *env,
+		    struct region *lcs,
+		    int line1, int count1, int line2, int count2)
+{
+	int b_ptr;
+	int sz, ret = -1;
+	struct histindex index;
+
+	memset(&index, 0, sizeof(index));
+
+	index.env = env;
+	index.xpp = xpp;
+
+	index.records = NULL;
+	index.line_map = NULL;
+	/* in case of early xdl_cha_free() */
+	index.rcha.head = NULL;
+
+	index.table_bits = xdl_hashbits(count1);
+	sz = index.records_size = 1 << index.table_bits;
+	sz *= sizeof(struct record *);
+	if (!(index.records = (struct record **) xdl_malloc(sz)))
+		goto cleanup;
+	memset(index.records, 0, sz);
+
+	sz = index.line_map_size = count1;
+	sz *= sizeof(struct record *);
+	if (!(index.line_map = (struct record **) xdl_malloc(sz)))
+		goto cleanup;
+	memset(index.line_map, 0, sz);
+
+	sz = index.line_map_size;
+	sz *= sizeof(unsigned int);
+	if (!(index.next_ptrs = (unsigned int *) xdl_malloc(sz)))
+		goto cleanup;
+	memset(index.next_ptrs, 0, sz);
+
+	/* lines / 4 + 1 comes from xprepare.c:xdl_prepare_ctx() */
+	if (xdl_cha_init(&index.rcha, sizeof(struct record), count1 / 4 + 1) < 0)
+		goto cleanup;
+
+	index.ptr_shift = line1;
+	index.max_chain_length = 64;
+
+	if (scanA(&index, line1, count1))
+		goto cleanup;
+
+	index.cnt = index.max_chain_length + 1;
+
+	for (b_ptr = line2; b_ptr <= LINE_END(2); )
+		b_ptr = try_lcs(&index, lcs, b_ptr, line1, count1, line2, count2);
+
+	if (index.has_common && index.max_chain_length < index.cnt)
+		ret = 1;
+	else
+		ret = 0;
+
+cleanup:
+	free_index(&index);
+	return ret;
+}
+
+static int histogram_diff(xpparam_t const *xpp, xdfenv_t *env,
+	int line1, int count1, int line2, int count2)
+{
+	struct region lcs;
+	int lcs_found;
+	int result;
+redo:
+	result = -1;
+
+	if (count1 <= 0 && count2 <= 0)
+		return 0;
+
+	if (LINE_END(1) >= MAX_PTR)
+		return -1;
+
+	if (!count1) {
+		while(count2--)
+			env->xdf2.rchg[line2++ - 1] = 1;
+		return 0;
+	} else if (!count2) {
+		while(count1--)
+			env->xdf1.rchg[line1++ - 1] = 1;
+		return 0;
+	}
+
+	memset(&lcs, 0, sizeof(lcs));
+	lcs_found = find_lcs(xpp, env, &lcs, line1, count1, line2, count2);
+	if (lcs_found < 0)
+		goto out;
+	else if (lcs_found)
+		result = fall_back_to_classic_diff(xpp, env, line1, count1, line2, count2);
+	else {
+		if (lcs.begin1 == 0 && lcs.begin2 == 0) {
+			while (count1--)
+				env->xdf1.rchg[line1++ - 1] = 1;
+			while (count2--)
+				env->xdf2.rchg[line2++ - 1] = 1;
+			result = 0;
+		} else {
+			result = histogram_diff(xpp, env,
+						line1, lcs.begin1 - line1,
+						line2, lcs.begin2 - line2);
+			if (result)
+				goto out;
+			/*
+			 * result = histogram_diff(xpp, env,
+			 *            lcs.end1 + 1, LINE_END(1) - lcs.end1,
+			 *            lcs.end2 + 1, LINE_END(2) - lcs.end2);
+			 * but let's optimize tail recursion ourself:
+			*/
+			count1 = LINE_END(1) - lcs.end1;
+			line1 = lcs.end1 + 1;
+			count2 = LINE_END(2) - lcs.end2;
+			line2 = lcs.end2 + 1;
+			goto redo;
+		}
+	}
+out:
+	return result;
+}
+
+int xdl_do_histogram_diff(mmfile_t *file1, mmfile_t *file2,
+	xpparam_t const *xpp, xdfenv_t *env)
+{
+	if (xdl_prepare_env(file1, file2, xpp, env) < 0)
+		return -1;
+
+	return histogram_diff(xpp, env,
+		env->xdf1.dstart + 1, env->xdf1.dend - env->xdf1.dstart + 1,
+		env->xdf2.dstart + 1, env->xdf2.dend - env->xdf2.dstart + 1);
+}
diff --git a/src/nvim/xdiff/xinclude.h b/src/nvim/xdiff/xinclude.h
new file mode 100644
index 0000000000..46b8608314
--- /dev/null
+++ b/src/nvim/xdiff/xinclude.h
@@ -0,0 +1,61 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+/* defines HAVE_ATTRIBUTE_UNUSED */
+#ifdef HAVE_CONFIG_H
+# include "../auto/config.h"
+#endif
+
+/* Mark unused function arguments with UNUSED, so that gcc -Wunused-parameter
+ * can be used to check for mistakes. */
+#ifdef HAVE_ATTRIBUTE_UNUSED
+# define UNUSED __attribute__((unused))
+#else
+# define UNUSED
+#endif
+
+#if defined(_MSC_VER)
+# define inline __inline
+#endif
+
+#if !defined(XINCLUDE_H)
+#define XINCLUDE_H
+
+#include <ctype.h>
+#include <stdio.h>
+#include <stdlib.h>
+#if !defined(_WIN32)
+#include <unistd.h>
+#endif
+#include <string.h>
+#include <limits.h>
+
+#include "xmacros.h"
+#include "xdiff.h"
+#include "xtypes.h"
+#include "xutils.h"
+#include "xprepare.h"
+#include "xdiffi.h"
+#include "xemit.h"
+
+
+#endif /* #if !defined(XINCLUDE_H) */
diff --git a/src/nvim/xdiff/xmacros.h b/src/nvim/xdiff/xmacros.h
new file mode 100644
index 0000000000..2809a28ca9
--- /dev/null
+++ b/src/nvim/xdiff/xmacros.h
@@ -0,0 +1,54 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#if !defined(XMACROS_H)
+#define XMACROS_H
+
+
+
+
+#define XDL_MIN(a, b) ((a) < (b) ? (a): (b))
+#define XDL_MAX(a, b) ((a) > (b) ? (a): (b))
+#define XDL_ABS(v) ((v) >= 0 ? (v): -(v))
+#define XDL_ISDIGIT(c) ((c) >= '0' && (c) <= '9')
+#define XDL_ISSPACE(c) (isspace((unsigned char)(c)))
+#define XDL_ADDBITS(v,b)	((v) + ((v) >> (b)))
+#define XDL_MASKBITS(b)		((1UL << (b)) - 1)
+#define XDL_HASHLONG(v,b)	(XDL_ADDBITS((unsigned long)(v), b) & XDL_MASKBITS(b))
+#define XDL_PTRFREE(p) do { if (p) { xdl_free(p); (p) = NULL; } } while (0)
+#define XDL_LE32_PUT(p, v) \
+do { \
+	unsigned char *__p = (unsigned char *) (p); \
+	*__p++ = (unsigned char) (v); \
+	*__p++ = (unsigned char) ((v) >> 8); \
+	*__p++ = (unsigned char) ((v) >> 16); \
+	*__p = (unsigned char) ((v) >> 24); \
+} while (0)
+#define XDL_LE32_GET(p, v) \
+do { \
+	unsigned char const *__p = (unsigned char const *) (p); \
+	(v) = (unsigned long) __p[0] | ((unsigned long) __p[1]) << 8 | \
+		((unsigned long) __p[2]) << 16 | ((unsigned long) __p[3]) << 24; \
+} while (0)
+
+
+#endif /* #if !defined(XMACROS_H) */
diff --git a/src/nvim/xdiff/xpatience.c b/src/nvim/xdiff/xpatience.c
new file mode 100644
index 0000000000..2c65aac386
--- /dev/null
+++ b/src/nvim/xdiff/xpatience.c
@@ -0,0 +1,393 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003-2016 Davide Libenzi, Johannes E. Schindelin
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+#include "xinclude.h"
+#include "xtypes.h"
+#include "xdiff.h"
+
+/*
+ * The basic idea of patience diff is to find lines that are unique in
+ * both files.  These are intuitively the ones that we want to see as
+ * common lines.
+ *
+ * The maximal ordered sequence of such line pairs (where ordered means
+ * that the order in the sequence agrees with the order of the lines in
+ * both files) naturally defines an initial set of common lines.
+ *
+ * Now, the algorithm tries to extend the set of common lines by growing
+ * the line ranges where the files have identical lines.
+ *
+ * Between those common lines, the patience diff algorithm is applied
+ * recursively, until no unique line pairs can be found; these line ranges
+ * are handled by the well-known Myers algorithm.
+ */
+
+#define NON_UNIQUE ULONG_MAX
+
+/*
+ * This is a hash mapping from line hash to line numbers in the first and
+ * second file.
+ */
+struct hashmap {
+	int nr, alloc;
+	struct entry {
+		unsigned long hash;
+		/*
+		 * 0 = unused entry, 1 = first line, 2 = second, etc.
+		 * line2 is NON_UNIQUE if the line is not unique
+		 * in either the first or the second file.
+		 */
+		unsigned long line1, line2;
+		/*
+		 * "next" & "previous" are used for the longest common
+		 * sequence;
+		 * initially, "next" reflects only the order in file1.
+		 */
+		struct entry *next, *previous;
+
+		/*
+		 * If 1, this entry can serve as an anchor. See
+		 * Documentation/diff-options.txt for more information.
+		 */
+		unsigned anchor : 1;
+	} *entries, *first, *last;
+	/* were common records found? */
+	unsigned long has_matches;
+	mmfile_t *file1, *file2;
+	xdfenv_t *env;
+	xpparam_t const *xpp;
+};
+
+static int is_anchor(xpparam_t const *xpp, const char *line)
+{
+	size_t i;
+	for (i = 0; i < xpp->anchors_nr; i++) {
+		if (!strncmp(line, xpp->anchors[i], strlen(xpp->anchors[i])))
+			return 1;
+	}
+	return 0;
+}
+
+/* The argument "pass" is 1 for the first file, 2 for the second. */
+static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
+			  int pass)
+{
+	xrecord_t **records = pass == 1 ?
+		map->env->xdf1.recs : map->env->xdf2.recs;
+	xrecord_t *record = records[line - 1], *other;
+	/*
+	 * After xdl_prepare_env() (or more precisely, due to
+	 * xdl_classify_record()), the "ha" member of the records (AKA lines)
+	 * is _not_ the hash anymore, but a linearized version of it.  In
+	 * other words, the "ha" member is guaranteed to start with 0 and
+	 * the second record's ha can only be 0 or 1, etc.
+	 *
+	 * So we multiply ha by 2 in the hope that the hashing was
+	 * "unique enough".
+	 */
+	int index = (int)((record->ha << 1) % map->alloc);
+
+	while (map->entries[index].line1) {
+		other = map->env->xdf1.recs[map->entries[index].line1 - 1];
+		if (map->entries[index].hash != record->ha ||
+				!xdl_recmatch(record->ptr, record->size,
+					other->ptr, other->size,
+					map->xpp->flags)) {
+			if (++index >= map->alloc)
+				index = 0;
+			continue;
+		}
+		if (pass == 2)
+			map->has_matches = 1;
+		if (pass == 1 || map->entries[index].line2)
+			map->entries[index].line2 = NON_UNIQUE;
+		else
+			map->entries[index].line2 = line;
+		return;
+	}
+	if (pass == 2)
+		return;
+	map->entries[index].line1 = line;
+	map->entries[index].hash = record->ha;
+	map->entries[index].anchor = is_anchor(xpp, map->env->xdf1.recs[line - 1]->ptr);
+	if (!map->first)
+		map->first = map->entries + index;
+	if (map->last) {
+		map->last->next = map->entries + index;
+		map->entries[index].previous = map->last;
+	}
+	map->last = map->entries + index;
+	map->nr++;
+}
+
+/*
+ * This function has to be called for each recursion into the inter-hunk
+ * parts, as previously non-unique lines can become unique when being
+ * restricted to a smaller part of the files.
+ *
+ * It is assumed that env has been prepared using xdl_prepare().
+ */
+static int fill_hashmap(mmfile_t *file1, mmfile_t *file2,
+		xpparam_t const *xpp, xdfenv_t *env,
+		struct hashmap *result,
+		int line1, int count1, int line2, int count2)
+{
+	result->file1 = file1;
+	result->file2 = file2;
+	result->xpp = xpp;
+	result->env = env;
+
+	/* We know exactly how large we want the hash map */
+	result->alloc = count1 * 2;
+	result->entries = (struct entry *)
+		xdl_malloc(result->alloc * sizeof(struct entry));
+	if (!result->entries)
+		return -1;
+	memset(result->entries, 0, result->alloc * sizeof(struct entry));
+
+	/* First, fill with entries from the first file */
+	while (count1--)
+		insert_record(xpp, line1++, result, 1);
+
+	/* Then search for matches in the second file */
+	while (count2--)
+		insert_record(xpp, line2++, result, 2);
+
+	return 0;
+}
+
+/*
+ * Find the longest sequence with a smaller last element (meaning a smaller
+ * line2, as we construct the sequence with entries ordered by line1).
+ */
+static int binary_search(struct entry **sequence, int longest,
+		struct entry *entry)
+{
+	int left = -1, right = longest;
+
+	while (left + 1 < right) {
+		int middle = left + (right - left) / 2;
+		/* by construction, no two entries can be equal */
+		if (sequence[middle]->line2 > entry->line2)
+			right = middle;
+		else
+			left = middle;
+	}
+	/* return the index in "sequence", _not_ the sequence length */
+	return left;
+}
+
+/*
+ * The idea is to start with the list of common unique lines sorted by
+ * the order in file1.  For each of these pairs, the longest (partial)
+ * sequence whose last element's line2 is smaller is determined.
+ *
+ * For efficiency, the sequences are kept in a list containing exactly one
+ * item per sequence length: the sequence with the smallest last
+ * element (in terms of line2).
+ */
+static struct entry *find_longest_common_sequence(struct hashmap *map)
+{
+	struct entry **sequence = (struct entry **)xdl_malloc(map->nr * sizeof(struct entry *));
+	int longest = 0, i;
+	struct entry *entry;
+	/*
+	 * If not -1, this entry in sequence must never be overridden.
+	 * Therefore, overriding entries before this has no effect, so
+	 * do not do that either.
+	 */
+	int anchor_i = -1;
+
+	/* Added to silence Coverity. */
+	if (sequence == NULL)
+		return map->first;
+
+	for (entry = map->first; entry; entry = entry->next) {
+		if (!entry->line2 || entry->line2 == NON_UNIQUE)
+			continue;
+		i = binary_search(sequence, longest, entry);
+		entry->previous = i < 0 ? NULL : sequence[i];
+		++i;
+		if (i <= anchor_i)
+			continue;
+		sequence[i] = entry;
+		if (entry->anchor) {
+			anchor_i = i;
+			longest = anchor_i + 1;
+		} else if (i == longest) {
+			longest++;
+		}
+	}
+
+	/* No common unique lines were found */
+	if (!longest) {
+		xdl_free(sequence);
+		return NULL;
+	}
+
+	/* Iterate starting at the last element, adjusting the "next" members */
+	entry = sequence[longest - 1];
+	entry->next = NULL;
+	while (entry->previous) {
+		entry->previous->next = entry;
+		entry = entry->previous;
+	}
+	xdl_free(sequence);
+	return entry;
+}
+
+static int match(struct hashmap *map, int line1, int line2)
+{
+	xrecord_t *record1 = map->env->xdf1.recs[line1 - 1];
+	xrecord_t *record2 = map->env->xdf2.recs[line2 - 1];
+	return xdl_recmatch(record1->ptr, record1->size,
+		record2->ptr, record2->size, map->xpp->flags);
+}
+
+static int patience_diff(mmfile_t *file1, mmfile_t *file2,
+		xpparam_t const *xpp, xdfenv_t *env,
+		int line1, int count1, int line2, int count2);
+
+static int walk_common_sequence(struct hashmap *map, struct entry *first,
+		int line1, int count1, int line2, int count2)
+{
+	int end1 = line1 + count1, end2 = line2 + count2;
+	int next1, next2;
+
+	for (;;) {
+		/* Try to grow the line ranges of common lines */
+		if (first) {
+			next1 = first->line1;
+			next2 = first->line2;
+			while (next1 > line1 && next2 > line2 &&
+					match(map, next1 - 1, next2 - 1)) {
+				next1--;
+				next2--;
+			}
+		} else {
+			next1 = end1;
+			next2 = end2;
+		}
+		while (line1 < next1 && line2 < next2 &&
+				match(map, line1, line2)) {
+			line1++;
+			line2++;
+		}
+
+		/* Recurse */
+		if (next1 > line1 || next2 > line2) {
+			struct hashmap submap;
+
+			memset(&submap, 0, sizeof(submap));
+			if (patience_diff(map->file1, map->file2,
+					map->xpp, map->env,
+					line1, next1 - line1,
+					line2, next2 - line2))
+				return -1;
+		}
+
+		if (!first)
+			return 0;
+
+		while (first->next &&
+				first->next->line1 == first->line1 + 1 &&
+				first->next->line2 == first->line2 + 1)
+			first = first->next;
+
+		line1 = first->line1 + 1;
+		line2 = first->line2 + 1;
+
+		first = first->next;
+	}
+}
+
+static int fall_back_to_classic_diff(struct hashmap *map,
+		int line1, int count1, int line2, int count2)
+{
+	xpparam_t xpp;
+	xpp.flags = map->xpp->flags & ~XDF_DIFF_ALGORITHM_MASK;
+
+	return xdl_fall_back_diff(map->env, &xpp,
+				  line1, count1, line2, count2);
+}
+
+/*
+ * Recursively find the longest common sequence of unique lines,
+ * and if none was found, ask xdl_do_diff() to do the job.
+ *
+ * This function assumes that env was prepared with xdl_prepare_env().
+ */
+static int patience_diff(mmfile_t *file1, mmfile_t *file2,
+		xpparam_t const *xpp, xdfenv_t *env,
+		int line1, int count1, int line2, int count2)
+{
+	struct hashmap map;
+	struct entry *first;
+	int result = 0;
+
+	/* trivial case: one side is empty */
+	if (!count1) {
+		while(count2--)
+			env->xdf2.rchg[line2++ - 1] = 1;
+		return 0;
+	} else if (!count2) {
+		while(count1--)
+			env->xdf1.rchg[line1++ - 1] = 1;
+		return 0;
+	}
+
+	memset(&map, 0, sizeof(map));
+	if (fill_hashmap(file1, file2, xpp, env, &map,
+			line1, count1, line2, count2))
+		return -1;
+
+	/* are there any matching lines at all? */
+	if (!map.has_matches) {
+		while(count1--)
+			env->xdf1.rchg[line1++ - 1] = 1;
+		while(count2--)
+			env->xdf2.rchg[line2++ - 1] = 1;
+		xdl_free(map.entries);
+		return 0;
+	}
+
+	first = find_longest_common_sequence(&map);
+	if (first)
+		result = walk_common_sequence(&map, first,
+			line1, count1, line2, count2);
+	else
+		result = fall_back_to_classic_diff(&map,
+			line1, count1, line2, count2);
+
+	xdl_free(map.entries);
+	return result;
+}
+
+int xdl_do_patience_diff(mmfile_t *file1, mmfile_t *file2,
+		xpparam_t const *xpp, xdfenv_t *env)
+{
+	if (xdl_prepare_env(file1, file2, xpp, env) < 0)
+		return -1;
+
+	/* environment is cleaned up in xdl_diff() */
+	return patience_diff(file1, file2, xpp, env,
+			1, env->xdf1.nrec, 1, env->xdf2.nrec);
+}
diff --git a/src/nvim/xdiff/xprepare.c b/src/nvim/xdiff/xprepare.c
new file mode 100644
index 0000000000..abeb8fb84e
--- /dev/null
+++ b/src/nvim/xdiff/xprepare.c
@@ -0,0 +1,483 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#include "xinclude.h"
+
+
+#define XDL_KPDIS_RUN 4
+#define XDL_MAX_EQLIMIT 1024
+#define XDL_SIMSCAN_WINDOW 100
+#define XDL_GUESS_NLINES1 256
+#define XDL_GUESS_NLINES2 20
+
+
+typedef struct s_xdlclass {
+	struct s_xdlclass *next;
+	unsigned long ha;
+	char const *line;
+	long size;
+	long idx;
+	long len1, len2;
+} xdlclass_t;
+
+typedef struct s_xdlclassifier {
+	unsigned int hbits;
+	long hsize;
+	xdlclass_t **rchash;
+	chastore_t ncha;
+	xdlclass_t **rcrecs;
+	long alloc;
+	long count;
+	long flags;
+} xdlclassifier_t;
+
+
+
+
+static int xdl_init_classifier(xdlclassifier_t *cf, long size, long flags);
+static void xdl_free_classifier(xdlclassifier_t *cf);
+static int xdl_classify_record(unsigned int pass, xdlclassifier_t *cf, xrecord_t **rhash,
+			       unsigned int hbits, xrecord_t *rec);
+static int xdl_prepare_ctx(unsigned int pass, mmfile_t *mf, long narec, xpparam_t const *xpp,
+			   xdlclassifier_t *cf, xdfile_t *xdf);
+static void xdl_free_ctx(xdfile_t *xdf);
+static int xdl_clean_mmatch(char const *dis, long i, long s, long e);
+static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xdf2);
+static int xdl_trim_ends(xdfile_t *xdf1, xdfile_t *xdf2);
+static int xdl_optimize_ctxs(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xdf2);
+
+
+
+
+static int xdl_init_classifier(xdlclassifier_t *cf, long size, long flags) {
+	cf->flags = flags;
+
+	cf->hbits = xdl_hashbits((unsigned int) size);
+	cf->hsize = 1 << cf->hbits;
+
+	if (xdl_cha_init(&cf->ncha, sizeof(xdlclass_t), size / 4 + 1) < 0) {
+
+		return -1;
+	}
+	if (!(cf->rchash = (xdlclass_t **) xdl_malloc(cf->hsize * sizeof(xdlclass_t *)))) {
+
+		xdl_cha_free(&cf->ncha);
+		return -1;
+	}
+	memset(cf->rchash, 0, cf->hsize * sizeof(xdlclass_t *));
+
+	cf->alloc = size;
+	if (!(cf->rcrecs = (xdlclass_t **) xdl_malloc(cf->alloc * sizeof(xdlclass_t *)))) {
+
+		xdl_free(cf->rchash);
+		xdl_cha_free(&cf->ncha);
+		return -1;
+	}
+
+	cf->count = 0;
+
+	return 0;
+}
+
+
+static void xdl_free_classifier(xdlclassifier_t *cf) {
+
+	xdl_free(cf->rcrecs);
+	xdl_free(cf->rchash);
+	xdl_cha_free(&cf->ncha);
+}
+
+
+static int xdl_classify_record(unsigned int pass, xdlclassifier_t *cf, xrecord_t **rhash,
+			       unsigned int hbits, xrecord_t *rec) {
+	long hi;
+	char const *line;
+	xdlclass_t *rcrec;
+	xdlclass_t **rcrecs;
+
+	line = rec->ptr;
+	hi = (long) XDL_HASHLONG(rec->ha, cf->hbits);
+	for (rcrec = cf->rchash[hi]; rcrec; rcrec = rcrec->next)
+		if (rcrec->ha == rec->ha &&
+				xdl_recmatch(rcrec->line, rcrec->size,
+					rec->ptr, rec->size, cf->flags))
+			break;
+
+	if (!rcrec) {
+		if (!(rcrec = xdl_cha_alloc(&cf->ncha))) {
+
+			return -1;
+		}
+		rcrec->idx = cf->count++;
+		if (cf->count > cf->alloc) {
+			cf->alloc *= 2;
+			if (!(rcrecs = (xdlclass_t **) xdl_realloc(cf->rcrecs, cf->alloc * sizeof(xdlclass_t *)))) {
+
+				return -1;
+			}
+			cf->rcrecs = rcrecs;
+		}
+		cf->rcrecs[rcrec->idx] = rcrec;
+		rcrec->line = line;
+		rcrec->size = rec->size;
+		rcrec->ha = rec->ha;
+		rcrec->len1 = rcrec->len2 = 0;
+		rcrec->next = cf->rchash[hi];
+		cf->rchash[hi] = rcrec;
+	}
+
+	(pass == 1) ? rcrec->len1++ : rcrec->len2++;
+
+	rec->ha = (unsigned long) rcrec->idx;
+
+	hi = (long) XDL_HASHLONG(rec->ha, hbits);
+	rec->next = rhash[hi];
+	rhash[hi] = rec;
+
+	return 0;
+}
+
+
+static int xdl_prepare_ctx(unsigned int pass, mmfile_t *mf, long narec, xpparam_t const *xpp,
+			   xdlclassifier_t *cf, xdfile_t *xdf) {
+	unsigned int hbits;
+	long nrec, hsize, bsize;
+	unsigned long hav;
+	char const *blk, *cur, *top, *prev;
+	xrecord_t *crec;
+	xrecord_t **recs, **rrecs;
+	xrecord_t **rhash;
+	unsigned long *ha;
+	char *rchg;
+	long *rindex;
+
+	ha = NULL;
+	rindex = NULL;
+	rchg = NULL;
+	rhash = NULL;
+	recs = NULL;
+
+	if (xdl_cha_init(&xdf->rcha, sizeof(xrecord_t), narec / 4 + 1) < 0)
+		goto abort;
+	if (!(recs = (xrecord_t **) xdl_malloc(narec * sizeof(xrecord_t *))))
+		goto abort;
+
+	if (XDF_DIFF_ALG(xpp->flags) == XDF_HISTOGRAM_DIFF)
+		hbits = hsize = 0;
+	else {
+		hbits = xdl_hashbits((unsigned int) narec);
+		hsize = 1 << hbits;
+		if (!(rhash = (xrecord_t **) xdl_malloc(hsize * sizeof(xrecord_t *))))
+			goto abort;
+		memset(rhash, 0, hsize * sizeof(xrecord_t *));
+	}
+
+	nrec = 0;
+	if ((cur = blk = xdl_mmfile_first(mf, &bsize)) != NULL) {
+		for (top = blk + bsize; cur < top; ) {
+			prev = cur;
+			hav = xdl_hash_record(&cur, top, xpp->flags);
+			if (nrec >= narec) {
+				narec *= 2;
+				if (!(rrecs = (xrecord_t **) xdl_realloc(recs, narec * sizeof(xrecord_t *))))
+					goto abort;
+				recs = rrecs;
+			}
+			if (!(crec = xdl_cha_alloc(&xdf->rcha)))
+				goto abort;
+			crec->ptr = prev;
+			crec->size = (long) (cur - prev);
+			crec->ha = hav;
+			recs[nrec++] = crec;
+
+			if ((XDF_DIFF_ALG(xpp->flags) != XDF_HISTOGRAM_DIFF) &&
+			    xdl_classify_record(pass, cf, rhash, hbits, crec) < 0)
+				goto abort;
+		}
+	}
+
+	if (!(rchg = (char *) xdl_malloc((nrec + 2) * sizeof(char))))
+		goto abort;
+	memset(rchg, 0, (nrec + 2) * sizeof(char));
+
+	if (!(rindex = (long *) xdl_malloc((nrec + 1) * sizeof(long))))
+		goto abort;
+	if (!(ha = (unsigned long *) xdl_malloc((nrec + 1) * sizeof(unsigned long))))
+		goto abort;
+
+	xdf->nrec = nrec;
+	xdf->recs = recs;
+	xdf->hbits = hbits;
+	xdf->rhash = rhash;
+	xdf->rchg = rchg + 1;
+	xdf->rindex = rindex;
+	xdf->nreff = 0;
+	xdf->ha = ha;
+	xdf->dstart = 0;
+	xdf->dend = nrec - 1;
+
+	return 0;
+
+abort:
+	xdl_free(ha);
+	xdl_free(rindex);
+	xdl_free(rchg);
+	xdl_free(rhash);
+	xdl_free(recs);
+	xdl_cha_free(&xdf->rcha);
+	return -1;
+}
+
+
+static void xdl_free_ctx(xdfile_t *xdf) {
+
+	xdl_free(xdf->rhash);
+	xdl_free(xdf->rindex);
+	xdl_free(xdf->rchg - 1);
+	xdl_free(xdf->ha);
+	xdl_free(xdf->recs);
+	xdl_cha_free(&xdf->rcha);
+}
+
+
+int xdl_prepare_env(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+		    xdfenv_t *xe) {
+	long enl1, enl2, sample;
+	xdlclassifier_t cf;
+
+	memset(&cf, 0, sizeof(cf));
+
+	/*
+	 * For histogram diff, we can afford a smaller sample size and
+	 * thus a poorer estimate of the number of lines, as the hash
+	 * table (rhash) won't be filled up/grown. The number of lines
+	 * (nrecs) will be updated correctly anyway by
+	 * xdl_prepare_ctx().
+	 */
+	sample = (XDF_DIFF_ALG(xpp->flags) == XDF_HISTOGRAM_DIFF
+		  ? XDL_GUESS_NLINES2 : XDL_GUESS_NLINES1);
+
+	enl1 = xdl_guess_lines(mf1, sample) + 1;
+	enl2 = xdl_guess_lines(mf2, sample) + 1;
+
+	if (XDF_DIFF_ALG(xpp->flags) != XDF_HISTOGRAM_DIFF &&
+	    xdl_init_classifier(&cf, enl1 + enl2 + 1, xpp->flags) < 0)
+		return -1;
+
+	if (xdl_prepare_ctx(1, mf1, enl1, xpp, &cf, &xe->xdf1) < 0) {
+
+		xdl_free_classifier(&cf);
+		return -1;
+	}
+	if (xdl_prepare_ctx(2, mf2, enl2, xpp, &cf, &xe->xdf2) < 0) {
+
+		xdl_free_ctx(&xe->xdf1);
+		xdl_free_classifier(&cf);
+		return -1;
+	}
+
+	if ((XDF_DIFF_ALG(xpp->flags) != XDF_PATIENCE_DIFF) &&
+	    (XDF_DIFF_ALG(xpp->flags) != XDF_HISTOGRAM_DIFF) &&
+	    xdl_optimize_ctxs(&cf, &xe->xdf1, &xe->xdf2) < 0) {
+
+		xdl_free_ctx(&xe->xdf2);
+		xdl_free_ctx(&xe->xdf1);
+		xdl_free_classifier(&cf);
+		return -1;
+	}
+
+	if (XDF_DIFF_ALG(xpp->flags) != XDF_HISTOGRAM_DIFF)
+		xdl_free_classifier(&cf);
+
+	return 0;
+}
+
+
+void xdl_free_env(xdfenv_t *xe) {
+
+	xdl_free_ctx(&xe->xdf2);
+	xdl_free_ctx(&xe->xdf1);
+}
+
+
+static int xdl_clean_mmatch(char const *dis, long i, long s, long e) {
+	long r, rdis0, rpdis0, rdis1, rpdis1;
+
+	/*
+	 * Limits the window the is examined during the similar-lines
+	 * scan. The loops below stops when dis[i - r] == 1 (line that
+	 * has no match), but there are corner cases where the loop
+	 * proceed all the way to the extremities by causing huge
+	 * performance penalties in case of big files.
+	 */
+	if (i - s > XDL_SIMSCAN_WINDOW)
+		s = i - XDL_SIMSCAN_WINDOW;
+	if (e - i > XDL_SIMSCAN_WINDOW)
+		e = i + XDL_SIMSCAN_WINDOW;
+
+	/*
+	 * Scans the lines before 'i' to find a run of lines that either
+	 * have no match (dis[j] == 0) or have multiple matches (dis[j] > 1).
+	 * Note that we always call this function with dis[i] > 1, so the
+	 * current line (i) is already a multimatch line.
+	 */
+	for (r = 1, rdis0 = 0, rpdis0 = 1; (i - r) >= s; r++) {
+		if (!dis[i - r])
+			rdis0++;
+		else if (dis[i - r] == 2)
+			rpdis0++;
+		else
+			break;
+	}
+	/*
+	 * If the run before the line 'i' found only multimatch lines, we
+	 * return 0 and hence we don't make the current line (i) discarded.
+	 * We want to discard multimatch lines only when they appear in the
+	 * middle of runs with nomatch lines (dis[j] == 0).
+	 */
+	if (rdis0 == 0)
+		return 0;
+	for (r = 1, rdis1 = 0, rpdis1 = 1; (i + r) <= e; r++) {
+		if (!dis[i + r])
+			rdis1++;
+		else if (dis[i + r] == 2)
+			rpdis1++;
+		else
+			break;
+	}
+	/*
+	 * If the run after the line 'i' found only multimatch lines, we
+	 * return 0 and hence we don't make the current line (i) discarded.
+	 */
+	if (rdis1 == 0)
+		return 0;
+	rdis1 += rdis0;
+	rpdis1 += rpdis0;
+
+	return rpdis1 * XDL_KPDIS_RUN < (rpdis1 + rdis1);
+}
+
+
+/*
+ * Try to reduce the problem complexity, discard records that have no
+ * matches on the other file. Also, lines that have multiple matches
+ * might be potentially discarded if they happear in a run of discardable.
+ */
+static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xdf2) {
+	long i, nm, nreff, mlim;
+	xrecord_t **recs;
+	xdlclass_t *rcrec;
+	char *dis, *dis1, *dis2;
+
+	if (!(dis = (char *) xdl_malloc(xdf1->nrec + xdf2->nrec + 2))) {
+
+		return -1;
+	}
+	memset(dis, 0, xdf1->nrec + xdf2->nrec + 2);
+	dis1 = dis;
+	dis2 = dis1 + xdf1->nrec + 1;
+
+	if ((mlim = xdl_bogosqrt(xdf1->nrec)) > XDL_MAX_EQLIMIT)
+		mlim = XDL_MAX_EQLIMIT;
+	for (i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart]; i <= xdf1->dend; i++, recs++) {
+		rcrec = cf->rcrecs[(*recs)->ha];
+		nm = rcrec ? rcrec->len2 : 0;
+		dis1[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;
+	}
+
+	if ((mlim = xdl_bogosqrt(xdf2->nrec)) > XDL_MAX_EQLIMIT)
+		mlim = XDL_MAX_EQLIMIT;
+	for (i = xdf2->dstart, recs = &xdf2->recs[xdf2->dstart]; i <= xdf2->dend; i++, recs++) {
+		rcrec = cf->rcrecs[(*recs)->ha];
+		nm = rcrec ? rcrec->len1 : 0;
+		dis2[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;
+	}
+
+	for (nreff = 0, i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart];
+	     i <= xdf1->dend; i++, recs++) {
+		if (dis1[i] == 1 ||
+		    (dis1[i] == 2 && !xdl_clean_mmatch(dis1, i, xdf1->dstart, xdf1->dend))) {
+			xdf1->rindex[nreff] = i;
+			xdf1->ha[nreff] = (*recs)->ha;
+			nreff++;
+		} else
+			xdf1->rchg[i] = 1;
+	}
+	xdf1->nreff = nreff;
+
+	for (nreff = 0, i = xdf2->dstart, recs = &xdf2->recs[xdf2->dstart];
+	     i <= xdf2->dend; i++, recs++) {
+		if (dis2[i] == 1 ||
+		    (dis2[i] == 2 && !xdl_clean_mmatch(dis2, i, xdf2->dstart, xdf2->dend))) {
+			xdf2->rindex[nreff] = i;
+			xdf2->ha[nreff] = (*recs)->ha;
+			nreff++;
+		} else
+			xdf2->rchg[i] = 1;
+	}
+	xdf2->nreff = nreff;
+
+	xdl_free(dis);
+
+	return 0;
+}
+
+
+/*
+ * Early trim initial and terminal matching records.
+ */
+static int xdl_trim_ends(xdfile_t *xdf1, xdfile_t *xdf2) {
+	long i, lim;
+	xrecord_t **recs1, **recs2;
+
+	recs1 = xdf1->recs;
+	recs2 = xdf2->recs;
+	for (i = 0, lim = XDL_MIN(xdf1->nrec, xdf2->nrec); i < lim;
+	     i++, recs1++, recs2++)
+		if ((*recs1)->ha != (*recs2)->ha)
+			break;
+
+	xdf1->dstart = xdf2->dstart = i;
+
+	recs1 = xdf1->recs + xdf1->nrec - 1;
+	recs2 = xdf2->recs + xdf2->nrec - 1;
+	for (lim -= i, i = 0; i < lim; i++, recs1--, recs2--)
+		if ((*recs1)->ha != (*recs2)->ha)
+			break;
+
+	xdf1->dend = xdf1->nrec - i - 1;
+	xdf2->dend = xdf2->nrec - i - 1;
+
+	return 0;
+}
+
+
+static int xdl_optimize_ctxs(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xdf2) {
+
+	if (xdl_trim_ends(xdf1, xdf2) < 0 ||
+	    xdl_cleanup_records(cf, xdf1, xdf2) < 0) {
+
+		return -1;
+	}
+
+	return 0;
+}
diff --git a/src/nvim/xdiff/xprepare.h b/src/nvim/xdiff/xprepare.h
new file mode 100644
index 0000000000..947d9fc1bb
--- /dev/null
+++ b/src/nvim/xdiff/xprepare.h
@@ -0,0 +1,34 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#if !defined(XPREPARE_H)
+#define XPREPARE_H
+
+
+
+int xdl_prepare_env(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+		    xdfenv_t *xe);
+void xdl_free_env(xdfenv_t *xe);
+
+
+
+#endif /* #if !defined(XPREPARE_H) */
diff --git a/src/nvim/xdiff/xtypes.h b/src/nvim/xdiff/xtypes.h
new file mode 100644
index 0000000000..8442bd436e
--- /dev/null
+++ b/src/nvim/xdiff/xtypes.h
@@ -0,0 +1,67 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#if !defined(XTYPES_H)
+#define XTYPES_H
+
+
+
+typedef struct s_chanode {
+	struct s_chanode *next;
+	long icurr;
+} chanode_t;
+
+typedef struct s_chastore {
+	chanode_t *head, *tail;
+	long isize, nsize;
+	chanode_t *ancur;
+	chanode_t *sncur;
+	long scurr;
+} chastore_t;
+
+typedef struct s_xrecord {
+	struct s_xrecord *next;
+	char const *ptr;
+	long size;
+	unsigned long ha;
+} xrecord_t;
+
+typedef struct s_xdfile {
+	chastore_t rcha;
+	long nrec;
+	unsigned int hbits;
+	xrecord_t **rhash;
+	long dstart, dend;
+	xrecord_t **recs;
+	char *rchg;
+	long *rindex;
+	long nreff;
+	unsigned long *ha;
+} xdfile_t;
+
+typedef struct s_xdfenv {
+	xdfile_t xdf1, xdf2;
+} xdfenv_t;
+
+
+
+#endif /* #if !defined(XTYPES_H) */
diff --git a/src/nvim/xdiff/xutils.c b/src/nvim/xdiff/xutils.c
new file mode 100644
index 0000000000..25a090fb73
--- /dev/null
+++ b/src/nvim/xdiff/xutils.c
@@ -0,0 +1,425 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003	Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#include <limits.h>
+#include <assert.h>
+#include "xinclude.h"
+
+
+
+
+long xdl_bogosqrt(long n) {
+	long i;
+
+	/*
+	 * Classical integer square root approximation using shifts.
+	 */
+	for (i = 1; n > 0; n >>= 2)
+		i <<= 1;
+
+	return i;
+}
+
+
+int xdl_emit_diffrec(char const *rec, long size, char const *pre, long psize,
+		     xdemitcb_t *ecb) {
+	int i = 2;
+	mmbuffer_t mb[3];
+
+	mb[0].ptr = (char *) pre;
+	mb[0].size = psize;
+	mb[1].ptr = (char *) rec;
+	mb[1].size = size;
+	if (size > 0 && rec[size - 1] != '\n') {
+		mb[2].ptr = (char *) "\n\\ No newline at end of file\n";
+		mb[2].size = (long)strlen(mb[2].ptr);
+		i++;
+	}
+	if (ecb->outf(ecb->priv, mb, i) < 0) {
+
+		return -1;
+	}
+
+	return 0;
+}
+
+void *xdl_mmfile_first(mmfile_t *mmf, long *size)
+{
+	*size = mmf->size;
+	return mmf->ptr;
+}
+
+
+long xdl_mmfile_size(mmfile_t *mmf)
+{
+	return mmf->size;
+}
+
+
+int xdl_cha_init(chastore_t *cha, long isize, long icount) {
+
+	cha->head = cha->tail = NULL;
+	cha->isize = isize;
+	cha->nsize = icount * isize;
+	cha->ancur = cha->sncur = NULL;
+	cha->scurr = 0;
+
+	return 0;
+}
+
+
+void xdl_cha_free(chastore_t *cha) {
+	chanode_t *cur, *tmp;
+
+	for (cur = cha->head; (tmp = cur) != NULL;) {
+		cur = cur->next;
+		xdl_free(tmp);
+	}
+}
+
+
+void *xdl_cha_alloc(chastore_t *cha) {
+	chanode_t *ancur;
+	void *data;
+
+	if (!(ancur = cha->ancur) || ancur->icurr == cha->nsize) {
+		if (!(ancur = (chanode_t *) xdl_malloc(sizeof(chanode_t) + cha->nsize))) {
+
+			return NULL;
+		}
+		ancur->icurr = 0;
+		ancur->next = NULL;
+		if (cha->tail)
+			cha->tail->next = ancur;
+		if (!cha->head)
+			cha->head = ancur;
+		cha->tail = ancur;
+		cha->ancur = ancur;
+	}
+
+	data = (char *) ancur + sizeof(chanode_t) + ancur->icurr;
+	ancur->icurr += cha->isize;
+
+	return data;
+}
+
+long xdl_guess_lines(mmfile_t *mf, long sample) {
+	long nl = 0, size, tsize = 0;
+	char const *data, *cur, *top;
+
+	if ((cur = data = xdl_mmfile_first(mf, &size)) != NULL) {
+		for (top = data + size; nl < sample && cur < top; ) {
+			nl++;
+			if (!(cur = memchr(cur, '\n', top - cur)))
+				cur = top;
+			else
+				cur++;
+		}
+		tsize += (long) (cur - data);
+	}
+
+	if (nl && tsize)
+		nl = xdl_mmfile_size(mf) / (tsize / nl);
+
+	return nl + 1;
+}
+
+int xdl_blankline(const char *line, long size, long flags)
+{
+	long i;
+
+	if (!(flags & XDF_WHITESPACE_FLAGS))
+		return (size <= 1);
+
+	for (i = 0; i < size && XDL_ISSPACE(line[i]); i++)
+		;
+
+	return (i == size);
+}
+
+/*
+ * Have we eaten everything on the line, except for an optional
+ * CR at the very end?
+ */
+static int ends_with_optional_cr(const char *l, long s, long i)
+{
+	int complete = s && l[s-1] == '\n';
+
+	if (complete)
+		s--;
+	if (s == i)
+		return 1;
+	/* do not ignore CR at the end of an incomplete line */
+	if (complete && s == i + 1 && l[i] == '\r')
+		return 1;
+	return 0;
+}
+
+int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
+{
+	int i1, i2;
+
+	if (s1 == s2 && !memcmp(l1, l2, s1))
+		return 1;
+	if (!(flags & XDF_WHITESPACE_FLAGS))
+		return 0;
+
+	i1 = 0;
+	i2 = 0;
+
+	/*
+	 * -w matches everything that matches with -b, and -b in turn
+	 * matches everything that matches with --ignore-space-at-eol,
+	 * which in turn matches everything that matches with --ignore-cr-at-eol.
+	 *
+	 * Each flavor of ignoring needs different logic to skip whitespaces
+	 * while we have both sides to compare.
+	 */
+	if (flags & XDF_IGNORE_WHITESPACE) {
+		goto skip_ws;
+		while (i1 < s1 && i2 < s2) {
+			if (l1[i1++] != l2[i2++])
+				return 0;
+		skip_ws:
+			while (i1 < s1 && XDL_ISSPACE(l1[i1]))
+				i1++;
+			while (i2 < s2 && XDL_ISSPACE(l2[i2]))
+				i2++;
+		}
+	} else if (flags & XDF_IGNORE_WHITESPACE_CHANGE) {
+		while (i1 < s1 && i2 < s2) {
+			if (XDL_ISSPACE(l1[i1]) && XDL_ISSPACE(l2[i2])) {
+				/* Skip matching spaces and try again */
+				while (i1 < s1 && XDL_ISSPACE(l1[i1]))
+					i1++;
+				while (i2 < s2 && XDL_ISSPACE(l2[i2]))
+					i2++;
+				continue;
+			}
+			if (l1[i1++] != l2[i2++])
+				return 0;
+		}
+	} else if (flags & XDF_IGNORE_WHITESPACE_AT_EOL) {
+		while (i1 < s1 && i2 < s2 && l1[i1] == l2[i2]) {
+			i1++;
+			i2++;
+		}
+	} else if (flags & XDF_IGNORE_CR_AT_EOL) {
+		/* Find the first difference and see how the line ends */
+		while (i1 < s1 && i2 < s2 && l1[i1] == l2[i2]) {
+			i1++;
+			i2++;
+		}
+		return (ends_with_optional_cr(l1, s1, i1) &&
+			ends_with_optional_cr(l2, s2, i2));
+	}
+
+	/*
+	 * After running out of one side, the remaining side must have
+	 * nothing but whitespace for the lines to match.  Note that
+	 * ignore-whitespace-at-eol case may break out of the loop
+	 * while there still are characters remaining on both lines.
+	 */
+	if (i1 < s1) {
+		while (i1 < s1 && XDL_ISSPACE(l1[i1]))
+			i1++;
+		if (s1 != i1)
+			return 0;
+	}
+	if (i2 < s2) {
+		while (i2 < s2 && XDL_ISSPACE(l2[i2]))
+			i2++;
+		return (s2 == i2);
+	}
+	return 1;
+}
+
+static unsigned long xdl_hash_record_with_whitespace(char const **data,
+		char const *top, long flags) {
+	unsigned long ha = 5381;
+	char const *ptr = *data;
+	int cr_at_eol_only = (flags & XDF_WHITESPACE_FLAGS) == XDF_IGNORE_CR_AT_EOL;
+
+	for (; ptr < top && *ptr != '\n'; ptr++) {
+		if (cr_at_eol_only) {
+			/* do not ignore CR at the end of an incomplete line */
+			if (*ptr == '\r' &&
+			    (ptr + 1 < top && ptr[1] == '\n'))
+				continue;
+		}
+		else if (XDL_ISSPACE(*ptr)) {
+			const char *ptr2 = ptr;
+			int at_eol;
+			while (ptr + 1 < top && XDL_ISSPACE(ptr[1])
+					&& ptr[1] != '\n')
+				ptr++;
+			at_eol = (top <= ptr + 1 || ptr[1] == '\n');
+			if (flags & XDF_IGNORE_WHITESPACE)
+				; /* already handled */
+			else if (flags & XDF_IGNORE_WHITESPACE_CHANGE
+				 && !at_eol) {
+				ha += (ha << 5);
+				ha ^= (unsigned long) ' ';
+			}
+			else if (flags & XDF_IGNORE_WHITESPACE_AT_EOL
+				 && !at_eol) {
+				while (ptr2 != ptr + 1) {
+					ha += (ha << 5);
+					ha ^= (unsigned long) *ptr2;
+					ptr2++;
+				}
+			}
+			continue;
+		}
+		ha += (ha << 5);
+		ha ^= (unsigned long) *ptr;
+	}
+	*data = ptr < top ? ptr + 1: ptr;
+
+	return ha;
+}
+
+unsigned long xdl_hash_record(char const **data, char const *top, long flags) {
+	unsigned long ha = 5381;
+	char const *ptr = *data;
+
+	if (flags & XDF_WHITESPACE_FLAGS)
+		return xdl_hash_record_with_whitespace(data, top, flags);
+
+	for (; ptr < top && *ptr != '\n'; ptr++) {
+		ha += (ha << 5);
+		ha ^= (unsigned long) *ptr;
+	}
+	*data = ptr < top ? ptr + 1: ptr;
+
+	return ha;
+}
+
+unsigned int xdl_hashbits(unsigned int size) {
+	unsigned int val = 1, bits = 0;
+
+	for (; val < size && bits < CHAR_BIT * sizeof(unsigned int); val <<= 1, bits++);
+	return bits ? bits: 1;
+}
+
+
+int xdl_num_out(char *out, long val) {
+	char *ptr, *str = out;
+	char buf[32];
+
+	ptr = buf + sizeof(buf) - 1;
+	*ptr = '\0';
+	if (val < 0) {
+		*--ptr = '-';
+		val = -val;
+	}
+	for (; val && ptr > buf; val /= 10)
+		*--ptr = "0123456789"[val % 10];
+	if (*ptr)
+		for (; *ptr; ptr++, str++)
+			*str = *ptr;
+	else
+		*str++ = '0';
+	*str = '\0';
+
+	return str - out;
+}
+
+int xdl_emit_hunk_hdr(long s1, long c1, long s2, long c2,
+		      const char *func, long funclen, xdemitcb_t *ecb) {
+	int nb = 0;
+	mmbuffer_t mb;
+	char buf[128];
+
+	memcpy(buf, "@@ -", 4);
+	nb += 4;
+
+	nb += xdl_num_out(buf + nb, c1 ? s1: s1 - 1);
+
+	if (c1 != 1) {
+		memcpy(buf + nb, ",", 1);
+		nb += 1;
+
+		nb += xdl_num_out(buf + nb, c1);
+	}
+
+	memcpy(buf + nb, " +", 2);
+	nb += 2;
+
+	nb += xdl_num_out(buf + nb, c2 ? s2: s2 - 1);
+
+	if (c2 != 1) {
+		memcpy(buf + nb, ",", 1);
+		nb += 1;
+
+		nb += xdl_num_out(buf + nb, c2);
+	}
+
+	memcpy(buf + nb, " @@", 3);
+	nb += 3;
+	if (func && funclen) {
+		buf[nb++] = ' ';
+		if (funclen > (long)sizeof(buf) - nb - 1)
+			funclen = sizeof(buf) - nb - 1;
+		memcpy(buf + nb, func, funclen);
+		nb += funclen;
+	}
+	buf[nb++] = '\n';
+
+	mb.ptr = buf;
+	mb.size = nb;
+	if (ecb->outf(ecb->priv, &mb, 1) < 0)
+		return -1;
+
+	return 0;
+}
+
+int xdl_fall_back_diff(xdfenv_t *diff_env, xpparam_t const *xpp,
+		int line1, int count1, int line2, int count2)
+{
+	/*
+	 * This probably does not work outside Git, since
+	 * we have a very simple mmfile structure.
+	 *
+	 * Note: ideally, we would reuse the prepared environment, but
+	 * the libxdiff interface does not (yet) allow for diffing only
+	 * ranges of lines instead of the whole files.
+	 */
+	mmfile_t subfile1, subfile2;
+	xdfenv_t env;
+
+	subfile1.ptr = (char *)diff_env->xdf1.recs[line1 - 1]->ptr;
+	subfile1.size = diff_env->xdf1.recs[line1 + count1 - 2]->ptr +
+		diff_env->xdf1.recs[line1 + count1 - 2]->size - subfile1.ptr;
+	subfile2.ptr = (char *)diff_env->xdf2.recs[line2 - 1]->ptr;
+	subfile2.size = diff_env->xdf2.recs[line2 + count2 - 2]->ptr +
+		diff_env->xdf2.recs[line2 + count2 - 2]->size - subfile2.ptr;
+	if (xdl_do_diff(&subfile1, &subfile2, xpp, &env) < 0)
+		return -1;
+
+	memcpy(diff_env->xdf1.rchg + line1 - 1, env.xdf1.rchg, count1);
+	memcpy(diff_env->xdf2.rchg + line2 - 1, env.xdf2.rchg, count2);
+
+	xdl_free_env(&env);
+
+	return 0;
+}
diff --git a/src/nvim/xdiff/xutils.h b/src/nvim/xdiff/xutils.h
new file mode 100644
index 0000000000..fba7bae03c
--- /dev/null
+++ b/src/nvim/xdiff/xutils.h
@@ -0,0 +1,47 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+#if !defined(XUTILS_H)
+#define XUTILS_H
+
+
+
+long xdl_bogosqrt(long n);
+int xdl_emit_diffrec(char const *rec, long size, char const *pre, long psize,
+		     xdemitcb_t *ecb);
+int xdl_cha_init(chastore_t *cha, long isize, long icount);
+void xdl_cha_free(chastore_t *cha);
+void *xdl_cha_alloc(chastore_t *cha);
+long xdl_guess_lines(mmfile_t *mf, long sample);
+int xdl_blankline(const char *line, long size, long flags);
+int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags);
+unsigned long xdl_hash_record(char const **data, char const *top, long flags);
+unsigned int xdl_hashbits(unsigned int size);
+int xdl_num_out(char *out, long val);
+int xdl_emit_hunk_hdr(long s1, long c1, long s2, long c2,
+		      const char *func, long funclen, xdemitcb_t *ecb);
+int xdl_fall_back_diff(xdfenv_t *diff_env, xpparam_t const *xpp,
+		       int line1, int count1, int line2, int count2);
+
+
+
+#endif /* #if !defined(XUTILS_H) */
diff --git a/test/functional/ui/diff_spec.lua b/test/functional/ui/diff_spec.lua
new file mode 100644
index 0000000000..8e6756e550
--- /dev/null
+++ b/test/functional/ui/diff_spec.lua
@@ -0,0 +1,959 @@
+local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
+
+local feed = helpers.feed
+local clear = helpers.clear
+local write_file = helpers.write_file
+
+describe('Diff mode screen', function()
+  local fname = 'Xtest-functional-diff-screen-1'
+  local fname_2 = fname .. '.2'
+  local screen
+
+  local reread = function()
+    feed(':e<cr><c-w>w:e<cr><c-w>w')
+  end
+
+  setup(function()
+    clear()
+    os.remove(fname)
+    os.remove(fname_2)
+  end)
+
+  teardown(function()
+    os.remove(fname)
+    os.remove(fname_2)
+  end)
+
+  before_each(function()
+    clear()
+    feed(':e ' .. fname_2 .. '<cr>')
+    feed(':vnew ' .. fname .. '<cr>')
+    feed(':diffthis<cr>')
+    feed('<c-w>w:diffthis<cr><c-w>w')
+
+    screen = Screen.new(40, 16)
+    screen:attach()
+    screen:set_default_attr_ids({
+      [1] = {foreground = Screen.colors.DarkBlue, background = Screen.colors.WebGray},
+      [2] = {background = Screen.colors.LightCyan1, bold = true, foreground = Screen.colors.Blue1},
+      [3] = {reverse = true},
+      [4] = {background = Screen.colors.LightBlue},
+      [5] = {foreground = Screen.colors.DarkBlue, background = Screen.colors.LightGrey},
+      [6] = {bold = true, foreground = Screen.colors.Blue1},
+      [7] = {bold = true, reverse = true},
+      [8] = {bold = true, background = Screen.colors.Red},
+      [9] = {background = Screen.colors.LightMagenta},
+    })
+  end)
+
+  it('Add a line in beginning of file 2', function()
+    write_file(fname, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", false)
+    write_file(fname_2, "0\n1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", false)
+    reread()
+
+    feed(':set diffopt=filler<cr>')
+    screen:expect([[
+      {1:  }{2:------------------}{3:│}{1:  }{4:0                }|
+      {1:  }^1                 {3:│}{1:  }1                |
+      {1:  }2                 {3:│}{1:  }2                |
+      {1:  }3                 {3:│}{1:  }3                |
+      {1:  }4                 {3:│}{1:  }4                |
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:+ }{5:+--  4 lines: 7···}{3:│}{1:+ }{5:+--  4 lines: 7··}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler                     |
+    ]])
+
+    feed(':set diffopt+=internal<cr>')
+    screen:expect([[
+      {1:  }{2:------------------}{3:│}{1:  }{4:0                }|
+      {1:  }^1                 {3:│}{1:  }1                |
+      {1:  }2                 {3:│}{1:  }2                |
+      {1:  }3                 {3:│}{1:  }3                |
+      {1:  }4                 {3:│}{1:  }4                |
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:+ }{5:+--  4 lines: 7···}{3:│}{1:+ }{5:+--  4 lines: 7··}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  it('Add a line in beginning of file 1', function()
+    write_file(fname, "0\n1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", false)
+    write_file(fname_2, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", false)
+    reread()
+
+    feed(":set diffopt=filler<cr>")
+    screen:expect([[
+      {1:  }{4:^0                 }{3:│}{1:  }{2:-----------------}|
+      {1:  }1                 {3:│}{1:  }1                |
+      {1:  }2                 {3:│}{1:  }2                |
+      {1:  }3                 {3:│}{1:  }3                |
+      {1:  }4                 {3:│}{1:  }4                |
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:+ }{5:+--  4 lines: 7···}{3:│}{1:+ }{5:+--  4 lines: 7··}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler                     |
+    ]])
+
+    feed(":set diffopt+=internal<cr>")
+    screen:expect([[
+      {1:  }{4:^0                 }{3:│}{1:  }{2:-----------------}|
+      {1:  }1                 {3:│}{1:  }1                |
+      {1:  }2                 {3:│}{1:  }2                |
+      {1:  }3                 {3:│}{1:  }3                |
+      {1:  }4                 {3:│}{1:  }4                |
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:+ }{5:+--  4 lines: 7···}{3:│}{1:+ }{5:+--  4 lines: 7··}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  it('Add a line at the end of file 2', function()
+    write_file(fname, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", false)
+    write_file(fname_2, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n", false)
+    reread()
+
+    feed(":set diffopt=filler<cr>")
+    screen:expect([[
+      {1:+ }{5:^+--  4 lines: 1···}{3:│}{1:+ }{5:+--  4 lines: 1··}|
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:  }7                 {3:│}{1:  }7                |
+      {1:  }8                 {3:│}{1:  }8                |
+      {1:  }9                 {3:│}{1:  }9                |
+      {1:  }10                {3:│}{1:  }10               |
+      {1:  }{2:------------------}{3:│}{1:  }{4:11               }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler                     |
+    ]])
+
+    feed(":set diffopt+=internal<cr>")
+    screen:expect([[
+      {1:+ }{5:^+--  4 lines: 1···}{3:│}{1:+ }{5:+--  4 lines: 1··}|
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:  }7                 {3:│}{1:  }7                |
+      {1:  }8                 {3:│}{1:  }8                |
+      {1:  }9                 {3:│}{1:  }9                |
+      {1:  }10                {3:│}{1:  }10               |
+      {1:  }{2:------------------}{3:│}{1:  }{4:11               }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  it('Add a line at the end of file 1', function()
+    write_file(fname, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n", false)
+    write_file(fname_2, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", false)
+    reread()
+
+    feed(":set diffopt=filler<cr>")
+    screen:expect([[
+      {1:+ }{5:^+--  4 lines: 1···}{3:│}{1:+ }{5:+--  4 lines: 1··}|
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:  }7                 {3:│}{1:  }7                |
+      {1:  }8                 {3:│}{1:  }8                |
+      {1:  }9                 {3:│}{1:  }9                |
+      {1:  }10                {3:│}{1:  }10               |
+      {1:  }{4:11                }{3:│}{1:  }{2:-----------------}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler                     |
+    ]])
+
+    feed(":set diffopt+=internal<cr>")
+    screen:expect([[
+      {1:+ }{5:^+--  4 lines: 1···}{3:│}{1:+ }{5:+--  4 lines: 1··}|
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:  }7                 {3:│}{1:  }7                |
+      {1:  }8                 {3:│}{1:  }8                |
+      {1:  }9                 {3:│}{1:  }9                |
+      {1:  }10                {3:│}{1:  }10               |
+      {1:  }{4:11                }{3:│}{1:  }{2:-----------------}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  it('Add a line in the middle of file 2, remove on at the end of file 1', function()
+    write_file(fname, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n", false)
+    write_file(fname_2, "1\n2\n3\n4\n4\n5\n6\n7\n8\n9\n10\n", false)
+    reread()
+
+    feed(':set diffopt=filler<cr>')
+    screen:expect([[
+      {1:  }^1                 {3:│}{1:  }1                |
+      {1:  }2                 {3:│}{1:  }2                |
+      {1:  }3                 {3:│}{1:  }3                |
+      {1:  }4                 {3:│}{1:  }4                |
+      {1:  }{2:------------------}{3:│}{1:  }{4:4                }|
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:  }7                 {3:│}{1:  }7                |
+      {1:  }8                 {3:│}{1:  }8                |
+      {1:  }9                 {3:│}{1:  }9                |
+      {1:  }10                {3:│}{1:  }10               |
+      {1:  }{4:11                }{3:│}{1:  }{2:-----------------}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler                     |
+    ]])
+
+    feed(':set diffopt+=internal<cr>')
+    screen:expect([[
+      {1:  }^1                 {3:│}{1:  }1                |
+      {1:  }2                 {3:│}{1:  }2                |
+      {1:  }3                 {3:│}{1:  }3                |
+      {1:  }4                 {3:│}{1:  }4                |
+      {1:  }{2:------------------}{3:│}{1:  }{4:4                }|
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:  }7                 {3:│}{1:  }7                |
+      {1:  }8                 {3:│}{1:  }8                |
+      {1:  }9                 {3:│}{1:  }9                |
+      {1:  }10                {3:│}{1:  }10               |
+      {1:  }{4:11                }{3:│}{1:  }{2:-----------------}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  it('Add a line in the middle of file 1, remove on at the end of file 2', function()
+    write_file(fname, "1\n2\n3\n4\n4\n5\n6\n7\n8\n9\n10\n", false)
+    write_file(fname_2, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n", false)
+    reread()
+
+    feed(':set diffopt=filler<cr>')
+    screen:expect([[
+      {1:  }^1                 {3:│}{1:  }1                |
+      {1:  }2                 {3:│}{1:  }2                |
+      {1:  }3                 {3:│}{1:  }3                |
+      {1:  }4                 {3:│}{1:  }4                |
+      {1:  }{4:4                 }{3:│}{1:  }{2:-----------------}|
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:  }7                 {3:│}{1:  }7                |
+      {1:  }8                 {3:│}{1:  }8                |
+      {1:  }9                 {3:│}{1:  }9                |
+      {1:  }10                {3:│}{1:  }10               |
+      {1:  }{2:------------------}{3:│}{1:  }{4:11               }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler                     |
+    ]])
+
+    feed(':set diffopt+=internal<cr>')
+    screen:expect([[
+      {1:  }^1                 {3:│}{1:  }1                |
+      {1:  }2                 {3:│}{1:  }2                |
+      {1:  }3                 {3:│}{1:  }3                |
+      {1:  }4                 {3:│}{1:  }4                |
+      {1:  }{4:4                 }{3:│}{1:  }{2:-----------------}|
+      {1:  }5                 {3:│}{1:  }5                |
+      {1:  }6                 {3:│}{1:  }6                |
+      {1:  }7                 {3:│}{1:  }7                |
+      {1:  }8                 {3:│}{1:  }8                |
+      {1:  }9                 {3:│}{1:  }9                |
+      {1:  }10                {3:│}{1:  }10               |
+      {1:  }{2:------------------}{3:│}{1:  }{4:11               }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  describe('normal/patience/histogram diff algorithm', function()
+    setup(function()
+      local f1 = [[#include <stdio.h>
+
+// Frobs foo heartily
+int frobnitz(int foo)
+{
+    int i;
+    for(i = 0; i < 10; i++)
+    {
+        printf("Your answer is: ");
+        printf("%d\n", foo);
+    }
+}
+
+int fact(int n)
+{
+    if(n > 1)
+    {
+        return fact(n-1) * n;
+    }
+    return 1;
+}
+
+int main(int argc, char **argv)
+{
+    frobnitz(fact(10));
+}]]
+      write_file(fname, f1, false)
+      local f2 = [[#include <stdio.h>
+
+int fib(int n)
+{
+    if(n > 2)
+    {
+        return fib(n-1) + fib(n-2);
+    }
+    return 1;
+}
+
+// Frobs foo heartily
+int frobnitz(int foo)
+{
+    int i;
+    for(i = 0; i < 10; i++)
+    {
+        printf("%d\n", foo);
+    }
+}
+
+int main(int argc, char **argv)
+{
+    frobnitz(fib(10));
+}]]
+      write_file(fname_2, f2, false)
+    end)
+
+    it('diffopt=+algorithm:myers', function()
+      reread()
+      feed(':set diffopt=internal,filler<cr>')
+      screen:expect([[
+        {1:  }^#include <stdio.h>{3:│}{1:  }#include <stdio.h|
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }{8:// Frobs foo heart}{3:│}{1:  }{8:int fib(int n)}{9:   }|
+        {1:  }{4:int frobnitz(int f}{3:│}{1:  }{2:-----------------}|
+        {1:  }{                 {3:│}{1:  }{                |
+        {1:  }{9:    i}{8:nt i;}{9:        }{3:│}{1:  }{9:    i}{8:f(n > 2)}{9:    }|
+        {1:  }{4:    for(i = 0; i <}{3:│}{1:  }{2:-----------------}|
+        {1:  }    {             {3:│}{1:  }    {            |
+        {1:  }{9:        }{8:printf("Yo}{3:│}{1:  }{9:        }{8:return fi}|
+        {1:  }{4:        printf("%d}{3:│}{1:  }{2:-----------------}|
+        {1:  }    }             {3:│}{1:  }    }            |
+        {1:  }{2:------------------}{3:│}{1:  }{4:    return 1;    }|
+        {1:  }}                 {3:│}{1:  }}                |
+        {1:  }                  {3:│}{1:  }                 |
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :set diffopt=internal,filler            |
+      ]])
+
+      feed('G')
+      screen:expect([[
+        {1:  }{2:------------------}{3:│}{1:  }{4:int frobnitz(int }|
+        {1:  }{                 {3:│}{1:  }{                |
+        {1:  }{9:    i}{8:f(n > 1)}{9:     }{3:│}{1:  }{9:    i}{8:nt i;}{9:       }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    for(i = 0; i }|
+        {1:  }    {             {3:│}{1:  }    {            |
+        {1:  }{9:        }{8:return fac}{3:│}{1:  }{9:        }{8:printf("%}|
+        {1:  }    }             {3:│}{1:  }    }            |
+        {1:  }{4:    return 1;     }{3:│}{1:  }{2:-----------------}|
+        {1:  }}                 {3:│}{1:  }}                |
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }int main(int argc,{3:│}{1:  }int main(int argc|
+        {1:  }{                 {3:│}{1:  }{                |
+        {1:  }{9:    frobnitz(f}{8:act}{9:(}{3:│}{1:  }{9:    frobnitz(f}{8:ib}{9:(}|
+        {1:  }^}                 {3:│}{1:  }}                |
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :set diffopt=internal,filler            |
+      ]])
+    end)
+
+    it('diffopt+=algorithm:patience', function()
+      reread()
+      feed(':set diffopt=internal,filler,algorithm:patience<cr>')
+      screen:expect([[
+        {1:  }^#include <stdio.h>{3:│}{1:  }#include <stdio.h|
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }{2:------------------}{3:│}{1:  }{4:int fib(int n)   }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:{                }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    if(n > 2)    }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    {            }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:        return fi}|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    }            }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    return 1;    }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:}                }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:                 }|
+        {1:  }// Frobs foo heart{3:│}{1:  }// Frobs foo hear|
+        {1:  }int frobnitz(int f{3:│}{1:  }int frobnitz(int |
+        {1:  }{                 {3:│}{1:  }{                |
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+                                                |
+      ]])
+
+      feed('G')
+      screen:expect([[
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }{4:int fact(int n)   }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:{                 }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:    if(n > 1)     }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:    {             }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:        return fac}{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:    }             }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:    return 1;     }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:}                 }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:                  }{3:│}{1:  }{2:-----------------}|
+        {1:  }int main(int argc,{3:│}{1:  }int main(int argc|
+        {1:  }{                 {3:│}{1:  }{                |
+        {1:  }{9:    frobnitz(f}{8:act}{9:(}{3:│}{1:  }{9:    frobnitz(f}{8:ib}{9:(}|
+        {1:  }^}                 {3:│}{1:  }}                |
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+                                                |
+      ]])
+    end)
+
+    it('diffopt+=algorithm:histogram', function()
+      reread()
+      feed(':set diffopt=internal,filler,algorithm:histogram<cr>')
+      screen:expect([[
+        {1:  }^#include <stdio.h>{3:│}{1:  }#include <stdio.h|
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }{2:------------------}{3:│}{1:  }{4:int fib(int n)   }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:{                }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    if(n > 2)    }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    {            }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:        return fi}|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    }            }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    return 1;    }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:}                }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:                 }|
+        {1:  }// Frobs foo heart{3:│}{1:  }// Frobs foo hear|
+        {1:  }int frobnitz(int f{3:│}{1:  }int frobnitz(int |
+        {1:  }{                 {3:│}{1:  }{                |
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+                                                |
+      ]])
+
+      feed('G')
+      screen:expect([[
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }{4:int fact(int n)   }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:{                 }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:    if(n > 1)     }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:    {             }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:        return fac}{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:    }             }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:    return 1;     }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:}                 }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:                  }{3:│}{1:  }{2:-----------------}|
+        {1:  }int main(int argc,{3:│}{1:  }int main(int argc|
+        {1:  }{                 {3:│}{1:  }{                |
+        {1:  }{9:    frobnitz(f}{8:act}{9:(}{3:│}{1:  }{9:    frobnitz(f}{8:ib}{9:(}|
+        {1:  }^}                 {3:│}{1:  }}                |
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+                                                |
+      ]])
+    end)
+  end)
+
+  describe('diffopt+=indent-heuristic', function()
+    setup(function()
+      local f1 = [[
+  def finalize(values)
+
+    values.each do |v|
+      v.finalize
+    end]]
+      write_file(fname, f1, false)
+      local f2 = [[
+  def finalize(values)
+
+    values.each do |v|
+      v.prepare
+    end
+
+    values.each do |v|
+      v.finalize
+    end]]
+      write_file(fname_2, f2, false)
+      feed(':diffupdate!<cr>')
+    end)
+
+    it('internal', function()
+      reread()
+      feed(":set diffopt=internal,filler<cr>")
+      screen:expect([[
+        {1:  }^def finalize(value{3:│}{1:  }def finalize(valu|
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }  values.each do |{3:│}{1:  }  values.each do |
+        {1:  }{2:------------------}{3:│}{1:  }{4:    v.prepare    }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:  end            }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:                 }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:  values.each do }|
+        {1:  }    v.finalize    {3:│}{1:  }    v.finalize   |
+        {1:  }  end             {3:│}{1:  }  end            |
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :set diffopt=internal,filler            |
+      ]])
+    end)
+
+    it('indent-heuristic', function()
+      reread()
+      feed(':set diffopt=internal,filler,indent-heuristic<cr>')
+      screen:expect([[
+        {1:  }^def finalize(value{3:│}{1:  }def finalize(valu|
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }{2:------------------}{3:│}{1:  }{4:  values.each do }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    v.prepare    }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:  end            }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:                 }|
+        {1:  }  values.each do |{3:│}{1:  }  values.each do |
+        {1:  }    v.finalize    {3:│}{1:  }    v.finalize   |
+        {1:  }  end             {3:│}{1:  }  end            |
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+                                                |
+      ]])
+    end)
+
+    it('indent-heuristic random order', function()
+      reread()
+      feed(':set diffopt=internal,filler,indent-heuristic,algorithm:patience<cr>')
+      feed(':<cr>')
+      screen:expect([[
+        {1:  }^def finalize(value{3:│}{1:  }def finalize(valu|
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }{2:------------------}{3:│}{1:  }{4:  values.each do }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:    v.prepare    }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:  end            }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:                 }|
+        {1:  }  values.each do |{3:│}{1:  }  values.each do |
+        {1:  }    v.finalize    {3:│}{1:  }    v.finalize   |
+        {1:  }  end             {3:│}{1:  }  end            |
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :                                       |
+      ]])
+    end)
+  end)
+
+  it('Diff the same file', function()
+    write_file(fname, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", false)
+    write_file(fname_2, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", false)
+    reread()
+
+    feed(':set diffopt=filler<cr>')
+    screen:expect([[
+      {1:+ }{5:^+-- 10 lines: 1···}{3:│}{1:+ }{5:+-- 10 lines: 1··}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler                     |
+    ]])
+
+    feed(':set diffopt+=internal<cr>')
+    screen:expect([[
+      {1:+ }{5:^+-- 10 lines: 1···}{3:│}{1:+ }{5:+-- 10 lines: 1··}|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  it('Diff an empty file', function()
+    write_file(fname, "", false)
+    write_file(fname_2, "", false)
+    reread()
+
+    feed(':set diffopt=filler<cr>')
+    screen:expect([[
+      {1:- }^                  {3:│}{1:- }                 |
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler                     |
+    ]])
+
+    feed(':set diffopt+=internal<cr>')
+    screen:expect([[
+      {1:- }^                  {3:│}{1:- }                 |
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  it('diffopt+=icase', function()
+    write_file(fname, "a\nb\ncd\n", false)
+    write_file(fname_2, "A\nb\ncDe\n", false)
+    reread()
+
+    feed(':set diffopt=filler,icase<cr>')
+    screen:expect([[
+      {1:  }^a                 {3:│}{1:  }A                |
+      {1:  }b                 {3:│}{1:  }b                |
+      {1:  }{9:cd                }{3:│}{1:  }{9:cD}{8:e}{9:              }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt=filler,icase               |
+    ]])
+
+    feed(':set diffopt+=internal<cr>')
+    screen:expect([[
+      {1:  }^a                 {3:│}{1:  }A                |
+      {1:  }b                 {3:│}{1:  }b                |
+      {1:  }{9:cd                }{3:│}{1:  }{9:cD}{8:e}{9:              }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+      {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+      :set diffopt+=internal                  |
+    ]])
+  end)
+
+  describe('diffopt+=iwhite', function()
+    setup(function()
+      local f1 = 'int main()\n{\n   printf("Hello, World!");\n   return 0;\n}\n'
+      write_file(fname, f1, false)
+      local f2 = 'int main()\n{\n   if (0)\n   {\n      printf("Hello, World!");\n      return 0;\n   }\n}\n'
+      write_file(fname_2, f2, false)
+      feed(':diffupdate!<cr>')
+    end)
+
+    it('external', function()
+      reread()
+      feed(':set diffopt=filler,iwhite<cr>')
+      screen:expect([[
+        {1:  }^int main()        {3:│}{1:  }int main()       |
+        {1:  }{                 {3:│}{1:  }{                |
+        {1:  }{2:------------------}{3:│}{1:  }{4:   if (0)        }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:   {             }|
+        {1:  }   printf("Hello, {3:│}{1:  }      printf("Hel|
+        {1:  }   return 0;      {3:│}{1:  }      return 0;  |
+        {1:  }{2:------------------}{3:│}{1:  }{4:   }             }|
+        {1:  }}                 {3:│}{1:  }}                |
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :set diffopt=filler,iwhite              |
+      ]])
+    end)
+
+    it('internal', function()
+      reread()
+      feed(':set diffopt=filler,iwhite,internal<cr>')
+      screen:expect([[
+        {1:  }^int main()        {3:│}{1:  }int main()       |
+        {1:  }{                 {3:│}{1:  }{                |
+        {1:  }{2:------------------}{3:│}{1:  }{4:   if (0)        }|
+        {1:  }{2:------------------}{3:│}{1:  }{4:   {             }|
+        {1:  }   printf("Hello, {3:│}{1:  }      printf("Hel|
+        {1:  }   return 0;      {3:│}{1:  }      return 0;  |
+        {1:  }{2:------------------}{3:│}{1:  }{4:   }             }|
+        {1:  }}                 {3:│}{1:  }}                |
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :set diffopt=filler,iwhite,internal     |
+      ]])
+    end)
+  end)
+
+  describe('diffopt+=iblank', function()
+    setup(function()
+      write_file(fname, 'a\n\n \ncd\nef\nxxx\n', false)
+      write_file(fname_2, 'a\ncd\n\nef\nyyy\n', false)
+      feed(':diffupdate!<cr>')
+    end)
+
+    it('generic', function()
+      reread()
+      feed(':set diffopt=internal,filler,iblank<cr>')
+      screen:expect([[
+        {1:  }^a                 {3:│}{1:  }a                |
+        {1:  }{4:                  }{3:│}{1:  }{2:-----------------}|
+        {1:  }{4:                  }{3:│}{1:  }{2:-----------------}|
+        {1:  }cd                {3:│}{1:  }cd               |
+        {1:  }ef                {3:│}{1:  }                 |
+        {1:  }{8:xxx}{9:               }{3:│}{1:  }ef               |
+        {1:  }{6:~                 }{3:│}{1:  }{8:yyy}{9:              }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :set diffopt=internal,filler,iblank     |
+      ]])
+    end)
+
+    it('diffopt+=iwhite', function()
+      reread()
+      feed(':set diffopt=internal,filler,iblank,iwhite<cr>')
+      feed(':<cr>')
+      screen:expect([[
+        {1:  }^a                 {3:│}{1:  }a                |
+        {1:  }                  {3:│}{1:  }cd               |
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }cd                {3:│}{1:  }ef               |
+        {1:  }ef                {3:│}{1:  }{8:yyy}{9:              }|
+        {1:  }{8:xxx}{9:               }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :                                       |
+      ]])
+    end)
+
+    it('diffopt+=iwhiteall', function()
+      reread()
+      feed(':set diffopt=internal,filler,iblank,iwhiteall<cr>')
+      feed(':<cr>')
+      screen:expect([[
+        {1:  }^a                 {3:│}{1:  }a                |
+        {1:  }                  {3:│}{1:  }cd               |
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }cd                {3:│}{1:  }ef               |
+        {1:  }ef                {3:│}{1:  }{8:yyy}{9:              }|
+        {1:  }{8:xxx}{9:               }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :                                       |
+      ]])
+    end)
+
+    it('diffopt+=iwhiteeol', function()
+      reread()
+      feed(':set diffopt=internal,filler,iblank,iwhiteeol<cr>')
+      feed(':<cr>')
+      screen:expect([[
+        {1:  }^a                 {3:│}{1:  }a                |
+        {1:  }                  {3:│}{1:  }cd               |
+        {1:  }                  {3:│}{1:  }                 |
+        {1:  }cd                {3:│}{1:  }ef               |
+        {1:  }ef                {3:│}{1:  }{8:yyy}{9:              }|
+        {1:  }{8:xxx}{9:               }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :                                       |
+      ]])
+    end)
+  end)
+
+  describe('diffopt+=iwhite{eol,all}', function()
+    setup(function()
+      write_file(fname, 'a \nx\ncd\nef\nxx  xx\nfoo\nbar\n', false)
+      write_file(fname_2, 'a\nx\nc d\n ef\nxx xx\nfoo\n\nbar\n', false)
+      feed(':diffupdate!<cr>')
+    end)
+
+    it('diffopt+=iwhiteeol', function()
+      reread()
+      feed(':set diffopt=internal,filler,iwhiteeol<cr>')
+      feed(':<cr>')
+      screen:expect([[
+        {1:  }^a                 {3:│}{1:  }a                |
+        {1:  }x                 {3:│}{1:  }x                |
+        {1:  }{9:cd                }{3:│}{1:  }{9:c}{8: }{9:d              }|
+        {1:  }{9:ef                }{3:│}{1:  }{8: }{9:ef              }|
+        {1:  }{9:xx }{8: }{9:xx            }{3:│}{1:  }{9:xx xx            }|
+        {1:  }foo               {3:│}{1:  }foo              |
+        {1:  }{2:------------------}{3:│}{1:  }{4:                 }|
+        {1:  }bar               {3:│}{1:  }bar              |
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :                                       |
+      ]])
+    end)
+
+    it('diffopt+=iwhiteall', function()
+      reread()
+      feed(':set diffopt=internal,filler,iwhiteall<cr>')
+      feed(':<cr>')
+      screen:expect([[
+        {1:  }^a                 {3:│}{1:  }a                |
+        {1:  }x                 {3:│}{1:  }x                |
+        {1:  }cd                {3:│}{1:  }c d              |
+        {1:  }ef                {3:│}{1:  } ef              |
+        {1:  }xx  xx            {3:│}{1:  }xx xx            |
+        {1:  }foo               {3:│}{1:  }foo              |
+        {1:  }{2:------------------}{3:│}{1:  }{4:                 }|
+        {1:  }bar               {3:│}{1:  }bar              |
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {1:  }{6:~                 }{3:│}{1:  }{6:~                }|
+        {7:<onal-diff-screen-1  }{3:<l-diff-screen-1.2 }|
+        :                                       |
+      ]])
+    end)
+  end)
+end)