3 files changed, 177 insertions, 118 deletions

diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 20c188ed55..b22f799da0 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -7577,10 +7577,11 @@ static void ex_bang(exarg_T *eap)
  */
 static void ex_undo(exarg_T *eap)
 {
-  if (eap->addr_count == 1)         /* :undo 123 */
-    undo_time(eap->line2, FALSE, FALSE, TRUE);
-  else
+  if (eap->addr_count == 1) {       // :undo 123
+    undo_time(eap->line2, false, false, true);
+  } else {
     u_undo(1);
+  }
 }
 
 static void ex_wundo(exarg_T *eap)
@@ -7613,8 +7614,8 @@ static void ex_redo(exarg_T *eap)
 static void ex_later(exarg_T *eap)
 {
   long count = 0;
-  int sec = FALSE;
-  int file = FALSE;
+  bool sec = false;
+  bool file = false;
   char_u      *p = eap->arg;
 
   if (*p == NUL)
@@ -7622,11 +7623,11 @@ static void ex_later(exarg_T *eap)
   else if (isdigit(*p)) {
     count = getdigits_long(&p);
     switch (*p) {
-    case 's': ++p; sec = TRUE; break;
-    case 'm': ++p; sec = TRUE; count *= 60; break;
-    case 'h': ++p; sec = TRUE; count *= 60 * 60; break;
-    case 'd': ++p; sec = TRUE; count *= 24 * 60 * 60; break;
-    case 'f': ++p; file = TRUE; break;
+    case 's': ++p; sec = true; break;
+    case 'm': ++p; sec = true; count *= 60; break;
+    case 'h': ++p; sec = true; count *= 60 * 60; break;
+    case 'd': ++p; sec = true; count *= 24 * 60 * 60; break;
+    case 'f': ++p; file = true; break;
     }
   }
 
@@ -7634,7 +7635,7 @@ static void ex_later(exarg_T *eap)
     EMSG2(_(e_invarg2), eap->arg);
   else
     undo_time(eap->cmdidx == CMD_earlier ? -count : count,
-        sec, file, FALSE);
+              sec, file, false);
 }
 
 /*
diff --git a/src/nvim/testdir/test_undo.vim b/src/nvim/testdir/test_undo.vim
index f31499607b..83ede1dc37 100644
--- a/src/nvim/testdir/test_undo.vim
+++ b/src/nvim/testdir/test_undo.vim
@@ -390,3 +390,47 @@ funct Test_undofile()
 
   set undodir&
 endfunc
+
+func Test_undo_0()
+  new
+  set ul=100
+  normal i1
+  undo
+  normal i2
+  undo
+  normal i3
+
+  undo 0
+  let d = undotree()
+  call assert_equal('', getline(1))
+  call assert_equal(0, d.seq_cur)
+
+  redo
+  let d = undotree()
+  call assert_equal('3', getline(1))
+  call assert_equal(3, d.seq_cur)
+
+  undo 2
+  undo 0
+  let d = undotree()
+  call assert_equal('', getline(1))
+  call assert_equal(0, d.seq_cur)
+
+  redo
+  let d = undotree()
+  call assert_equal('2', getline(1))
+  call assert_equal(2, d.seq_cur)
+
+  undo 1
+  undo 0
+  let d = undotree()
+  call assert_equal('', getline(1))
+  call assert_equal(0, d.seq_cur)
+
+  redo
+  let d = undotree()
+  call assert_equal('1', getline(1))
+  call assert_equal(1, d.seq_cur)
+
+  bwipe!
+endfunc
diff --git a/src/nvim/undo.c b/src/nvim/undo.c
index f4eb50b3b5..e15b9ec796 100644
--- a/src/nvim/undo.c
+++ b/src/nvim/undo.c
@@ -122,7 +122,7 @@ static long u_newcount, u_oldcount;
  * When 'u' flag included in 'cpoptions', we behave like vi.  Need to remember
  * the action that "u" should do.
  */
-static int undo_undoes = FALSE;
+static bool undo_undoes = false;
 
 static int lastmark = 0;
 
@@ -591,7 +591,7 @@ int u_savecommon(linenr_T top, linenr_T bot, linenr_T newbot, int reload)
   uep->ue_next = curbuf->b_u_newhead->uh_entry;
   curbuf->b_u_newhead->uh_entry = uep;
   curbuf->b_u_synced = false;
-  undo_undoes = FALSE;
+  undo_undoes = false;
 
 #ifdef U_DEBUG
   u_check(FALSE);
@@ -1675,10 +1675,11 @@ void u_undo(int count)
     count = 1;
   }
 
-  if (vim_strchr(p_cpo, CPO_UNDO) == NULL)
-    undo_undoes = TRUE;
-  else
+  if (vim_strchr(p_cpo, CPO_UNDO) == NULL) {
+    undo_undoes = true;
+  } else {
     undo_undoes = !undo_undoes;
+  }
   u_doit(count, false, true);
 }
 
@@ -1804,31 +1805,29 @@ static void u_doit(int startcount, bool quiet, bool do_buf_event)
   u_undo_end(undo_undoes, false, quiet);
 }
 
-/*
- * Undo or redo over the timeline.
- * When "step" is negative go back in time, otherwise goes forward in time.
- * When "sec" is FALSE make "step" steps, when "sec" is TRUE use "step" as
- * seconds.
- * When "file" is TRUE use "step" as a number of file writes.
- * When "absolute" is TRUE use "step" as the sequence number to jump to.
- * "sec" must be FALSE then.
- */
-void undo_time(long step, int sec, int file, int absolute)
+// Undo or redo over the timeline.
+// When "step" is negative go back in time, otherwise goes forward in time.
+// When "sec" is false make "step" steps, when "sec" is true use "step" as
+// seconds.
+// When "file" is true use "step" as a number of file writes.
+// When "absolute" is true use "step" as the sequence number to jump to.
+// "sec" must be false then.
+void undo_time(long step, bool sec, bool file, bool absolute)
 {
   long target;
   long closest;
   long closest_start;
   long closest_seq = 0;
   long val;
-  u_header_T      *uhp;
+  u_header_T      *uhp = NULL;
   u_header_T      *last;
   int mark;
   int nomark;
   int round;
-  int dosec = sec;
-  int dofile = file;
-  int above = FALSE;
-  int did_undo = TRUE;
+  bool dosec = sec;
+  bool dofile = file;
+  bool above = false;
+  bool did_undo = true;
 
   /* First make sure the current undoable change is synced. */
   if (curbuf->b_u_synced == false)
@@ -1842,13 +1841,7 @@ void undo_time(long step, int sec, int file, int absolute)
   /* "target" is the node below which we want to be.
    * Init "closest" to a value we can't reach. */
   if (absolute) {
-    if (step == 0) {
-      // target 0 does not exist, got to 1 and above it.
-      target = 1;
-      above = true;
-    } else {
-      target = step;
-    }
+    target = step;
     closest = -1;
   } else {
     if (dosec) {
@@ -1873,7 +1866,7 @@ void undo_time(long step, int sec, int file, int absolute)
         if (target <= 0)
           /* Go to before first write: before the oldest change. Use
            * the sequence number for that. */
-          dofile = FALSE;
+          dofile = false;
       } else {
         /* Moving forward to a newer write. */
         target = curbuf->b_u_save_nr_cur + step;
@@ -1881,7 +1874,7 @@ void undo_time(long step, int sec, int file, int absolute)
           /* Go to after last write: after the latest change. Use
            * the sequence number for that. */
           target = curbuf->b_u_seq_last + 1;
-          dofile = FALSE;
+          dofile = false;
         }
       }
     } else
@@ -1906,6 +1899,11 @@ void undo_time(long step, int sec, int file, int absolute)
   closest_start = closest;
   closest_seq = curbuf->b_u_seq_cur;
 
+  // When "target" is 0; Back to origin.
+  if (target == 0) {
+    goto found;
+  }
+
   /*
    * May do this twice:
    * 1. Search for "target", update "closest" to the best match found.
@@ -2015,17 +2013,17 @@ void undo_time(long step, int sec, int file, int absolute)
     }
 
     target = closest_seq;
-    dosec = FALSE;
-    dofile = FALSE;
-    if (step < 0)
-      above = TRUE;             /* stop above the header */
+    dosec = false;
+    dofile = false;
+    if (step < 0) {
+      above = true;             // stop above the header
+    }
   }
 
-  /* If we found it: Follow the path to go to where we want to be. */
-  if (uhp != NULL) {
-    /*
-     * First go up the tree as much as needed.
-     */
+found:
+  // If we found it: Follow the path to go to where we want to be.
+  if (uhp != NULL || target == 0) {
+    // First go up the tree as much as needed.
     while (!got_int) {
       /* Do the change warning now, for the same reason as above. */
       change_warning(0);
@@ -2035,83 +2033,97 @@ void undo_time(long step, int sec, int file, int absolute)
         uhp = curbuf->b_u_newhead;
       else
         uhp = uhp->uh_next.ptr;
-      if (uhp == NULL || uhp->uh_walk != mark
-          || (uhp->uh_seq == target && !above))
+      if (uhp == NULL
+          || (target > 0 && uhp->uh_walk != mark)
+          || (uhp->uh_seq == target && !above)) {
         break;
+      }
       curbuf->b_u_curhead = uhp;
       u_undoredo(true, true);
-      uhp->uh_walk = nomark;            // don't go back down here
+      if (target > 0) {
+        uhp->uh_walk = nomark;          // don't go back down here
+      }
     }
 
-    /*
-     * And now go down the tree (redo), branching off where needed.
-     */
-    while (!got_int) {
-      /* Do the change warning now, for the same reason as above. */
-      change_warning(0);
+    // When back to origin, redo is not needed.
+    if (target > 0) {
+      // And now go down the tree (redo), branching off where needed.
+      while (!got_int) {
+        // Do the change warning now, for the same reason as above.
+        change_warning(0);
 
-      uhp = curbuf->b_u_curhead;
-      if (uhp == NULL)
-        break;
-
-      /* Go back to the first branch with a mark. */
-      while (uhp->uh_alt_prev.ptr != NULL
-             && uhp->uh_alt_prev.ptr->uh_walk == mark)
-        uhp = uhp->uh_alt_prev.ptr;
+        uhp = curbuf->b_u_curhead;
+        if (uhp == NULL) {
+          break;
+        }
 
-      /* Find the last branch with a mark, that's the one. */
-      last = uhp;
-      while (last->uh_alt_next.ptr != NULL
-             && last->uh_alt_next.ptr->uh_walk == mark)
-        last = last->uh_alt_next.ptr;
-      if (last != uhp) {
-        /* Make the used branch the first entry in the list of
-         * alternatives to make "u" and CTRL-R take this branch. */
-        while (uhp->uh_alt_prev.ptr != NULL)
+        // Go back to the first branch with a mark.
+        while (uhp->uh_alt_prev.ptr != NULL
+               && uhp->uh_alt_prev.ptr->uh_walk == mark) {
           uhp = uhp->uh_alt_prev.ptr;
-        if (last->uh_alt_next.ptr != NULL)
-          last->uh_alt_next.ptr->uh_alt_prev.ptr =
-            last->uh_alt_prev.ptr;
-        last->uh_alt_prev.ptr->uh_alt_next.ptr = last->uh_alt_next.ptr;
-        last->uh_alt_prev.ptr = NULL;
-        last->uh_alt_next.ptr = uhp;
-        uhp->uh_alt_prev.ptr = last;
-
-        if (curbuf->b_u_oldhead == uhp)
-          curbuf->b_u_oldhead = last;
-        uhp = last;
-        if (uhp->uh_next.ptr != NULL)
-          uhp->uh_next.ptr->uh_prev.ptr = uhp;
-      }
-      curbuf->b_u_curhead = uhp;
+        }
 
-      if (uhp->uh_walk != mark)
-        break;              /* must have reached the target */
+        // Find the last branch with a mark, that's the one.
+        last = uhp;
+        while (last->uh_alt_next.ptr != NULL
+               && last->uh_alt_next.ptr->uh_walk == mark) {
+          last = last->uh_alt_next.ptr;
+        }
+        if (last != uhp) {
+          // Make the used branch the first entry in the list of
+          // alternatives to make "u" and CTRL-R take this branch.
+          while (uhp->uh_alt_prev.ptr != NULL) {
+            uhp = uhp->uh_alt_prev.ptr;
+          }
+          if (last->uh_alt_next.ptr != NULL) {
+            last->uh_alt_next.ptr->uh_alt_prev.ptr = last->uh_alt_prev.ptr;
+          }
+          last->uh_alt_prev.ptr->uh_alt_next.ptr = last->uh_alt_next.ptr;
+          last->uh_alt_prev.ptr = NULL;
+          last->uh_alt_next.ptr = uhp;
+          uhp->uh_alt_prev.ptr = last;
 
-      /* Stop when going backwards in time and didn't find the exact
-       * header we were looking for. */
-      if (uhp->uh_seq == target && above) {
-        curbuf->b_u_seq_cur = target - 1;
-        break;
-      }
+          if (curbuf->b_u_oldhead == uhp) {
+            curbuf->b_u_oldhead = last;
+          }
+          uhp = last;
+          if (uhp->uh_next.ptr != NULL) {
+            uhp->uh_next.ptr->uh_prev.ptr = uhp;
+          }
+        }
+        curbuf->b_u_curhead = uhp;
 
-      u_undoredo(false, true);
+        if (uhp->uh_walk != mark) {
+          break;            // must have reached the target
+        }
 
-      /* Advance "curhead" to below the header we last used.  If it
-      * becomes NULL then we need to set "newhead" to this leaf. */
-      if (uhp->uh_prev.ptr == NULL)
-        curbuf->b_u_newhead = uhp;
-      curbuf->b_u_curhead = uhp->uh_prev.ptr;
-      did_undo = FALSE;
+        // Stop when going backwards in time and didn't find the exact
+        // header we were looking for.
+        if (uhp->uh_seq == target && above) {
+          curbuf->b_u_seq_cur = target - 1;
+          break;
+        }
 
-      if (uhp->uh_seq == target)        /* found it! */
-        break;
+        u_undoredo(false, true);
 
-      uhp = uhp->uh_prev.ptr;
-      if (uhp == NULL || uhp->uh_walk != mark) {
-        // Need to redo more but can't find it...
-        internal_error("undo_time()");
-        break;
+        // Advance "curhead" to below the header we last used.  If it
+        // becomes NULL then we need to set "newhead" to this leaf.
+        if (uhp->uh_prev.ptr == NULL) {
+          curbuf->b_u_newhead = uhp;
+        }
+        curbuf->b_u_curhead = uhp->uh_prev.ptr;
+        did_undo = false;
+
+        if (uhp->uh_seq == target) {    // found it!
+          break;
+        }
+
+        uhp = uhp->uh_prev.ptr;
+        if (uhp == NULL || uhp->uh_walk != mark) {
+          // Need to redo more but can't find it...
+          internal_error("undo_time()");
+          break;
+        }
       }
     }
   }
@@ -2375,8 +2387,8 @@ static void u_undoredo(int undo, bool do_buf_event)
 /// Otherwise, report the number of changes (this may be incorrect
 /// in some cases, but it's better than nothing).
 static void u_undo_end(
-    int did_undo,     ///< just did an undo
-    int absolute,     ///< used ":undo N"
+    bool did_undo,    ///< just did an undo
+    bool absolute,    ///< used ":undo N"
     bool quiet)
 {
   char        *msgstr;
@@ -2416,13 +2428,15 @@ static void u_undo_end(
     /* For ":undo N" we prefer a "after #N" message. */
     if (absolute && curbuf->b_u_curhead->uh_next.ptr != NULL) {
       uhp = curbuf->b_u_curhead->uh_next.ptr;
-      did_undo = FALSE;
-    } else if (did_undo)
+      did_undo = false;
+    } else if (did_undo) {
       uhp = curbuf->b_u_curhead;
-    else
+    } else {
       uhp = curbuf->b_u_curhead->uh_next.ptr;
-  } else
+    }
+  } else {
     uhp = curbuf->b_u_newhead;
+  }
 
   if (uhp == NULL)
     *msgbuf = NUL;