vim-patch:9.1.1009: diff feature can be improved

Problem:  diff feature can be improved
Solution: include the linematch diff alignment algorithm
          (Jonathon)

closes: vim/vim#9661

https://github.com/vim/vim/commit/7c7a4e6d1ad50d5b25b42aa2d5a33a8d04a4cc8a

Co-authored-by: Jonathon <jonathonwhite@protonmail.com>

1 files changed, 17 insertions, 5 deletions

diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index 99f70793b3..4ead58c74f 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -1820,7 +1820,8 @@ static void find_top_diff_block(diff_T **thistopdiff, diff_T **nextblockblock, i
       topdiffchange = 0;
     }
 
-    // check if the fromwin topline is matched by the current diff. if so, set it to the top of the diff block
+    // check if the fromwin topline is matched by the current diff. if so,
+    // set it to the top of the diff block
     if (topline >= topdiff->df_lnum[fromidx] && topline <=
         (topdiff->df_lnum[fromidx] + topdiff->df_count[fromidx])) {
       // this line is inside the current diff block, so we will save the
@@ -2021,10 +2022,15 @@ static void run_linematch_algorithm(diff_T *dp)
   size_t ndiffs = 0;
   for (int i = 0; i < DB_COUNT; i++) {
     if (curtab->tp_diffbuf[i] != NULL) {
-      // write the contents of the entire buffer to
-      // diffbufs_mm[diffbuffers_count]
-      diff_write_buffer(curtab->tp_diffbuf[i], &diffbufs_mm[ndiffs],
-                        dp->df_lnum[i], dp->df_lnum[i] + dp->df_count[i] - 1);
+      if (dp->df_count[i] > 0) {
+        // write the contents of the entire buffer to
+        // diffbufs_mm[diffbuffers_count]
+        diff_write_buffer(curtab->tp_diffbuf[i], &diffbufs_mm[ndiffs],
+                          dp->df_lnum[i], dp->df_lnum[i] + dp->df_count[i] - 1);
+      } else {
+        diffbufs_mm[ndiffs].size = 0;
+        diffbufs_mm[ndiffs].ptr = NULL;
+      }
 
       diffbufs[ndiffs] = &diffbufs_mm[ndiffs];
 
@@ -2060,6 +2066,12 @@ static void run_linematch_algorithm(diff_T *dp)
 /// Returns > 0 for inserting that many filler lines above it (never happens
 /// when 'diffopt' doesn't contain "filler").
 /// This should only be used for windows where 'diff' is set.
+/// When diffopt contains linematch, a changed/added/deleted line
+/// may also have filler lines above it. In such a case, the possibilities
+/// are no longer mutually exclusive. The number of filler lines is
+/// returned from diff_check, and the integer 'linestatus' passed by
+/// pointer is set to -1 to indicate a changed line, and -2 to indicate an
+/// added line
 ///
 /// @param wp
 /// @param lnum