fold.c: more edge-cases when updating (#6207)

When foldUpdateIEMSRecurse() re-uses an existing fold, it misses the
case where the existing fold spans from before startlnum to after
firstlnum, the new fold does not span this range, and there is no
"forced start" of a fold. We add a case for this in.

Ensure that if there was no forced break in folds, we merge folds that
now touch each other.

Include testing for a tricky foldmethod=expr case that has never been a
bug. This case works at the moment because of some effects that are not
obvious when reading the code.
A test for this could be useful to ensure a regression doesn't happen.

vim-patch:8.0.0408

1 files changed, 50 insertions, 21 deletions

diff --git a/src/nvim/fold.c b/src/nvim/fold.c
index 36a5b0efd7..d810aee0ce 100644
--- a/src/nvim/fold.c
+++ b/src/nvim/fold.c
@@ -2232,32 +2232,51 @@ static linenr_T foldUpdateIEMSRecurse(garray_T *gap, int level,
              * before where we started looking, extend it.  If it
              * starts at another line, update nested folds to keep
              * their position, compensating for the new fd_top. */
-            if (fp->fd_top >= startlnum && fp->fd_top != firstlnum) {
-              if (fp->fd_top > firstlnum)
-                /* like lines are inserted */
+            if (fp->fd_top == firstlnum) {
+              // We have found a fold beginning exactly where we want one.
+            } else if (fp->fd_top >= startlnum) {
+              if (fp->fd_top > firstlnum) {
+                // We will move the start of this fold up, hence we move all
+                // nested folds (with relative line numbers) down.
                 foldMarkAdjustRecurse(&fp->fd_nested,
-                    (linenr_T)0, (linenr_T)MAXLNUM,
-                    (long)(fp->fd_top - firstlnum), 0L);
-              else
-                /* like lines are deleted */
+                                      (linenr_T)0, (linenr_T)MAXLNUM,
+                                      (long)(fp->fd_top - firstlnum), 0L);
+              } else {
+                // Will move fold down, move nested folds relatively up.
                 foldMarkAdjustRecurse(&fp->fd_nested,
-                    (linenr_T)0,
-                    (long)(firstlnum - fp->fd_top - 1),
-                    (linenr_T)MAXLNUM,
-                    (long)(fp->fd_top - firstlnum));
+                                      (linenr_T)0,
+                                      (long)(firstlnum - fp->fd_top - 1),
+                                      (linenr_T)MAXLNUM,
+                                      (long)(fp->fd_top - firstlnum));
+              }
               fp->fd_len += fp->fd_top - firstlnum;
               fp->fd_top = firstlnum;
-              fold_changed = TRUE;
-            } else if (flp->start != 0 && lvl == level
-                       && fp->fd_top != firstlnum) {
-              /* Existing fold that includes startlnum must stop
-               * if we find the start of a new fold at the same
-               * level.  Split it.  Delete contained folds at
-               * this point to split them too. */
-              foldRemove(&fp->fd_nested, flp->lnum - fp->fd_top,
-                  flp->lnum - fp->fd_top);
+              fold_changed = true;
+            } else if ((flp->start != 0 && lvl == level)
+                       || (firstlnum != startlnum)) {
+              // Before there was a fold spanning from above startlnum to below
+              // firstlnum. This fold is valid above startlnum (because we are
+              // not updating that range), but there is now a break in it.
+              // If the break is because we are now forced to start a new fold
+              // at the level "level" at line fline->lnum, then we need to
+              // split the fold at fline->lnum.
+              // If the break is because the range [startlnum, firstlnum) is
+              // now at a lower indent than "level", we need to split the fold
+              // in this range.
+              // Any splits have to be done recursively.
+              linenr_T breakstart;
+              linenr_T breakend;
+              if (firstlnum != startlnum) {
+                breakstart = startlnum;
+                breakend = firstlnum;
+              } else {
+                breakstart = flp->lnum;
+                breakend = flp->lnum;
+              }
+              foldRemove(&fp->fd_nested, breakstart - fp->fd_top,
+                         breakend - fp->fd_top);
               i = (int)(fp - (fold_T *)gap->ga_data);
-              foldSplit(gap, i, flp->lnum, flp->lnum - 1);
+              foldSplit(gap, i, breakstart, breakend - 1);
               fp = (fold_T *)gap->ga_data + i + 1;
               /* If using the "marker" or "syntax" method, we
                * need to continue until the end of the fold is
@@ -2267,6 +2286,16 @@ static linenr_T foldUpdateIEMSRecurse(garray_T *gap, int level,
                   || getlevel == foldlevelSyntax)
                 finish = TRUE;
             }
+            if (fp->fd_top == startlnum && concat) {
+              i = (int)(fp - (fold_T *)gap->ga_data);
+              if (i != 0) {
+                fp2 = fp - 1;
+                if (fp2->fd_top + fp2->fd_len == fp->fd_top) {
+                  foldMerge(fp2, gap, fp);
+                  fp = fp2;
+                }
+              }
+            }
             break;
           }
           if (fp->fd_top >= startlnum) {