Robustly handle folds during a :move command

In order to re-order marks according to the :move command, do_move()
uses mark_adjust() in a non-standard manner. The non-standard action is
that it moves some marks *past* other marks. This doesn't matter for
marks, but mark_adjust() calls foldMarkAdjust() which simply changes
fold starts and lengths and doesn't have enough information to know that
other folds have to be checked and reordered.

The array of folds for each window are assumed to be in order of
increasing line number, and if this gets broken some folds can get
"lost".

There has been a previous patch to avoid this problem by deleting and
recalculating all folds in the window, but this comes at the cost of
closing all folds when executing :move, and doesn't cover the case of
manual folds.
This patch adds a new function foldMoveRange() specifically for the
:move command that handles reordering folds as well as simply moving
them. Additionally, we allow calling mark_adjust_nofold() that does the
same as mark_adjust() but doesn't affect any fold array.

Calling mark_adjust_nofold() should be done in the same manner as
calling mark_adjust(), but according changes to the fold arrays must be
done seperately by the calling function.

vim-patch:8.0.0457
vim-patch:8.0.0459
vim-patch:8.0.0461
vim-patch:8.0.0465

src/nvim/fold.c

@@ -2597,6 +2597,154 @@ static void foldRemove(garray_T *gap, linenr_T top, linenr_T bot)
   }
 }
 
+// foldMoveRange() {{{2
+static void reverse_fold_order(garray_T *gap, size_t start, size_t end)
+{
+  for (; start < end; start++, end--) {
+    fold_T *left = (fold_T *)gap->ga_data + start;
+    fold_T *right = (fold_T *)gap->ga_data + end;
+    fold_T tmp = *left;
+    *left = *right;
+    *right = tmp;
+  }
+}
+
+// Move folds within the inclusive range "line1" to "line2" to after "dest"
+// require "line1" <= "line2" <= "dest"
+//
+// There are the following situations for the first fold at or below line1 - 1.
+//       1  2  3  4
+//       1  2  3  4
+// line1    2  3  4
+//          2  3  4  5  6  7
+// line2       3  4  5  6  7
+//             3  4     6  7  8  9
+// dest           4        7  8  9
+//                4        7  8    10
+//                4        7  8    10
+//
+// In the following descriptions, "moved" means moving in the buffer, *and* in
+// the fold array.
+// Meanwhile, "shifted" just means moving in the buffer.
+// 1. not changed
+// 2. truncated above line1
+// 3. length reduced by  line2 - line1, folds starting between the end of 3 and
+//    dest are truncated and shifted up
+// 4. internal folds moved (from [line1, line2] to dest)
+// 5. moved to dest.
+// 6. truncated below line2 and moved.
+// 7. length reduced by line2 - dest, folds starting between line2 and dest are
+//    removed, top is moved down by move_len.
+// 8. truncated below dest and shifted up.
+// 9. shifted up
+// 10. not changed
+static void truncate_fold(fold_T *fp, linenr_T end)
+{
+  // I want to stop *at here*, foldRemove() stops *above* top
+  end += 1;
+  foldRemove(&fp->fd_nested, end - fp->fd_top, MAXLNUM);
+  fp->fd_len = end - fp->fd_top;
+}
+
+#define fold_end(fp) ((fp)->fd_top + (fp)->fd_len - 1)
+#define valid_fold(fp, gap) ((fp) < ((fold_T *)(gap)->ga_data + (gap)->ga_len))
+#define fold_index(fp, gap) ((size_t)(fp - ((fold_T *)(gap)->ga_data)))
+void foldMoveRange(garray_T *gap, const linenr_T line1, const linenr_T line2,
+                   const linenr_T dest)
+{
+  fold_T *fp;
+  const linenr_T range_len = line2 - line1 + 1;
+  const linenr_T move_len = dest - line2;
+  const bool at_start = foldFind(gap, line1 - 1, &fp);
+
+  if (at_start) {
+    if (fold_end(fp) > dest) {
+      // Case 4 -- don't have to change this fold, but have to move nested
+      // folds.
+      foldMoveRange(&fp->fd_nested, line1 - fp->fd_top, line2 -
+                    fp->fd_top, dest - fp->fd_top);
+      return;
+    } else if (fold_end(fp) > line2) {
+      // Case 3 -- Remove nested folds between line1 and line2 & reduce the
+      // length of fold by "range_len".
+      // Folds after this one must be dealt with.
+      foldMarkAdjustRecurse(&fp->fd_nested, line1 - fp->fd_top,
+                            line2 - fp->fd_top, MAXLNUM, -range_len);
+      fp->fd_len -= range_len;
+    } else {
+      // Case 2 -- truncate fold *above* line1.
+      // Folds after this one must be dealt with.
+      truncate_fold(fp, line1 - 1);
+    }
+    // Look at the next fold, and treat that one as if it were the first after
+    // "line1" (because now it is).
+    fp = fp + 1;
+  }
+
+  if (!valid_fold(fp, gap) || fp->fd_top > dest) {
+    // No folds after "line1" and before "dest"
+    // Case 10.
+    return;
+  } else if (fp->fd_top > line2) {
+    for (; valid_fold(fp, gap) && fold_end(fp) <= dest; fp++) {
+      // Case 9. (for all case 9's) -- shift up.
+      fp->fd_top -= range_len;
+    }
+    if (valid_fold(fp, gap) && fp->fd_top <= dest) {
+      // Case 8. -- ensure truncated at dest, shift up
+      truncate_fold(fp, dest);
+      fp->fd_top -= range_len;
+    }
+    return;
+  } else if (fold_end(fp) > dest) {
+    // Case 7 -- remove nested folds and shrink
+    foldMarkAdjustRecurse(&fp->fd_nested, line2 + 1 - fp->fd_top,
+                          dest - fp->fd_top, MAXLNUM, -move_len);
+    fp->fd_len -= move_len;
+    fp->fd_top += move_len;
+    return;
+  }
+
+  // Case 5 or 6: changes rely on whether there are folds between the end of
+  // this fold and "dest".
+  size_t move_start = fold_index(fp, gap);
+  size_t move_end = 0, dest_index = 0;
+  for (; valid_fold(fp, gap) && fp->fd_top <= dest; fp++) {
+    if (fp->fd_top <= line2) {
+      // 5, or 6
+      if (fold_end(fp) > line2) {
+        // 6, truncate before moving
+        truncate_fold(fp, line2);
+      }
+      fp->fd_top += move_len;
+      continue;
+    }
+
+    // Record index of the first fold after the moved range.
+    if (move_end == 0) {
+      move_end = fold_index(fp, gap);
+    }
+
+    if (fold_end(fp) > dest) {
+      truncate_fold(fp, dest);
+    }
+
+    fp->fd_top -= range_len;
+  }
+  dest_index = fold_index(fp, gap);
+
+  // All folds are now correct, but they are not necessarily in the correct
+  // order.
+  // We have to swap folds in the range [move_end, dest_index) with those in
+  // the range [move_start, move_end).
+  reverse_fold_order(gap, move_start, dest_index - 1);
+  reverse_fold_order(gap, move_start, move_start + dest_index - move_end - 1);
+  reverse_fold_order(gap, move_start + dest_index - move_end, dest_index - 1);
+}
+#undef fold_end
+#undef valid_fold
+#undef fold_index
+
 /* foldMerge() {{{2 */
 /*
  * Merge two adjacent folds (and the nested ones in them).