feat(colorcolchar): make colorcol qsort() stable by including index

1 files changed, 18 insertions, 5 deletions

diff --git a/src/nvim/window.c b/src/nvim/window.c
index cf5d0013a3..5e99cec015 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -7410,7 +7410,17 @@ static bool frame_check_width(const frame_T *topfrp, int width)
 /// Compares the column numbers
 static int colorcol_cmp(const void *a, const void *b)
 {
-  return ((const colorcol_T *)a)->col - ((const colorcol_T *)b)->col;
+  colorcol_T *ac = (colorcol_T*) a;
+  colorcol_T *bc = (colorcol_T*) b;
+  int ret = ac->col - bc->col;
+  if (ret == 0) {
+    // qsort() is not inherently stable, so to make it stable,
+    // the syn_attr field temporarily contains the original index.
+    // Comparing these will enforce stability.
+    return ac->syn_attr - bc->syn_attr;
+  } else {
+    return ret;
+  }
 }
 
 /// Handle setting 'colorcolumn' or 'textwidth' in window "wp".
@@ -7495,13 +7505,14 @@ char *check_colorcolumn(win_T *wp)
     }
 
     if (!do_skip) {
-      color_cols[count++] = (colorcol_T) {
-        .col = col - 1,      // 1-based to 0-based
-        .syn_attr = 0,
+      color_cols[count] = (colorcol_T) {
+        .col = col - 1,          // 1-based to 0-based
+        .syn_attr = (int) count, // Temporarily use this for stable sorting.
         .ch = ch,
         .syn_name = syn_name[0] == 0 ? NULL : xstrdup(syn_name),
         .flags = flags,
       };
+      count ++;
     }
 
     if (*s == NUL) {
@@ -7528,7 +7539,9 @@ char *check_colorcolumn(win_T *wp)
     for (unsigned int i = 0; i < count; i++) {
       // skip duplicates
       if (j == 0 || wp->w_p_cc_cols[j - 1].col != color_cols[i].col) {
-        wp->w_p_cc_cols[j++] = color_cols[i];
+        wp->w_p_cc_cols[j] = color_cols[i];
+        // Clear syn_attr, which was used for stable sorting.
+        wp->w_p_cc_cols[j ++].syn_attr = 0;
       }
     }
     memset(&wp->w_p_cc_cols[j], 0, sizeof(wp->w_p_cc_cols[j]));