From b5d3ad90e39a58967aa75aa08acd623c440aa97b Mon Sep 17 00:00:00 2001
From: Matthias Clasen <mclasen@redhat.com>
Date: Fri, 27 Aug 2021 23:54:59 -0400
Subject: Try harder to disect clusters

We need to take into account whether the characters
we are counting are actually cursor positions, else
we are bound to look for more carets than the font
has available, sometimes.

This change would benefit from passing log attrs into
pango_glyph_string_index_to_x.
---
 pango/glyphstring.c | 49 ++++++++++++++++++++++++++++++++++---------------
 1 file changed, 34 insertions(+), 15 deletions(-)

diff --git a/pango/glyphstring.c b/pango/glyphstring.c
index 30d68220..11eeffbf 100644
--- a/pango/glyphstring.c
+++ b/pango/glyphstring.c
@@ -404,6 +404,9 @@ pango_glyph_string_index_to_x (PangoGlyphString *glyphs,
 
   const char *p;
 
+  int attrs_len;
+  PangoLogAttr *attrs = NULL;
+
   g_return_if_fail (glyphs != NULL);
   g_return_if_fail (length >= 0);
   g_return_if_fail (length == 0 || text != NULL);
@@ -495,10 +498,20 @@ pango_glyph_string_index_to_x (PangoGlyphString *glyphs,
 
   /* Calculate offset of character within cluster */
 
-  for (p = text + start_index;
+  if (g_utf8_next_char (text + start_index) != text + end_index)
+    {
+      /* FIXME: should allow log attrs to be passed in */
+      attrs_len = g_utf8_strlen (text, length) + 1;
+      attrs = g_new0 (PangoLogAttr, attrs_len + 1);
+      pango_default_break (text, length, analysis, attrs, attrs_len);
+    }
+
+  for (p = text + start_index, i = g_utf8_pointer_to_offset (text, text + start_index);
        p < text + end_index;
-       p = g_utf8_next_char (p))
+       p = g_utf8_next_char (p), i++)
     {
+      if (!attrs[i].is_cursor_position)
+        continue;
       if (p < text + index)
         cluster_offset++;
       cluster_chars++;
@@ -526,6 +539,7 @@ pango_glyph_string_index_to_x (PangoGlyphString *glyphs,
       hb_position_t caret;
       unsigned int caret_count = 1;
       int glyph_pos;
+      int num_carets;
 
       hb_font = pango_font_get_hb_font (analysis->font);
 
@@ -544,7 +558,7 @@ pango_glyph_string_index_to_x (PangoGlyphString *glyphs,
                 {
                   if (glyph_pos != -1)
                     {
-                      /* multiple non-mark glyphs in cluster, giving up */
+                      g_print ("multiple non-mark glyphs in cluster, giving up\n");
                       goto fallback;
                     }
                   glyph_pos = i;
@@ -552,24 +566,29 @@ pango_glyph_string_index_to_x (PangoGlyphString *glyphs,
             }
           if (glyph_pos == -1)
             {
-              /* no non-mark glyph in a multi-glyph cluster, giving up */
+              g_print ("no non-mark glyph in a multi-glyph cluster, giving up\n");
               goto fallback;
             }
         }
 
-      hb_ot_layout_get_ligature_carets (hb_font,
-                                        (analysis->level % 2) ? HB_DIRECTION_RTL : HB_DIRECTION_LTR,
-                                        glyphs->glyphs[glyph_pos].glyph,
-                                        cluster_offset - 1, &caret_count, &caret);
-      if (caret_count > 0)
+      num_carets = hb_ot_layout_get_ligature_carets (hb_font,
+                                                     (analysis->level % 2) ? HB_DIRECTION_RTL : HB_DIRECTION_LTR,
+                                                      glyphs->glyphs[glyph_pos].glyph,
+                                                      cluster_offset - 1, &caret_count, &caret);
+      if (caret_count == 0)
         {
-          if (analysis->level % 2) /* Right to left */
-            *x_pos = end_xpos + caret;
-          else
-            *x_pos = start_xpos + caret;
-          *x_pos += glyphs->glyphs[glyph_pos].geometry.x_offset;
-          return;
+          g_print ("no ligature caret information found for glyph %d, caret %d / %d. font has %d ligature carets\n", glyphs->glyphs[glyph_pos].glyph, cluster_offset - 1, cluster_chars, num_carets);
+          goto fallback;
         }
+
+      g_print ("using ligature caret information for glyph %d, caret %d / %d. font has %d ligature carets\n", glyphs->glyphs[glyph_pos].glyph, cluster_offset - 1, cluster_chars, num_carets);
+
+      if (analysis->level % 2) /* Right to left */
+        *x_pos = end_xpos + caret;
+      else
+        *x_pos = start_xpos + caret;
+      *x_pos += glyphs->glyphs[glyph_pos].geometry.x_offset;
+      return;
     }
 
 fallback:
-- 
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