Make CoreText shaping engine more robust

This commit makes the CoreText shaping engine much more robust against
the output from the CoreText typesetter. Refer to the source code
comments for exhaustive details.

Furthermore, this patch fixing cursor navigation in strings with
zero-width spaces.

1 files changed, 338 insertions, 79 deletions

diff --git a/modules/basic/basic-coretext.c b/modules/basic/basic-coretext.c
index 991743e4..baacec61 100644
--- a/modules/basic/basic-coretext.c
+++ b/modules/basic/basic-coretext.c
@@ -3,6 +3,7 @@
  *
  * Copyright (C) 2005 Imendio AB
  * Copyright (C) 2010  Kristian Rietveld  <kris@gtk.org>
+ * Copyright (C) 2012  Kristian Rietveld  <kris@lanedo.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ -68,36 +69,136 @@ set_glyph (PangoFont        *font,
   glyphs->glyphs[i].geometry.width = logical_rect.width;
 }
 
-static void
-basic_engine_shape (PangoEngineShape    *engine,
-		    PangoFont           *font,
-		    const char          *text,
-		    gint                 length,
-		    const PangoAnalysis *analysis,
-		    PangoGlyphString    *glyphs)
+
+/* The "RunIterator" helps us to iterate over the array of runs that is obtained from
+ * the CoreText type setter. Even though Pango considers the string that is passed to
+ * the shaping engine a single run, CoreText might consider it to consist out of
+ * multiple runs. Because of this, we have an interface around the CoreText array of
+ * runs that works like iterating a single array, which makes our job in the shaping
+ * engine function easier.
+ */
+
+struct RunIterator
 {
-  char *copy;
   CTLineRef line;
   CFStringRef cstr;
+  CFArrayRef runs;
+  CFIndex glyph_count;
+
+  CFIndex total_ct_i;
+  CFIndex ct_i;
+
+  int current_run_number;
+  CTRunRef current_run;
+  CFIndex *current_indices;
+  const CGGlyph *current_cgglyphs;
+  CTRunStatus current_run_status;
+};
+
+static void
+run_iterator_free_current_run (struct RunIterator *iter)
+{
+  iter->current_run_number = -1;
+  iter->current_run = NULL;
+  iter->current_cgglyphs = NULL;
+  if (iter->current_indices)
+    free (iter->current_indices);
+  iter->current_indices = NULL;
+}
+
+static void
+run_iterator_set_current_run (struct RunIterator *iter,
+                              const int           run_number)
+{
+  CFIndex ct_glyph_count;
+
+  run_iterator_free_current_run (iter);
+
+  iter->current_run_number = run_number;
+  iter->current_run = CFArrayGetValueAtIndex (iter->runs, run_number);
+  iter->current_run_status = CTRunGetStatus (iter->current_run);
+  iter->current_cgglyphs = CTRunGetGlyphsPtr (iter->current_run);
+
+  ct_glyph_count = CTRunGetGlyphCount (iter->current_run);
+  iter->current_indices = malloc (sizeof (CFIndex *) * ct_glyph_count);
+  CTRunGetStringIndices (iter->current_run, CFRangeMake (0, ct_glyph_count),
+                         iter->current_indices);
+
+  iter->ct_i = 0;
+}
+
+static CFIndex
+run_iterator_get_glyph_count (struct RunIterator *iter)
+{
+  CFIndex accumulator = 0;
+  CFIndex i;
+
+  for (i = 0; i < CFArrayGetCount (iter->runs); i++)
+    accumulator += CTRunGetGlyphCount (CFArrayGetValueAtIndex (iter->runs, i));
+
+  return accumulator;
+}
+
+static gboolean
+run_iterator_is_rtl (struct RunIterator *iter)
+{
+  /* Assume run status is equal for all runs? */
+  CTRunStatus run_status = CTRunGetStatus (CFArrayGetValueAtIndex (iter->runs, 0));
+
+  return run_status & kCTRunStatusRightToLeft;
+}
+
+static gboolean
+run_iterator_run_is_non_monotonic (struct RunIterator *iter)
+{
+  CTRunStatus run_status = CTRunGetStatus (iter->current_run);
+
+  return run_status & kCTRunStatusNonMonotonic;
+}
+
+static gunichar
+run_iterator_get_character (struct RunIterator *iter)
+{
+  return CFStringGetCharacterAtIndex (iter->cstr, iter->current_indices[iter->ct_i]);
+}
+
+static CGGlyph
+run_iterator_get_cgglyph (struct RunIterator *iter)
+{
+  return iter->current_cgglyphs[iter->ct_i];
+}
+
+static CFIndex
+run_iterator_get_index (struct RunIterator *iter)
+{
+  return iter->current_indices[iter->ct_i];
+}
+
+static void
+run_iterator_create (struct RunIterator *iter,
+                     const char         *text,
+                     const gint          length,
+                     CTFontRef           ctfont)
+{
+  char *copy;
   CFDictionaryRef attributes;
   CFAttributedStringRef attstr;
-  PangoCoreTextFont *cfont = PANGO_CORE_TEXT_FONT (font);
-  PangoCoverage *coverage;
-  CFArrayRef runs;
-  CTRunRef run;
-  CTRunStatus run_status;
-  CFIndex i, glyph_count;
-  CFIndex *indices = NULL;
-  const CGGlyph *cgglyphs;
 
   CFTypeRef keys[] = {
       (CFTypeRef) kCTFontAttributeName
   };
 
   CFTypeRef values[] = {
-      pango_core_text_font_get_ctfont (cfont)
+      ctfont
   };
 
+  /* Initialize RunIterator structure */
+  iter->current_run_number = -1;
+  iter->current_run = NULL;
+  iter->current_indices = NULL;
+  iter->current_cgglyphs = NULL;
+
+  /* Create CTLine */
   attributes = CFDictionaryCreate (kCFAllocatorDefault,
                                    (const void **)keys,
                                    (const void **)values,
@@ -108,100 +209,258 @@ basic_engine_shape (PangoEngineShape    *engine,
   copy = g_strndup (text, length + 1);
   copy[length] = 0;
 
-  cstr = CFStringCreateWithCString (kCFAllocatorDefault, copy,
-                                    kCFStringEncodingUTF8);
+  iter->cstr = CFStringCreateWithCString (kCFAllocatorDefault, copy,
+                                          kCFStringEncodingUTF8);
   g_free (copy);
 
   attstr = CFAttributedStringCreate (kCFAllocatorDefault,
-                                     cstr,
+                                     iter->cstr,
                                      attributes);
 
-  line = CTLineCreateWithAttributedString (attstr);
+  iter->line = CTLineCreateWithAttributedString (attstr);
+  iter->runs = CTLineGetGlyphRuns (iter->line);
 
-  runs = CTLineGetGlyphRuns (line);
+  CFRelease (attstr);
+  CFRelease (attributes);
+
+  iter->total_ct_i = 0;
+  iter->glyph_count = run_iterator_get_glyph_count (iter);
+  run_iterator_set_current_run (iter, 0);
+}
 
-  /* Since Pango divides things into runs already, we assume there is
-   * only a single run in this line.
+static void
+run_iterator_free (struct RunIterator *iter)
+{
+  run_iterator_free_current_run (iter);
+
+  CFRelease (iter->line);
+  CFRelease (iter->cstr);
+}
+
+static gboolean
+run_iterator_at_end (struct RunIterator *iter)
+{
+  if (iter->total_ct_i == -1)
+    return TRUE;
+
+  return FALSE;
+}
+
+static void
+run_iterator_advance (struct RunIterator *iter)
+{
+  if (iter->total_ct_i >= iter->glyph_count - 1)
+    {
+      run_iterator_free_current_run (iter);
+      iter->ct_i = iter->total_ct_i = -1;
+    }
+  else
+    {
+      iter->total_ct_i++;
+      iter->ct_i++;
+
+      if (iter->total_ct_i < iter->glyph_count &&
+          iter->ct_i >= CTRunGetGlyphCount (iter->current_run))
+        {
+          iter->current_run_number++;
+          run_iterator_set_current_run (iter, iter->current_run_number);
+        }
+    }
+}
+
+
+
+struct GlyphInfo
+{
+  CFIndex index;
+  CGGlyph cgglyph;
+  gunichar wc;
+};
+
+static gint
+glyph_info_compare_func (gconstpointer a, gconstpointer b)
+{
+  const struct GlyphInfo *gi_a = a;
+  const struct GlyphInfo *gi_b = b;
+
+  if (gi_a->index < gi_b->index)
+    return -1;
+  else if (gi_a->index > gi_b->index)
+    return 1;
+  /* else */
+  return 0;
+}
+
+static void
+glyph_info_free (gpointer data, gpointer user_data)
+{
+  g_slice_free (struct GlyphInfo, data);
+}
+
+static GSList *
+create_core_text_glyph_list (const char *text,
+                             gint        length,
+                             CTFontRef   ctfont)
+{
+  GSList *glyph_list = NULL;
+  struct RunIterator riter;
+
+  run_iterator_create (&riter, text, length, ctfont);
+
+  while (!run_iterator_at_end (&riter))
+    {
+      struct GlyphInfo *gi;
+
+      gi = g_slice_new (struct GlyphInfo);
+      gi->index = run_iterator_get_index (&riter);
+      gi->cgglyph = run_iterator_get_cgglyph (&riter);
+      gi->wc = run_iterator_get_character (&riter);
+
+      glyph_list = g_slist_prepend (glyph_list, gi);
+
+      run_iterator_advance (&riter);
+    }
+
+  glyph_list = g_slist_sort (glyph_list, glyph_info_compare_func);
+
+  run_iterator_free (&riter);
+
+  return glyph_list;
+}
+
+
+static void
+basic_engine_shape (PangoEngineShape    *engine,
+		    PangoFont           *font,
+		    const char          *text,
+		    gint                 length,
+		    const PangoAnalysis *analysis,
+		    PangoGlyphString    *glyphs)
+{
+  const char *p;
+  gulong n_chars, gs_i, gs_prev_i;
+  PangoCoreTextFont *cfont = PANGO_CORE_TEXT_FONT (font);
+  PangoCoverage *coverage;
+  GSList *glyph_list;
+  GSList *glyph_iter;
+
+  /* We first fully iterate over the glyph sequence generated by CoreText and
+   * store this into a list, which is sorted after the iteration. We make a pass
+   * over the sorted linked list to build up the PangoGlyphString.
+   *
+   * We have to do this in order to properly handle a bunch of characteristics of the
+   * glyph sequence generated by the CoreText typesetter:
+   *   # E.g. zero-width spaces do not end up in the CoreText glyph sequence. We have
+   *     to manually account for the gap in the character indices.
+   *   # Sometimes, CoreText generates two glyph for the same character index. We
+   *     currently handle this "properly" as in we do not crash or corrupt memory,
+   *     but that's about it.
+   *   # Due to mismatches in size, the CoreText glyph sequence can either be longer or
+   *     shorter than the PangoGlyphString. Note that the size of the PangoGlyphString
+   *     should match the number of characters in "text".
+   *
+   * If performance becomes a problem, it is certainly possible to use a faster code
+   * that only does a single iteration over the string for "simple cases". Simple cases
+   * could include these that only consist out of one run (simple Latin text), which
+   * don't have gaps in the glyph sequence and which are monotonically
+   * increasing/decreasing.
+   *
+   * FIXME items for future fixing:
+   *   # CoreText strings are UTF16, and the indices *often* refer to characters,
+   *     but not *always*. Notable exception is when a character is encoded using
+   *     two UTF16 code points. This are two characters in a CFString. At this point
+   *     advancing a single character in the CFString and advancing a single character
+   *     using g_utf8_next_char in the const char string goes out of sync.
+   *   # We currently don't bother about LTR, Pango core appears to fix this up for us.
+   *     (Even when we cared warnings were generated that strings were in the wrong
+   *     order, this should be investigated).
+   *   # When CoreText generates two glyphs for one character, only one is stored.
+   *     This breaks the example strings for e.g. Georgian and Gothic.
    */
-  run = CFArrayGetValueAtIndex (runs, 0);
-  run_status = CTRunGetStatus (run);
-  glyph_count = CTRunGetGlyphCount (run);
-  cgglyphs = CTRunGetGlyphsPtr (run);
 
-  indices = malloc (sizeof (CFIndex *) * glyph_count);
-  CTRunGetStringIndices (run, CFRangeMake (0, glyph_count), indices);
+  glyph_list = create_core_text_glyph_list (text, length,
+                                            pango_core_text_font_get_ctfont (cfont));
+
+  /* Translate the glyph list to a PangoGlyphString */
+  n_chars = g_utf8_strlen (text, length);
+  pango_glyph_string_set_size (glyphs, n_chars);
+
+  glyph_iter = glyph_list;
 
-  pango_glyph_string_set_size (glyphs, glyph_count);
   coverage = pango_font_get_coverage (PANGO_FONT (cfont),
                                       analysis->language);
 
-  for (i = 0; i < glyph_count; i++)
+  for (gs_prev_i = -1, gs_i = 0, p = text; gs_i < n_chars;
+       gs_prev_i = gs_i, gs_i++, p = g_utf8_next_char (p))
     {
-      CFIndex real_i, prev_i;
-      gunichar wc;
-      gunichar mirrored_ch;
-      PangoCoverageLevel result;
+      struct GlyphInfo *gi = glyph_iter != NULL ? glyph_iter->data : NULL;
 
-      if (run_status & kCTRunStatusRightToLeft)
+      if (gi == NULL || gi->index > gs_i)
         {
-          real_i = glyph_count - i - 1;
-          prev_i = real_i + 1;
+          /* gs_i is behind, insert empty glyph */
+          set_glyph (font, glyphs, gs_i, p - text, PANGO_GLYPH_EMPTY);
+          continue;
         }
-      else
+      else if (gi->index < gs_i)
         {
-          real_i = i;
-          prev_i = real_i - 1;
+          while (gi && gi->index < gs_i)
+            {
+              glyph_iter = g_slist_next (glyph_iter);
+              if (glyph_iter)
+                gi = glyph_iter->data;
+              else
+                gi = NULL;
+            }
         }
 
-      wc = CFStringGetCharacterAtIndex (cstr, indices[real_i]);
-
-      if (analysis->level % 2)
-	if (pango_get_mirror_char (wc, &mirrored_ch))
-	  wc = mirrored_ch;
+      if (gi != NULL && gi->index == gs_i)
+        {
+          gunichar mirrored_ch;
+          PangoCoverageLevel result;
 
-      if (wc == 0xa0)	/* non-break-space */
-	wc = 0x20;
+          if (analysis->level % 2)
+            if (g_unichar_get_mirror_char (gi->wc, &mirrored_ch))
+              gi->wc = mirrored_ch;
 
-      result = pango_coverage_get (coverage, wc);
+          if (gi->wc == 0xa0)	/* non-break-space */
+            gi->wc = 0x20;
 
-      if (result != PANGO_COVERAGE_NONE)
-        {
-          set_glyph (font, glyphs, real_i,
-                     g_utf8_offset_to_pointer (text, indices[real_i]) - text,
-                     cgglyphs[real_i]);
+          result = pango_coverage_get (coverage, gi->wc);
 
-          if (g_unichar_type (wc) == G_UNICODE_NON_SPACING_MARK)
+          if (result != PANGO_COVERAGE_NONE)
             {
-              if (i > 0)
+              set_glyph (font, glyphs, gs_i, p - text, gi->cgglyph);
+
+              if (g_unichar_type (gi->wc) == G_UNICODE_NON_SPACING_MARK)
                 {
-                  PangoRectangle logical_rect, ink_rect;
-
-                  glyphs->glyphs[real_i].geometry.width = MAX (glyphs->glyphs[prev_i].geometry.width,
-                                                               glyphs->glyphs[real_i].geometry.width);
-                  glyphs->glyphs[prev_i].geometry.width = 0;
-                  glyphs->log_clusters[real_i] = glyphs->log_clusters[prev_i];
-
-                  /* Some heuristics to try to guess how overstrike glyphs are
-                   * done and compensate
-                   */
-                  pango_font_get_glyph_extents (font, glyphs->glyphs[real_i].glyph, &ink_rect, &logical_rect);
-                  if (logical_rect.width == 0 && ink_rect.x == 0)
-                    glyphs->glyphs[real_i].geometry.x_offset = (glyphs->glyphs[real_i].geometry.width - ink_rect.width) / 2;
+                  if (gi->index > 0)
+                    {
+                      PangoRectangle logical_rect, ink_rect;
+
+                      glyphs->glyphs[gs_i].geometry.width = MAX (glyphs->glyphs[gs_prev_i].geometry.width,
+                                                                 glyphs->glyphs[gs_i].geometry.width);
+                      glyphs->glyphs[gs_prev_i].geometry.width = 0;
+                      glyphs->log_clusters[gs_i] = glyphs->log_clusters[gs_prev_i];
+
+                      /* Some heuristics to try to guess how overstrike glyphs are
+                       * done and compensate
+                       */
+                      pango_font_get_glyph_extents (font, glyphs->glyphs[gs_i].glyph, &ink_rect, &logical_rect);
+                      if (logical_rect.width == 0 && ink_rect.x == 0)
+                        glyphs->glyphs[gs_i].geometry.x_offset = (glyphs->glyphs[gs_i].geometry.width - ink_rect.width) / 2;
+                    }
                 }
             }
+          else
+            set_glyph (font, glyphs, gs_i, p - text, PANGO_GET_UNKNOWN_GLYPH (gi->wc));
+
+          glyph_iter = g_slist_next (glyph_iter);
         }
-      else
-        set_glyph (font, glyphs, real_i,
-                   g_utf8_offset_to_pointer (text, indices[real_i]) - text,
-                   PANGO_GET_UNKNOWN_GLYPH (wc));
     }
 
-  free (indices);
-  CFRelease (line);
-  CFRelease (attstr);
-  CFRelease (cstr);
-  CFRelease (attributes);
   pango_coverage_unref (coverage);
+  g_slist_foreach (glyph_list, glyph_info_free, NULL);
+  g_slist_free (glyph_list);
 }
 
 static void