[pango] Fix offset calculation for justify

[Behdad Esfahbod <behdad src gnome org>]

commit bf500a961f0cbdb1f5b3ac9bb71080e7613df23d
Author: Behdad Esfahbod <behdad behdad org>
Date:   Fri Aug 21 13:35:32 2015 +0100

    Fix offset calculation for justify
    
    Bug 753772 - Justify option inserts spaces in wrong positions
    https://bugzilla.gnome.org/show_bug.cgi?id=753772
    
    Went unnoticed for 8 years...

 pango/pango-layout.c |   34 ++++++++++++++++++++++++++++------
 1 files changed, 28 insertions(+), 6 deletions(-)
---
diff --git a/pango/pango-layout.c b/pango/pango-layout.c
index 6d6e0c1..3874b2e 100644
--- a/pango/pango-layout.c
+++ b/pango/pango-layout.c
@@ -5170,7 +5170,6 @@ justify_clusters (PangoLayoutLine *line,
   const gchar *text = line->layout->text;
   const PangoLogAttr *log_attrs = line->layout->log_attrs;
 
-  int offset;
   int total_remaining_width, total_gaps = 0;
   int added_so_far, gaps_so_far;
   gboolean is_hinted;
@@ -5192,7 +5191,6 @@ justify_clusters (PangoLayoutLine *line,
       added_so_far = 0;
       gaps_so_far = 0;
 
-      offset = state->line_start_offset;
       for (run_iter = line->runs; run_iter; run_iter = run_iter->next)
        {
          PangoLayoutRun *run = run_iter->data;
@@ -5200,6 +5198,20 @@ justify_clusters (PangoLayoutLine *line,
          gboolean is_first_gap = TRUE;
          PangoGlyphItemIter cluster_iter;
          gboolean have_cluster;
+         int offset;
+
+         /* We need character offset of the start of the run.  We don't have this.
+          * Compute by counting from the beginning of the line.  The naming is
+          * confusing.  Note that:
+          *
+          * run->item->offset        is byte offset of start of run in layout->text.
+          * state->line_start_index  is byte offset of start of line in layout->text.
+          * state->line_start_offset is character offset of start of line in layout->text.
+          */
+         g_assert (run->item->offset >= state->line_start_index);
+         offset = state->line_start_offset
+                + pango_utf8_strlen (text + state->line_start_index,
+                                     run->item->offset - state->line_start_index);
 
          for (have_cluster = pango_glyph_item_iter_init_start (&cluster_iter, run, text);
               have_cluster;
@@ -5278,7 +5290,6 @@ justify_words (PangoLayoutLine *line,
   const gchar *text = line->layout->text;
   const PangoLogAttr *log_attrs = line->layout->log_attrs;
 
-  int offset;
   int total_remaining_width, total_space_width = 0;
   int added_so_far, spaces_so_far;
   gboolean is_hinted;
@@ -5300,13 +5311,26 @@ justify_words (PangoLayoutLine *line,
       added_so_far = 0;
       spaces_so_far = 0;
 
-      offset = state->line_start_offset;
       for (run_iter = line->runs; run_iter; run_iter = run_iter->next)
        {
          PangoLayoutRun *run = run_iter->data;
          PangoGlyphString *glyphs = run->glyphs;
          PangoGlyphItemIter cluster_iter;
          gboolean have_cluster;
+         int offset;
+
+         /* We need character offset of the start of the run.  We don't have this.
+          * Compute by counting from the beginning of the line.  The naming is
+          * confusing.  Note that:
+          *
+          * run->item->offset        is byte offset of start of run in layout->text.
+          * state->line_start_index  is byte offset of start of line in layout->text.
+          * state->line_start_offset is character offset of start of line in layout->text.
+          */
+         g_assert (run->item->offset >= state->line_start_index);
+         offset = state->line_start_offset
+                + pango_utf8_strlen (text + state->line_start_index,
+                                     run->item->offset - state->line_start_index);
 
          for (have_cluster = pango_glyph_item_iter_init_start (&cluster_iter, run, text);
               have_cluster;
@@ -5341,8 +5365,6 @@ justify_words (PangoLayoutLine *line,
                    }
                }
            }
-
-         offset += glyphs->num_glyphs;
        }
 
       if (mode == MEASURE)