[pango/avoid-overlong-lines: 6/6] Try harder to not produce overlong lines

[Matthias Clasen <matthiasc src gnome org>]

commit cc60ad4cb7bceef90f6d90930df294abee73d33a
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Nov 11 19:58:21 2021 -0500

    Try harder to not produce overlong lines
    
    Our accounting for run lengths is imperfect (mainly
    due to log widths for clusters being evenly distributed),
    so it can happen that after reshaping the split item,
    we find that it does not actually fit in the remaining
    width. Previously, we would just use the split run
    at that point and produce an overlong line. Instead,
    undo the split, disable the breakpoint we used, and
    try again.

 pango/pango-layout.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)
---
diff --git a/pango/pango-layout.c b/pango/pango-layout.c
index 35e9ba62..57534492 100644
--- a/pango/pango-layout.c
+++ b/pango/pango-layout.c
@@ -3525,6 +3525,29 @@ shape_tab (PangoLayoutLine  *line,
     }
 }
 
+#define DISABLE_BREAKPOINT_FLAG (1 << 16)
+
+static inline void
+disable_breakpoint (PangoLayout *layout,
+                    int          offset)
+{
+  layout->log_attrs[offset].reserved |= DISABLE_BREAKPOINT_FLAG;
+}
+
+static inline gboolean
+breakpoint_is_disabled (PangoLayout *layout,
+                        int          offset)
+{
+  return (layout->log_attrs[offset].reserved & DISABLE_BREAKPOINT_FLAG) != 0;
+}
+
+static void
+clear_breakpoint_flags (PangoLayout *layout)
+{
+  for (int i = 0; i < layout->n_chars + 1; i++)
+    layout->log_attrs[i].reserved = 0;
+}
+
 static inline gboolean
 can_break_at (PangoLayout *layout,
               gint         offset,
@@ -3542,6 +3565,8 @@ can_break_at (PangoLayout *layout,
 
   if (offset == layout->n_chars)
     return TRUE;
+  else if (breakpoint_is_disabled (layout, offset))
+    return FALSE;
   else if (wrap == PANGO_WRAP_WORD)
     return layout->log_attrs[offset].is_line_break;
   else if (wrap == PANGO_WRAP_CHAR)
@@ -3990,7 +4015,27 @@ process_item (PangoLayout     *layout,
               /* Add the width back, to the line, reshape, subtract the new width */
               state->remaining_width += break_width;
               insert_run (line, state, new_item, FALSE);
+
               break_width = pango_glyph_string_get_width (((PangoGlyphItem *)(line->runs->data))->glyphs);
+              if (break_width > state->remaining_width &&
+                  !breakpoint_is_disabled (layout, break_num_chars))
+                {
+                  /* Unsplit the item, disable the breakpoint, try again */
+
+                  uninsert_run (line);
+                  pango_item_free (new_item);
+                  pango_item_unsplit (item, length, break_num_chars);
+
+                  disable_breakpoint (layout, break_num_chars);
+
+                  num_chars = item->num_chars;
+                  width = orig_width;
+                  break_num_chars = num_chars;
+                  break_width = width;
+
+                  goto retry_break;
+                }
+
               state->remaining_width -= break_width;
 
               state->log_widths_offset += break_num_chars;
@@ -4598,6 +4643,8 @@ pango_layout_check_lines (PangoLayout *layout)
     }
   while (!done);
 
+  clear_breakpoint_flags (layout);
+
   g_free (state.log_widths);
   g_list_free_full (state.baseline_shifts, g_free);