[AAT] Use a ring buffer for ligature stack

I think Apple does very similarly, but probably with a stack size of 16.
We do it with a stack size that is currently set to 64.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1531

diff --git a/src/hb-aat-layout-morx-table.hh b/src/hb-aat-layout-morx-table.hh
index 6e7bb03..c2a75a8 100644 (file)
--- a/src/hb-aat-layout-morx-table.hh
+++ b/src/hb-aat-layout-morx-table.hh
@@ -461,14 +461,11 @@ struct LigatureSubtable
       DEBUG_MSG (APPLY, nullptr, "Ligature transition at %u", buffer->idx);
       if (entry.flags & LigatureEntryT::SetComponent)
       {
-        if (unlikely (match_length >= ARRAY_LENGTH (match_positions)))
-         match_length = 0; /* TODO Use a ring buffer instead. */
-
        /* Never mark same index twice, in case DontAdvance was used... */
-       if (match_length && match_positions[match_length - 1] == buffer->out_len)
+       if (match_length && match_positions[(match_length - 1u) % ARRAY_LENGTH (match_positions)] == buffer->out_len)
          match_length--;
 
-       match_positions[match_length++] = buffer->out_len;
+       match_positions[match_length++ % ARRAY_LENGTH (match_positions)] = buffer->out_len;
        DEBUG_MSG (APPLY, nullptr, "Set component at %u", buffer->out_len);
       }
 
@@ -502,7 +499,7 @@ struct LigatureSubtable
          }
 
          DEBUG_MSG (APPLY, nullptr, "Moving to stack position %u", cursor - 1);
-         buffer->move_to (match_positions[--cursor]);
+         buffer->move_to (match_positions[--cursor % ARRAY_LENGTH (match_positions)]);
 
          if (unlikely (!actionData->sanitize (&c->sanitizer))) break;
          action = *actionData;
@@ -530,17 +527,17 @@ struct LigatureSubtable
            DEBUG_MSG (APPLY, nullptr, "Produced ligature %u", lig);
            buffer->replace_glyph (lig);
 
-           unsigned int lig_end = match_positions[match_length - 1] + 1;
+           unsigned int lig_end = match_positions[(match_length - 1) % ARRAY_LENGTH (match_positions)] + 1;
            /* Now go and delete all subsequent components. */
            while (match_length - 1 > cursor)
            {
              DEBUG_MSG (APPLY, nullptr, "Skipping ligature component");
-             buffer->move_to (match_positions[--match_length]);
+             buffer->move_to (match_positions[--match_length % ARRAY_LENGTH (match_positions)]);
              buffer->replace_glyph (DELETED_GLYPH);
            }
 
            buffer->move_to (lig_end);
-           buffer->merge_out_clusters (match_positions[cursor], buffer->out_len);
+           buffer->merge_out_clusters (match_positions[cursor % ARRAY_LENGTH (match_positions)], buffer->out_len);
          }
 
          actionData++;