[SingleSubstFormat2] Speed up closure

src/OT/Layout/GSUB/SingleSubstFormat2.hh

@@ -36,8 +36,24 @@ struct SingleSubstFormat2_4
 
   void closure (hb_closure_context_t *c) const
   {
-    + hb_zip (this+coverage, substitute)
-    | hb_filter (c->parent_active_glyphs (), hb_first)
+    auto &cov = this+coverage;
+    auto &glyph_set = c->parent_active_glyphs ();
+
+    if (substitute.len > glyph_set.get_population () * 4)
+    {
+      for (auto g : glyph_set)
+      {
+	unsigned i = cov.get_coverage (g);
+	if (i == NOT_COVERED || i >= substitute.len)
+	  continue;
+	c->output->add (substitute.arrayZ[i]);
+      }
+
+      return;
+    }
+
+    + hb_zip (cov, substitute)
+    | hb_filter (glyph_set, hb_first)
     | hb_map (hb_second)
     | hb_sink (c->output)
     ;