[map] Add TODO

src/hb-map.hh

@@ -46,6 +46,10 @@ struct hb_hashmap_t
   static_assert (hb_is_integer (K) || hb_is_pointer (K), "");
   static_assert (hb_is_integer (V) || hb_is_pointer (V), "");
 
+  /* TODO If key type is a pointer, keep hash in item_t and use to:
+   * 1. avoid rehashing when resizing table, and
+   * 2. compare hash before comparing keys, for speed.
+   */
   struct item_t
   {
     K key;
@@ -248,6 +252,10 @@ struct hb_hashmap_t
   }
 };
 
+/*
+ * hb_map_t
+ */
+
 struct hb_map_t : hb_hashmap_t<hb_codepoint_t,
 			       hb_codepoint_t,
 			       HB_MAP_VALUE_INVALID,