[set] Use better hash

src/hb-bit-page.hh

@@ -104,10 +104,7 @@ struct hb_bit_page_t
   }
   uint32_t hash () const
   {
-    return
-    + hb_iter (v)
-    | hb_reduce ([] (uint32_t h, const elt_t &_) { return h * 31 + hb_hash (_); }, (uint32_t) 0u)
-    ;
+    return hb_bytes_t ((const char *) &v, sizeof (v)).hash ();
   }
 
   void add (hb_codepoint_t g) { elt (g) |= mask (g); }

src/hb-bit-set.hh

@@ -134,7 +134,11 @@ struct hb_bit_set_t
   {
     uint32_t h = 0;
     for (auto &map : page_map)
-      h = h * 31 + hb_hash (map.major) + hb_hash (pages[map.index]);
+    {
+      auto &page = pages.arrayZ[map.index];
+      if (unlikely (page.is_empty ())) continue;
+      h = h * 31 + hb_hash (map.major) + hb_hash (page);
+    }
     return h;
   }