use murmur3 finisher to improve font hash efficiency

src/core/SkChecksum.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkChecksum_DEFINED
 #define SkChecksum_DEFINED
 
@@ skipping 34 matching lines @@
     static uint32_t Mix(uint32_t hash) {
         hash ^= hash >> 16;
         hash *= 0x85ebca6b;
         hash ^= hash >> 13;
         hash *= 0xc2b2ae35;
         hash ^= hash >> 16;
         return hash;
     }
 
     /**
+     * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you
+     * suspect its low bits aren't well mixed.
+     *
+     *  This version is 2-lines cheaper than Mix, but seems to be sufficient for the font cache.
+     */
+    static uint32_t CheapMix(uint32_t hash) {
+        hash ^= hash >> 16;
+        hash *= 0x85ebca6b;
+        hash ^= hash >> 16;
+        return hash;
+    }
+
+    /**
      * Calculate 32-bit Murmur hash (murmur3).
      * This should take 2-3x longer than SkChecksum::Compute, but is a considerably better hash.
      * See en.wikipedia.org/wiki/MurmurHash.
      *
      *  @param data Memory address of the data block to be processed. Must be 32-bit aligned.
      *  @param size Size of the data block in bytes. Must be a multiple of 4.
      *  @param seed Initial hash seed. (optional)
      *  @return hash result
      */
     static uint32_t Murmur3(const uint32_t* data, size_t bytes, uint32_t seed=0) {
@@ skipping 81 matching lines @@
          *  define.
          */
         if (8 == sizeof(result)) {
             result ^= result >> HALFBITS;
         }
         return static_cast<uint32_t>(result);
     }
 };
 
 #endif