Add SkChecksum::Murmur3.

include/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 18 matching lines @@
         ROTR = 17,
         ROTL = sizeof(uintptr_t) * 8 - ROTR,
         HALFBITS = sizeof(uintptr_t) * 4
     };
 
     static inline uintptr_t Mash(uintptr_t total, uintptr_t value) {
         return ((total >> ROTR) | (total << ROTL)) ^ value;
     }
 
 public:
+
+    /**
+     * 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) {
+        SkASSERT(SkIsAlign4(bytes));
+        const size_t words = bytes/4;
+
+        uint32_t hash = seed;
+        for (size_t i = 0; i < words; i++) {
+            uint32_t k = data[i];
+            k *= 0xcc9e2d51;
+            k = (k << 15) | (k >> 17);
+            k *= 0x1b873593;
+
+            hash ^= k;
+            hash = (hash << 13) | (hash >> 19);
+            hash *= 5;
+            hash += 0xe6546b64;
+        }
+        hash ^= bytes;
+        hash ^= hash >> 16;
+        hash *= 0x85ebca6b;
+        hash ^= hash >> 13;
+        hash *= 0xc2b2ae35;
+        hash ^= hash >> 16;
+        return hash;
+    }
+
     /**
      *  Compute a 32-bit checksum for a given data block
      *
      *  WARNING: this algorithm is tuned for efficiency, not backward/forward
      *  compatibility.  It may change at any time, so a checksum generated with
      *  one version of the Skia code may not match a checksum generated with
      *  a different version of the Skia code.
      *
      *  @param data Memory address of the data block to be processed. Must be
      *              32-bit aligned.
@@ skipping 41 matching lines @@
          *  define.
          */
         if (8 == sizeof(result)) {
             result ^= result >> HALFBITS;
         }
         return static_cast<uint32_t>(result);
     }
 };
 
 #endif