Move SkChecksum::Murmur3 out of the header.

src/core/SkChecksum.cpp

Index: src/core/SkChecksum.cpp
diff --git a/src/core/SkChecksum.cpp b/src/core/SkChecksum.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..4457eb4f9976a6719a7b5bd03481fb868be363de
--- /dev/null
+++ b/src/core/SkChecksum.cpp
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkChecksum.h"
+
+uint32_t SkChecksum::Murmur3(const void* data, size_t bytes, uint32_t seed) {
+    // Use may_alias to remind the compiler we're intentionally violating strict aliasing,
+    // and so not to apply strict-aliasing-based optimizations.
+    typedef uint32_t SK_ATTRIBUTE(may_alias) aliased_uint32_t;
+    typedef uint8_t SK_ATTRIBUTE(may_alias) aliased_uint8_t;
+
+    // Handle 4 bytes at a time while possible.
+    const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data;
+    const size_t words = bytes/4;
+    uint32_t hash = seed;
+    for (size_t i = 0; i < words; i++) {
+        uint32_t k = safe_data[i];
+        k *= 0xcc9e2d51;
+        k = (k << 15) | (k >> 17);
+        k *= 0x1b873593;
+
+        hash ^= k;
+        hash = (hash << 13) | (hash >> 19);
+        hash *= 5;
+        hash += 0xe6546b64;
+    }
+
+    // Handle last 0-3 bytes.
+    const aliased_uint8_t* safe_tail = (const uint8_t*)(safe_data + words);
+    uint32_t k = 0;
+    switch (bytes & 3) {
+        case 3: k ^= safe_tail[2] << 16;
+        case 2: k ^= safe_tail[1] <<  8;
+        case 1: k ^= safe_tail[0] <<  0;
+                k *= 0xcc9e2d51;
+                k = (k << 15) | (k >> 17);
+                k *= 0x1b873593;
+                hash ^= k;
+    }
+
+    hash ^= bytes;
+    return SkChecksum::Mix(hash);
+}