diff -r 000000000000 -r 6474c204b198 gfx/skia/trunk/src/utils/SkBitmapHasher.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/skia/trunk/src/utils/SkBitmapHasher.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,64 @@
+/*
+ * Copyright 2012 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkBitmap.h"
+#include "SkBitmapHasher.h"
+#include "SkEndian.h"
+#include "SkImageEncoder.h"
+
+#include "SkMD5.h"
+
+/**
+ * Write an int32 value to a stream in little-endian order.
+ */
+static void write_int32_to_buffer(uint32_t val, SkWStream* out) {
+    val = SkEndian_SwapLE32(val);
+    for (size_t byte = 0; byte < 4; ++byte) {
+        out->write8((uint8_t)(val & 0xff));
+        val = val >> 8;
+    }
+}
+
+/**
+ * Return the first 8 bytes of a bytearray, encoded as a little-endian uint64.
+ */
+static inline uint64_t first_8_bytes_as_uint64(const uint8_t *bytearray) {
+    return SkEndian_SwapLE64(*(reinterpret_cast<const uint64_t *>(bytearray)));
+}
+
+/*static*/ bool SkBitmapHasher::ComputeDigestInternal(const SkBitmap& bitmap, uint64_t *result) {
+    SkMD5 out;
+
+    // start with the x/y dimensions
+    write_int32_to_buffer(SkToU32(bitmap.width()), &out);
+    write_int32_to_buffer(SkToU32(bitmap.height()), &out);
+
+    // add all the pixel data
+    SkAutoTDelete<SkImageEncoder> enc(CreateARGBImageEncoder());
+    if (!enc->encodeStream(&out, bitmap, SkImageEncoder::kDefaultQuality)) {
+        return false;
+    }
+
+    SkMD5::Digest digest;
+    out.finish(digest);
+    *result = first_8_bytes_as_uint64(digest.data);
+    return true;
+}
+
+/*static*/ bool SkBitmapHasher::ComputeDigest(const SkBitmap& bitmap, uint64_t *result) {
+    if (ComputeDigestInternal(bitmap, result)) {
+        return true;
+    }
+
+    // Hmm, that didn't work. Maybe if we create a new
+    // kARGB_8888_Config version of the bitmap it will work better?
+    SkBitmap copyBitmap;
+    if (!bitmap.copyTo(&copyBitmap, kPMColor_SkColorType)) {
+        return false;
+    }
+    return ComputeDigestInternal(copyBitmap, result);
+}