Refactor BlockDifference() to use MediaInitializer().

media/video/capture/screen/differ_block.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_VIDEO_CAPTURE_SCREEN_DIFFER_BLOCK_H_
 #define MEDIA_VIDEO_CAPTURE_SCREEN_DIFFER_BLOCK_H_
 
 #include "base/basictypes.h"
 #include "media/base/media_export.h"
 
 namespace media {
 
-// Size (in pixels) of each square block used for diffing. This must be a
-// multiple of sizeof(uint64)/8.
-const int kBlockSize = 32;
+enum {
+  // Size (in pixels) of each square block used for diffing. This must be a
+  // multiple of sizeof(uint64)/8.
+  kBlockSize = 32,
 
-// Format: BGRA 32 bit.
-const int kBytesPerPixel = 4;
+  // Format: BGRA 32 bit.
+  kBytesPerPixel = 4,
+};
+
+// Selects the optimal CPU specific BlockDifference() method at runtime.  Must
+// be called prior to using BlockDifference.  Called during media library init;
+// most users should not need to call this function.
+MEDIA_EXPORT void InitializeCPUSpecificBlockDifference();
 
 // Low level functions to compare 2 blocks of pixels. Zero means the blocks
 // are identical. One - the blocks are different.
 MEDIA_EXPORT int BlockDifference(const uint8* image1,
                                  const uint8* image2,
                                  int stride);
 
 }  // namespace media
 
 #endif  // MEDIA_VIDEO_CAPTURE_SCREEN_DIFFER_BLOCK_H_