diff --git a/metadata.h b/metadata.h
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--- /dev/null
+++ b/metadata.h
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+// Copyright (c) 2010 The Chromium OS 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 CHROMEOS_PLATFORM_UPDATE_ENGINE_METADATA_H__
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_METADATA_H__
+
+#include "update_engine/delta_diff_generator.h"
+#include "update_engine/graph_types.h"
+
+namespace chromeos_update_engine {
+
+class Metadata {
+ public:
+  // Reads metadata from old image and new image and determines
+  // the smallest way to encode the metadata for the diff.
+  // If there's no change in the metadata, it creates a MOVE
+  // operation. If there is a change, the smallest of REPLACE, REPLACE_BZ,
+  // or BSDIFF wins. It writes the diff to data_fd and updates data_file_size
+  // accordingly. It also adds the required operation to the graph and adds the
+  // metadata extents to blocks.
+  // Returns true on success.
+  static bool DeltaReadMetadata(Graph* graph,
+                                std::vector<DeltaDiffGenerator::Block>* blocks,
+                                const std::string& old_image,
+                                const std::string& new_image,
+                                int data_fd,
+                                off_t* data_file_size);
+
+ private:
+  // This should never be constructed.
+  DISALLOW_IMPLICIT_CONSTRUCTORS(Metadata);
+};
+
+};  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_METADATA_H__