diff --git a/split_file_writer.h b/split_file_writer.h
new file mode 100644
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--- /dev/null
+++ b/split_file_writer.h
@@ -0,0 +1,63 @@
+// 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_SPLIT_FILE_WRITER_H__
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_SPLIT_FILE_WRITER_H__
+
+#include "update_engine/file_writer.h"
+
+// SplitFileWriter is an implementation of FileWriter suited to our
+// full autoupdate format. The first 8 bytes read are assumed to be a
+// big-endian number describing how many of the next following bytes
+// go to the first FileWriter. After that, the rest of the bytes all
+// go to the second FileWriter.
+
+namespace chromeos_update_engine {
+
+class SplitFileWriter : public FileWriter {
+ public:
+  SplitFileWriter(FileWriter* first_file_writer, FileWriter* second_file_writer)
+      : first_file_writer_(first_file_writer),
+        first_length_(0),
+        first_path_(NULL),
+        first_flags_(0),
+        first_mode_(0),
+        second_file_writer_(second_file_writer),
+        bytes_received_(0) {}
+  
+  void SetFirstOpenArgs(const char* path, int flags, mode_t mode) {
+    first_path_ = path;
+    first_flags_ = flags;
+    first_mode_ = mode;
+  }
+  
+  // If both succeed, returns the return value from the second Open() call.
+  // On error, both files will be left closed.
+  virtual int Open(const char* path, int flags, mode_t mode);
+
+  virtual ssize_t Write(const void* bytes, size_t count);
+
+  virtual int Close();
+
+ private:
+  // Data for the first file writer.
+  FileWriter* const first_file_writer_;
+  off_t first_length_;
+  const char* first_path_;
+  int first_flags_;
+  mode_t first_mode_;
+  
+  // The scond file writeer.
+  FileWriter* const second_file_writer_;
+
+  // Bytes written thus far
+  off_t bytes_received_;
+  char first_length_buf_[sizeof(uint64_t)];
+
+  DISALLOW_COPY_AND_ASSIGN(SplitFileWriter);
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_SPLIT_FILE_WRITER_H__