Adding partially circular memory buffer wrapper.

content/common/partial_circular_buffer.h

Index: content/common/partial_circular_buffer.h
diff --git a/content/common/partial_circular_buffer.h b/content/common/partial_circular_buffer.h
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+// Copyright (c) 2013 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 CONTENT_COMMON_PARTIAL_CIRCULAR_BUFFER_H_
+#define CONTENT_COMMON_PARTIAL_CIRCULAR_BUFFER_H_
+
+#include "base/basictypes.h"
+#include "base/gtest_prod_util.h"
+#include "content/common/content_export.h"
+
+namespace content {
+
+// A wrapper around a memory buffer that allows circular read and write with a
+// selectable wrapping position. Buffer layout (after wrap; H is header):
+// -----------------------------------------------------------
+// | H | Beginning           | End           | Middle        |
+// -----------------------------------------------------------
+//  ^---- Non-wrapping -----^ ^--------- Wrapping ----------^
+// The non-wrapping part is never overwritten. The wrapping part will be
+// circular. The very first part is the header (see the BufferData struct
+// below). It consists of the following information:
+// - Length written to the buffer (not including header).
+// - Wrapping position.
+// - End position of buffer. (If the last byte is at x, this will be x + 1.)
+// Users of wrappers around the same underlying buffer must ensure that writing
+// is finished before reading is started.
+class CONTENT_EXPORT PartialCircularBuffer {
+ public:
+  // Use for reading. |buffer_size| is in bytes and must be larger than the
+  // header size (see above).
+  PartialCircularBuffer(void* buffer, uint32 buffer_size);
+
+  // Use for writing. |buffer_size| is in bytes and must be larger than the
+  // header size (see above).
+  PartialCircularBuffer(void* buffer,
+                        uint32 buffer_size,
+                        uint32 wrap_position);
+
+  uint32 Read(void* buffer, uint32 buffer_size);
+  void Write(const void* buffer, uint32 buffer_size);
+
+ private:
+  friend class PartialCircularBufferTest;
+
+#pragma pack(push)
+#pragma pack(4)
+  struct BufferData {
+    uint32 total_written;
+    uint32 wrap_position;
+    uint32 end_position;
+    uint8 data[1];
+  };
+#pragma pack(pop)
+
+  void DoWrite(void* dest, const void* src, uint32 num);
+
+  // Used for reading and writing.
+  BufferData* buffer_data_;
+  uint32 memory_buffer_size_;
+  uint32 data_size_;
+  uint32 position_;
+
+  // Used for reading.
+  uint32 total_read_;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_PARTIAL_CIRCULAR_BUFFER_H_