cc: add RingBuffer class for timestamp storing in FrameRateCounter

cc/ring_buffer.h

+// Copyright 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 CC_RING_BUFFER_H_
+#define CC_RING_BUFFER_H_
+
+#include "base/logging.h"
+
+namespace cc {
+
+template<typename T, int size>
+class RingBuffer {
+ public:
+  explicit RingBuffer()
+    : current_index_(0) {
+  }
+
+  int BufferSize() const {
+    return size;
+  }
+
+  int CurrentIndex() const {
+    return current_index_;
+  }
+
+  // tests if a value was saved to this index
+  bool IsFilledIndex(int n) const {
+    DCHECK(n >= 0);

[egraether, 2013/01/10 23:38:29]

Prevents using negative indices.

+    return BufferIndex(n) < current_index_;
+  }
+
+  // n = 0 returns the oldest value and
+  // n = bufferSize() - 1 returns the most recent value.
+  T ReadBuffer(int n) const {
+    DCHECK(IsFilledIndex(n));

[egraether, 2013/01/10 23:38:29]

Forbids access to uninitialized indices.

+    return buffer_[BufferIndex(n)];
+  }
+
+  void SaveToBuffer(T value) {
+    buffer_[BufferIndex(0)] = value;
+    current_index_++;
+  }
+
+ private:
+  inline int BufferIndex(int n) const {
+    return (current_index_ + n) % size;
+  }
+
+  T buffer_[size];
+  int current_index_;
+};
+
+}  // namespace cc
+
+#endif  // CC_RING_BUFFER_H_