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_
+
+namespace cc {
+
+template<typename T, unsigned int size>
+class RingBuffer {
+ public:
+  explicit RingBuffer()
+    : current_index_(0) {
+  }
+
+  unsigned int BufferSize() const {
+    return size;
+  }
+
+  unsigned int CurrentIndex() const {
+    return current_index_;
+  }
+
+  // tests if a value was saved to this index
+  bool IsFilledIndex(unsigned int n) const {

[shawnsingh, 2013/01/10 21:13:01]

I just noticed that the style guide asks us never to use unsigned int.  So we
have to use normal int for this whole class, and right here we should add a
DCHECK that it is >= 0.

[danakj, 2013/01/10 23:31:40]

What, are you sure it doesn't mean to use "unsigned" instead of "unsigned int"?

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

[shawnsingh, 2013/01/10 21:13:01]

Can we DCHECK here that n is always >= 0 and strictly < size ?

Perhaps we also want to DCHECK(IsFilledIndex(n)) here?

+  }
+
+  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];
+  unsigned int current_index_;
+};
+
+}  // namespace cc
+
+#endif  // CC_RING_BUFFER_H_