cc: add RingBuffer class for timestamp storing in FrameRateCounter

cc/ring_buffer.h

+// Copyright 2012 The Chromium Authors. All rights reserved.

[shawnsingh, 2013/01/09 20:55:09]

2013 now =)

+// 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_

[shawnsingh, 2013/01/09 20:55:09]

As I understand, we are trying to make all new files in cc/ to follow chromium
style, even though many files are currently still using the old way from
back-in-the-day when the code was in WebKit.

So for this entire file let's do:
 - indentation of 2 spaces, not 4
 - m_buffer --> buffer_
 - m_currentIndex --> currentIndex_
 - functions that use camelCase should have first letter also capitalized -->
BufferSize(), ReadBuffer(), etc.

I'm still coming to grips with chromium style too, so I may have missed some

+#define CC_RING_BUFFER_H_
+
+namespace cc {
+
+template<typename T, int size>
+class RingBuffer {
+public:
+    int bufferSize() const
+    {
+        return size;
+    }
+
+    // n = 0 returns the oldest value and
+    // n = bufferSize() - 1 returns the most recent value.
+    T readBuffer(int n) const
+    {
+        return m_buffer[bufferIndex(m_currentIndex + n)];
+    }
+
+protected:
+    explicit RingBuffer()
+        : m_currentIndex(0)
+    {
+    }
+
+    void saveToBuffer(T value)
+    {
+        m_buffer[bufferIndex(m_currentIndex)] = value;
+        m_currentIndex++;
+    }
+
+    inline int bufferIndex(int n) const
+    {
+        return (n % size + size) % size;

[shawnsingh, 2013/01/09 20:55:09]

Wouldn't this be equivalent to (n+size) % size,  without the need for the extra
mod operation?    If I'm not mistaken, that would be more efficient, I believe
mod operators are often pretty expensive in terms of # CPU cycles.  Or am I
missing something subtle here? =)

+    }
+
+    T m_buffer[size];
+    int m_currentIndex;
+};
+
+}  // namespace cc
+
+#endif  // CC_RING_BUFFER_H_