Introduce cadence based VideoRendererAlgorithm.

media/base/moving_average.cc

Index: media/base/moving_average.cc
diff --git a/media/base/moving_average.cc b/media/base/moving_average.cc
new file mode 100644
index 0000000000000000000000000000000000000000..22f9b8299e4e37082de7a1c13d96d301a97415a8
--- /dev/null
+++ b/media/base/moving_average.cc
@@ -0,0 +1,41 @@
+// Copyright 2015 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.
+
+#include "media/base/moving_average.h"
+
+#include <algorithm>
+
+namespace media {
+
+MovingAverage::MovingAverage(size_t depth)
+    : depth_(depth),
+      count_(0),
+      samples_(new base::TimeDelta[depth]) {
+}
+
+MovingAverage::~MovingAverage() {}
+
+void MovingAverage::AddSample(base::TimeDelta sample) {
+  if (count_ < depth_) {
+    samples_[count_++] = sample;
+    total_ += sample;
+    return;
+  }

[xhwang, 2015/04/28 16:01:07]

If you initialize |samples_| with all zeros (which is the default value), we
don't need this "if" block.

[DaleCurtis, 2015/04/28 21:45:23]

Done.

+
+  base::TimeDelta& oldest = samples_[count_++ % depth_];
+  total_ += sample - oldest;
+  oldest = sample;
+}
+
+base::TimeDelta MovingAverage::Average() const {
+  DCHECK_GT(count_, 0u);
+  return total_ / std::min(depth_, count_);

[xhwang, 2015/04/28 16:01:07]

nit: If we always choose a |depth_| of a power of 2 (e.g. 32, which should be
close enough to 25), then in most cases we can use bit shift instead of
division. But it's a bit tricky since we are dealing with TimeDelta instead of
integer here.

[DaleCurtis, 2015/04/28 21:45:24]

Hmm, I feel that's a bit of premature optimization :) I've added a TODO() to do
some testing to see if this is a hot spot.

+}
+
+void MovingAverage::Reset() {
+  count_ = 0;
+  total_ = base::TimeDelta();
+}
+
+}  // namespace media