Fix Bug: Video with Variable Frame Rate plays at incorrect speed.

media/base/moving_average.cc

 // 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_(depth_) {
-}
+    : depth_(depth), count_(0), samples_(depth_), square_sum_us_(0) {}
 
 MovingAverage::~MovingAverage() {
 }
 
 void MovingAverage::AddSample(base::TimeDelta sample) {
   // |samples_| is zero-initialized, so |oldest| is also zero before |count_|
   // exceeds |depth_|.
   base::TimeDelta& oldest = samples_[count_++ % depth_];
   total_ += sample - oldest;
+  square_sum_us_ += sample.InMicroseconds() * sample.InMicroseconds() -
+                    oldest.InMicroseconds() * oldest.InMicroseconds();
   oldest = sample;
 }
 
 base::TimeDelta MovingAverage::Average() const {
   DCHECK_GT(count_, 0u);
 
   // TODO(dalecurtis): Consider limiting |depth| to powers of two so that we can
   // replace the integer divide with a bit shift operation.
 
   return total_ / std::min(static_cast<uint64_t>(depth_), count_);
 }
 
+base::TimeDelta MovingAverage::Deviation() const {
+  DCHECK_GT(count_, 0u);
+
+  const double size = std::min(static_cast<uint64_t>(depth_), count_);
+  const double total_us = total_.InMicroseconds();
+  double sqr_deviation_us =
+      (square_sum_us_ - total_us / size * total_us) / size;

[DaleCurtis, 2015/12/01 22:42:19]

I think your old code w/ size as a double is better, pedantically this uses two
divides instead of a single one.

[qiangchen, 2015/12/01 23:35:33]

Done.

+  if (sqr_deviation_us < 0)
+    sqr_deviation_us = 0;
+
+  return base::TimeDelta::FromMicroseconds(sqrt(sqr_deviation_us));
+}
+
 void MovingAverage::Reset() {
   count_ = 0;
   total_ = base::TimeDelta();
+  square_sum_us_ = 0;
   std::fill(samples_.begin(), samples_.end(), base::TimeDelta());
 }
 
 }  // namespace media