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 int size = std::min(static_cast<uint64_t>(depth_), count_);
+  const int64_t average_us = Average().InMicroseconds();
+  const int64_t sqr_deviation_us =
+      square_sum_us_ / size - average_us * average_us;

[DaleCurtis, 2015/12/01 20:11:28]

Cast upper or lower to double and store as double? The multiplies won't lose
precision unless there's an overflow, but the divide + sqrt might lose something
meaningful.

[qiangchen, 2015/12/01 22:37:07]

Done.

Not sure are you fine with implicit cast. Anyway, in float computation, No
DCHECK_GE can be applied, because theoretically average square sum is larger
than or equal to square average. In float computation it is possible to have a
value like -1e-14, in the case that all frames have exactly same length.

+  DCHECK_GE(sqr_deviation_us, 0);
+
+  return base::TimeDelta::FromMicroseconds(
+      static_cast<int64_t>(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