[TO M49] Use double microseconds for tracking back/front timestamp in AudioClock.

media/filters/audio_clock.h

 // Copyright 2014 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 MEDIA_FILTERS_AUDIO_CLOCK_H_
 #define MEDIA_FILTERS_AUDIO_CLOCK_H_
 
 #include <stdint.h>
 
+#include <cmath>
 #include <deque>
 
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "media/base/media_export.h"
 
 namespace media {
 
 // Models a queue of buffered audio in a playback pipeline for use with
 // estimating the amount of delay in wall clock time. Takes changes in playback
@@ skipping 62 matching lines @@
   // +-----------------------+-----------------------+-----------------------+
   // |   10 frames silence   |   20 frames @ 1.0x    |   20 frames @ 0.5x    |
   // |      = 250 ms (wall)  |      = 500 ms (wall)  |      = 500 ms (wall)  |
   // |      =   0 ms (media) |      = 500 ms (media) |      = 250 ms (media) |
   // +-----------------------+-----------------------+-----------------------+
   // ^                                                                       ^
   // front_timestamp() is equal to               back_timestamp() is equal to
   // |start_timestamp| since no                  amount of media frames tracked
   // media data has been played yet.             by AudioClock, which would be
   //                                             1000 + 500 + 250 = 1750 ms.
-  base::TimeDelta front_timestamp() const { return front_timestamp_; }
-  base::TimeDelta back_timestamp() const { return back_timestamp_; }
+  base::TimeDelta front_timestamp() const {
+    return base::TimeDelta::FromMicroseconds(
+        std::round(front_timestamp_micros_));
+  }
+  base::TimeDelta back_timestamp() const {
+    return base::TimeDelta::FromMicroseconds(
+        std::round(back_timestamp_micros_));
+  }
 
   // Returns the amount of wall time until |timestamp| will be played by the
   // audio hardware.
   //
   // |timestamp| must be within front_timestamp() and back_timestamp().
   base::TimeDelta TimeUntilPlayback(base::TimeDelta timestamp) const;
 
   void ContiguousAudioDataBufferedForTesting(
       base::TimeDelta* total,
       base::TimeDelta* same_rate_total) const;
 
  private:
   // Even with a ridiculously high sample rate of 256kHz, using 64 bits will
   // permit tracking up to 416999965 days worth of time (that's 1141 millenia).
   //
   // 32 bits on the other hand would top out at measly 2 hours and 20 minutes.
   struct AudioData {
     AudioData(int64_t frames, double playback_rate);
 
     int64_t frames;
     double playback_rate;
   };
 
   // Helpers for operating on |buffered_|.
   void PushBufferedAudioData(int64_t frames, double playback_rate);
   void PopBufferedAudioData(int64_t frames);
-  base::TimeDelta ComputeBufferedMediaDuration() const;
+  double ComputeBufferedMediaDurationMicros() const;
 
   const base::TimeDelta start_timestamp_;
   const double microseconds_per_frame_;
 
   std::deque<AudioData> buffered_;
   int64_t total_buffered_frames_;
 
-  base::TimeDelta front_timestamp_;
-  base::TimeDelta back_timestamp_;
+  // Use double rather than TimeDelta to avoid loss of partial microseconds when
+  // converting between frames-written/delayed and time-passed (see conversion
+  // in WroteAudio()). Particularly for |back_timestamp|, which accumulates more
+  // time with each call to WroteAudio(), the loss of precision can accumulate
+  // to create noticeable audio/video sync drift for longer (2-3 hr) videos.
+  // See http://crbug.com/564604.
+  double front_timestamp_micros_;
+  double back_timestamp_micros_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioClock);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_FILTERS_AUDIO_CLOCK_H_