Switch GetWallClockTime to using vectors for input and output.

media/base/time_source.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_BASE_TIME_SOURCE_H_
 #define MEDIA_BASE_TIME_SOURCE_H_
 
+#include <vector>
+
+#include "base/callback.h"
 #include "base/time/time.h"
 #include "media/base/media_export.h"
 
 namespace media {
 
 // A TimeSource is capable of providing the current media time.
 class MEDIA_EXPORT TimeSource {
  public:
+  // Helper alias for converting media timestamps into a wall clock timestamps.
+  using WallClockTimeCB =
+      base::Callback<bool(const std::vector<base::TimeDelta>&,
+                          std::vector<base::TimeTicks>*)>;
+
   TimeSource() {}
   virtual ~TimeSource() {}
 
   // Signal the time source to start ticking. It is expected that values from
   // CurrentMediaTime() will start increasing.
   virtual void StartTicking() = 0;
 
   // Signal the time source to stop ticking. It is expected that values from
   // CurrentMediaTime() will remain constant.
   virtual void StopTicking() = 0;
 
   // Updates the current playback rate. It is expected that values from
   // CurrentMediaTime() will eventually reflect the new playback rate (e.g., the
-  // media time will advance at half speed if the rate was set to 0.5f).
+  // media time will advance at half speed if the rate was set to 0.5).
   virtual void SetPlaybackRate(double playback_rate) = 0;
 
   // Sets the media time to start ticking from. Only valid to call while the
   // time source is not ticking.
   virtual void SetMediaTime(base::TimeDelta time) = 0;
 
   // Returns the current media timestamp relative to the timestamp set by
   // SetMediaTime().
   //
   // Values returned are intended for informational purposes, such as displaying
   // UI with the current minute and second count. While it is guaranteed values
   // will never go backwards, the frequency at which they update may be low.
   virtual base::TimeDelta CurrentMediaTime() = 0;
 
-  // Converts a media timestamp into a wall clock time. If the media time is
-  // stopped, returns a null TimeTicks.
+  // Converts a vector of media timestamps into a vector of wall clock times. If
+  // the media time is stopped, returns false and does not modify the output
+  // vector. Returns true and converts all timestamps otherwise. Guarantees that
+  // wall clock time does not go backwards for monotonically increasing media

[xhwang, 2015/05/12 04:46:53]

Q for "does not go backwards": Can f(a) == f(b) for a < b? 

[DaleCurtis, 2015/05/12 05:35:42]

Not unless there's a rounding error, f(a) != f(b) if a != b.

[xhwang, 2015/05/12 05:45:42]

nit: "does not go backwards" means a >= b, which should be a > b in this case.
Sorry for not being clear.

[DaleCurtis, 2015/05/12 16:50:04]

I'm still not clear on your nit here :) Within a single run, the code will
ensure f(a) > f(b) for a > b, f(a) < (b)  for a < b, and f(a) == f(b) for a ==
b.

Is your comment asking me to clarify that more exactly here?

[xhwang, 2015/05/12 17:20:55]

Just say the wall clock time is a strictly increasing function of media
timestamps in one GetWallClockTime call?
(http://en.wikipedia.org/wiki/Monotonic_function)

+  // timestamps.
   //
-  // |media_time| values too far ahead of the current media time will return an
-  // estimated value; as such, these values may go backwards in time slightly.
+  // Each timestamp converted from |media_timestamps| will be pushed into
+  // |wall_clock_times| such that after all timestamps are converted, the two
+  // vectors are parallel (media_timestamps[i] -> wall_clock_times[i]).
   //
-  // |media_time| values behind the current media time may be significantly
-  // incorrect if the playback rate has changed recently. The only guarantee is
-  // that the returned time will be less than the current wall clock time.
-  virtual base::TimeTicks GetWallClockTime(base::TimeDelta media_time) = 0;
+  // |media_timestamps| values too far ahead of the current media time will
+  // be converted to an estimated value; as such, these values may go backwards
+  // in time slightly between calls to GetWallClockTime().
+  //
+  // |media_timestamps| values behind the current media time may be
+  // significantly incorrect if the playback rate has changed recently. The only
+  // guarantee is that the returned time will be less than the current wall
+  // clock time.
+  virtual bool GetWallClockTime(

[xhwang, 2015/05/12 04:46:53]

bike shedding: should it be GetWallClockTime_s_?

[DaleCurtis, 2015/05/12 05:35:42]

Done.

+      const std::vector<base::TimeDelta>& media_timestamps,
+      std::vector<base::TimeTicks>* wall_clock_times) = 0;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_TIME_SOURCE_H_