[Cast] Repair receiver playout time calculations and frame skip logic.

media/cast/base/clock_drift_smoother.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_CAST_BASE_CLOCK_DRIFT_SMOOTHER_H_
+#define MEDIA_CAST_BASE_CLOCK_DRIFT_SMOOTHER_H_
+
+#include "base/time/time.h"
+
+namespace media {
+namespace cast {
+
+// Tracks the jitter and drift between clocks, providing a smoothed offset.
+// Internally, a Simple IIR filter is used to maintain a running average that
+// moves at a rate based on the passage of time.
+class ClockDriftSmoother {
+ public:
+  // |time_constant| is the amount of time an impulse signal takes to decay by
+  // ~62.6%.  Interpretation: If the value passed to several Update() calls is
+  // held constant for T seconds, then the running average will have moved
+  // towards the value by ~62.6% from where it started.
+  explicit ClockDriftSmoother(base::TimeDelta time_constant);
+  ~ClockDriftSmoother();
+
+  // Returns the current offset.
+  base::TimeDelta Current() const;
+
+  // Discard all history and reset to exactly |offset|, measured |now|.
+  void Reset(base::TimeTicks now, base::TimeDelta offset);
+
+  // Update the current offset, which was measured |now|.  The weighting that
+  // |measured_offset| will have on the running average is influenced by how
+  // much time has passed since the last call to this method (or Reset()).
+  // |now| should be monotonically non-decreasing over successive calls of this
+  // method.
+  void Update(base::TimeTicks now, base::TimeDelta measured_offset);
+
+  // Returns a time constant suitable for most use cases, where the clocks
+  // are expected to drift very little with respect to each other, and the
+  // jitter caused by clock imprecision is effectively canceled out.
+  static base::TimeDelta GetDefaultTimeConstant();
+
+ private:
+  const base::TimeDelta time_constant_;
+  base::TimeTicks last_update_time_;
+  double estimate_us_;
+};
+
+}  // namespace cast
+}  // namespace media
+
+#endif  // MEDIA_CAST_BASE_CLOCK_DRIFT_SMOOTHER_H_