FeedbackSignalAccumulator utility class for averaging feedback signals

content/browser/media/capture/feedback_signal_accumulator.h

+// Copyright (c) 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.
+
+#ifndef CONTENT_BROWSER_MEDIA_CAPTURE_FEEDBACK_SIGNAL_ACCUMULATOR_H_
+#define CONTENT_BROWSER_MEDIA_CAPTURE_FEEDBACK_SIGNAL_ACCUMULATOR_H_
+
+#include "base/time/time.h"
+#include "content/common/content_export.h"
+
+namespace content {
+
+// Utility class for maintaining an exponentially-decaying average of feedback
+// signal values whose updates occur at undetermined, possibly irregular time
+// intervals.
+//
+// Usage note: Reset() must be called at least once before the first call to
+// Update().
+class CONTENT_EXPORT FeedbackSignalAccumulator {
+ public:
+  // |half_life| is the amount of time that must pass between two data points to
+  // move the accumulated average value halfway in-between.  Example: If
+  // |half_life| is one second, then calling Reset(0.0, t=0s) and then
+  // Update(1.0, t=1s) will result in an accumulated average value of 0.5.
+  explicit FeedbackSignalAccumulator(base::TimeDelta half_life);
+
+  // Erase all memory of historical values, re-starting with the given
+  // |starting_value|.
+  void Reset(double starting_value, base::TimeTicks timestamp);
+  base::TimeTicks reset_time() const { return reset_time_; }
+
+  // Apply the given |value|, which was observed at the given |timestamp|, to
+  // the accumulated average.  If the timestamp is in chronological order, the
+  // update succeeds and this method returns true.  Otherwise the update has no
+  // effect and false is returned.  If there are two or more updates at the same
+  // |timestamp|, only the one with the greatest value will be accounted for.

[hubbe, 2015/05/12 19:10:07]

Document why "greatest value" makes sense.

[miu, 2015/05/12 20:58:37]

Done.  (Documented at class-level comments.)

+  bool Update(double value, base::TimeTicks timestamp);
+  base::TimeTicks update_time() const { return update_time_; }
+
+  // Returns the current accumulated average value.
+  double current() const { return average_; }
+
+ private:
+  // In conjunction with the |update_time_| and |prior_update_time_|, this is
+  // used to compute the weight of the current update value versus the prior
+  // accumulated average.
+  const base::TimeDelta half_life_;
+
+  base::TimeTicks reset_time_;  // |timestamp| passed in last call to Reset().
+  double average_;  // Current accumulated average.
+  double update_value_;  // Latest |value| accepted by Update().
+  base::TimeTicks update_time_;  // Latest |timestamp| accepted by Update().
+  double prior_average_;  // Accumulated average before last call to Update().
+  base::TimeTicks prior_update_time_;  // |timestamp| in prior call to Update().
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_MEDIA_CAPTURE_FEEDBACK_SIGNAL_ACCUMULATOR_H_