[WIP] Move media/capture to device/capture

media/capture/content/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 MEDIA_CAPTURE_CONTENT_FEEDBACK_SIGNAL_ACCUMULATOR_H_
-#define MEDIA_CAPTURE_CONTENT_FEEDBACK_SIGNAL_ACCUMULATOR_H_
-
-#include "base/time/time.h"
-#include "media/capture/capture_export.h"
-
-namespace media {
-
-// Utility class for maintaining an exponentially-decaying average of feedback
-// signal values whose updates occur at undetermined, possibly irregular time
-// intervals.
-//
-// Feedback signals can be made by multiple sources.  Meaning, there can be
-// several values provided for the same timestamp.  In this case, the greatest
-// value is retained and used to re-compute the average.  Therefore, the values
-// provided to this class' methods should be appropriately translated with this
-// in mind.  For example, an "fraction available" metric should be translated
-// into a "fraction utilized" one.
-//
-// Usage note: Reset() must be called at least once before the first call to
-// Update().
-//
-// This template class supports data points that are timestamped using either
-// |base::TimeDelta| or |base::TimeTicks|.
-template <typename TimeType>
-class 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)
-      : half_life_(half_life), average_(NAN) {
-    DCHECK(half_life_ > base::TimeDelta());
-  }
-
-  // Erase all memory of historical values, re-starting with the given
-  // |starting_value|.
-  void Reset(double starting_value, TimeType timestamp) {
-    average_ = update_value_ = prior_average_ = starting_value;
-    reset_time_ = update_time_ = prior_update_time_ = timestamp;
-  }
-
-  TimeType 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
-  // (see class comments for elaboration).
-  bool Update(double value, TimeType timestamp) {
-    DCHECK(!std::isnan(average_)) << "Reset() must be called once.";
-
-    if (timestamp < update_time_) {
-      return false;  // Not in chronological order.
-    } else if (timestamp == update_time_) {
-      if (timestamp == reset_time_) {
-        // Edge case: Multiple updates at reset timestamp.
-        average_ = update_value_ = prior_average_ =
-            std::max(value, update_value_);
-        return true;
-      }
-      if (value <= update_value_)
-        return true;
-      update_value_ = value;
-    } else {
-      prior_average_ = average_;
-      prior_update_time_ = update_time_;
-      update_value_ = value;
-      update_time_ = timestamp;
-    }
-
-    const double elapsed_us = static_cast<double>(
-        (update_time_ - prior_update_time_).InMicroseconds());
-    const double weight =
-        elapsed_us / (elapsed_us + half_life_.InMicroseconds());
-    average_ = weight * update_value_ + (1.0 - weight) * prior_average_;
-    DCHECK(std::isfinite(average_));
-
-    return true;
-  }
-
-  TimeType 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_;
-
-  TimeType reset_time_;   // |timestamp| passed in last call to Reset().
-  double average_;        // Current accumulated average.
-  double update_value_;   // Latest |value| accepted by Update().
-  TimeType update_time_;  // Latest |timestamp| accepted by Update().
-  double prior_average_;  // Accumulated average before last call to Update().
-  TimeType prior_update_time_;  // |timestamp| in prior call to Update().
-};
-
-}  // namespace media
-
-#endif  // MEDIA_CAPTURE_CONTENT_FEEDBACK_SIGNAL_ACCUMULATOR_H_