Add a new overuse estimator for the delay based BWE behind experiment.

webrtc/modules/congestion_controller/trendline_estimator.h

Index: webrtc/modules/congestion_controller/trendline_estimator.h
diff --git a/webrtc/modules/congestion_controller/trendline_estimator.h b/webrtc/modules/congestion_controller/trendline_estimator.h
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+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#ifndef WEBRTC_MODULES_CONGESTION_CONTROLLER_TRENDLINE_ESTIMATOR_H_
+#define WEBRTC_MODULES_CONGESTION_CONTROLLER_TRENDLINE_ESTIMATOR_H_
+
+#include <list>
+#include <utility>
+
+#include "webrtc/base/constructormagic.h"
+
+namespace webrtc {
+
+class TrendlineEstimator {
+ public:
+  // |window_size| is the number of points required to compute a trend line.
+  // |smoothing_coef| controls how much we smooth out the delay before fitting
+  // the trend line. |threshold_gain| is used to scale the trendline slope for
+  // comparison to the old threshold. Once the old estimator has been removed
+  // (or the thresholds been merged into the estimators), we can just set the
+  // threshold instead of setting a gain.
+  TrendlineEstimator(size_t window_size,
+                     double smoothing_coef,
+                     double threshold_gain);
+  ~TrendlineEstimator();
+
+  // Update the estimator with a new sample. The deltas should represent deltas
+  // between timestamp groups as defined by the InterArrival class.
+  void Update(double recv_delta_ms, double send_delta_ms, double now_ms);
+
+  // Returns the estimated trend k multiplied by some gain.
+  // 0 < k < 1   ->  the delay increases, queues are filling up
+  //   k == 0    ->  the delay does not change
+  //   k < 0     ->  the delay decreases, queues are being emptied
+  double trendline_slope() const { return trendline_ * threshold_gain_; }
+
+  // Returns the number of deltas which the current estimator state is based on.
+  unsigned int num_of_deltas() const { return num_of_deltas_; }
+
+ private:
+  // Parameters.
+  const size_t window_size_;
+  const double smoothing_coef_;
+  const double threshold_gain_;
+  // Used by the existing threshold.
+  unsigned int num_of_deltas_;
+  // Exponential backoff filtering.
+  double accumulated_delay_;
+  double smoothed_delay_;
+  // Linear least squares regression.
+  std::list<std::pair<double, double>> delay_hist_;
+  double trendline_;
+
+  RTC_DISALLOW_COPY_AND_ASSIGN(TrendlineEstimator);
+};
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_CONGESTION_CONTROLLER_TRENDLINE_ESTIMATOR_H_