Avoid precision loss in TrendlineEstimator from int64_t -> double conversion

webrtc/modules/congestion_controller/trendline_estimator.h

 /*
  *  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 <stddef.h>
+#include <stdint.h>
+
 #include <list>
 #include <utility>
 
 #include "webrtc/base/constructormagic.h"
-#include "webrtc/common_types.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);
+  void Update(double recv_delta_ms,
+              double send_delta_ms,
+              int64_t arrival_time_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_;
+  // Keep the arrival times small by using the change from the first packet.
+  int64_t first_arrival_time_ms;
   // 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_