Update RateStatistics to handle too-little-data case.

webrtc/base/rate_statistics.h

 /*
  *  Copyright (c) 2013 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_BASE_RATE_STATISTICS_H_
 #define WEBRTC_BASE_RATE_STATISTICS_H_
 
 #include <memory>
 
+#include "webrtc/base/optional.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
 
 class RateStatistics {
  public:
-  // window_size = window size in ms for the rate estimation
+  // max_window_size_ms = Maximum window size in ms for the rate estimation.
+  //                      Initial window size is set to this, but may be changed
+  //                      to something lower by calling SetWindowSize().
   // scale = coefficient to convert counts/ms to desired units,
   //         ex: if counts represents bytes, use 8*1000 to go to bits/s
-  RateStatistics(uint32_t window_size_ms, float scale);
+  RateStatistics(int64_t max_window_size_ms, float scale);
   ~RateStatistics();
 
   void Reset();
   void Update(size_t count, int64_t now_ms);
-  uint32_t Rate(int64_t now_ms);
+  rtc::Optional<uint32_t> Rate(int64_t now_ms);
+  bool SetWindowSize(int64_t window_size_ms, int64_t now_ms);
 
  private:
   void EraseOld(int64_t now_ms);
 
   // Counters are kept in buckets (circular buffer), with one bucket
   // per millisecond.
-  const int num_buckets_;
-  std::unique_ptr<size_t[]> buckets_;
+  struct Bucket {
+    size_t sum;      // Sum of all samples in this bucket.
+    size_t samples;  // Number of samples in this bucket.
+  };
+  std::unique_ptr<Bucket[]> buckets_;
 
   // Total count recorded in buckets.
   size_t accumulated_count_;
 
+  // The total number of samples in the buckets.
+  size_t num_samples_;
+
   // Oldest time recorded in buckets.
   int64_t oldest_time_;
 
   // Bucket index of oldest counter recorded in buckets.
-  int oldest_index_;
+  uint32_t oldest_index_;
 
   // To convert counts/ms to desired units
   const float scale_;
+
+  // The window sizes, in ms, over which the rate is calculated.
+  const int64_t max_window_size_ms_;
+  int64_t current_window_size_ms_;
 };
 }  // namespace webrtc
 
 #endif  // WEBRTC_BASE_RATE_STATISTICS_H_