Refactor NACK bitrate allocation

webrtc/base/rate_statistics.h

Index: webrtc/base/rate_statistics.h
diff --git a/webrtc/base/rate_statistics.h b/webrtc/base/rate_statistics.h
index 3e913cc1bbc310ba9d6b122bf93ea499a592ebfc..a6f8802ee4da3f94d6f900c7f66342ca38bc0683 100644
--- a/webrtc/base/rate_statistics.h
+++ b/webrtc/base/rate_statistics.h
@@ -20,17 +20,35 @@ namespace webrtc {
 
 class RateStatistics {
  public:
+  static constexpr float kBpsScale = 8000.0f;
+
   // 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
+  // scale = coefficient to convert counts/ms to desired unit
+  //         ex: kBpsScale (8000) for bits/s if count represents bytes.
   RateStatistics(int64_t max_window_size_ms, float scale);
   ~RateStatistics();
 
+  // Reset instance to original state.
   void Reset();
+
+  // Update rate with a new data point, moving averaging window as needed.
   void Update(size_t count, int64_t now_ms);
+
+  // Poll the current rate, moving averaging window as needed.
   rtc::Optional<uint32_t> Rate(int64_t now_ms);
+
+  // Utility method for polling rate from a  const function. Note that this
+  // still updates the internal state (moves averaging window), but it doesn't
+  // make any alterations that are observable from the other methods, as long as
+  // supplied timestamps are from a monotonic clock. Ie, it doesn't matter if
+  // this call moves the window, since any subsequent call to Update or Rate
+  // would still have moved the window as much or more.
+  rtc::Optional<uint32_t> Rate(int64_t now_ms) const;
+
+  // Update the size of the averaging window. The maximum allowed value for
+  // window_size_ms is max_window_size_ms as supplied in the constructor.
   bool SetWindowSize(int64_t window_size_ms, int64_t now_ms);
 
  private: