Webrtc 码率统计 RateStatistics

RateStatistics

用于webrtc的码率统计，

用法说明

• RateStatistics::RateStatistics(int64_t window_size_ms, float scale)；

window_size_ms 统计窗口的大小，scale 缩放参数用于单位转换，比如byte转bit就是8000；

• void RateStatistics::Update(size_t count, int64_t now_ms)；每次数据来时进行更新，count是收到数据的大小，now_ms当前时刻；

• rtc::Optional<uint32_t> RateStatistics::Rate(int64_t now_ms) const获取当前时间的码率；

  算法说明

RateStatistics::RateStatistics(int64_t window_size_ms, float scale)
    : buckets_(new Bucket[window_size_ms]()),
      accumulated_count_(0),
      num_samples_(0),
      oldest_time_(-window_size_ms),
      oldest_index_(0),
      scale_(scale),
      max_window_size_ms_(window_size_ms),
      current_window_size_ms_(max_window_size_ms_) {}

window_size_ms初始化buckets_，如果是1秒就是1000ms，就是1000个bucket。

void RateStatistics::Update(size_t count, int64_t now_ms) {
  if (now_ms < oldest_time_) {
    // Too old data is ignored.
    return;
  }
 //删除超出窗口老的数据
  EraseOld(now_ms);

  // First ever sample, reset window to start now.
  if (!IsInitialized())
    oldest_time_ = now_ms;

  //更新数据  
  //根据当前时刻now_ms，计算相对window_size_ms_的偏移量，根据偏移量计算出index
  uint32_t now_offset = static_cast<uint32_t>(now_ms - oldest_time_);
  RTC_DCHECK_LT(now_offset, max_window_size_ms_);  
  uint32_t index = oldest_index_ + now_offset;
  if (index >= max_window_size_ms_)
    index -= max_window_size_ms_;
   
  buckets_[index].sum += count;
  ++buckets_[index].samples;
  accumulated_count_ += count;
  ++num_samples_;
}

收到数据时进行更新

rtc::Optional<uint32_t> RateStatistics::Rate(int64_t now_ms) const {
  // Yeah, this const_cast ain't pretty, but the alternative is to declare most
  // of the members as mutable...
  const_cast<RateStatistics*>(this)->EraseOld(now_ms);

  // If window is a single bucket or there is only one sample in a data set that
  // has not grown to the full window size, treat this as rate unavailable.
  int64_t active_window_size = now_ms - oldest_time_ + 1;
  if (num_samples_ == 0 || active_window_size <= 1 ||
      (num_samples_ <= 1 && active_window_size < current_window_size_ms_)) {
    return rtc::Optional<uint32_t>();
  }

  float scale = scale_ / active_window_size;
  return rtc::Optional<uint32_t>(
      static_cast<uint32_t>(accumulated_count_ * scale + 0.5f));
}

获取码率： rate = scale*accumulated_count_/active_window_size