text: // Copyright 2013 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  // The purpose of this file is determine what bitrate to use for mirroring.
  // Ideally this should be as much as possible, without causing any frames to
  // arrive late.
  
  // The current algorithm is to measure how much bandwidth we've been using
  // recently. We also keep track of how much data has been queued up for sending
  // in a virtual "buffer" (this virtual buffer represents all the buffers between
  // the sender and the receiver, including retransmissions and so forth.)
  // If we estimate that our virtual buffer is mostly empty, we try to use
  // more bandwidth than our recent usage, otherwise we use less.
  
  #include "media/cast/congestion_control/congestion_control.h"
  
  #include "base/logging.h"
  #include "media/cast/cast_config.h"
  #include "media/cast/cast_defines.h"
  
  namespace media {
  namespace cast {
  
  // This means that we *try* to keep our buffer 90% empty.
  // If it is less full, we increase the bandwidth, if it is more
  // we decrease the bandwidth. Making this smaller makes the
  // congestion control more aggressive.
  static const double kTargetEmptyBufferFraction = 0.9;
  
  // This is the size of our history in frames. Larger values makes the
  // congestion control adapt slower.
  static const size_t kHistorySize = 100;
  
  CongestionControl::FrameStats::FrameStats() : frame_size(0) {
  }
  
  CongestionControl::CongestionControl(base::TickClock* clock,
                                       uint32 max_bitrate_configured,
                                       uint32 min_bitrate_configured,
                                       size_t max_unacked_frames)
      : clock_(clock),
        max_bitrate_configured_(max_bitrate_configured),
        min_bitrate_configured_(min_bitrate_configured),
        last_frame_stats_(static_cast<uint32>(-1)),
        last_acked_frame_(static_cast<uint32>(-1)),
        last_encoded_frame_(static_cast<uint32>(-1)),
        history_size_(max_unacked_frames + kHistorySize),
        acked_bits_in_history_(0) {
    DCHECK_GE(max_bitrate_configured, min_bitrate_configured) << "Invalid config";
    frame_stats_.resize(2);
    base::TimeTicks now = clock->NowTicks();
    frame_stats_[0].ack_time = now;
    frame_stats_[0].sent_time = now;
    frame_stats_[1].ack_time = now;
    DCHECK(!frame_stats_[0].ack_time.is_null());
  }
  
  CongestionControl::~CongestionControl() {}
  
  void CongestionControl::UpdateRtt(base::TimeDelta rtt) {
    rtt_ = base::TimeDelta::FromSecondsD(
        (rtt_.InSecondsF() * 7 + rtt.InSecondsF()) / 8);
  }
  
  // Calculate how much "dead air" there is between two frames.
  base::TimeDelta CongestionControl::DeadTime(const FrameStats& a,
                                              const FrameStats& b) {
    if (b.sent_time > a.ack_time) {
      return b.sent_time - a.ack_time;
    } else {
      return base::TimeDelta();
    }
  }
  
  double CongestionControl::CalculateSafeBitrate() {
    double transmit_time =
        (GetFrameStats(last_acked_frame_)->ack_time -
         frame_stats_.front().sent_time - dead_time_in_history_).InSecondsF();
  
    if (acked_bits_in_history_ == 0 || transmit_time <= 0.0) {
      return min_bitrate_configured_;
    }
    return acked_bits_in_history_ / std::max(transmit_time, 1E-3);
  }
  
  CongestionControl::FrameStats* CongestionControl::GetFrameStats(
      uint32 frame_id) {
    int32 offset = static_cast<int32>(frame_id - last_frame_stats_);
    DCHECK_LT(offset, static_cast<int32>(kHistorySize));
    if (offset > 0) {
      frame_stats_.resize(frame_stats_.size() + offset);
      last_frame_stats_ += offset;
      offset = 0;
    }
    while (frame_stats_.size() > history_size_) {
      DCHECK_GT(frame_stats_.size(), 1UL);
      DCHECK(!frame_stats_[0].ack_time.is_null());
      acked_bits_in_history_ -= frame_stats_[0].frame_size;
      dead_time_in_history_ -= DeadTime(frame_stats_[0], frame_stats_[1]);
      DCHECK_GE(acked_bits_in_history_, 0UL);
      VLOG(2) << "DT: " << dead_time_in_history_.InSecondsF();
      DCHECK_GE(dead_time_in_history_.InSecondsF(), 0.0);
      frame_stats_.pop_front();
    }
    offset += frame_stats_.size() - 1;
    if (offset < 0 || offset >= static_cast<int32>(frame_stats_.size())) {
      return NULL;
    }
    return &frame_stats_[offset];
  }
  
  void CongestionControl::AckFrame(uint32 frame_id, base::TimeTicks when) {
    FrameStats* frame_stats = GetFrameStats(last_acked_frame_);
    while (IsNewerFrameId(frame_id, last_acked_frame_)) {
      FrameStats* last_frame_stats = frame_stats;
      last_acked_frame_++;
      frame_stats = GetFrameStats(last_acked_frame_);
      DCHECK(frame_stats);
      frame_stats->ack_time = when;
      acked_bits_in_history_ += frame_stats->frame_size;
      dead_time_in_history_ += DeadTime(*last_frame_stats, *frame_stats);
    }
  }
  
  void CongestionControl::SendFrameToTransport(uint32 frame_id,
                                               size_t frame_size,
                                               base::TimeTicks when) {
    last_encoded_frame_ = frame_id;
    FrameStats* frame_stats = GetFrameStats(frame_id);
    DCHECK(frame_stats);
    frame_stats->frame_size = frame_size;
    frame_stats->sent_time = when;
  }
  
  base::TimeTicks CongestionControl::EstimatedAckTime(uint32 frame_id,
                                                      double bitrate) {
    FrameStats* frame_stats = GetFrameStats(frame_id);
    DCHECK(frame_stats);
    if (frame_stats->ack_time.is_null()) {
      DCHECK(frame_stats->frame_size) << "frame_id: " << frame_id;
      base::TimeTicks ret = EstimatedSendingTime(frame_id, bitrate);
      ret += base::TimeDelta::FromSecondsD(frame_stats->frame_size / bitrate);
      ret += rtt_;
      base::TimeTicks now = clock_->NowTicks();
      if (ret < now) {
        // This is a little counter-intuitive, but it seems to work.
        // Basically, when we estimate that the ACK should have already happened,
        // we figure out how long ago it should have happened and guess that the
        // ACK will happen half of that time in the future. This will cause some
        // over-estimation when acks are late, which is actually what we want.
        return now + (now - ret) / 2;
      } else {
        return ret;
      }
    } else {
      return frame_stats->ack_time;
    }
  }
  
  base::TimeTicks CongestionControl::EstimatedSendingTime(uint32 frame_id,
                                                          double bitrate) {
    FrameStats* frame_stats = GetFrameStats(frame_id);
    DCHECK(frame_stats);
    base::TimeTicks ret = EstimatedAckTime(frame_id - 1, bitrate) - rtt_;
    if (frame_stats->sent_time.is_null()) {
      // Not sent yet, but we can't start sending it in the past.
      return std::max(ret, clock_->NowTicks());
    } else {
      return std::max(ret, frame_stats->sent_time);
    }
  }
  
  uint32 CongestionControl::GetBitrate(base::TimeTicks playout_time,
                                       base::TimeDelta playout_delay) {
    double safe_bitrate = CalculateSafeBitrate();
    // Estimate when we might start sending the next frame.
    base::TimeDelta time_to_catch_up =
        playout_time -
        EstimatedSendingTime(last_encoded_frame_ + 1, safe_bitrate);
  
    double empty_buffer_fraction =
        time_to_catch_up.InSecondsF() / playout_delay.InSecondsF();
    empty_buffer_fraction = std::min(empty_buffer_fraction, 1.0);
    empty_buffer_fraction = std::max(empty_buffer_fraction, 0.0);
  
    uint32 bits_per_second = static_cast<uint32>(
        safe_bitrate * empty_buffer_fraction / kTargetEmptyBufferFraction);
    VLOG(3) << " FBR:" << (bits_per_second / 1E6)
            << " EBF:" << empty_buffer_fraction
            << " SBR:" << (safe_bitrate / 1E6);
    bits_per_second = std::max(bits_per_second, min_bitrate_configured_);
    bits_per_second = std::min(bits_per_second, max_bitrate_configured_);
    return bits_per_second;
  }
  
  }  // namespace cast
  }  // namespace media
  