source/rtcp_packet/transport_feedback.cc

/*
 *  Copyright (c) 2015 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.
 */

#include "modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h"

#include <algorithm>

#include "modules/include/module_common_types.h"
#include "modules/rtp_rtcp/source/byte_io.h"
#include "modules/rtp_rtcp/source/rtcp_packet/common_header.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
#include "rtc_base/trace_event.h"

namespace webrtc {
namespace rtcp {
namespace {
// Header size:
// * 4 bytes Common RTCP Packet Header
// * 8 bytes Common Packet Format for RTCP Feedback Messages
// * 8 bytes FeedbackPacket header
constexpr size_t kTransportFeedbackHeaderSizeBytes = 4 + 8 + 8;
constexpr size_t kChunkSizeBytes = 2;
// TODO(sprang): Add support for dynamic max size for easier fragmentation,
// eg. set it to what's left in the buffer or IP_PACKET_SIZE.
// Size constraint imposed by RTCP common header: 16bit size field interpreted
// as number of four byte words minus the first header word.
constexpr size_t kMaxSizeBytes = (1 << 16) * 4;
// Payload size:
// * 8 bytes Common Packet Format for RTCP Feedback Messages
// * 8 bytes FeedbackPacket header.
// * 2 bytes for one chunk.
constexpr size_t kMinPayloadSizeBytes = 8 + 8 + 2;
constexpr int kBaseScaleFactor =
    TransportFeedback::kDeltaScaleFactor * (1 << 8);
constexpr int64_t kTimeWrapPeriodUs = (1ll << 24) * kBaseScaleFactor;

//    Message format
//
//     0                   1                   2                   3
//     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//    |V=2|P|  FMT=15 |    PT=205     |           length              |
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//  0 |                     SSRC of packet sender                     |
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//  4 |                      SSRC of media source                     |
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//  8 |      base sequence number     |      packet status count      |
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// 12 |                 reference time                | fb pkt. count |
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// 16 |          packet chunk         |         packet chunk          |
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//    .                                                               .
//    .                                                               .
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//    |         packet chunk          |  recv delta   |  recv delta   |
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//    .                                                               .
//    .                                                               .
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//    |           recv delta          |  recv delta   | zero padding  |
//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
}  // namespace
constexpr uint8_t TransportFeedback::kFeedbackMessageType;
constexpr size_t TransportFeedback::kMaxReportedPackets;

// Keep delta_sizes that can be encoded into single chunk if it is last chunk.
class TransportFeedback::LastChunk {
 public:
  using DeltaSize = TransportFeedback::DeltaSize;

  LastChunk();

  bool Empty() const;
  void Clear();
  // Return if delta sizes still can be encoded into single chunk with added
  // |delta_size|.
  bool CanAdd(DeltaSize delta_size) const;
  // Add |delta_size|, assumes |CanAdd(delta_size)|,
  void Add(DeltaSize delta_size);

  // Encode chunk as large as possible removing encoded delta sizes.
  // Assume CanAdd() == false for some valid delta_size.
  uint16_t Emit();
  // Encode all stored delta_sizes into single chunk, pad with 0s if needed.
  uint16_t EncodeLast() const;

  // Decode up to |max_size| delta sizes from |chunk|.
  void Decode(uint16_t chunk, size_t max_size);
  // Appends content of the Lastchunk to |deltas|.
  void AppendTo(std::vector<DeltaSize>* deltas) const;

 private:
  static constexpr size_t kMaxRunLengthCapacity = 0x1fff;
  static constexpr size_t kMaxOneBitCapacity = 14;
  static constexpr size_t kMaxTwoBitCapacity = 7;
  static constexpr size_t kMaxVectorCapacity = kMaxOneBitCapacity;
  static constexpr DeltaSize kLarge = 2;

  uint16_t EncodeOneBit() const;
  void DecodeOneBit(uint16_t chunk, size_t max_size);

  uint16_t EncodeTwoBit(size_t size) const;
  void DecodeTwoBit(uint16_t chunk, size_t max_size);

  uint16_t EncodeRunLength() const;
  void DecodeRunLength(uint16_t chunk, size_t max_size);

  DeltaSize delta_sizes_[kMaxVectorCapacity];
  size_t size_;
  bool all_same_;
  bool has_large_delta_;
};
constexpr size_t TransportFeedback::LastChunk::kMaxRunLengthCapacity;
constexpr size_t TransportFeedback::LastChunk::kMaxOneBitCapacity;
constexpr size_t TransportFeedback::LastChunk::kMaxTwoBitCapacity;
constexpr size_t TransportFeedback::LastChunk::kMaxVectorCapacity;

TransportFeedback::LastChunk::LastChunk() {
  Clear();
}

bool TransportFeedback::LastChunk::Empty() const {
  return size_ == 0;
}

void TransportFeedback::LastChunk::Clear() {
  size_ = 0;
  all_same_ = true;
  has_large_delta_ = false;
}

bool TransportFeedback::LastChunk::CanAdd(DeltaSize delta_size) const {
  RTC_DCHECK_LE(delta_size, 2);
  if (size_ < kMaxTwoBitCapacity)
    return true;
  if (size_ < kMaxOneBitCapacity && !has_large_delta_ && delta_size != kLarge)
    return true;
  if (size_ < kMaxRunLengthCapacity && all_same_ &&
      delta_sizes_[0] == delta_size)
    return true;
  return false;
}

void TransportFeedback::LastChunk::Add(DeltaSize delta_size) {
  RTC_DCHECK(CanAdd(delta_size));
  if (size_ < kMaxVectorCapacity)
    delta_sizes_[size_] = delta_size;
  size_++;
  all_same_ = all_same_ && delta_size == delta_sizes_[0];
  has_large_delta_ = has_large_delta_ || delta_size == kLarge;
}

uint16_t TransportFeedback::LastChunk::Emit() {
  RTC_DCHECK(!CanAdd(0) || !CanAdd(1) || !CanAdd(2));
  if (all_same_) {
    uint16_t chunk = EncodeRunLength();
    Clear();
    return chunk;
  }
  if (size_ == kMaxOneBitCapacity) {
    uint16_t chunk = EncodeOneBit();
    Clear();
    return chunk;
  }
  RTC_DCHECK_GE(size_, kMaxTwoBitCapacity);
  uint16_t chunk = EncodeTwoBit(kMaxTwoBitCapacity);
  // Remove |kMaxTwoBitCapacity| encoded delta sizes:
  // Shift remaining delta sizes and recalculate all_same_ && has_large_delta_.
  size_ -= kMaxTwoBitCapacity;
  all_same_ = true;
  has_large_delta_ = false;
  for (size_t i = 0; i < size_; ++i) {
    DeltaSize delta_size = delta_sizes_[kMaxTwoBitCapacity + i];
    delta_sizes_[i] = delta_size;
    all_same_ = all_same_ && delta_size == delta_sizes_[0];
    has_large_delta_ = has_large_delta_ || delta_size == kLarge;
  }

  return chunk;
}

uint16_t TransportFeedback::LastChunk::EncodeLast() const {
  RTC_DCHECK_GT(size_, 0);
  if (all_same_)
    return EncodeRunLength();
  if (size_ <= kMaxTwoBitCapacity)
    return EncodeTwoBit(size_);
  return EncodeOneBit();
}

// Appends content of the Lastchunk to |deltas|.
void TransportFeedback::LastChunk::AppendTo(
    std::vector<DeltaSize>* deltas) const {
  if (all_same_) {
    deltas->insert(deltas->end(), size_, delta_sizes_[0]);
  } else {
    deltas->insert(deltas->end(), delta_sizes_, delta_sizes_ + size_);
  }
}

void TransportFeedback::LastChunk::Decode(uint16_t chunk, size_t max_size) {
  if ((chunk & 0x8000) == 0) {
    DecodeRunLength(chunk, max_size);
  } else if ((chunk & 0x4000) == 0) {
    DecodeOneBit(chunk, max_size);
  } else {
    DecodeTwoBit(chunk, max_size);
  }
}

//  One Bit Status Vector Chunk
//
//  0                   1
//  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//  |T|S|       symbol list         |
//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//
//  T = 1
//  S = 0
//  Symbol list = 14 entries where 0 = not received, 1 = received 1-byte delta.
uint16_t TransportFeedback::LastChunk::EncodeOneBit() const {
  RTC_DCHECK(!has_large_delta_);
  RTC_DCHECK_LE(size_, kMaxOneBitCapacity);
  uint16_t chunk = 0x8000;
  for (size_t i = 0; i < size_; ++i)
    chunk |= delta_sizes_[i] << (kMaxOneBitCapacity - 1 - i);
  return chunk;
}

void TransportFeedback::LastChunk::DecodeOneBit(uint16_t chunk,
                                                size_t max_size) {
  RTC_DCHECK_EQ(chunk & 0xc000, 0x8000);
  size_ = std::min(kMaxOneBitCapacity, max_size);
  has_large_delta_ = false;
  all_same_ = false;
  for (size_t i = 0; i < size_; ++i)
    delta_sizes_[i] = (chunk >> (kMaxOneBitCapacity - 1 - i)) & 0x01;
}

//  Two Bit Status Vector Chunk
//
//  0                   1
//  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//  |T|S|       symbol list         |
//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//
//  T = 1
//  S = 1
//  symbol list = 7 entries of two bits each.
uint16_t TransportFeedback::LastChunk::EncodeTwoBit(size_t size) const {
  RTC_DCHECK_LE(size, size_);
  uint16_t chunk = 0xc000;
  for (size_t i = 0; i < size; ++i)
    chunk |= delta_sizes_[i] << 2 * (kMaxTwoBitCapacity - 1 - i);
  return chunk;
}

void TransportFeedback::LastChunk::DecodeTwoBit(uint16_t chunk,
                                                size_t max_size) {
  RTC_DCHECK_EQ(chunk & 0xc000, 0xc000);
  size_ = std::min(kMaxTwoBitCapacity, max_size);
  has_large_delta_ = true;
  all_same_ = false;
  for (size_t i = 0; i < size_; ++i)
    delta_sizes_[i] = (chunk >> 2 * (kMaxTwoBitCapacity - 1 - i)) & 0x03;
}

//  Run Length Status Vector Chunk
//
//  0                   1
//  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//  |T| S |       Run Length        |
//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//
//  T = 0
//  S = symbol
//  Run Length = Unsigned integer denoting the run length of the symbol
uint16_t TransportFeedback::LastChunk::EncodeRunLength() const {
  RTC_DCHECK(all_same_);
  RTC_DCHECK_LE(size_, kMaxRunLengthCapacity);
  return (delta_sizes_[0] << 13) | static_cast<uint16_t>(size_);
}

void TransportFeedback::LastChunk::DecodeRunLength(uint16_t chunk,
                                                   size_t max_count) {
  RTC_DCHECK_EQ(chunk & 0x8000, 0);
  size_ = std::min<size_t>(chunk & 0x1fff, max_count);
  DeltaSize delta_size = (chunk >> 13) & 0x03;
  has_large_delta_ = delta_size >= kLarge;
  all_same_ = true;
  // To make it consistent with Add function, populate delta_sizes_ beyound 1st.
  for (size_t i = 0; i < std::min<size_t>(size_, kMaxVectorCapacity); ++i)
    delta_sizes_[i] = delta_size;
}

TransportFeedback::TransportFeedback()
    : base_seq_no_(0),
      num_seq_no_(0),
      base_time_ticks_(0),
      feedback_seq_(0),
      last_timestamp_us_(0),
      last_chunk_(new LastChunk()),
      size_bytes_(kTransportFeedbackHeaderSizeBytes) {}

TransportFeedback::~TransportFeedback() {}

void TransportFeedback::SetBase(uint16_t base_sequence,
                                int64_t ref_timestamp_us) {
  RTC_DCHECK_EQ(num_seq_no_, 0);
  RTC_DCHECK_GE(ref_timestamp_us, 0);
  base_seq_no_ = base_sequence;
  base_time_ticks_ = (ref_timestamp_us % kTimeWrapPeriodUs) / kBaseScaleFactor;
  last_timestamp_us_ = GetBaseTimeUs();
}

void TransportFeedback::SetFeedbackSequenceNumber(uint8_t feedback_sequence) {
  feedback_seq_ = feedback_sequence;
}

bool TransportFeedback::AddReceivedPacket(uint16_t sequence_number,
                                          int64_t timestamp_us) {
  // Convert to ticks and round.
  int64_t delta_full = (timestamp_us - last_timestamp_us_) % kTimeWrapPeriodUs;
  if (delta_full > kTimeWrapPeriodUs / 2)
    delta_full -= kTimeWrapPeriodUs;
  delta_full +=
      delta_full < 0 ? -(kDeltaScaleFactor / 2) : kDeltaScaleFactor / 2;
  delta_full /= kDeltaScaleFactor;

  int16_t delta = static_cast<int16_t>(delta_full);
  // If larger than 16bit signed, we can't represent it - need new fb packet.
  if (delta != delta_full) {
    RTC_LOG(LS_WARNING) << "Delta value too large ( >= 2^16 ticks )";
    return false;
  }

  uint16_t next_seq_no = base_seq_no_ + num_seq_no_;
  if (sequence_number != next_seq_no) {
    uint16_t last_seq_no = next_seq_no - 1;
    if (!IsNewerSequenceNumber(sequence_number, last_seq_no))
      return false;
    for (; next_seq_no != sequence_number; ++next_seq_no)
      if (!AddDeltaSize(0))
        return false;
  }

  DeltaSize delta_size = (delta >= 0 && delta <= 0xff) ? 1 : 2;
  if (!AddDeltaSize(delta_size))
    return false;

  packets_.emplace_back(sequence_number, delta);
  last_timestamp_us_ += delta * kDeltaScaleFactor;
  size_bytes_ += delta_size;
  return true;
}

const std::vector<TransportFeedback::ReceivedPacket>&
TransportFeedback::GetReceivedPackets() const {
  return packets_;
}

uint16_t TransportFeedback::GetBaseSequence() const {
  return base_seq_no_;
}

int64_t TransportFeedback::GetBaseTimeUs() const {
  return static_cast<int64_t>(base_time_ticks_) * kBaseScaleFactor;
}

// De-serialize packet.
bool TransportFeedback::Parse(const CommonHeader& packet) {
  RTC_DCHECK_EQ(packet.type(), kPacketType);
  RTC_DCHECK_EQ(packet.fmt(), kFeedbackMessageType);
  TRACE_EVENT0("webrtc", "TransportFeedback::Parse");

  if (packet.payload_size_bytes() < kMinPayloadSizeBytes) {
    RTC_LOG(LS_WARNING) << "Buffer too small (" << packet.payload_size_bytes()
                        << " bytes) to fit a "
                           "FeedbackPacket. Minimum size = "
                        << kMinPayloadSizeBytes;
    return false;
  }

  const uint8_t* const payload = packet.payload();
  ParseCommonFeedback(payload);

  base_seq_no_ = ByteReader<uint16_t>::ReadBigEndian(&payload[8]);
  uint16_t status_count = ByteReader<uint16_t>::ReadBigEndian(&payload[10]);
  base_time_ticks_ = ByteReader<int32_t, 3>::ReadBigEndian(&payload[12]);
  feedback_seq_ = payload[15];
  Clear();
  size_t index = 16;
  const size_t end_index = packet.payload_size_bytes();

  if (status_count == 0) {
    RTC_LOG(LS_WARNING) << "Empty feedback messages not allowed.";
    return false;
  }

  std::vector<uint8_t> delta_sizes;
  delta_sizes.reserve(status_count);
  while (delta_sizes.size() < status_count) {
    if (index + kChunkSizeBytes > end_index) {
      RTC_LOG(LS_WARNING) << "Buffer overflow while parsing packet.";
      Clear();
      return false;
    }

    uint16_t chunk = ByteReader<uint16_t>::ReadBigEndian(&payload[index]);
    index += kChunkSizeBytes;
    encoded_chunks_.push_back(chunk);
    last_chunk_->Decode(chunk, status_count - delta_sizes.size());
    last_chunk_->AppendTo(&delta_sizes);
  }
  // Last chunk is stored in the |last_chunk_|.
  encoded_chunks_.pop_back();
  RTC_DCHECK_EQ(delta_sizes.size(), status_count);
  num_seq_no_ = status_count;

  uint16_t seq_no = base_seq_no_;
  for (size_t delta_size : delta_sizes) {
    if (index + delta_size > end_index) {
      RTC_LOG(LS_WARNING) << "Buffer overflow while parsing packet.";
      Clear();
      return false;
    }
    switch (delta_size) {
      case 0:
        break;
      case 1: {
        int16_t delta = payload[index];
        packets_.emplace_back(seq_no, delta);
        last_timestamp_us_ += delta * kDeltaScaleFactor;
        index += delta_size;
        break;
      }
      case 2: {
        int16_t delta = ByteReader<int16_t>::ReadBigEndian(&payload[index]);
        packets_.emplace_back(seq_no, delta);
        last_timestamp_us_ += delta * kDeltaScaleFactor;
        index += delta_size;
        break;
      }
      case 3:
        Clear();
        RTC_LOG(LS_WARNING) << "Invalid delta_size for seq_no " << seq_no;
        return false;
      default:
        RTC_NOTREACHED();
        break;
    }
    ++seq_no;
  }
  size_bytes_ = RtcpPacket::kHeaderLength + index;
  RTC_DCHECK_LE(index, end_index);
  return true;
}

std::unique_ptr<TransportFeedback> TransportFeedback::ParseFrom(
    const uint8_t* buffer,
    size_t length) {
  CommonHeader header;
  if (!header.Parse(buffer, length))
    return nullptr;
  if (header.type() != kPacketType || header.fmt() != kFeedbackMessageType)
    return nullptr;
  std::unique_ptr<TransportFeedback> parsed(new TransportFeedback);
  if (!parsed->Parse(header))
    return nullptr;
  return parsed;
}

bool TransportFeedback::IsConsistent() const {
  size_t packet_size = kTransportFeedbackHeaderSizeBytes;
  std::vector<DeltaSize> delta_sizes;
  LastChunk chunk_decoder;
  for (uint16_t chunk : encoded_chunks_) {
    chunk_decoder.Decode(chunk, kMaxReportedPackets);
    chunk_decoder.AppendTo(&delta_sizes);
    packet_size += kChunkSizeBytes;
  }
  if (!last_chunk_->Empty()) {
    last_chunk_->AppendTo(&delta_sizes);
    packet_size += kChunkSizeBytes;
  }
  if (num_seq_no_ != delta_sizes.size()) {
    RTC_LOG(LS_ERROR) << delta_sizes.size() << " packets encoded. Expected "
                      << num_seq_no_;
    return false;
  }
  int64_t timestamp_us = base_time_ticks_ * kBaseScaleFactor;
  auto packet_it = packets_.begin();
  uint16_t seq_no = base_seq_no_;
  for (DeltaSize delta_size : delta_sizes) {
    if (delta_size > 0) {
      if (packet_it == packets_.end()) {
        RTC_LOG(LS_ERROR) << "Failed to find delta for seq_no " << seq_no;
        return false;
      }
      if (packet_it->sequence_number() != seq_no) {
        RTC_LOG(LS_ERROR) << "Expected to find delta for seq_no " << seq_no
                          << ". Next delta is for "
                          << packet_it->sequence_number();
        return false;
      }
      if (delta_size == 1 &&
          (packet_it->delta_ticks() < 0 || packet_it->delta_ticks() > 0xff)) {
        RTC_LOG(LS_ERROR) << "Delta " << packet_it->delta_ticks()
                          << " for seq_no " << seq_no
                          << " doesn't fit into one byte";
        return false;
      }
      timestamp_us += packet_it->delta_us();
      ++packet_it;
    }
    packet_size += delta_size;
    ++seq_no;
  }
  if (packet_it != packets_.end()) {
    RTC_LOG(LS_ERROR) << "Unencoded delta for seq_no "
                      << packet_it->sequence_number();
    return false;
  }
  if (timestamp_us != last_timestamp_us_) {
    RTC_LOG(LS_ERROR) << "Last timestamp mismatch. Calculated: " << timestamp_us
                      << ". Saved: " << last_timestamp_us_;
    return false;
  }
  if (size_bytes_ != packet_size) {
    RTC_LOG(LS_ERROR) << "Rtcp packet size mismatch. Calculated: "
                      << packet_size << ". Saved: " << size_bytes_;
    return false;
  }
  return true;
}

size_t TransportFeedback::BlockLength() const {
  // Round size_bytes_ up to multiple of 32bits.
  return (size_bytes_ + 3) & (~static_cast<size_t>(3));
}

// Serialize packet.
bool TransportFeedback::Create(uint8_t* packet,
                               size_t* position,
                               size_t max_length,
                               PacketReadyCallback* callback) const {
  if (num_seq_no_ == 0)
    return false;

  while (*position + BlockLength() > max_length) {
    if (!OnBufferFull(packet, position, callback))
      return false;
  }
  const size_t position_end = *position + BlockLength();

  CreateHeader(kFeedbackMessageType, kPacketType, HeaderLength(), packet,
               position);
  CreateCommonFeedback(packet + *position);
  *position += kCommonFeedbackLength;

  ByteWriter<uint16_t>::WriteBigEndian(&packet[*position], base_seq_no_);
  *position += 2;

  ByteWriter<uint16_t>::WriteBigEndian(&packet[*position], num_seq_no_);
  *position += 2;

  ByteWriter<int32_t, 3>::WriteBigEndian(&packet[*position], base_time_ticks_);
  *position += 3;

  packet[(*position)++] = feedback_seq_;

  for (uint16_t chunk : encoded_chunks_) {
    ByteWriter<uint16_t>::WriteBigEndian(&packet[*position], chunk);
    *position += 2;
  }
  if (!last_chunk_->Empty()) {
    uint16_t chunk = last_chunk_->EncodeLast();
    ByteWriter<uint16_t>::WriteBigEndian(&packet[*position], chunk);
    *position += 2;
  }

  for (const auto& received_packet : packets_) {
    int16_t delta = received_packet.delta_ticks();
    if (delta >= 0 && delta <= 0xFF) {
      packet[(*position)++] = delta;
    } else {
      ByteWriter<int16_t>::WriteBigEndian(&packet[*position], delta);
      *position += 2;
    }
  }

  while ((*position % 4) != 0)
    packet[(*position)++] = 0;

  RTC_DCHECK_EQ(*position, position_end);
  return true;
}

void TransportFeedback::Clear() {
  num_seq_no_ = 0;
  last_timestamp_us_ = GetBaseTimeUs();
  packets_.clear();
  encoded_chunks_.clear();
  last_chunk_->Clear();
  size_bytes_ = kTransportFeedbackHeaderSizeBytes;
}

bool TransportFeedback::AddDeltaSize(DeltaSize delta_size) {
  if (num_seq_no_ == kMaxReportedPackets)
    return false;
  size_t add_chunk_size = last_chunk_->Empty() ? kChunkSizeBytes : 0;
  if (size_bytes_ + delta_size + add_chunk_size > kMaxSizeBytes)
    return false;

  if (last_chunk_->CanAdd(delta_size)) {
    size_bytes_ += add_chunk_size;
    last_chunk_->Add(delta_size);
    ++num_seq_no_;
    return true;
  }
  if (size_bytes_ + delta_size + kChunkSizeBytes > kMaxSizeBytes)
    return false;

  encoded_chunks_.push_back(last_chunk_->Emit());
  size_bytes_ += kChunkSizeBytes;
  last_chunk_->Add(delta_size);
  ++num_seq_no_;
  return true;
}

}  // namespace rtcp
}  // namespace webrtc