modules/rtp_rtcp/source/flexfec_header_reader_writer.cc - src - Git at Google

 /*
  *  Copyright (c) 2023 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/flexfec_header_reader_writer.h"

 #include <string.h>

 #include "api/scoped_refptr.h"
 #include "modules/rtp_rtcp/source/byte_io.h"
 #include "modules/rtp_rtcp/source/forward_error_correction_internal.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"

 namespace webrtc {

 namespace {

 // Maximum number of media packets that can be protected in one batch.
 constexpr size_t kMaxMediaPackets = 48;  // Since we are reusing ULPFEC masks.

 // Maximum number of media packets tracked by FEC decoder.
 // Maintain a sufficiently larger tracking window than `kMaxMediaPackets`
 // to account for packet reordering in pacer/ network.
 constexpr size_t kMaxTrackedMediaPackets = 4 * kMaxMediaPackets;

 // Maximum number of FEC packets stored inside ForwardErrorCorrection.
 constexpr size_t kMaxFecPackets = kMaxMediaPackets;

 // Size (in bytes) of packet masks, given number of K bits set.
 constexpr size_t kFlexfecPacketMaskSizes[] = {2, 6, 14};

 // Size (in bytes) of part of header which is not packet mask specific.
 constexpr size_t kBaseHeaderSize = 8;

 // Size (in bytes) of part of header which is stream specific.
 constexpr size_t kStreamSpecificHeaderSize = 2;

 // Size (in bytes) of header, given the single stream packet mask size, i.e.
 // the number of K-bits set.
 constexpr size_t kHeaderSizes[] = {
     kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[0],
     kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[1],
     kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[2]};

 // Here we count the K-bits as belonging to the packet mask.
 // This can be used in conjunction with FlexfecHeaderWriter::MinPacketMaskSize,
 // which calculates a bound on the needed packet mask size including K-bits,
 // given a packet mask without K-bits.
 size_t FlexfecHeaderSize(size_t packet_mask_size) {
   RTC_DCHECK_LE(packet_mask_size, kFlexfecPacketMaskSizes[2]);
   if (packet_mask_size <= kFlexfecPacketMaskSizes[0]) {
     return kHeaderSizes[0];
   } else if (packet_mask_size <= kFlexfecPacketMaskSizes[1]) {
     return kHeaderSizes[1];
   }
   return kHeaderSizes[2];
 }

 }  // namespace

 FlexfecHeaderReader::FlexfecHeaderReader()
     : FecHeaderReader(kMaxTrackedMediaPackets, kMaxFecPackets) {}

 FlexfecHeaderReader::~FlexfecHeaderReader() = default;

 // TODO(brandtr): Update this function when we support flexible masks,
 // and retransmissions.
 bool FlexfecHeaderReader::ReadFecHeader(
     ForwardErrorCorrection::ReceivedFecPacket* fec_packet) const {
   // Protected ssrcs should already be populated from RTP header.
   if (fec_packet->protected_streams.empty()) {
     RTC_LOG(LS_WARNING)
         << "Discarding FlexFEC packet with no protected sources.";
     return false;
   }
   if (fec_packet->pkt->data.size() <=
       kBaseHeaderSize + kStreamSpecificHeaderSize) {
     RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
     return false;
   }
   uint8_t* const data = fec_packet->pkt->data.MutableData();
   bool r_bit = (data[0] & 0x80) != 0;
   if (r_bit) {
     RTC_LOG(LS_INFO)
         << "FlexFEC packet with retransmission bit set. We do not yet "
            "support this, thus discarding the packet.";
     return false;
   }
   bool f_bit = (data[0] & 0x40) != 0;
   if (f_bit) {
     RTC_LOG(LS_INFO)
         << "FlexFEC packet with inflexible generator matrix. We do "
            "not yet support this, thus discarding packet.";
     return false;
   }

   // First seq_num will be in byte index 8
   // (See FEC header schematic in flexfec_header_reader_writer.h.)
   size_t byte_index = 8;
   for (size_t i = 0; i < fec_packet->protected_streams.size(); ++i) {
     if (fec_packet->pkt->data.size() < byte_index + kStreamSpecificHeaderSize) {
       RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
       return false;
     }

     fec_packet->protected_streams[i].seq_num_base =
         ByteReader<uint16_t>::ReadBigEndian(&data[byte_index]);
     byte_index += kStreamSpecificHeaderSize;

     // Parse the FlexFEC packet mask and remove the interleaved K-bits.
     // (See FEC header schematic in flexfec_header_reader_writer.h.)
     // We store the packed packet mask in-band, which "destroys" the standards
     // compliance of the header. That is fine though, since the code that
     // reads from the header (from this point and onwards) is aware of this.
     // TODO(brandtr): When the FEC packet classes have been refactored, store
     // the packed packet masks out-of-band, thus leaving the FlexFEC header as
     // is.
     //
     // We treat the mask parts as unsigned integers with host order endianness
     // in order to simplify the bit shifting between bytes.
     if (fec_packet->pkt->data.size() <
         (byte_index + kFlexfecPacketMaskSizes[0])) {
       RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
       return false;
     }
     fec_packet->protected_streams[i].packet_mask_offset = byte_index;
     bool k_bit0 = (data[byte_index] & 0x80) != 0;
     uint16_t mask_part0 =
         ByteReader<uint16_t>::ReadBigEndian(&data[byte_index]);
     // Shift away K-bit 0, implicitly clearing the last bit.
     mask_part0 <<= 1;
     ByteWriter<uint16_t>::WriteBigEndian(&data[byte_index], mask_part0);
     byte_index += kFlexfecPacketMaskSizes[0];
     if (k_bit0) {
       // The first K-bit is set, and the packet mask is thus only 2 bytes long.
       // We have finished reading the properties for current ssrc.
       fec_packet->protected_streams[i].packet_mask_size =
           kFlexfecPacketMaskSizes[0];
     } else {
       if (fec_packet->pkt->data.size() <
           (byte_index + kFlexfecPacketMaskSizes[1] -
            kFlexfecPacketMaskSizes[0])) {
         return false;
       }
       bool k_bit1 = (data[byte_index] & 0x80) != 0;
       // We have already shifted the first two bytes of the packet mask one step
       // to the left, thus removing K-bit 0. We will now shift the next four
       // bytes of the packet mask two steps to the left. (One step for the
       // removed K-bit 0, and one step for the to be removed K-bit 1).
       uint8_t bit15 = (data[byte_index] >> 6) & 0x01;
       data[byte_index - 1] |= bit15;
       uint32_t mask_part1 =
           ByteReader<uint32_t>::ReadBigEndian(&data[byte_index]);
       // Shift away K-bit 1 and bit 15, implicitly clearing the last two bits.
       mask_part1 <<= 2;
       ByteWriter<uint32_t>::WriteBigEndian(&data[byte_index], mask_part1);
       byte_index += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
       if (k_bit1) {
         // The first K-bit is clear, but the second K-bit is set. The packet
         // mask is thus 6 bytes long. We have finished reading the properties
         // for current ssrc.
         fec_packet->protected_streams[i].packet_mask_size =
             kFlexfecPacketMaskSizes[1];
       } else {
         if (fec_packet->pkt->data.size() <
             (byte_index + kFlexfecPacketMaskSizes[2] -
              kFlexfecPacketMaskSizes[1])) {
           RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
           return false;
         }
         fec_packet->protected_streams[i].packet_mask_size =
             kFlexfecPacketMaskSizes[2];
         // At this point, K-bits 0 and 1 have been removed, and the front-most
         // part of the FlexFEC packet mask has been packed accordingly. We will
         // now shift the remaining part of the packet mask two steps to
         // the left. This corresponds to the (in total) two K-bits, which
         // have been removed.
         uint8_t tail_bits = (data[byte_index] >> 6) & 0x03;
         data[byte_index - 1] |= tail_bits;
         uint64_t mask_part2 =
             ByteReader<uint64_t>::ReadBigEndian(&data[byte_index]);
         // Shift away bit 46, and bit 47, which were copied to the previous
         // part of the mask, implicitly clearing the last two bits.
         mask_part2 <<= 2;
         ByteWriter<uint64_t>::WriteBigEndian(&data[byte_index], mask_part2);
         byte_index += kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1];
       }
     }
   }

   fec_packet->fec_header_size = byte_index;

   // In FlexFEC, all media packets are protected in their entirety.
   fec_packet->protection_length =
       fec_packet->pkt->data.size() - fec_packet->fec_header_size;

   return true;
 }

 FlexfecHeaderWriter::FlexfecHeaderWriter()
     : FecHeaderWriter(kMaxMediaPackets, kMaxFecPackets, kHeaderSizes[2]) {}

 FlexfecHeaderWriter::~FlexfecHeaderWriter() = default;

 size_t FlexfecHeaderWriter::MinPacketMaskSize(const uint8_t* packet_mask,
                                               size_t packet_mask_size) const {
   if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear &&
       (packet_mask[1] & 0x01) == 0) {
     // Packet mask is 16 bits long, with bit 15 clear.
     // It can be used as is.
     return kFlexfecPacketMaskSizes[0];
   } else if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear) {
     // Packet mask is 16 bits long, with bit 15 set.
     // We must expand the packet mask with zeros in the FlexFEC header.
     return kFlexfecPacketMaskSizes[1];
   } else if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet &&
              (packet_mask[5] & 0x03) == 0) {
     // Packet mask is 48 bits long, with bits 46 and 47 clear.
     // It can be used as is.
     return kFlexfecPacketMaskSizes[1];
   } else if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet) {
     // Packet mask is 48 bits long, with at least one of bits 46 and 47 set.
     // We must expand it with zeros.
     return kFlexfecPacketMaskSizes[2];
   }
   RTC_DCHECK_NOTREACHED() << "Incorrect packet mask size: " << packet_mask_size
                           << ".";
   return kFlexfecPacketMaskSizes[2];
 }

 size_t FlexfecHeaderWriter::FecHeaderSize(size_t packet_mask_size) const {
   return FlexfecHeaderSize(packet_mask_size);
 }

 // This function adapts the precomputed ULPFEC packet masks to the
 // FlexFEC header standard. Note that the header size is computed by
 // FecHeaderSize(), so in this function we can be sure that we are
 // writing in space that is intended for the header.
 //
 // TODO(brandtr): Update this function when we support offset-based masks
 // and retransmissions.
 void FlexfecHeaderWriter::FinalizeFecHeader(
     rtc::ArrayView<const ProtectedStream> protected_streams,
     ForwardErrorCorrection::Packet& fec_packet) const {
   uint8_t* data = fec_packet.data.MutableData();
   *data &= 0x7f;  // Clear R bit.
   *data &= 0xbf;  // Clear F bit.

   // First seq_num will be in byte index 8
   // (See FEC header schematic in flexfec_header_reader_writer.h.)
   uint8_t* write_at = data + 8;
   for (const ProtectedStream& protected_stream : protected_streams) {
     ByteWriter<uint16_t>::WriteBigEndian(write_at,
                                          protected_stream.seq_num_base);
     write_at += kStreamSpecificHeaderSize;
     // Adapt ULPFEC packet mask to FlexFEC header.
     //
     // We treat the mask parts as unsigned integers with host order endianness
     // in order to simplify the bit shifting between bytes.
     if (protected_stream.packet_mask.size() == kUlpfecPacketMaskSizeLBitSet) {
       // The packet mask is 48 bits long.
       uint16_t tmp_mask_part0 =
           ByteReader<uint16_t>::ReadBigEndian(&protected_stream.packet_mask[0]);
       uint32_t tmp_mask_part1 =
           ByteReader<uint32_t>::ReadBigEndian(&protected_stream.packet_mask[2]);

       tmp_mask_part0 >>= 1;  // Shift, thus clearing K-bit 0.
       ByteWriter<uint16_t>::WriteBigEndian(write_at, tmp_mask_part0);
       write_at += kFlexfecPacketMaskSizes[0];
       tmp_mask_part1 >>= 2;  // Shift, thus clearing K-bit 1 and bit 15.
       ByteWriter<uint32_t>::WriteBigEndian(write_at, tmp_mask_part1);

       bool bit15 = (protected_stream.packet_mask[1] & 0x01) != 0;
       if (bit15)
         *write_at |= 0x40;  // Set bit 15.

       bool bit46 = (protected_stream.packet_mask[5] & 0x02) != 0;
       bool bit47 = (protected_stream.packet_mask[5] & 0x01) != 0;
       if (!bit46 && !bit47) {
         *write_at |= 0x80;  // Set K-bit 1.
         write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
       } else {
         write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
         // Clear all trailing bits.
         memset(write_at, 0,
                kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1]);
         if (bit46)
           *write_at |= 0x80;  // Set bit 46.
         if (bit47)
           *write_at |= 0x40;  // Set bit 47.
         write_at += kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1];
       }
     } else if (protected_stream.packet_mask.size() ==
                kUlpfecPacketMaskSizeLBitClear) {
       // The packet mask is 16 bits long.
       uint16_t tmp_mask_part0 =
           ByteReader<uint16_t>::ReadBigEndian(&protected_stream.packet_mask[0]);

       tmp_mask_part0 >>= 1;  // Shift, thus clearing K-bit 0.
       ByteWriter<uint16_t>::WriteBigEndian(write_at, tmp_mask_part0);
       bool bit15 = (protected_stream.packet_mask[1] & 0x01) != 0;
       if (!bit15) {
         *write_at |= 0x80;  // Set K-bit 0.
         write_at += kFlexfecPacketMaskSizes[0];
       } else {
         write_at += kFlexfecPacketMaskSizes[0];
         // Clear all trailing bits.
         memset(write_at, 0U,
                kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0]);
         *write_at |= 0x80;  // Set K-bit 1.
         *write_at |= 0x40;  // Set bit 15.
         write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
       }
     } else {
       RTC_DCHECK_NOTREACHED() << "Incorrect packet mask size: "
                               << protected_stream.packet_mask.size() << ".";
     }
   }
 }

 }  // namespace webrtc
	/*
	* Copyright (c) 2023 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/flexfec_header_reader_writer.h"

	#include <string.h>

	#include "api/scoped_refptr.h"
	#include "modules/rtp_rtcp/source/byte_io.h"
	#include "modules/rtp_rtcp/source/forward_error_correction_internal.h"
	#include "rtc_base/checks.h"
	#include "rtc_base/logging.h"

	namespace webrtc {

	namespace {

	// Maximum number of media packets that can be protected in one batch.
	constexpr size_t kMaxMediaPackets = 48; // Since we are reusing ULPFEC masks.

	// Maximum number of media packets tracked by FEC decoder.
	// Maintain a sufficiently larger tracking window than `kMaxMediaPackets`
	// to account for packet reordering in pacer/ network.
	constexpr size_t kMaxTrackedMediaPackets = 4 * kMaxMediaPackets;

	// Maximum number of FEC packets stored inside ForwardErrorCorrection.
	constexpr size_t kMaxFecPackets = kMaxMediaPackets;

	// Size (in bytes) of packet masks, given number of K bits set.
	constexpr size_t kFlexfecPacketMaskSizes[] = {2, 6, 14};

	// Size (in bytes) of part of header which is not packet mask specific.
	constexpr size_t kBaseHeaderSize = 8;

	// Size (in bytes) of part of header which is stream specific.
	constexpr size_t kStreamSpecificHeaderSize = 2;

	// Size (in bytes) of header, given the single stream packet mask size, i.e.
	// the number of K-bits set.
	constexpr size_t kHeaderSizes[] = {
	kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[0],
	kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[1],
	kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[2]};

	// Here we count the K-bits as belonging to the packet mask.
	// This can be used in conjunction with FlexfecHeaderWriter::MinPacketMaskSize,
	// which calculates a bound on the needed packet mask size including K-bits,
	// given a packet mask without K-bits.
	size_t FlexfecHeaderSize(size_t packet_mask_size) {
	RTC_DCHECK_LE(packet_mask_size, kFlexfecPacketMaskSizes[2]);
	if (packet_mask_size <= kFlexfecPacketMaskSizes[0]) {
	return kHeaderSizes[0];
	} else if (packet_mask_size <= kFlexfecPacketMaskSizes[1]) {
	return kHeaderSizes[1];
	}
	return kHeaderSizes[2];
	}

	} // namespace

	FlexfecHeaderReader::FlexfecHeaderReader()
	: FecHeaderReader(kMaxTrackedMediaPackets, kMaxFecPackets) {}

	FlexfecHeaderReader::~FlexfecHeaderReader() = default;

	// TODO(brandtr): Update this function when we support flexible masks,
	// and retransmissions.
	bool FlexfecHeaderReader::ReadFecHeader(
	ForwardErrorCorrection::ReceivedFecPacket* fec_packet) const {
	// Protected ssrcs should already be populated from RTP header.
	if (fec_packet->protected_streams.empty()) {
	RTC_LOG(LS_WARNING)
	<< "Discarding FlexFEC packet with no protected sources.";
	return false;
	}
	if (fec_packet->pkt->data.size() <=
	kBaseHeaderSize + kStreamSpecificHeaderSize) {
	RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
	return false;
	}
	uint8_t* const data = fec_packet->pkt->data.MutableData();
	bool r_bit = (data[0] & 0x80) != 0;
	if (r_bit) {
	RTC_LOG(LS_INFO)
	<< "FlexFEC packet with retransmission bit set. We do not yet "
	"support this, thus discarding the packet.";
	return false;
	}
	bool f_bit = (data[0] & 0x40) != 0;
	if (f_bit) {
	RTC_LOG(LS_INFO)
	<< "FlexFEC packet with inflexible generator matrix. We do "
	"not yet support this, thus discarding packet.";
	return false;
	}

	// First seq_num will be in byte index 8
	// (See FEC header schematic in flexfec_header_reader_writer.h.)
	size_t byte_index = 8;
	for (size_t i = 0; i < fec_packet->protected_streams.size(); ++i) {
	if (fec_packet->pkt->data.size() < byte_index + kStreamSpecificHeaderSize) {
	RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
	return false;
	}

	fec_packet->protected_streams[i].seq_num_base =
	ByteReader<uint16_t>::ReadBigEndian(&data[byte_index]);
	byte_index += kStreamSpecificHeaderSize;

	// Parse the FlexFEC packet mask and remove the interleaved K-bits.
	// (See FEC header schematic in flexfec_header_reader_writer.h.)
	// We store the packed packet mask in-band, which "destroys" the standards
	// compliance of the header. That is fine though, since the code that
	// reads from the header (from this point and onwards) is aware of this.
	// TODO(brandtr): When the FEC packet classes have been refactored, store
	// the packed packet masks out-of-band, thus leaving the FlexFEC header as
	// is.
	//
	// We treat the mask parts as unsigned integers with host order endianness
	// in order to simplify the bit shifting between bytes.
	if (fec_packet->pkt->data.size() <
	(byte_index + kFlexfecPacketMaskSizes[0])) {
	RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
	return false;
	}
	fec_packet->protected_streams[i].packet_mask_offset = byte_index;
	bool k_bit0 = (data[byte_index] & 0x80) != 0;
	uint16_t mask_part0 =
	ByteReader<uint16_t>::ReadBigEndian(&data[byte_index]);
	// Shift away K-bit 0, implicitly clearing the last bit.
	mask_part0 <<= 1;
	ByteWriter<uint16_t>::WriteBigEndian(&data[byte_index], mask_part0);
	byte_index += kFlexfecPacketMaskSizes[0];
	if (k_bit0) {
	// The first K-bit is set, and the packet mask is thus only 2 bytes long.
	// We have finished reading the properties for current ssrc.
	fec_packet->protected_streams[i].packet_mask_size =
	kFlexfecPacketMaskSizes[0];
	} else {
	if (fec_packet->pkt->data.size() <
	(byte_index + kFlexfecPacketMaskSizes[1] -
	kFlexfecPacketMaskSizes[0])) {
	return false;
	}
	bool k_bit1 = (data[byte_index] & 0x80) != 0;
	// We have already shifted the first two bytes of the packet mask one step
	// to the left, thus removing K-bit 0. We will now shift the next four
	// bytes of the packet mask two steps to the left. (One step for the
	// removed K-bit 0, and one step for the to be removed K-bit 1).
	uint8_t bit15 = (data[byte_index] >> 6) & 0x01;
	data[byte_index - 1] \|= bit15;
	uint32_t mask_part1 =
	ByteReader<uint32_t>::ReadBigEndian(&data[byte_index]);
	// Shift away K-bit 1 and bit 15, implicitly clearing the last two bits.
	mask_part1 <<= 2;
	ByteWriter<uint32_t>::WriteBigEndian(&data[byte_index], mask_part1);
	byte_index += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
	if (k_bit1) {
	// The first K-bit is clear, but the second K-bit is set. The packet
	// mask is thus 6 bytes long. We have finished reading the properties
	// for current ssrc.
	fec_packet->protected_streams[i].packet_mask_size =
	kFlexfecPacketMaskSizes[1];
	} else {
	if (fec_packet->pkt->data.size() <
	(byte_index + kFlexfecPacketMaskSizes[2] -
	kFlexfecPacketMaskSizes[1])) {
	RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
	return false;
	}
	fec_packet->protected_streams[i].packet_mask_size =
	kFlexfecPacketMaskSizes[2];
	// At this point, K-bits 0 and 1 have been removed, and the front-most
	// part of the FlexFEC packet mask has been packed accordingly. We will
	// now shift the remaining part of the packet mask two steps to
	// the left. This corresponds to the (in total) two K-bits, which
	// have been removed.
	uint8_t tail_bits = (data[byte_index] >> 6) & 0x03;
	data[byte_index - 1] \|= tail_bits;
	uint64_t mask_part2 =
	ByteReader<uint64_t>::ReadBigEndian(&data[byte_index]);
	// Shift away bit 46, and bit 47, which were copied to the previous
	// part of the mask, implicitly clearing the last two bits.
	mask_part2 <<= 2;
	ByteWriter<uint64_t>::WriteBigEndian(&data[byte_index], mask_part2);
	byte_index += kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1];
	}
	}
	}

	fec_packet->fec_header_size = byte_index;

	// In FlexFEC, all media packets are protected in their entirety.
	fec_packet->protection_length =
	fec_packet->pkt->data.size() - fec_packet->fec_header_size;

	return true;
	}

	FlexfecHeaderWriter::FlexfecHeaderWriter()
	: FecHeaderWriter(kMaxMediaPackets, kMaxFecPackets, kHeaderSizes[2]) {}

	FlexfecHeaderWriter::~FlexfecHeaderWriter() = default;

	size_t FlexfecHeaderWriter::MinPacketMaskSize(const uint8_t* packet_mask,
	size_t packet_mask_size) const {
	if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear &&
	(packet_mask[1] & 0x01) == 0) {
	// Packet mask is 16 bits long, with bit 15 clear.
	// It can be used as is.
	return kFlexfecPacketMaskSizes[0];
	} else if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear) {
	// Packet mask is 16 bits long, with bit 15 set.
	// We must expand the packet mask with zeros in the FlexFEC header.
	return kFlexfecPacketMaskSizes[1];
	} else if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet &&
	(packet_mask[5] & 0x03) == 0) {
	// Packet mask is 48 bits long, with bits 46 and 47 clear.
	// It can be used as is.
	return kFlexfecPacketMaskSizes[1];
	} else if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet) {
	// Packet mask is 48 bits long, with at least one of bits 46 and 47 set.
	// We must expand it with zeros.
	return kFlexfecPacketMaskSizes[2];
	}
	RTC_DCHECK_NOTREACHED() << "Incorrect packet mask size: " << packet_mask_size
	<< ".";
	return kFlexfecPacketMaskSizes[2];
	}

	size_t FlexfecHeaderWriter::FecHeaderSize(size_t packet_mask_size) const {
	return FlexfecHeaderSize(packet_mask_size);
	}

	// This function adapts the precomputed ULPFEC packet masks to the
	// FlexFEC header standard. Note that the header size is computed by
	// FecHeaderSize(), so in this function we can be sure that we are
	// writing in space that is intended for the header.
	//
	// TODO(brandtr): Update this function when we support offset-based masks
	// and retransmissions.
	void FlexfecHeaderWriter::FinalizeFecHeader(
	rtc::ArrayView<const ProtectedStream> protected_streams,
	ForwardErrorCorrection::Packet& fec_packet) const {
	uint8_t* data = fec_packet.data.MutableData();
	*data &= 0x7f; // Clear R bit.
	*data &= 0xbf; // Clear F bit.

	// First seq_num will be in byte index 8
	// (See FEC header schematic in flexfec_header_reader_writer.h.)
	uint8_t* write_at = data + 8;
	for (const ProtectedStream& protected_stream : protected_streams) {
	ByteWriter<uint16_t>::WriteBigEndian(write_at,
	protected_stream.seq_num_base);
	write_at += kStreamSpecificHeaderSize;
	// Adapt ULPFEC packet mask to FlexFEC header.
	//
	// We treat the mask parts as unsigned integers with host order endianness
	// in order to simplify the bit shifting between bytes.
	if (protected_stream.packet_mask.size() == kUlpfecPacketMaskSizeLBitSet) {
	// The packet mask is 48 bits long.
	uint16_t tmp_mask_part0 =
	ByteReader<uint16_t>::ReadBigEndian(&protected_stream.packet_mask[0]);
	uint32_t tmp_mask_part1 =
	ByteReader<uint32_t>::ReadBigEndian(&protected_stream.packet_mask[2]);

	tmp_mask_part0 >>= 1; // Shift, thus clearing K-bit 0.
	ByteWriter<uint16_t>::WriteBigEndian(write_at, tmp_mask_part0);
	write_at += kFlexfecPacketMaskSizes[0];
	tmp_mask_part1 >>= 2; // Shift, thus clearing K-bit 1 and bit 15.
	ByteWriter<uint32_t>::WriteBigEndian(write_at, tmp_mask_part1);

	bool bit15 = (protected_stream.packet_mask[1] & 0x01) != 0;
	if (bit15)
	*write_at \|= 0x40; // Set bit 15.

	bool bit46 = (protected_stream.packet_mask[5] & 0x02) != 0;
	bool bit47 = (protected_stream.packet_mask[5] & 0x01) != 0;
	if (!bit46 && !bit47) {
	*write_at \|= 0x80; // Set K-bit 1.
	write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
	} else {
	write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
	// Clear all trailing bits.
	memset(write_at, 0,
	kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1]);
	if (bit46)
	*write_at \|= 0x80; // Set bit 46.
	if (bit47)
	*write_at \|= 0x40; // Set bit 47.
	write_at += kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1];
	}
	} else if (protected_stream.packet_mask.size() ==
	kUlpfecPacketMaskSizeLBitClear) {
	// The packet mask is 16 bits long.
	uint16_t tmp_mask_part0 =
	ByteReader<uint16_t>::ReadBigEndian(&protected_stream.packet_mask[0]);

	tmp_mask_part0 >>= 1; // Shift, thus clearing K-bit 0.
	ByteWriter<uint16_t>::WriteBigEndian(write_at, tmp_mask_part0);
	bool bit15 = (protected_stream.packet_mask[1] & 0x01) != 0;
	if (!bit15) {
	*write_at \|= 0x80; // Set K-bit 0.
	write_at += kFlexfecPacketMaskSizes[0];
	} else {
	write_at += kFlexfecPacketMaskSizes[0];
	// Clear all trailing bits.
	memset(write_at, 0U,
	kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0]);
	*write_at \|= 0x80; // Set K-bit 1.
	*write_at \|= 0x40; // Set bit 15.
	write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
	}
	} else {
	RTC_DCHECK_NOTREACHED() << "Incorrect packet mask size: "
	<< protected_stream.packet_mask.size() << ".";
	}
	}
	}

	} // namespace webrtc