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https://github.com/facebookincubator/mvfst.git
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00e67c1bf9
Reviewed By: lnicco Differential Revision: D33587012 fbshipit-source-id: 972eb440f0156c9c04aa6e8787561b18295c1a97
66 lines
2.2 KiB
C++
66 lines
2.2 KiB
C++
/*
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* Copyright (c) Meta Platforms, Inc. and affiliates.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*/
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#include <quic/codec/PacketNumber.h>
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namespace quic {
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PacketNumEncodingResult::PacketNumEncodingResult(
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PacketNum resultIn,
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size_t lengthIn)
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: result(resultIn), length(lengthIn) {}
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PacketNumEncodingResult encodePacketNumber(
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PacketNum packetNum,
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PacketNum largestAckedPacketNum) {
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PacketNum twiceDistance = (packetNum - largestAckedPacketNum) * 2;
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// The number of bits we need to mask all set bits in twiceDistance.
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// This is 1 + floor(log2(x)).
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size_t lengthInBits = folly::findLastSet(twiceDistance);
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// Round up to bytes
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size_t lengthInBytes = lengthInBits == 0 ? 1 : (lengthInBits + 7) >> 3;
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if (lengthInBytes > 4) {
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throw QuicInternalException(
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folly::to<std::string>(
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"Impossible to encode PacketNum=",
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packetNum,
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", largestAcked=",
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largestAckedPacketNum),
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LocalErrorCode::PACKET_NUMBER_ENCODING);
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}
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// We need a mask that's all 1 for lengthInBytes bytes. Left shift a 1 by that
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// many bits and then -1 will give us that. Or if lengthInBytes is 8, then ~0
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// will just do it.
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DCHECK_NE(lengthInBytes, 8);
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int64_t mask = (1ULL << lengthInBytes * 8) - 1;
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return PacketNumEncodingResult(packetNum & mask, lengthInBytes);
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}
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PacketNum decodePacketNumber(
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uint64_t encodedPacketNum,
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size_t packetNumBytes,
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PacketNum expectedNextPacketNum) {
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CHECK(packetNumBytes <= 4);
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size_t packetNumBits = 8 * packetNumBytes;
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PacketNum packetNumWin = 1ULL << packetNumBits;
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PacketNum packetNumHalfWin = packetNumWin >> 1;
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PacketNum mask = packetNumWin - 1;
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PacketNum candidate = (expectedNextPacketNum & ~mask) | encodedPacketNum;
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if (expectedNextPacketNum > packetNumHalfWin &&
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candidate <= expectedNextPacketNum - packetNumHalfWin &&
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candidate < (1ULL << 62) - packetNumWin) {
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return candidate + packetNumWin;
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}
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if (candidate > expectedNextPacketNum + packetNumHalfWin &&
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candidate >= packetNumWin) {
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return candidate - packetNumWin;
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}
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return candidate;
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}
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} // namespace quic
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