| // Copyright 2016 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. |
| // |
| // Decoder for strings encoded using the HPACK Huffman Code (see |
| // https://httpwg.github.io/specs/rfc7541.html#huffman.code). |
| // |
| // This implementation is inspired by the One-Shift algorithm described in |
| // "On the Implementation of Minimum Redundancy Prefix Codes", by Alistair |
| // Moffat and Andrew Turpin, 1997. |
| // See also https://en.wikipedia.org/wiki/Canonical_Huffman_code for background |
| // on canonical Huffman codes. |
| // |
| // This decoder differs from that in .../spdy/hpack/hpack_huffman_table.cc |
| // as follows: |
| // 1) It decodes only the code described in RFC7541, where as the older |
| // implementation supported any canonical Huffman code provided at run |
| // time. |
| // 2) It uses a fixed amount of memory allocated at build time; it doesn't |
| // construct a tree of of decoding tables based on an encoding |
| // table provided at run time. |
| // 3) In benchmarks it runs from 10% to 70% faster, based on the length |
| // of the strings (faster for longer strings). Some of the improvements |
| // could be back ported, but others are fundamental to the approach. |
| |
| #include "net/spdy/hpack/hpack_huffman_decoder.h" |
| |
| #include <bitset> |
| #include <limits> |
| #include <utility> |
| |
| #include "base/logging.h" |
| #include "net/spdy/hpack/hpack_input_stream.h" |
| |
| namespace net { |
| namespace { |
| |
| typedef HpackHuffmanDecoder::HuffmanWord HuffmanWord; |
| typedef HpackHuffmanDecoder::HuffmanCodeLength HuffmanCodeLength; |
| |
| const HuffmanCodeLength kHuffmanWordLength = |
| std::numeric_limits<HuffmanWord>::digits; |
| |
| const HuffmanCodeLength kMinCodeLength = 5; |
| const HuffmanCodeLength kMaxCodeLength = 30; |
| |
| const HuffmanWord kInvalidLJCode = ~static_cast<HuffmanWord>(0); |
| // Length of a code in bits to the first code with that length, left-justified. |
| // Note that this can be computed from kLengthToFirstCanonical. |
| const HuffmanWord kLengthToFirstLJCode[] = { |
| kInvalidLJCode, // There are no codes of length 0. |
| kInvalidLJCode, // There are no codes of length 1. |
| kInvalidLJCode, // There are no codes of length 2. |
| kInvalidLJCode, // There are no codes of length 3. |
| kInvalidLJCode, // There are no codes of length 4. |
| 0x00000000, // Length 5. |
| 0x50000000, // Length 6. |
| 0xb8000000, // Length 7. |
| 0xf8000000, // Length 8. |
| kInvalidLJCode, // There are no codes of length 9. |
| 0xfe000000, // Length 10. |
| 0xff400000, // Length 11. |
| 0xffa00000, // Length 12. |
| 0xffc00000, // Length 13. |
| 0xfff00000, // Length 14. |
| 0xfff80000, // Length 15. |
| kInvalidLJCode, // There are no codes of length 16. |
| kInvalidLJCode, // There are no codes of length 17. |
| kInvalidLJCode, // There are no codes of length 18. |
| 0xfffe0000, // Length 19. |
| 0xfffe6000, // Length 20. |
| 0xfffee000, // Length 21. |
| 0xffff4800, // Length 22. |
| 0xffffb000, // Length 23. |
| 0xffffea00, // Length 24. |
| 0xfffff600, // Length 25. |
| 0xfffff800, // Length 26. |
| 0xfffffbc0, // Length 27. |
| 0xfffffe20, // Length 28. |
| kInvalidLJCode, // There are no codes of length 29. |
| 0xfffffff0, // Length 30. |
| }; |
| |
| // TODO(jamessynge): Determine the performance impact of different types for |
| // the elements of this array (i.e. a larger type uses more cache, yet might |
| // better on some architectures). |
| const uint8_t kInvalidCanonical = 255; |
| // Maps from length of a code to the first 'canonical symbol' with that length. |
| const uint8_t kLengthToFirstCanonical[] = { |
| kInvalidCanonical, // Length 0, 0 codes. |
| kInvalidCanonical, // Length 1, 0 codes. |
| kInvalidCanonical, // Length 2, 0 codes. |
| kInvalidCanonical, // Length 3, 0 codes. |
| kInvalidCanonical, // Length 4, 0 codes. |
| 0, // Length 5, 10 codes. |
| 10, // Length 6, 26 codes. |
| 36, // Length 7, 32 codes. |
| 68, // Length 8, 6 codes. |
| kInvalidCanonical, // Length 9, 0 codes. |
| 74, // Length 10, 5 codes. |
| 79, // Length 11, 3 codes. |
| 82, // Length 12, 2 codes. |
| 84, // Length 13, 6 codes. |
| 90, // Length 14, 2 codes. |
| 92, // Length 15, 3 codes. |
| kInvalidCanonical, // Length 16, 0 codes. |
| kInvalidCanonical, // Length 17, 0 codes. |
| kInvalidCanonical, // Length 18, 0 codes. |
| 95, // Length 19, 3 codes. |
| 98, // Length 20, 8 codes. |
| 106, // Length 21, 13 codes. |
| 119, // Length 22, 26 codes. |
| 145, // Length 23, 29 codes. |
| 174, // Length 24, 12 codes. |
| 186, // Length 25, 4 codes. |
| 190, // Length 26, 15 codes. |
| 205, // Length 27, 19 codes. |
| 224, // Length 28, 29 codes. |
| kInvalidCanonical, // Length 29, 0 codes. |
| 253, // Length 30, 4 codes. |
| }; |
| |
| // Mapping from canonical symbol (0 to 255) to actual symbol. |
| // clang-format off |
| const uint8_t kCanonicalToSymbol[] = { |
| '0', '1', '2', 'a', 'c', 'e', 'i', 'o', |
| 's', 't', 0x20, '%', '-', '.', '/', '3', |
| '4', '5', '6', '7', '8', '9', '=', 'A', |
| '_', 'b', 'd', 'f', 'g', 'h', 'l', 'm', |
| 'n', 'p', 'r', 'u', ':', 'B', 'C', 'D', |
| 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', |
| 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', |
| 'U', 'V', 'W', 'Y', 'j', 'k', 'q', 'v', |
| 'w', 'x', 'y', 'z', '&', '*', ',', ';', |
| 'X', 'Z', '!', '\"', '(', ')', '?', '\'', |
| '+', '|', '#', '>', 0x00, '$', '@', '[', |
| ']', '~', '^', '}', '<', '`', '{', '\\', |
| 0xc3, 0xd0, 0x80, 0x82, 0x83, 0xa2, 0xb8, 0xc2, |
| 0xe0, 0xe2, 0x99, 0xa1, 0xa7, 0xac, 0xb0, 0xb1, |
| 0xb3, 0xd1, 0xd8, 0xd9, 0xe3, 0xe5, 0xe6, 0x81, |
| 0x84, 0x85, 0x86, 0x88, 0x92, 0x9a, 0x9c, 0xa0, |
| 0xa3, 0xa4, 0xa9, 0xaa, 0xad, 0xb2, 0xb5, 0xb9, |
| 0xba, 0xbb, 0xbd, 0xbe, 0xc4, 0xc6, 0xe4, 0xe8, |
| 0xe9, 0x01, 0x87, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, |
| 0x8f, 0x93, 0x95, 0x96, 0x97, 0x98, 0x9b, 0x9d, |
| 0x9e, 0xa5, 0xa6, 0xa8, 0xae, 0xaf, 0xb4, 0xb6, |
| 0xb7, 0xbc, 0xbf, 0xc5, 0xe7, 0xef, 0x09, 0x8e, |
| 0x90, 0x91, 0x94, 0x9f, 0xab, 0xce, 0xd7, 0xe1, |
| 0xec, 0xed, 0xc7, 0xcf, 0xea, 0xeb, 0xc0, 0xc1, |
| 0xc8, 0xc9, 0xca, 0xcd, 0xd2, 0xd5, 0xda, 0xdb, |
| 0xee, 0xf0, 0xf2, 0xf3, 0xff, 0xcb, 0xcc, 0xd3, |
| 0xd4, 0xd6, 0xdd, 0xde, 0xdf, 0xf1, 0xf4, 0xf5, |
| 0xf6, 0xf7, 0xf8, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, |
| 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x0b, |
| 0x0c, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, |
| 0x15, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, |
| 0x1e, 0x1f, 0x7f, 0xdc, 0xf9, 0x0a, 0x0d, 0x16, |
| }; |
| // clang-format on |
| |
| #if DCHECK_IS_ON() |
| |
| // Only used in DLOG. |
| bool IsEOSPrefix(HuffmanWord bits, HuffmanCodeLength bits_available) { |
| if (bits_available == 0) { |
| return true; |
| } |
| // We expect all the bits below the high order |bits_available| bits |
| // to be cleared. |
| HuffmanWord expected = HuffmanWord(0xffffffff) << (32 - bits_available); |
| return bits == expected; |
| } |
| |
| #endif // DCHECK_IS_ON() |
| |
| } // namespace |
| |
| // TODO(jamessynge): Should we read these magic numbers from |
| // kLengthToFirstLJCode? Would that reduce cache consumption? Slow decoding? |
| // TODO(jamessynge): Is this being inlined by the compiler? Should we inline |
| // into DecodeString the tests for code lengths 5 through 8 (> 99% of codes |
| // according to the HPACK spec)? |
| HpackHuffmanDecoder::HuffmanCodeLength HpackHuffmanDecoder::CodeLengthOfPrefix( |
| HpackHuffmanDecoder::HuffmanWord value) { |
| HuffmanCodeLength length; |
| if (value < 0xb8000000) { |
| if (value < 0x50000000) { |
| length = 5; |
| } else { |
| length = 6; |
| } |
| } else { |
| if (value < 0xfe000000) { |
| if (value < 0xf8000000) { |
| length = 7; |
| } else { |
| length = 8; |
| } |
| } else { |
| if (value < 0xffc00000) { |
| if (value < 0xffa00000) { |
| if (value < 0xff400000) { |
| length = 10; |
| } else { |
| length = 11; |
| } |
| } else { |
| length = 12; |
| } |
| } else { |
| if (value < 0xfffe0000) { |
| if (value < 0xfff80000) { |
| if (value < 0xfff00000) { |
| length = 13; |
| } else { |
| length = 14; |
| } |
| } else { |
| length = 15; |
| } |
| } else { |
| if (value < 0xffff4800) { |
| if (value < 0xfffee000) { |
| if (value < 0xfffe6000) { |
| length = 19; |
| } else { |
| length = 20; |
| } |
| } else { |
| length = 21; |
| } |
| } else { |
| if (value < 0xffffea00) { |
| if (value < 0xffffb000) { |
| length = 22; |
| } else { |
| length = 23; |
| } |
| } else { |
| if (value < 0xfffffbc0) { |
| if (value < 0xfffff800) { |
| if (value < 0xfffff600) { |
| length = 24; |
| } else { |
| length = 25; |
| } |
| } else { |
| length = 26; |
| } |
| } else { |
| if (value < 0xfffffff0) { |
| if (value < 0xfffffe20) { |
| length = 27; |
| } else { |
| length = 28; |
| } |
| } else { |
| length = 30; |
| } |
| } |
| } |
| } |
| } |
| } |
| } |
| } |
| return length; |
| } |
| |
| HuffmanWord HpackHuffmanDecoder::DecodeToCanonical( |
| HuffmanCodeLength code_length, |
| HuffmanWord bits) { |
| DCHECK_LE(kMinCodeLength, code_length); |
| DCHECK_LE(code_length, kMaxCodeLength); |
| |
| // What is the first left-justified code of length |code_length|? |
| HuffmanWord first_lj_code = kLengthToFirstLJCode[code_length]; |
| DCHECK_NE(kInvalidLJCode, first_lj_code); |
| |
| // Which canonical symbol corresponds to the high order |code_length| |
| // bits of |first_lj_code|? |
| HuffmanWord first_canonical = kLengthToFirstCanonical[code_length]; |
| DCHECK_NE(kInvalidCanonical, first_canonical); |
| |
| // What is the position of the canonical symbol being decoded within |
| // the canonical symbols of length |code_length|? |
| HuffmanWord ordinal_in_length = |
| ((bits - first_lj_code) >> (kHuffmanWordLength - code_length)); |
| |
| // Combined these two to produce the position of the canonical symbol |
| // being decoded within all of the canonical symbols. |
| return first_canonical + ordinal_in_length; |
| } |
| |
| char HpackHuffmanDecoder::CanonicalToSource(HuffmanWord canonical) { |
| DCHECK_LT(canonical, 256u); |
| return static_cast<char>(kCanonicalToSymbol[canonical]); |
| } |
| |
| // TODO(jamessynge): Maybe further refactorings, including just passing in a |
| // SpdyStringPiece instead of an HpackInputStream, thus avoiding the PeekBits |
| // calls, and also allowing us to separate the code into portions dealing with |
| // long strings, and a later portion dealing with the last few bytes of strings. |
| // TODO(jamessynge): Determine if that is worth it by adding some counters to |
| // measure the distribution of string sizes seen in practice. |
| bool HpackHuffmanDecoder::DecodeString(HpackInputStream* in, std::string* out) { |
| out->clear(); |
| |
| // Load |bits| with the leading bits of the input stream, left justified |
| // (i.e. the bits of the first byte are the high-order bits of |bits|, |
| // and the bits of the fourth byte are the low-order bits of |bits|). |
| // |peeked_success| if there are more bits in |*in| (i.e. the encoding |
| // of the string to be decoded is more than 4 bytes). |
| |
| auto bits_available_and_bits = in->InitializePeekBits(); |
| HuffmanCodeLength bits_available = bits_available_and_bits.first; |
| HuffmanWord bits = bits_available_and_bits.second; |
| |
| // |peeked_success| tracks whether the previous PeekBits call was able to |
| // store any new bits into |bits|. For the first pass through the loop below |
| // the value false is appropriate: |
| // If we have 32 bits (i.e. the input has at least 4 bytes), then: |
| // |peeked_sucess| is not examined because |code_length| is |
| // at most 30 in the HPACK Huffman Code. |
| // If we have at most 24 bits (i.e. the input has at most 3 bytes), then: |
| // It is possible that the very first |code_length| is greater than |
| // |bits_available|, in which case we need to read peeked_success to |
| // determine whether we should try to read more input, or have already |
| // loaded |bits| with the final bits of the input. |
| // After the first loop |peeked_success| has been set by a call to PeekBits. |
| bool peeked_success = false; |
| |
| while (true) { |
| const HuffmanCodeLength code_length = CodeLengthOfPrefix(bits); |
| DCHECK_LE(kMinCodeLength, code_length); |
| DCHECK_LE(code_length, kMaxCodeLength); |
| DVLOG(2) << "bits: 0b" << std::bitset<32>(bits) |
| << " (avail=" << bits_available << ")" |
| << " prefix length: " << code_length |
| << (code_length > bits_available ? " *****" : ""); |
| if (code_length > bits_available) { |
| if (!peeked_success) { |
| // Unable to read enough input for a match. If only a portion of |
| // the last byte remains, this is a successful EOS condition. |
| // Note that this does NOT check whether the available bits are all |
| // set to 1, which the encoder is required to set at EOS, and the |
| // decoder is required to check. |
| // TODO(jamessynge): Discuss whether we should enforce this check, |
| // as required by the RFC, presumably flag guarded so that we can |
| // disable it should it occur a lot. From my testing it appears that |
| // our encoder may be doing this wrong. Sigh. |
| // TODO(jamessynge): Add a counter for how often the remaining bits |
| // are non-zero. |
| in->ConsumeByteRemainder(); |
| DLOG_IF(WARNING, |
| (in->HasMoreData() || !IsEOSPrefix(bits, bits_available))) |
| << "bits: 0b" << std::bitset<32>(bits) |
| << " (avail=" << bits_available << ")" |
| << " prefix length: " << code_length |
| << " HasMoreData: " << in->HasMoreData(); |
| return !in->HasMoreData(); |
| } |
| // We're dealing with a long code. It *might* be useful to add a special |
| // method to HpackInputStream for getting more than "at most 8" bits |
| // at a time. |
| do { |
| peeked_success = in->PeekBits(&bits_available, &bits); |
| } while (peeked_success && bits_available < 32); |
| } else { |
| // Convert from the prefix code of length |code_length| to the |
| // canonical symbol (i.e. where the input symbols (bytes) are ordered by |
| // increasing code length and then by their increasing uint8 value). |
| HuffmanWord canonical = DecodeToCanonical(code_length, bits); |
| |
| bits = bits << code_length; |
| bits_available -= code_length; |
| in->ConsumeBits(code_length); |
| |
| if (canonical < 256) { |
| out->push_back(CanonicalToSource(canonical)); |
| } else { |
| // Encoder is not supposed to explicity encode the EOS symbol (30 |
| // 1-bits). |
| // TODO(jamessynge): Discuss returning false here, as required by HPACK. |
| DCHECK(false) << "EOS explicitly encoded!\n" |
| << "bits: 0b" << std::bitset<32>(bits) |
| << " (avail=" << bits_available << ")" |
| << " prefix length: " << code_length |
| << " canonical: " << canonical; |
| } |
| // Get some more bits for decoding (up to 8). |peeked_success| is true |
| // if we got any bits. |
| peeked_success = in->PeekBits(&bits_available, &bits); |
| } |
| DLOG_IF(WARNING, (VLOG_IS_ON(2) && bits_available < 32 && !peeked_success)) |
| << "no more peeking possible"; |
| } |
| } |
| |
| } // namespace net |
