// Copyright 2014 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. package hpack import ( "bytes" "errors" "io" "sync" ) var bufPool = sync.Pool{ New: func() interface{} { return new(bytes.Buffer) }, } // HuffmanDecode decodes the string in v and writes the expanded // result to w, returning the number of bytes written to w and the // Write call's return value. At most one Write call is made. func HuffmanDecode(w io.Writer, v []byte) (int, error) { buf := bufPool.Get().(*bytes.Buffer) buf.Reset() defer bufPool.Put(buf) if err := huffmanDecode(buf, 0, v); err != nil { return 0, err } return w.Write(buf.Bytes()) } // HuffmanDecodeToString decodes the string in v. func HuffmanDecodeToString(v []byte) (string, error) { buf := bufPool.Get().(*bytes.Buffer) buf.Reset() defer bufPool.Put(buf) if err := huffmanDecode(buf, 0, v); err != nil { return "", err } return buf.String(), nil } // ErrInvalidHuffman is returned for errors found decoding // Huffman-encoded strings. var ErrInvalidHuffman = errors.New("hpack: invalid Huffman-encoded data") // huffmanDecode decodes v to buf. // If maxLen is greater than 0, attempts to write more to buf than // maxLen bytes will return ErrStringLength. func huffmanDecode(buf *bytes.Buffer, maxLen int, v []byte) error { rootHuffmanNode := getRootHuffmanNode() n := rootHuffmanNode // cur is the bit buffer that has not been fed into n. // cbits is the number of low order bits in cur that are valid. // sbits is the number of bits of the symbol prefix being decoded. cur, cbits, sbits := uint(0), uint8(0), uint8(0) for _, b := range v { cur = cur<<8 | uint(b) cbits += 8 sbits += 8 for cbits >= 8 { idx := byte(cur >> (cbits - 8)) n = n.children[idx] if n == nil { return ErrInvalidHuffman } if n.children == nil { if maxLen != 0 && buf.Len() == maxLen { return ErrStringLength } buf.WriteByte(n.sym) cbits -= n.codeLen n = rootHuffmanNode sbits = cbits } else { cbits -= 8 } } } for cbits > 0 { n = n.children[byte(cur<<(8-cbits))] if n == nil { return ErrInvalidHuffman } if n.children != nil || n.codeLen > cbits { break } if maxLen != 0 && buf.Len() == maxLen { return ErrStringLength } buf.WriteByte(n.sym) cbits -= n.codeLen n = rootHuffmanNode sbits = cbits } if sbits > 7 { // Either there was an incomplete symbol, or overlong padding. // Both are decoding errors per RFC 7541 section 5.2. return ErrInvalidHuffman } if mask := uint(1<<cbits - 1); cur&mask != mask { // Trailing bits must be a prefix of EOS per RFC 7541 section 5.2. return ErrInvalidHuffman } return nil } // incomparable is a zero-width, non-comparable type. Adding it to a struct // makes that struct also non-comparable, and generally doesn't add // any size (as long as it's first). type incomparable [0]func() type node struct { _ incomparable // children is non-nil for internal nodes children *[256]*node // The following are only valid if children is nil: codeLen uint8 // number of bits that led to the output of sym sym byte // output symbol } func newInternalNode() *node { return &node{children: new([256]*node)} } var ( buildRootOnce sync.Once lazyRootHuffmanNode *node ) func getRootHuffmanNode() *node { buildRootOnce.Do(buildRootHuffmanNode) return lazyRootHuffmanNode } func buildRootHuffmanNode() { if len(huffmanCodes) != 256 { panic("unexpected size") } lazyRootHuffmanNode = newInternalNode() // allocate a leaf node for each of the 256 symbols leaves := new([256]node) for sym, code := range huffmanCodes { codeLen := huffmanCodeLen[sym] cur := lazyRootHuffmanNode for codeLen > 8 { codeLen -= 8 i := uint8(code >> codeLen) if cur.children[i] == nil { cur.children[i] = newInternalNode() } cur = cur.children[i] } shift := 8 - codeLen start, end := int(uint8(code<<shift)), int(1<<shift) leaves[sym].sym = byte(sym) leaves[sym].codeLen = codeLen for i := start; i < start+end; i++ { cur.children[i] = &leaves[sym] } } } // AppendHuffmanString appends s, as encoded in Huffman codes, to dst // and returns the extended buffer. func AppendHuffmanString(dst []byte, s string) []byte { // This relies on the maximum huffman code length being 30 (See tables.go huffmanCodeLen array) // So if a uint64 buffer has less than 32 valid bits can always accommodate another huffmanCode. var ( x uint64 // buffer n uint // number valid of bits present in x ) for i := 0; i < len(s); i++ { c := s[i] n += uint(huffmanCodeLen[c]) x <<= huffmanCodeLen[c] % 64 x |= uint64(huffmanCodes[c]) if n >= 32 { n %= 32 // Normally would be -= 32 but %= 32 informs compiler 0 <= n <= 31 for upcoming shift y := uint32(x >> n) // Compiler doesn't combine memory writes if y isn't uint32 dst = append(dst, byte(y>>24), byte(y>>16), byte(y>>8), byte(y)) } } // Add padding bits if necessary if over := n % 8; over > 0 { const ( eosCode = 0x3fffffff eosNBits = 30 eosPadByte = eosCode >> (eosNBits - 8) ) pad := 8 - over x = (x << pad) | (eosPadByte >> over) n += pad // 8 now divides into n exactly } // n in (0, 8, 16, 24, 32) switch n / 8 { case 0: return dst case 1: return append(dst, byte(x)) case 2: y := uint16(x) return append(dst, byte(y>>8), byte(y)) case 3: y := uint16(x >> 8) return append(dst, byte(y>>8), byte(y), byte(x)) } // case 4: y := uint32(x) return append(dst, byte(y>>24), byte(y>>16), byte(y>>8), byte(y)) } // HuffmanEncodeLength returns the number of bytes required to encode // s in Huffman codes. The result is round up to byte boundary. func HuffmanEncodeLength(s string) uint64 { n := uint64(0) for i := 0; i < len(s); i++ { n += uint64(huffmanCodeLen[s[i]]) } return (n + 7) / 8 }