// 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 { 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 } type node struct { // children is non-nil for internal nodes children []*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: make([]*node, 256)} } var rootHuffmanNode = newInternalNode() func init() { if len(huffmanCodes) != 256 { panic("unexpected size") } for i, code := range huffmanCodes { addDecoderNode(byte(i), code, huffmanCodeLen[i]) } } func addDecoderNode(sym byte, code uint32, codeLen uint8) { cur := rootHuffmanNode 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) for i := start; i < start+end; i++ { cur.children[i] = &node{sym: sym, codeLen: codeLen} } } // AppendHuffmanString appends s, as encoded in Huffman codes, to dst // and returns the extended buffer. func AppendHuffmanString(dst []byte, s string) []byte { rembits := uint8(8) for i := 0; i < len(s); i++ { if rembits == 8 { dst = append(dst, 0) } dst, rembits = appendByteToHuffmanCode(dst, rembits, s[i]) } if rembits < 8 { // special EOS symbol code := uint32(0x3fffffff) nbits := uint8(30) t := uint8(code >> (nbits - rembits)) dst[len(dst)-1] |= t } return dst } // 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 } // appendByteToHuffmanCode appends Huffman code for c to dst and // returns the extended buffer and the remaining bits in the last // element. The appending is not byte aligned and the remaining bits // in the last element of dst is given in rembits. func appendByteToHuffmanCode(dst []byte, rembits uint8, c byte) ([]byte, uint8) { code := huffmanCodes[c] nbits := huffmanCodeLen[c] for { if rembits > nbits { t := uint8(code << (rembits - nbits)) dst[len(dst)-1] |= t rembits -= nbits break } t := uint8(code >> (nbits - rembits)) dst[len(dst)-1] |= t nbits -= rembits rembits = 8 if nbits == 0 { break } dst = append(dst, 0) } return dst, rembits }