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// 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()
for i, code := range huffmanCodes {
addDecoderNode(byte(i), code, huffmanCodeLen[i])
}
}
func addDecoderNode(sym byte, code uint32, codeLen uint8) {
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)
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
}
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