distribution/vendor/github.com/bradfitz/http2/hpack/huffman.go

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// Copyright 2014 The Go Authors.
// See https://code.google.com/p/go/source/browse/CONTRIBUTORS
// Licensed under the same terms as Go itself:
// https://code.google.com/p/go/source/browse/LICENSE
package hpack
import (
"bytes"
"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)
n := rootHuffmanNode
cur, nbits := uint(0), uint8(0)
for _, b := range v {
cur = cur<<8 | uint(b)
nbits += 8
for nbits >= 8 {
n = n.children[byte(cur>>(nbits-8))]
if n.children == nil {
buf.WriteByte(n.sym)
nbits -= n.codeLen
n = rootHuffmanNode
} else {
nbits -= 8
}
}
}
for nbits > 0 {
n = n.children[byte(cur<<(8-nbits))]
if n.children != nil || n.codeLen > nbits {
break
}
buf.WriteByte(n.sym)
nbits -= n.codeLen
n = rootHuffmanNode
}
return w.Write(buf.Bytes())
}
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() {
for i, code := range huffmanCodes {
if i > 255 {
panic("too many huffman codes")
}
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
}