net/hpack/huffman.go

// 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}
	}
}