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Commit 5c78589b authored by Joe Tsai's avatar Joe Tsai Committed by Nigel Tao
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compress/flate: simplify inflate logic 

The flate library contains generator code, which is used to generate
the fixed huffman table. This is done so that fixed blocks can be
processed quicker since there is no need generate the decoder table
for fixed codes.

Instead, delete the precomputed table, and use sync.Once to generate
it at runtime when used.

Advantages:
* Reduces duplicated logic in flate package
* Reduces binary size by approximately 2KiB

Disadvantages:
* For the simplest possible program that simply decodes the fixed
block "\x03\x00" once, the modified code takes 4.7% longer for the
first decode. Compression performance for subsequent blocks afterwards
has no noticeable slow down.

Change-Id: I8f351218debf7d732118808859eda481b01011f6
Reviewed-on: https://go-review.googlesource.com/14181Reviewed-by: 's avatarNigel Tao <nigeltao@golang.org>
parent 9f0baca5
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Showing with 35 additions and 352 deletions
src/compress/flate/fixedhuff.go deleted 100644 → 0
View file @ 9f0baca5
// Copyright 2013 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 flate
// autogenerated by go run gen.go -output fixedhuff.go, DO NOT EDIT
var fixedHuffmanDecoder = huffmanDecoder{
7,
[huffmanNumChunks]uint32{
0x1007, 0x0508, 0x0108, 0x1188, 0x1107, 0x0708, 0x0308, 0x0c09,
0x1087, 0x0608, 0x0208, 0x0a09, 0x0008, 0x0808, 0x0408, 0x0e09,
0x1047, 0x0588, 0x0188, 0x0909, 0x1147, 0x0788, 0x0388, 0x0d09,
0x10c7, 0x0688, 0x0288, 0x0b09, 0x0088, 0x0888, 0x0488, 0x0f09,
0x1027, 0x0548, 0x0148, 0x11c8, 0x1127, 0x0748, 0x0348, 0x0c89,
0x10a7, 0x0648, 0x0248, 0x0a89, 0x0048, 0x0848, 0x0448, 0x0e89,
0x1067, 0x05c8, 0x01c8, 0x0989, 0x1167, 0x07c8, 0x03c8, 0x0d89,
0x10e7, 0x06c8, 0x02c8, 0x0b89, 0x00c8, 0x08c8, 0x04c8, 0x0f89,
0x1017, 0x0528, 0x0128, 0x11a8, 0x1117, 0x0728, 0x0328, 0x0c49,
0x1097, 0x0628, 0x0228, 0x0a49, 0x0028, 0x0828, 0x0428, 0x0e49,
0x1057, 0x05a8, 0x01a8, 0x0949, 0x1157, 0x07a8, 0x03a8, 0x0d49,
0x10d7, 0x06a8, 0x02a8, 0x0b49, 0x00a8, 0x08a8, 0x04a8, 0x0f49,
0x1037, 0x0568, 0x0168, 0x11e8, 0x1137, 0x0768, 0x0368, 0x0cc9,
0x10b7, 0x0668, 0x0268, 0x0ac9, 0x0068, 0x0868, 0x0468, 0x0ec9,
0x1077, 0x05e8, 0x01e8, 0x09c9, 0x1177, 0x07e8, 0x03e8, 0x0dc9,
0x10f7, 0x06e8, 0x02e8, 0x0bc9, 0x00e8, 0x08e8, 0x04e8, 0x0fc9,
0x1007, 0x0518, 0x0118, 0x1198, 0x1107, 0x0718, 0x0318, 0x0c29,
0x1087, 0x0618, 0x0218, 0x0a29, 0x0018, 0x0818, 0x0418, 0x0e29,
0x1047, 0x0598, 0x0198, 0x0929, 0x1147, 0x0798, 0x0398, 0x0d29,
0x10c7, 0x0698, 0x0298, 0x0b29, 0x0098, 0x0898, 0x0498, 0x0f29,
0x1027, 0x0558, 0x0158, 0x11d8, 0x1127, 0x0758, 0x0358, 0x0ca9,
0x10a7, 0x0658, 0x0258, 0x0aa9, 0x0058, 0x0858, 0x0458, 0x0ea9,
0x1067, 0x05d8, 0x01d8, 0x09a9, 0x1167, 0x07d8, 0x03d8, 0x0da9,
0x10e7, 0x06d8, 0x02d8, 0x0ba9, 0x00d8, 0x08d8, 0x04d8, 0x0fa9,
0x1017, 0x0538, 0x0138, 0x11b8, 0x1117, 0x0738, 0x0338, 0x0c69,
0x1097, 0x0638, 0x0238, 0x0a69, 0x0038, 0x0838, 0x0438, 0x0e69,
0x1057, 0x05b8, 0x01b8, 0x0969, 0x1157, 0x07b8, 0x03b8, 0x0d69,
0x10d7, 0x06b8, 0x02b8, 0x0b69, 0x00b8, 0x08b8, 0x04b8, 0x0f69,
0x1037, 0x0578, 0x0178, 0x11f8, 0x1137, 0x0778, 0x0378, 0x0ce9,
0x10b7, 0x0678, 0x0278, 0x0ae9, 0x0078, 0x0878, 0x0478, 0x0ee9,
0x1077, 0x05f8, 0x01f8, 0x09e9, 0x1177, 0x07f8, 0x03f8, 0x0de9,
0x10f7, 0x06f8, 0x02f8, 0x0be9, 0x00f8, 0x08f8, 0x04f8, 0x0fe9,
0x1007, 0x0508, 0x0108, 0x1188, 0x1107, 0x0708, 0x0308, 0x0c19,
0x1087, 0x0608, 0x0208, 0x0a19, 0x0008, 0x0808, 0x0408, 0x0e19,
0x1047, 0x0588, 0x0188, 0x0919, 0x1147, 0x0788, 0x0388, 0x0d19,
0x10c7, 0x0688, 0x0288, 0x0b19, 0x0088, 0x0888, 0x0488, 0x0f19,
0x1027, 0x0548, 0x0148, 0x11c8, 0x1127, 0x0748, 0x0348, 0x0c99,
0x10a7, 0x0648, 0x0248, 0x0a99, 0x0048, 0x0848, 0x0448, 0x0e99,
0x1067, 0x05c8, 0x01c8, 0x0999, 0x1167, 0x07c8, 0x03c8, 0x0d99,
0x10e7, 0x06c8, 0x02c8, 0x0b99, 0x00c8, 0x08c8, 0x04c8, 0x0f99,
0x1017, 0x0528, 0x0128, 0x11a8, 0x1117, 0x0728, 0x0328, 0x0c59,
0x1097, 0x0628, 0x0228, 0x0a59, 0x0028, 0x0828, 0x0428, 0x0e59,
0x1057, 0x05a8, 0x01a8, 0x0959, 0x1157, 0x07a8, 0x03a8, 0x0d59,
0x10d7, 0x06a8, 0x02a8, 0x0b59, 0x00a8, 0x08a8, 0x04a8, 0x0f59,
0x1037, 0x0568, 0x0168, 0x11e8, 0x1137, 0x0768, 0x0368, 0x0cd9,
0x10b7, 0x0668, 0x0268, 0x0ad9, 0x0068, 0x0868, 0x0468, 0x0ed9,
0x1077, 0x05e8, 0x01e8, 0x09d9, 0x1177, 0x07e8, 0x03e8, 0x0dd9,
0x10f7, 0x06e8, 0x02e8, 0x0bd9, 0x00e8, 0x08e8, 0x04e8, 0x0fd9,
0x1007, 0x0518, 0x0118, 0x1198, 0x1107, 0x0718, 0x0318, 0x0c39,
0x1087, 0x0618, 0x0218, 0x0a39, 0x0018, 0x0818, 0x0418, 0x0e39,
0x1047, 0x0598, 0x0198, 0x0939, 0x1147, 0x0798, 0x0398, 0x0d39,
0x10c7, 0x0698, 0x0298, 0x0b39, 0x0098, 0x0898, 0x0498, 0x0f39,
0x1027, 0x0558, 0x0158, 0x11d8, 0x1127, 0x0758, 0x0358, 0x0cb9,
0x10a7, 0x0658, 0x0258, 0x0ab9, 0x0058, 0x0858, 0x0458, 0x0eb9,
0x1067, 0x05d8, 0x01d8, 0x09b9, 0x1167, 0x07d8, 0x03d8, 0x0db9,
0x10e7, 0x06d8, 0x02d8, 0x0bb9, 0x00d8, 0x08d8, 0x04d8, 0x0fb9,
0x1017, 0x0538, 0x0138, 0x11b8, 0x1117, 0x0738, 0x0338, 0x0c79,
0x1097, 0x0638, 0x0238, 0x0a79, 0x0038, 0x0838, 0x0438, 0x0e79,
0x1057, 0x05b8, 0x01b8, 0x0979, 0x1157, 0x07b8, 0x03b8, 0x0d79,
0x10d7, 0x06b8, 0x02b8, 0x0b79, 0x00b8, 0x08b8, 0x04b8, 0x0f79,
0x1037, 0x0578, 0x0178, 0x11f8, 0x1137, 0x0778, 0x0378, 0x0cf9,
0x10b7, 0x0678, 0x0278, 0x0af9, 0x0078, 0x0878, 0x0478, 0x0ef9,
0x1077, 0x05f8, 0x01f8, 0x09f9, 0x1177, 0x07f8, 0x03f8, 0x0df9,
0x10f7, 0x06f8, 0x02f8, 0x0bf9, 0x00f8, 0x08f8, 0x04f8, 0x0ff9,
},
nil, 0,
}
src/compress/flate/gen.go deleted 100644 → 0
View file @ 9f0baca5
// Copyright 2012 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.
// +build ignore
// This program generates fixedhuff.go
// Invoke as
//
// go run gen.go -output fixedhuff.go
package main
import (
"bytes"
"flag"
"fmt"
"go/format"
"io/ioutil"
"log"
)
var filename = flag.String("output", "fixedhuff.go", "output file name")
const maxCodeLen = 16
// Note: the definition of the huffmanDecoder struct is copied from
// inflate.go, as it is private to the implementation.
// chunk & 15 is number of bits
// chunk >> 4 is value, including table link
const (
huffmanChunkBits = 9
huffmanNumChunks = 1 << huffmanChunkBits
huffmanCountMask = 15
huffmanValueShift = 4
)
type huffmanDecoder struct {
min int // the minimum code length
chunks [huffmanNumChunks]uint32 // chunks as described above
links [][]uint32 // overflow links
linkMask uint32 // mask the width of the link table
}
// Initialize Huffman decoding tables from array of code lengths.
// Following this function, h is guaranteed to be initialized into a complete
// tree (i.e., neither over-subscribed nor under-subscribed). The exception is a
// degenerate case where the tree has only a single symbol with length 1. Empty
// trees are permitted.
func (h *huffmanDecoder) init(bits []int) bool {
// Sanity enables additional runtime tests during Huffman
// table construction. It's intended to be used during
// development to supplement the currently ad-hoc unit tests.
const sanity = false
if h.min != 0 {
*h = huffmanDecoder{}
}
// Count number of codes of each length,
// compute min and max length.
var count [maxCodeLen]int
var min, max int
for _, n := range bits {
if n == 0 {
continue
}
if min == 0 || n < min {
min = n
}
if n > max {
max = n
}
count[n]++
}
// Empty tree. The decompressor.huffSym function will fail later if the tree
// is used. Technically, an empty tree is only valid for the HDIST tree and
// not the HCLEN and HLIT tree. However, a stream with an empty HCLEN tree
// is guaranteed to fail since it will attempt to use the tree to decode the
// codes for the HLIT and HDIST trees. Similarly, an empty HLIT tree is
// guaranteed to fail later since the compressed data section must be
// composed of at least one symbol (the end-of-block marker).
if max == 0 {
return true
}
code := 0
var nextcode [maxCodeLen]int
for i := min; i <= max; i++ {
code <<= 1
nextcode[i] = code
code += count[i]
}
// Check that the coding is complete (i.e., that we've
// assigned all 2-to-the-max possible bit sequences).
// Exception: To be compatible with zlib, we also need to
// accept degenerate single-code codings. See also
// TestDegenerateHuffmanCoding.
if code != 1<<uint(max) && !(code == 1 && max == 1) {
return false
}
h.min = min
if max > huffmanChunkBits {
numLinks := 1 << (uint(max) - huffmanChunkBits)
h.linkMask = uint32(numLinks - 1)
// create link tables
link := nextcode[huffmanChunkBits+1] >> 1
h.links = make([][]uint32, huffmanNumChunks-link)
for j := uint(link); j < huffmanNumChunks; j++ {
reverse := int(reverseByte[j>>8]) | int(reverseByte[j&0xff])<<8
reverse >>= uint(16 - huffmanChunkBits)
off := j - uint(link)
if sanity && h.chunks[reverse] != 0 {
panic("impossible: overwriting existing chunk")
}
h.chunks[reverse] = uint32(off<<huffmanValueShift | (huffmanChunkBits + 1))
h.links[off] = make([]uint32, numLinks)
}
}
for i, n := range bits {
if n == 0 {
continue
}
code := nextcode[n]
nextcode[n]++
chunk := uint32(i<<huffmanValueShift | n)
reverse := int(reverseByte[code>>8]) | int(reverseByte[code&0xff])<<8
reverse >>= uint(16 - n)
if n <= huffmanChunkBits {
for off := reverse; off < len(h.chunks); off += 1 << uint(n) {
// We should never need to overwrite
// an existing chunk. Also, 0 is
// never a valid chunk, because the
// lower 4 "count" bits should be
// between 1 and 15.
if sanity && h.chunks[off] != 0 {
panic("impossible: overwriting existing chunk")
}
h.chunks[off] = chunk
}
} else {
j := reverse & (huffmanNumChunks - 1)
if sanity && h.chunks[j]&huffmanCountMask != huffmanChunkBits+1 {
// Longer codes should have been
// associated with a link table above.
panic("impossible: not an indirect chunk")
}
value := h.chunks[j] >> huffmanValueShift
linktab := h.links[value]
reverse >>= huffmanChunkBits
for off := reverse; off < len(linktab); off += 1 << uint(n-huffmanChunkBits) {
if sanity && linktab[off] != 0 {
panic("impossible: overwriting existing chunk")
}
linktab[off] = chunk
}
}
}
if sanity {
// Above we've sanity checked that we never overwrote
// an existing entry. Here we additionally check that
// we filled the tables completely.
for i, chunk := range h.chunks {
if chunk == 0 {
// As an exception, in the degenerate
// single-code case, we allow odd
// chunks to be missing.
if code == 1 && i%2 == 1 {
continue
}
panic("impossible: missing chunk")
}
}
for _, linktab := range h.links {
for _, chunk := range linktab {
if chunk == 0 {
panic("impossible: missing chunk")
}
}
}
}
return true
}
func main() {
flag.Parse()
var h huffmanDecoder
var bits [288]int
initReverseByte()
for i := 0; i < 144; i++ {
bits[i] = 8
}
for i := 144; i < 256; i++ {
bits[i] = 9
}
for i := 256; i < 280; i++ {
bits[i] = 7
}
for i := 280; i < 288; i++ {
bits[i] = 8
}
h.init(bits[:])
if h.links != nil {
log.Fatal("Unexpected links table in fixed Huffman decoder")
}
var buf bytes.Buffer
fmt.Fprintf(&buf, `// Copyright 2013 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.`+"\n\n")
fmt.Fprintln(&buf, "package flate")
fmt.Fprintln(&buf)
fmt.Fprintln(&buf, "// autogenerated by go run gen.go -output fixedhuff.go, DO NOT EDIT")
fmt.Fprintln(&buf)
fmt.Fprintln(&buf, "var fixedHuffmanDecoder = huffmanDecoder{")
fmt.Fprintf(&buf, "\t%d,\n", h.min)
fmt.Fprintln(&buf, "\t[huffmanNumChunks]uint32{")
for i := 0; i < huffmanNumChunks; i++ {
if i&7 == 0 {
fmt.Fprintf(&buf, "\t\t")
} else {
fmt.Fprintf(&buf, " ")
}
fmt.Fprintf(&buf, "0x%04x,", h.chunks[i])
if i&7 == 7 {
fmt.Fprintln(&buf)
}
}
fmt.Fprintln(&buf, "\t},")
fmt.Fprintln(&buf, "\tnil, 0,")
fmt.Fprintln(&buf, "}")
data, err := format.Source(buf.Bytes())
if err != nil {
log.Fatal(err)
}
err = ioutil.WriteFile(*filename, data, 0644)
if err != nil {
log.Fatal(err)
}
}
var reverseByte [256]byte
func initReverseByte() {
for x := 0; x < 256; x++ {
var result byte
for i := uint(0); i < 8; i++ {
result |= byte(((x >> i) & 1) << (7 - i))
}
reverseByte[x] = result
}
}
src/compress/flate/inflate.go
View file @ 5c78589b
......@@ -2,8 +2,6 @@
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:generate go run gen.go -output fixedhuff.go
// Package flate implements the DEFLATE compressed data format, described in
// RFC 1951. The gzip and zlib packages implement access to DEFLATE-based file
// formats.
......@@ -13,6 +11,7 @@ import (
"bufio"
"io"
"strconv"
"sync"
)
const (
......@@ -26,6 +25,10 @@ const (
numCodes = 19 // number of codes in Huffman meta-code
)
// Initialize the fixedHuffmanDecoder only once upon first use.
var fixedOnce sync.Once
var fixedHuffmanDecoder huffmanDecoder
// A CorruptInputError reports the presence of corrupt input at a given offset.
type CorruptInputError int64
......@@ -67,10 +70,6 @@ type Resetter interface {
Reset(r io.Reader, dict []byte) error
}
// Note that much of the implementation of huffmanDecoder is also copied
// into gen.go (in package main) for the purpose of precomputing the
// fixed huffman tables so they can be included statically.
// The data structure for decoding Huffman tables is based on that of
// zlib. There is a lookup table of a fixed bit width (huffmanChunkBits),
// For codes smaller than the table width, there are multiple entries
......@@ -78,12 +77,15 @@ type Resetter interface {
// larger than the table width, the table contains a link to an overflow
// table. The width of each entry in the link table is the maximum code
// size minus the chunk width.
//
// Note that you can do a lookup in the table even without all bits
// filled. Since the extra bits are zero, and the DEFLATE Huffman codes
// have the property that shorter codes come before longer ones, the
// bit length estimate in the result is a lower bound on the actual
// number of bits.
//
// See the following:
// http://www.gzip.org/algorithm.txt
// chunk & 15 is number of bits
// chunk >> 4 is value, including table link
......@@ -760,6 +762,26 @@ func makeReader(r io.Reader) Reader {
return bufio.NewReader(r)
}
func fixedHuffmanDecoderInit() {
fixedOnce.Do(func() {
// These come from the RFC section 3.2.6.
var bits [288]int
for i := 0; i < 144; i++ {
bits[i] = 8
}
for i := 144; i < 256; i++ {
bits[i] = 9
}
for i := 256; i < 280; i++ {
bits[i] = 7
}
for i := 280; i < 288; i++ {
bits[i] = 8
}
fixedHuffmanDecoder.init(bits[:])
})
}
func (f *decompressor) Reset(r io.Reader, dict []byte) error {
*f = decompressor{
r: makeReader(r),
......@@ -783,11 +805,13 @@ func (f *decompressor) Reset(r io.Reader, dict []byte) error {
//
// The ReadCloser returned by NewReader also implements Resetter.
func NewReader(r io.Reader) io.ReadCloser {
fixedHuffmanDecoderInit()
var f decompressor
f.bits = new([maxNumLit + maxNumDist]int)
f.codebits = new([numCodes]int)
f.r = makeReader(r)
f.hist = new([maxHist]byte)
f.bits = new([maxNumLit + maxNumDist]int)
f.codebits = new([numCodes]int)
f.step = (*decompressor).nextBlock
return &f
}
......@@ -800,6 +824,8 @@ func NewReader(r io.Reader) io.ReadCloser {
//
// The ReadCloser returned by NewReader also implements Resetter.
func NewReaderDict(r io.Reader, dict []byte) io.ReadCloser {
fixedHuffmanDecoderInit()
var f decompressor
f.r = makeReader(r)
f.hist = new([maxHist]byte)
......
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