164 lines
3.5 KiB
Go
164 lines
3.5 KiB
Go
// Copyright 2015, Joe Tsai. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE.md file.
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// Package brotli implements the Brotli compressed data format.
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package brotli
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import "runtime"
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// Error is the wrapper type for errors specific to this library.
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type Error string
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func (e Error) Error() string { return "brotli: " + string(e) }
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var (
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ErrCorrupt error = Error("stream is corrupted")
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)
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func errRecover(err *error) {
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switch ex := recover().(type) {
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case nil:
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// Do nothing.
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case runtime.Error:
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panic(ex)
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case error:
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*err = ex
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default:
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panic(ex)
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}
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}
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var (
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reverseLUT [256]uint8
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mtfLUT [256]uint8
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)
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func init() {
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initLUTs()
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printLUTs() // Only occurs in debug mode
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}
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func initLUTs() {
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initCommonLUTs()
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initPrefixLUTs()
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initContextLUTs()
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initDictLUTs()
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}
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func initCommonLUTs() {
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for i := range mtfLUT {
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mtfLUT[i] = uint8(i)
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}
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for i := range reverseLUT {
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b := uint8(i)
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b = (b&0xaa)>>1 | (b&0x55)<<1
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b = (b&0xcc)>>2 | (b&0x33)<<2
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b = (b&0xf0)>>4 | (b&0x0f)<<4
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reverseLUT[i] = b
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}
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}
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// neededBits computes the minimum number of bits needed to encode n elements.
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func neededBits(n uint32) (nb uint) {
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for n -= 1; n > 0; n >>= 1 {
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nb++
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}
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return
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}
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// reverseUint32 reverses all bits of v.
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func reverseUint32(v uint32) (x uint32) {
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x |= uint32(reverseLUT[byte(v>>0)]) << 24
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x |= uint32(reverseLUT[byte(v>>8)]) << 16
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x |= uint32(reverseLUT[byte(v>>16)]) << 8
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x |= uint32(reverseLUT[byte(v>>24)]) << 0
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return x
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}
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// reverseBits reverses the lower n bits of v.
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func reverseBits(v uint32, n uint) uint32 {
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return reverseUint32(v << (32 - n))
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}
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// moveToFront is a data structure that allows for more efficient move-to-front
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// transformations (described in RFC section 7.3). Since most transformations
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// only involve a fairly low number of symbols, it can be quite expensive
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// filling out the dict with values 0..255 for every call. Instead, we remember
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// what part of the dict was altered and make sure we reset it at the beginning
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// of every encode and decode operation.
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type moveToFront struct {
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dict [256]uint8 // Mapping from indexes to values
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tail int // Number of tail bytes that are already ordered
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}
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func (m *moveToFront) Encode(vals []uint8) {
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// Reset dict to be identical to mtfLUT.
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copy(m.dict[:], mtfLUT[:256-m.tail])
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var max int
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for i, val := range vals {
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var idx uint8 // Reverse lookup idx in dict
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for di, dv := range m.dict {
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if dv == val {
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idx = uint8(di)
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break
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}
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}
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vals[i] = idx
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max |= int(idx)
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copy(m.dict[1:], m.dict[:idx])
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m.dict[0] = val
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}
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m.tail = 256 - max - 1
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}
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func (m *moveToFront) Decode(idxs []uint8) {
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// Reset dict to be identical to mtfLUT.
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copy(m.dict[:], mtfLUT[:256-m.tail])
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var max int
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for i, idx := range idxs {
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val := m.dict[idx] // Forward lookup val in dict
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idxs[i] = val
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max |= int(idx)
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copy(m.dict[1:], m.dict[:idx])
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m.dict[0] = val
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}
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m.tail = 256 - max - 1
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}
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func allocUint8s(s []uint8, n int) []uint8 {
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if cap(s) >= n {
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return s[:n]
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}
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return make([]uint8, n, n*3/2)
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}
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func allocUint32s(s []uint32, n int) []uint32 {
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if cap(s) >= n {
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return s[:n]
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}
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return make([]uint32, n, n*3/2)
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}
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func extendSliceUints32s(s [][]uint32, n int) [][]uint32 {
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if cap(s) >= n {
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return s[:n]
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}
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ss := make([][]uint32, n, n*3/2)
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copy(ss, s[:cap(s)])
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return ss
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}
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func extendDecoders(s []prefixDecoder, n int) []prefixDecoder {
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if cap(s) >= n {
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return s[:n]
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}
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ss := make([]prefixDecoder, n, n*3/2)
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copy(ss, s[:cap(s)])
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return ss
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}
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