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131 lines
3.3 KiB
131 lines
3.3 KiB
// 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 bzip2
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import "github.com/dsnet/compress/internal/errors"
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// moveToFront implements both the MTF and RLE stages of bzip2 at the same time.
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// Any runs of zeros in the encoded output will be replaced by a sequence of
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// RUNA and RUNB symbols are encode the length of the run.
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//
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// The RLE encoding used can actually be encoded to and decoded from using
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// normal two's complement arithmetic. The methodology for doing so is below.
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//
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// Assuming the following:
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// num: The value being encoded by RLE encoding.
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// run: A sequence of RUNA and RUNB symbols represented as a binary integer,
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// where RUNA is the 0 bit, RUNB is the 1 bit, and least-significant RUN
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// symbols are at the least-significant bit positions.
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// cnt: The number of RUNA and RUNB symbols.
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//
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// Then the RLE encoding used by bzip2 has this mathematical property:
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// num+1 == (1<<cnt) | run
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type moveToFront struct {
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dictBuf [256]uint8
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dictLen int
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vals []byte
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syms []uint16
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blkSize int
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}
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func (mtf *moveToFront) Init(dict []uint8, blkSize int) {
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if len(dict) > len(mtf.dictBuf) {
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panicf(errors.Internal, "alphabet too large")
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}
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copy(mtf.dictBuf[:], dict)
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mtf.dictLen = len(dict)
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mtf.blkSize = blkSize
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}
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func (mtf *moveToFront) Encode(vals []byte) (syms []uint16) {
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dict := mtf.dictBuf[:mtf.dictLen]
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syms = mtf.syms[:0]
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if len(vals) > mtf.blkSize {
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panicf(errors.Internal, "exceeded block size")
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}
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var lastNum uint32
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for _, val := range vals {
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// Normal move-to-front transform.
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var idx uint8 // Reverse lookup idx in dict
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for di, dv := range 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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copy(dict[1:], dict[:idx])
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dict[0] = val
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// Run-length encoding augmentation.
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if idx == 0 {
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lastNum++
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continue
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}
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if lastNum > 0 {
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for rc := lastNum + 1; rc != 1; rc >>= 1 {
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syms = append(syms, uint16(rc&1))
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}
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lastNum = 0
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}
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syms = append(syms, uint16(idx)+1)
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}
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if lastNum > 0 {
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for rc := lastNum + 1; rc != 1; rc >>= 1 {
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syms = append(syms, uint16(rc&1))
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}
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}
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mtf.syms = syms
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return syms
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}
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func (mtf *moveToFront) Decode(syms []uint16) (vals []byte) {
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dict := mtf.dictBuf[:mtf.dictLen]
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vals = mtf.vals[:0]
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var lastCnt uint
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var lastRun uint32
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for _, sym := range syms {
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// Run-length encoding augmentation.
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if sym < 2 {
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lastRun |= uint32(sym) << lastCnt
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lastCnt++
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continue
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}
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if lastCnt > 0 {
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cnt := int((1<<lastCnt)|lastRun) - 1
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if len(vals)+cnt > mtf.blkSize || lastCnt > 24 {
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panicf(errors.Corrupted, "run-length decoding exceeded block size")
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}
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for i := cnt; i > 0; i-- {
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vals = append(vals, dict[0])
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}
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lastCnt, lastRun = 0, 0
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}
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// Normal move-to-front transform.
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val := dict[sym-1] // Forward lookup val in dict
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copy(dict[1:], dict[:sym-1])
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dict[0] = val
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if len(vals) >= mtf.blkSize {
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panicf(errors.Corrupted, "run-length decoding exceeded block size")
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}
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vals = append(vals, val)
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}
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if lastCnt > 0 {
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cnt := int((1<<lastCnt)|lastRun) - 1
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if len(vals)+cnt > mtf.blkSize || lastCnt > 24 {
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panicf(errors.Corrupted, "run-length decoding exceeded block size")
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}
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for i := cnt; i > 0; i-- {
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vals = append(vals, dict[0])
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}
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}
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mtf.vals = vals
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return vals
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}
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