// Copyright 2011 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 bzip2 implements bzip2 decompression.
package bzip2
import "io"
// There's no RFC for bzip2. I used the Wikipedia page for reference and a lot
// of guessing: http://en.wikipedia.org/wiki/Bzip2
// The source code to pyflate was useful for debugging:
// http://www.paul.sladen.org/projects/pyflate
// A StructuralError is returned when the bzip2 data is found to be
// syntactically invalid.
type StructuralError string
func (s StructuralError) Error() string {
return "bzip2 data invalid: " + string(s)
}
// A reader decompresses bzip2 compressed data.
type reader struct {
br bitReader
fileCRC uint32
blockCRC uint32
wantBlockCRC uint32
setupDone bool // true if we have parsed the bzip2 header.
blockSize int // blockSize in bytes, i.e. 900 * 1000.
eof bool
c [256]uint // the `C' array for the inverse BWT.
tt []uint32 // mirrors the `tt' array in the bzip2 source and contains the P array in the upper 24 bits.
tPos uint32 // Index of the next output byte in tt.
preRLE []uint32 // contains the RLE data still to be processed.
preRLEUsed int // number of entries of preRLE used.
lastByte int // the last byte value seen.
byteRepeats uint // the number of repeats of lastByte seen.
repeats uint // the number of copies of lastByte to output.
}
// NewReader returns an io.Reader which decompresses bzip2 data from r.
// If r does not also implement io.ByteReader,
// the decompressor may read more data than necessary from r.
func NewReader(r io.Reader) io.Reader {
bz2 := new(reader)
bz2.br = newBitReader(r)
return bz2
}
const bzip2FileMagic = 0x425a // "BZ"
const bzip2BlockMagic = 0x314159265359
const bzip2FinalMagic = 0x177245385090
// setup parses the bzip2 header.
func (bz2 *reader) setup(needMagic bool) error {
br := &bz2.br
if needMagic {
magic := br.ReadBits(16)
if magic != bzip2FileMagic {
return StructuralError("bad magic value")
}
}
t := br.ReadBits(8)
if t != 'h' {
return StructuralError("non-Huffman entropy encoding")
}
level := br.ReadBits(8)
if level < '1' || level > '9' {
return StructuralError("invalid compression level")
}
bz2.fileCRC = 0
bz2.blockSize = 100 * 1000 * (level - '0')
if bz2.blockSize > len(bz2.tt) {
bz2.tt = make([]uint32, bz2.blockSize)
}
return nil
}
func (bz2 *reader) Read(buf []byte) (n int, err error) {
if bz2.eof {
return 0, io.EOF
}
if !bz2.setupDone {
err = bz2.setup(true)
brErr := bz2.br.Err()
if brErr != nil {
err = brErr
}
if err != nil {
return 0, err
}
bz2.setupDone = true
}
n, err = bz2.read(buf)
brErr := bz2.br.Err()
if brErr != nil {
err = brErr
}
return
}
func (bz2 *reader) readFromBlock(buf []byte) int {
// bzip2 is a block based compressor, except that it has a run-length
// preprocessing step. The block based nature means that we can
// preallocate fixed-size buffers and reuse them. However, the RLE
// preprocessing would require allocating huge buffers to store the
// maximum expansion. Thus we process blocks all at once, except for
// the RLE which we decompress as required.
n := 0
for (bz2.repeats > 0 || bz2.preRLEUsed < len(bz2.preRLE)) && n < len(buf) {
// We have RLE data pending.
// The run-length encoding works like this:
// Any sequence of four equal bytes is followed by a length
// byte which contains the number of repeats of that byte to
// include. (The number of repeats can be zero.) Because we are
// decompressing on-demand our state is kept in the reader
// object.
if bz2.repeats > 0 {
buf[n] = byte(bz2.lastByte)
n++
bz2.repeats--
if bz2.repeats == 0 {
bz2.lastByte = -1
}
continue
}
bz2.tPos = bz2.preRLE[bz2.tPos]
b := byte(bz2.tPos)
bz2.tPos >>= 8
bz2.preRLEUsed++
if bz2.byteRepeats == 3 {
bz2.repeats = uint(b)
bz2.byteRepeats = 0
continue
}
if bz2.lastByte == int(b) {
bz2.byteRepeats++
} else {
bz2.byteRepeats = 0
}
bz2.lastByte = int(b)
buf[n] = b
n++
}
return n
}
func (bz2 *reader) read(buf []byte) (int, error) {
for {
n := bz2.readFromBlock(buf)
if n > 0 {
bz2.blockCRC = updateCRC(bz2.blockCRC, buf[:n])
return n, nil
}
// End of block. Check CRC.
if bz2.blockCRC != bz2.wantBlockCRC {
bz2.br.err = StructuralError("block checksum mismatch")
return 0, bz2.br.err
}
// Find next block.
br := &bz2.br
switch br.ReadBits64(48) {
default:
return 0, StructuralError("bad magic value found")
case bzip2BlockMagic:
// Start of block.
err := bz2.readBlock()
if err != nil {
return 0, err
}
case bzip2FinalMagic:
// Check end-of-file CRC.
wantFileCRC := uint32(br.ReadBits64(32))
if br.err != nil {
return 0, br.err
}
if bz2.fileCRC != wantFileCRC {
br.err = StructuralError("file checksum mismatch")
return 0, br.err
}
// Skip ahead to byte boundary.
// Is there a file concatenated to this one?
// It would start with BZ.
if br.bits%8 != 0 {
br.ReadBits(br.bits % 8)
}
b, err := br.r.ReadByte()
if err == io.EOF {
br.err = io.EOF
bz2.eof = true
return 0, io.EOF
}
if err != nil {
br.err = err
return 0, err
}
z, err := br.r.ReadByte()
if err != nil {
if err == io.EOF {
err = io.ErrUnexpectedEOF
}
br.err = err
return 0, err
}
if b != 'B' || z != 'Z' {
return 0, StructuralError("bad magic value in continuation file")
}
if err := bz2.setup(false); err != nil {
return 0, err
}
}
}
}
// readBlock reads a bzip2 block. The magic number should already have been consumed.
func (bz2 *reader) readBlock() (err error) {
br := &bz2.br
bz2.wantBlockCRC = uint32(br.ReadBits64(32)) // skip checksum. TODO: check it if we can figure out what it is.
bz2.blockCRC = 0
bz2.fileCRC = (bz2.fileCRC<<1 | bz2.fileCRC>>31) ^ bz2.wantBlockCRC
randomized := br.ReadBits(1)
if randomized != 0 {
return StructuralError("deprecated randomized files")
}
origPtr := uint(br.ReadBits(24))
// If not every byte value is used in the block (i.e., it's text) then
// the symbol set is reduced. The symbols used are stored as a
// two-level, 16x16 bitmap.
symbolRangeUsedBitmap := br.ReadBits(16)
symbolPresent := make([]bool, 256)
numSymbols := 0
for symRange := uint(0); symRange < 16; symRange++ {
if symbolRangeUsedBitmap&(1<<(15-symRange)) != 0 {
bits := br.ReadBits(16)
for symbol := uint(0); symbol < 16; symbol++ {
if bits&(1<<(15-symbol)) != 0 {
symbolPresent[16*symRange+symbol] = true
numSymbols++
}
}
}
}
if numSymbols == 0 {
// There must be an EOF symbol.
return StructuralError("no symbols in input")
}
// A block uses between two and six different Huffman trees.
numHuffmanTrees := br.ReadBits(3)
if numHuffmanTrees < 2 || numHuffmanTrees > 6 {
return StructuralError("invalid number of Huffman trees")
}
// The Huffman tree can switch every 50 symbols so there's a list of
// tree indexes telling us which tree to use for each 50 symbol block.
numSelectors := br.ReadBits(15)
treeIndexes := make([]uint8, numSelectors)
// The tree indexes are move-to-front transformed and stored as unary
// numbers.
mtfTreeDecoder := newMTFDecoderWithRange(numHuffmanTrees)
for i := range treeIndexes {
c := 0
for {
inc := br.ReadBits(1)
if inc == 0 {
break
}
c++
}
if c >= numHuffmanTrees {
return StructuralError("tree index too large")
}
treeIndexes[i] = mtfTreeDecoder.Decode(c)
}
// The list of symbols for the move-to-front transform is taken from
// the previously decoded symbol bitmap.
symbols := make([]byte, numSymbols)
nextSymbol := 0
for i := 0; i < 256; i++ {
if symbolPresent[i] {
symbols[nextSymbol] = byte(i)
nextSymbol++
}
}
mtf := newMTFDecoder(symbols)
numSymbols += 2 // to account for RUNA and RUNB symbols
huffmanTrees := make([]huffmanTree, numHuffmanTrees)
// Now we decode the arrays of code-lengths for each tree.
lengths := make([]uint8, numSymbols)
for i := range huffmanTrees {
// The code lengths are delta encoded from a 5-bit base value.
length := br.ReadBits(5)
for j := range lengths {
for {
if length < 1 || length > 20 {
return StructuralError("Huffman length out of range")
}
if !br.ReadBit() {
break
}
if br.ReadBit() {
length--
} else {
length++
}
}
lengths[j] = uint8(length)
}
huffmanTrees[i], err = newHuffmanTree(lengths)
if err != nil {
return err
}
}
selectorIndex := 1 // the next tree index to use
if len(treeIndexes) == 0 {
return StructuralError("no tree selectors given")
}
if int(treeIndexes[0]) >= len(huffmanTrees) {
return StructuralError("tree selector out of range")
}
currentHuffmanTree := huffmanTrees[treeIndexes[0]]
bufIndex := 0 // indexes bz2.buf, the output buffer.
// The output of the move-to-front transform is run-length encoded and
// we merge the decoding into the Huffman parsing loop. These two
// variables accumulate the repeat count. See the Wikipedia page for
// details.
repeat := 0
repeatPower := 0
// The `C' array (used by the inverse BWT) needs to be zero initialized.
for i := range bz2.c {
bz2.c[i] = 0
}
decoded := 0 // counts the number of symbols decoded by the current tree.
for {
if decoded == 50 {
if selectorIndex >= numSelectors {
return StructuralError("insufficient selector indices for number of symbols")
}
if int(treeIndexes[selectorIndex]) >= len(huffmanTrees) {
return StructuralError("tree selector out of range")
}
currentHuffmanTree = huffmanTrees[treeIndexes[selectorIndex]]
selectorIndex++
decoded = 0
}
v := currentHuffmanTree.Decode(br)
decoded++
if v < 2 {
// This is either the RUNA or RUNB symbol.
if repeat == 0 {
repeatPower = 1
}
repeat += repeatPower << v
repeatPower <<= 1
// This limit of 2 million comes from the bzip2 source
// code. It prevents repeat from overflowing.
if repeat > 2*1024*1024 {
return StructuralError("repeat count too large")
}
continue
}
if repeat > 0 {
// We have decoded a complete run-length so we need to
// replicate the last output symbol.
if repeat > bz2.blockSize-bufIndex {
return StructuralError("repeats past end of block")
}
for i := 0; i < repeat; i++ {
b := mtf.First()
bz2.tt[bufIndex] = uint32(b)
bz2.c[b]++
bufIndex++
}
repeat = 0
}
if int(v) == numSymbols-1 {
// This is the EOF symbol. Because it's always at the
// end of the move-to-front list, and never gets moved
// to the front, it has this unique value.
break
}
// Since two metasymbols (RUNA and RUNB) have values 0 and 1,
// one would expect |v-2| to be passed to the MTF decoder.
// However, the front of the MTF list is never referenced as 0,
// it's always referenced with a run-length of 1. Thus 0
// doesn't need to be encoded and we have |v-1| in the next
// line.
b := mtf.Decode(int(v - 1))
if bufIndex >= bz2.blockSize {
return StructuralError("data exceeds block size")
}
bz2.tt[bufIndex] = uint32(b)
bz2.c[b]++
bufIndex++
}
if origPtr >= uint(bufIndex) {
return StructuralError("origPtr out of bounds")
}
// We have completed the entropy decoding. Now we can perform the
// inverse BWT and setup the RLE buffer.
bz2.preRLE = bz2.tt[:bufIndex]
bz2.preRLEUsed = 0
bz2.tPos = inverseBWT(bz2.preRLE, origPtr, bz2.c[:])
bz2.lastByte = -1
bz2.byteRepeats = 0
bz2.repeats = 0
return nil
}
// inverseBWT implements the inverse Burrows-Wheeler transform as described in
// http://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-124.pdf, section 4.2.
// In that document, origPtr is called `I' and c is the `C' array after the
// first pass over the data. It's an argument here because we merge the first
// pass with the Huffman decoding.
//
// This also implements the `single array' method from the bzip2 source code
// which leaves the output, still shuffled, in the bottom 8 bits of tt with the
// index of the next byte in the top 24-bits. The index of the first byte is
// returned.
func inverseBWT(tt []uint32, origPtr uint, c []uint) uint32 {
sum := uint(0)
for i := 0; i < 256; i++ {
sum += c[i]
c[i] = sum - c[i]
}
for i := range tt {
b := tt[i] & 0xff
tt[c[b]] |= uint32(i) << 8
c[b]++
}
return tt[origPtr] >> 8
}
// This is a standard CRC32 like in hash/crc32 except that all the shifts are reversed,
// causing the bits in the input to be processed in the reverse of the usual order.
var crctab [256]uint32
func init() {
const poly = 0x04C11DB7
for i := range crctab {
crc := uint32(i) << 24
for j := 0; j < 8; j++ {
if crc&0x80000000 != 0 {
crc = (crc << 1) ^ poly
} else {
crc <<= 1
}
}
crctab[i] = crc
}
}
// updateCRC updates the crc value to incorporate the data in b.
// The initial value is 0.
func updateCRC(val uint32, b []byte) uint32 {
crc := ^val
for _, v := range b {
crc = crctab[byte(crc>>24)^v] ^ (crc << 8)
}
return ^crc
}