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110 lines
3.0 KiB
110 lines
3.0 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 implements the BZip2 compressed data format.
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//
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// Canonical C implementation:
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// http://bzip.org
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//
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// Unofficial format specification:
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// https://github.com/dsnet/compress/blob/master/doc/bzip2-format.pdf
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package bzip2
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import (
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"fmt"
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"hash/crc32"
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"github.com/dsnet/compress/internal"
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"github.com/dsnet/compress/internal/errors"
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)
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// There does not exist a formal specification of the BZip2 format. As such,
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// much of this work is derived by either reverse engineering the original C
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// source code or using secondary sources.
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//
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// Significant amounts of fuzz testing is done to ensure that outputs from
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// this package is properly decoded by the C library. Furthermore, we test that
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// both this package and the C library agree about what inputs are invalid.
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//
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// Compression stack:
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// Run-length encoding 1 (RLE1)
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// Burrows-Wheeler transform (BWT)
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// Move-to-front transform (MTF)
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// Run-length encoding 2 (RLE2)
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// Prefix encoding (PE)
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//
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// References:
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// http://bzip.org/
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// https://en.wikipedia.org/wiki/Bzip2
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// https://code.google.com/p/jbzip2/
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const (
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BestSpeed = 1
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BestCompression = 9
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DefaultCompression = 6
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)
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const (
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hdrMagic = 0x425a // Hex of "BZ"
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blkMagic = 0x314159265359 // BCD of PI
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endMagic = 0x177245385090 // BCD of sqrt(PI)
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blockSize = 100000
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)
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func errorf(c int, f string, a ...interface{}) error {
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return errors.Error{Code: c, Pkg: "bzip2", Msg: fmt.Sprintf(f, a...)}
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}
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func panicf(c int, f string, a ...interface{}) {
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errors.Panic(errorf(c, f, a...))
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}
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// errWrap converts a lower-level errors.Error to be one from this package.
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// The replaceCode passed in will be used to replace the code for any errors
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// with the errors.Invalid code.
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//
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// For the Reader, set this to errors.Corrupted.
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// For the Writer, set this to errors.Internal.
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func errWrap(err error, replaceCode int) error {
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if cerr, ok := err.(errors.Error); ok {
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if errors.IsInvalid(cerr) {
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cerr.Code = replaceCode
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}
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err = errorf(cerr.Code, "%s", cerr.Msg)
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}
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return err
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}
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var errClosed = errorf(errors.Closed, "")
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// crc computes the CRC-32 used by BZip2.
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//
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// The CRC-32 computation in bzip2 treats bytes as having bits in big-endian
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// order. That is, the MSB is read before the LSB. Thus, we can use the
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// standard library version of CRC-32 IEEE with some minor adjustments.
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//
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// The byte array is used as an intermediate buffer to swap the bits of every
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// byte of the input.
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type crc struct {
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val uint32
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buf [256]byte
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}
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// update computes the CRC-32 of appending buf to c.
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func (c *crc) update(buf []byte) {
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cval := internal.ReverseUint32(c.val)
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for len(buf) > 0 {
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n := len(buf)
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if n > len(c.buf) {
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n = len(c.buf)
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}
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for i, b := range buf[:n] {
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c.buf[i] = internal.ReverseLUT[b]
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}
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cval = crc32.Update(cval, crc32.IEEETable, c.buf[:n])
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buf = buf[n:]
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}
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c.val = internal.ReverseUint32(cval)
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}
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