forgejo-federation/vendor/github.com/dsnet/compress/bzip2/common.go

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// Copyright 2015, Joe Tsai. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE.md file.
// Package bzip2 implements the BZip2 compressed data format.
//
// Canonical C implementation:
// http://bzip.org
//
// Unofficial format specification:
// https://github.com/dsnet/compress/blob/master/doc/bzip2-format.pdf
package bzip2
import (
"fmt"
"hash/crc32"
"github.com/dsnet/compress/internal"
"github.com/dsnet/compress/internal/errors"
)
// There does not exist a formal specification of the BZip2 format. As such,
// much of this work is derived by either reverse engineering the original C
// source code or using secondary sources.
//
// Significant amounts of fuzz testing is done to ensure that outputs from
// this package is properly decoded by the C library. Furthermore, we test that
// both this package and the C library agree about what inputs are invalid.
//
// Compression stack:
// Run-length encoding 1 (RLE1)
// Burrows-Wheeler transform (BWT)
// Move-to-front transform (MTF)
// Run-length encoding 2 (RLE2)
// Prefix encoding (PE)
//
// References:
// http://bzip.org/
// https://en.wikipedia.org/wiki/Bzip2
// https://code.google.com/p/jbzip2/
const (
BestSpeed = 1
BestCompression = 9
DefaultCompression = 6
)
const (
hdrMagic = 0x425a // Hex of "BZ"
blkMagic = 0x314159265359 // BCD of PI
endMagic = 0x177245385090 // BCD of sqrt(PI)
blockSize = 100000
)
func errorf(c int, f string, a ...interface{}) error {
return errors.Error{Code: c, Pkg: "bzip2", Msg: fmt.Sprintf(f, a...)}
}
func panicf(c int, f string, a ...interface{}) {
errors.Panic(errorf(c, f, a...))
}
// errWrap converts a lower-level errors.Error to be one from this package.
// The replaceCode passed in will be used to replace the code for any errors
// with the errors.Invalid code.
//
// For the Reader, set this to errors.Corrupted.
// For the Writer, set this to errors.Internal.
func errWrap(err error, replaceCode int) error {
if cerr, ok := err.(errors.Error); ok {
if errors.IsInvalid(cerr) {
cerr.Code = replaceCode
}
err = errorf(cerr.Code, "%s", cerr.Msg)
}
return err
}
var errClosed = errorf(errors.Closed, "")
// crc computes the CRC-32 used by BZip2.
//
// The CRC-32 computation in bzip2 treats bytes as having bits in big-endian
// order. That is, the MSB is read before the LSB. Thus, we can use the
// standard library version of CRC-32 IEEE with some minor adjustments.
//
// The byte array is used as an intermediate buffer to swap the bits of every
// byte of the input.
type crc struct {
val uint32
buf [256]byte
}
// update computes the CRC-32 of appending buf to c.
func (c *crc) update(buf []byte) {
cval := internal.ReverseUint32(c.val)
for len(buf) > 0 {
n := len(buf)
if n > len(c.buf) {
n = len(c.buf)
}
for i, b := range buf[:n] {
c.buf[i] = internal.ReverseLUT[b]
}
cval = crc32.Update(cval, crc32.IEEETable, c.buf[:n])
buf = buf[n:]
}
c.val = internal.ReverseUint32(cval)
}