crc32.go

// Package crc32itu implements the ITU I.363.5 CRC-32 algorithm (also known as
// AAL5 CRC). This algorithm was popularised by BZIP2 and is also used in ATM
// transmissions.
//
// This package is particularly useful for interoperability with PHP, as it
// produces the same output as PHP's hash('crc32', ...) function. Go's standard
// library hash/crc32 uses a different polynomial and bit ordering, making it
// incompatible with PHP's crc32 hash.
//
// The polynomial used is 0x04C11DB7, with reversed bit shifts compared to the
// IEEE polynomial used in hash/crc32.
//
// The algorithm is the same as that in POSIX 1003.2-1992 cksum, except cksum
// appends the message length to the CRC at the end.
package crc32itu

import "hash"

// 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.

// Code extracted from golang compress/bzip2

// Size is the size of a CRC-32 checksum in bytes.
const Size = 4

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
}

// Checksum returns the ITU I.363.5 checksum of data.
func Checksum(data []byte) uint32 {
	return updateCRC(0, data)
}

// Update returns the result of adding the bytes in p to the crc.
func Update(crc uint32, data []byte) uint32 {
	return updateCRC(crc, data)
}

// New returns a new instance of hash.Hash32 that can be used to compute CRC32
// values using this standard interface.
func New() hash.Hash32 {
	return &crc32digest{}
}

type crc32digest struct {
	crc uint32
}

func (d *crc32digest) Size() int { return Size }

func (d *crc32digest) BlockSize() int { return 1 }

func (d *crc32digest) Reset() { d.crc = 0 }

func (d *crc32digest) Sum32() uint32 { return d.crc }

func (d *crc32digest) Sum(in []byte) []byte {
	s := d.Sum32()
	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
}

func (d *crc32digest) Write(p []byte) (n int, err error) {
	d.crc = updateCRC(d.crc, p)
	return len(p), nil
}