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// Copyright 2009 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 crc32 implements the 32-bit cyclic redundancy check, or CRC-32,
// checksum. See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for
// information.
package crc32
import (
"hash"
"os"
)
// The size of a CRC-32 checksum in bytes.
const Size = 4
// Predefined polynomials.
const (
// Far and away the most common CRC-32 polynomial.
// Used by ethernet (IEEE 802.3), v.42, fddi, gzip, zip, png, mpeg-2, ...
IEEE = 0xedb88320
// Castagnoli's polynomial, used in iSCSI.
// Has better error detection characteristics than IEEE.
// http://dx.doi.org/10.1109/26.231911
Castagnoli = 0x82f63b78
// Koopman's polynomial.
// Also has better error detection characteristics than IEEE.
// http://dx.doi.org/10.1109/DSN.2002.1028931
Koopman = 0xeb31d82e
)
// Table is a 256-word table representing the polynomial for efficient processing.
type Table [256]uint32
// MakeTable returns the Table constructed from the specified polynomial.
func MakeTable(poly uint32) *Table {
t := new(Table)
for i := 0; i < 256; i++ {
crc := uint32(i)
for j := 0; j < 8; j++ {
if crc&1 == 1 {
crc = (crc >> 1) ^ poly
} else {
crc >>= 1
}
}
t[i] = crc
}
return t
}
// IEEETable is the table for the IEEE polynomial.
var IEEETable = MakeTable(IEEE)
// digest represents the partial evaluation of a checksum.
type digest struct {
crc uint32
tab *Table
}
// New creates a new hash.Hash32 computing the CRC-32 checksum
// using the polynomial represented by the Table.
func New(tab *Table) hash.Hash32 { return &digest{0, tab} }
// NewIEEE creates a new hash.Hash32 computing the CRC-32 checksum
// using the IEEE polynomial.
func NewIEEE() hash.Hash32 { return New(IEEETable) }
func (d *digest) Size() int { return Size }
func (d *digest) Reset() { d.crc = 0 }
func update(crc uint32, tab *Table, p []byte) uint32 {
crc = ^crc
for _, v := range p {
crc = tab[byte(crc)^v] ^ (crc >> 8)
}
return ^crc
}
// Update returns the result of adding the bytes in p to the crc.
func Update(crc uint32, tab *Table, p []byte) uint32 {
return update(crc, tab, p)
}
func (d *digest) Write(p []byte) (n int, err os.Error) {
d.crc = update(d.crc, d.tab, p)
return len(p), nil
}
func (d *digest) Sum32() uint32 { return d.crc }
func (d *digest) Sum() []byte {
p := make([]byte, 4)
s := d.Sum32()
p[0] = byte(s >> 24)
p[1] = byte(s >> 16)
p[2] = byte(s >> 8)
p[3] = byte(s)
return p
}
// Checksum returns the CRC-32 checksum of data
// using the polynomial represented by the Table.
func Checksum(data []byte, tab *Table) uint32 { return update(0, tab, data) }
// ChecksumIEEE returns the CRC-32 checksum of data
// using the IEEE polynomial.
func ChecksumIEEE(data []byte) uint32 { return update(0, IEEETable, data) }
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