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