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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.
// This package implements the 64-bit cyclic redundancy check, or CRC-64, checksum.
// See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for information.
package crc64
import (
"hash"
"os"
)
// The size of a CRC-64 checksum in bytes.
const Size = 8
// Predefined polynomials.
const (
// The ISO polynomial, defined in ISO 3309 and used in HDLC.
ISO = 0xD800000000000000
// The ECMA polynomial, defined in ECMA 182.
ECMA = 0xC96C5795D7870F42
)
// Table is a 256-word table representing the polynomial for efficient processing.
type Table [256]uint64
// MakeTable returns the Table constructed from the specified polynomial.
func MakeTable(poly uint64) *Table {
t := new(Table)
for i := 0; i < 256; i++ {
crc := uint64(i)
for j := 0; j < 8; j++ {
if crc&1 == 1 {
crc = (crc >> 1) ^ poly
} else {
crc >>= 1
}
}
t[i] = crc
}
return t
}
// digest represents the partial evaluation of a checksum.
type digest struct {
crc uint64
tab *Table
}
// New creates a new hash.Hash64 computing the CRC-64 checksum
// using the polynomial represented by the Table.
func New(tab *Table) hash.Hash64 { return &digest{0, tab} }
func (d *digest) Size() int { return Size }
func (d *digest) Reset() { d.crc = 0 }
func update(crc uint64, tab *Table, p []byte) uint64 {
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 uint64, tab *Table, p []byte) uint64 {
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) Sum64() uint64 { return d.crc }
func (d *digest) Sum() []byte {
p := make([]byte, 8)
s := d.Sum64()
p[0] = byte(s >> 56)
p[1] = byte(s >> 48)
p[2] = byte(s >> 40)
p[3] = byte(s >> 32)
p[4] = byte(s >> 24)
p[5] = byte(s >> 16)
p[6] = byte(s >> 8)
p[7] = byte(s)
return p
}
// Checksum returns the CRC-64 checksum of data
// using the polynomial represented by the Table.
func Checksum(data []byte, tab *Table) uint64 { return update(0, tab, data) }
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