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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 crc64 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"

// 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) BlockSize() int { return 1 }

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 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(in []byte) []byte {

        s := d.Sum64()

        in = append(in, byte(s>>56))

        in = append(in, byte(s>>48))

        in = append(in, byte(s>>40))

        in = append(in, byte(s>>32))

        in = append(in, byte(s>>24))

        in = append(in, byte(s>>16))

        in = append(in, byte(s>>8))

        in = append(in, byte(s))

        return in

}

// 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) }