Source file src/pkg/hash/crc64/crc64.go

     1	// Copyright 2009 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// Package crc64 implements the 64-bit cyclic redundancy check, or CRC-64,
     6	// checksum. See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for
     7	// information.
     8	package crc64
     9	
    10	import "hash"
    11	
    12	// The size of a CRC-64 checksum in bytes.
    13	const Size = 8
    14	
    15	// Predefined polynomials.
    16	const (
    17		// The ISO polynomial, defined in ISO 3309 and used in HDLC.
    18		ISO = 0xD800000000000000
    19	
    20		// The ECMA polynomial, defined in ECMA 182.
    21		ECMA = 0xC96C5795D7870F42
    22	)
    23	
    24	// Table is a 256-word table representing the polynomial for efficient processing.
    25	type Table [256]uint64
    26	
    27	// MakeTable returns the Table constructed from the specified polynomial.
    28	func MakeTable(poly uint64) *Table {
    29		t := new(Table)
    30		for i := 0; i < 256; i++ {
    31			crc := uint64(i)
    32			for j := 0; j < 8; j++ {
    33				if crc&1 == 1 {
    34					crc = (crc >> 1) ^ poly
    35				} else {
    36					crc >>= 1
    37				}
    38			}
    39			t[i] = crc
    40		}
    41		return t
    42	}
    43	
    44	// digest represents the partial evaluation of a checksum.
    45	type digest struct {
    46		crc uint64
    47		tab *Table
    48	}
    49	
    50	// New creates a new hash.Hash64 computing the CRC-64 checksum
    51	// using the polynomial represented by the Table.
    52	func New(tab *Table) hash.Hash64 { return &digest{0, tab} }
    53	
    54	func (d *digest) Size() int { return Size }
    55	
    56	func (d *digest) BlockSize() int { return 1 }
    57	
    58	func (d *digest) Reset() { d.crc = 0 }
    59	
    60	func update(crc uint64, tab *Table, p []byte) uint64 {
    61		crc = ^crc
    62		for _, v := range p {
    63			crc = tab[byte(crc)^v] ^ (crc >> 8)
    64		}
    65		return ^crc
    66	}
    67	
    68	// Update returns the result of adding the bytes in p to the crc.
    69	func Update(crc uint64, tab *Table, p []byte) uint64 {
    70		return update(crc, tab, p)
    71	}
    72	
    73	func (d *digest) Write(p []byte) (n int, err error) {
    74		d.crc = update(d.crc, d.tab, p)
    75		return len(p), nil
    76	}
    77	
    78	func (d *digest) Sum64() uint64 { return d.crc }
    79	
    80	func (d *digest) Sum(in []byte) []byte {
    81		s := d.Sum64()
    82		in = append(in, byte(s>>56))
    83		in = append(in, byte(s>>48))
    84		in = append(in, byte(s>>40))
    85		in = append(in, byte(s>>32))
    86		in = append(in, byte(s>>24))
    87		in = append(in, byte(s>>16))
    88		in = append(in, byte(s>>8))
    89		in = append(in, byte(s))
    90		return in
    91	}
    92	
    93	// Checksum returns the CRC-64 checksum of data
    94	// using the polynomial represented by the Table.
    95	func Checksum(data []byte, tab *Table) uint64 { return update(0, tab, data) }