libgo/go/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 // This package implements the 64-bit cyclic redundancy check, or CRC-64, checksum.
   6 // See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for information.
   7 package crc64
   8
   9 import (
  10         "hash"
  11         "os"
  12 )
  13
  14 // The size of a CRC-64 checksum in bytes.
  15 const Size = 8
  16
  17 // Predefined polynomials.
  18 const (
  19         // The ISO polynomial, defined in ISO 3309 and used in HDLC.
  20         ISO = 0xD800000000000000
  21
  22         // The ECMA polynomial, defined in ECMA 182.
  23         ECMA = 0xC96C5795D7870F42
  24 )
  25
  26 // Table is a 256-word table representing the polynomial for efficient processing.
  27 type Table [256]uint64
  28
  29 // MakeTable returns the Table constructed from the specified polynomial.
  30 func MakeTable(poly uint64) *Table {
  31         t := new(Table)
  32         for i := 0; i < 256; i++ {
  33                 crc := uint64(i)
  34                 for j := 0; j < 8; j++ {
  35                         if crc&1 == 1 {
  36                                 crc = (crc >> 1) ^ poly
  37                         } else {
  38                                 crc >>= 1
  39                         }
  40                 }
  41                 t[i] = crc
  42         }
  43         return t
  44 }
  45
  46 // digest represents the partial evaluation of a checksum.
  47 type digest struct {
  48         crc uint64
  49         tab *Table
  50 }
  51
  52 // New creates a new hash.Hash64 computing the CRC-64 checksum
  53 // using the polynomial represented by the Table.
  54 func New(tab *Table) hash.Hash64 { return &digest{0, tab} }
  55
  56 func (d *digest) Size() int { return Size }
  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 os.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() []byte {
  81         p := make([]byte, 8)
  82         s := d.Sum64()
  83         p[0] = byte(s >> 54)
  84         p[1] = byte(s >> 48)
  85         p[2] = byte(s >> 40)
  86         p[3] = byte(s >> 32)
  87         p[4] = byte(s >> 24)
  88         p[5] = byte(s >> 16)
  89         p[6] = byte(s >> 8)
  90         p[7] = byte(s)
  91         return p
  92 }
  93
  94 // Checksum returns the CRC-64 checksum of data
  95 // using the polynomial represented by the Table.
  96 func Checksum(data []byte, tab *Table) uint64 { return update(0, tab, data) }