Source file src/pkg/hash/crc32/crc32.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 crc32 implements the 32-bit cyclic redundancy check, or CRC-32, 6 // checksum. See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for 7 // information. 8 package crc32 9 10 import ( 11 "hash" 12 "sync" 13 ) 14 15 // The size of a CRC-32 checksum in bytes. 16 const Size = 4 17 18 // Predefined polynomials. 19 const ( 20 // Far and away the most common CRC-32 polynomial. 21 // Used by ethernet (IEEE 802.3), v.42, fddi, gzip, zip, png, mpeg-2, ... 22 IEEE = 0xedb88320 23 24 // Castagnoli's polynomial, used in iSCSI. 25 // Has better error detection characteristics than IEEE. 26 // http://dx.doi.org/10.1109/26.231911 27 Castagnoli = 0x82f63b78 28 29 // Koopman's polynomial. 30 // Also has better error detection characteristics than IEEE. 31 // http://dx.doi.org/10.1109/DSN.2002.1028931 32 Koopman = 0xeb31d82e 33 ) 34 35 // Table is a 256-word table representing the polynomial for efficient processing. 36 type Table [256]uint32 37 38 // castagnoliTable points to a lazily initialized Table for the Castagnoli 39 // polynomial. MakeTable will always return this value when asked to make a 40 // Castagnoli table so we can compare against it to find when the caller is 41 // using this polynomial. 42 var castagnoliTable *Table 43 var castagnoliOnce sync.Once 44 45 func castagnoliInit() { 46 castagnoliTable = makeTable(Castagnoli) 47 } 48 49 // IEEETable is the table for the IEEE polynomial. 50 var IEEETable = makeTable(IEEE) 51 52 // MakeTable returns the Table constructed from the specified polynomial. 53 func MakeTable(poly uint32) *Table { 54 switch poly { 55 case IEEE: 56 return IEEETable 57 case Castagnoli: 58 castagnoliOnce.Do(castagnoliInit) 59 return castagnoliTable 60 } 61 return makeTable(poly) 62 } 63 64 // makeTable returns the Table constructed from the specified polynomial. 65 func makeTable(poly uint32) *Table { 66 t := new(Table) 67 for i := 0; i < 256; i++ { 68 crc := uint32(i) 69 for j := 0; j < 8; j++ { 70 if crc&1 == 1 { 71 crc = (crc >> 1) ^ poly 72 } else { 73 crc >>= 1 74 } 75 } 76 t[i] = crc 77 } 78 return t 79 } 80 81 // digest represents the partial evaluation of a checksum. 82 type digest struct { 83 crc uint32 84 tab *Table 85 } 86 87 // New creates a new hash.Hash32 computing the CRC-32 checksum 88 // using the polynomial represented by the Table. 89 func New(tab *Table) hash.Hash32 { return &digest{0, tab} } 90 91 // NewIEEE creates a new hash.Hash32 computing the CRC-32 checksum 92 // using the IEEE polynomial. 93 func NewIEEE() hash.Hash32 { return New(IEEETable) } 94 95 func (d *digest) Size() int { return Size } 96 97 func (d *digest) BlockSize() int { return 1 } 98 99 func (d *digest) Reset() { d.crc = 0 } 100 101 func update(crc uint32, tab *Table, p []byte) uint32 { 102 crc = ^crc 103 for _, v := range p { 104 crc = tab[byte(crc)^v] ^ (crc >> 8) 105 } 106 return ^crc 107 } 108 109 // Update returns the result of adding the bytes in p to the crc. 110 func Update(crc uint32, tab *Table, p []byte) uint32 { 111 if tab == castagnoliTable { 112 return updateCastagnoli(crc, p) 113 } 114 return update(crc, tab, p) 115 } 116 117 func (d *digest) Write(p []byte) (n int, err error) { 118 d.crc = Update(d.crc, d.tab, p) 119 return len(p), nil 120 } 121 122 func (d *digest) Sum32() uint32 { return d.crc } 123 124 func (d *digest) Sum(in []byte) []byte { 125 s := d.Sum32() 126 in = append(in, byte(s>>24)) 127 in = append(in, byte(s>>16)) 128 in = append(in, byte(s>>8)) 129 in = append(in, byte(s)) 130 return in 131 } 132 133 // Checksum returns the CRC-32 checksum of data 134 // using the polynomial represented by the Table. 135 func Checksum(data []byte, tab *Table) uint32 { return Update(0, tab, data) } 136 137 // ChecksumIEEE returns the CRC-32 checksum of data 138 // using the IEEE polynomial. 139 func ChecksumIEEE(data []byte) uint32 { return update(0, IEEETable, data) }