crc64.go

Documentation: hash/crc64

		 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 https://en.wikipedia.org/wiki/Cyclic_redundancy_check for
		 7  // information.
		 8  package crc64
		 9  
		10  import (
		11  	"errors"
		12  	"hash"
		13  	"sync"
		14  )
		15  
		16  // The size of a CRC-64 checksum in bytes.
		17  const Size = 8
		18  
		19  // Predefined polynomials.
		20  const (
		21  	// The ISO polynomial, defined in ISO 3309 and used in HDLC.
		22  	ISO = 0xD800000000000000
		23  
		24  	// The ECMA polynomial, defined in ECMA 182.
		25  	ECMA = 0xC96C5795D7870F42
		26  )
		27  
		28  // Table is a 256-word table representing the polynomial for efficient processing.
		29  type Table [256]uint64
		30  
		31  var (
		32  	slicing8TablesBuildOnce sync.Once
		33  	slicing8TableISO				*[8]Table
		34  	slicing8TableECMA			 *[8]Table
		35  )
		36  
		37  func buildSlicing8TablesOnce() {
		38  	slicing8TablesBuildOnce.Do(buildSlicing8Tables)
		39  }
		40  
		41  func buildSlicing8Tables() {
		42  	slicing8TableISO = makeSlicingBy8Table(makeTable(ISO))
		43  	slicing8TableECMA = makeSlicingBy8Table(makeTable(ECMA))
		44  }
		45  
		46  // MakeTable returns a Table constructed from the specified polynomial.
		47  // The contents of this Table must not be modified.
		48  func MakeTable(poly uint64) *Table {
		49  	buildSlicing8TablesOnce()
		50  	switch poly {
		51  	case ISO:
		52  		return &slicing8TableISO[0]
		53  	case ECMA:
		54  		return &slicing8TableECMA[0]
		55  	default:
		56  		return makeTable(poly)
		57  	}
		58  }
		59  
		60  func makeTable(poly uint64) *Table {
		61  	t := new(Table)
		62  	for i := 0; i < 256; i++ {
		63  		crc := uint64(i)
		64  		for j := 0; j < 8; j++ {
		65  			if crc&1 == 1 {
		66  				crc = (crc >> 1) ^ poly
		67  			} else {
		68  				crc >>= 1
		69  			}
		70  		}
		71  		t[i] = crc
		72  	}
		73  	return t
		74  }
		75  
		76  func makeSlicingBy8Table(t *Table) *[8]Table {
		77  	var helperTable [8]Table
		78  	helperTable[0] = *t
		79  	for i := 0; i < 256; i++ {
		80  		crc := t[i]
		81  		for j := 1; j < 8; j++ {
		82  			crc = t[crc&0xff] ^ (crc >> 8)
		83  			helperTable[j][i] = crc
		84  		}
		85  	}
		86  	return &helperTable
		87  }
		88  
		89  // digest represents the partial evaluation of a checksum.
		90  type digest struct {
		91  	crc uint64
		92  	tab *Table
		93  }
		94  
		95  // New creates a new hash.Hash64 computing the CRC-64 checksum using the
		96  // polynomial represented by the Table. Its Sum method will lay the
		97  // value out in big-endian byte order. The returned Hash64 also
		98  // implements encoding.BinaryMarshaler and encoding.BinaryUnmarshaler to
		99  // marshal and unmarshal the internal state of the hash.
	 100  func New(tab *Table) hash.Hash64 { return &digest{0, tab} }
	 101  
	 102  func (d *digest) Size() int { return Size }
	 103  
	 104  func (d *digest) BlockSize() int { return 1 }
	 105  
	 106  func (d *digest) Reset() { d.crc = 0 }
	 107  
	 108  const (
	 109  	magic				 = "crc\x02"
	 110  	marshaledSize = len(magic) + 8 + 8
	 111  )
	 112  
	 113  func (d *digest) MarshalBinary() ([]byte, error) {
	 114  	b := make([]byte, 0, marshaledSize)
	 115  	b = append(b, magic...)
	 116  	b = appendUint64(b, tableSum(d.tab))
	 117  	b = appendUint64(b, d.crc)
	 118  	return b, nil
	 119  }
	 120  
	 121  func (d *digest) UnmarshalBinary(b []byte) error {
	 122  	if len(b) < len(magic) || string(b[:len(magic)]) != magic {
	 123  		return errors.New("hash/crc64: invalid hash state identifier")
	 124  	}
	 125  	if len(b) != marshaledSize {
	 126  		return errors.New("hash/crc64: invalid hash state size")
	 127  	}
	 128  	if tableSum(d.tab) != readUint64(b[4:]) {
	 129  		return errors.New("hash/crc64: tables do not match")
	 130  	}
	 131  	d.crc = readUint64(b[12:])
	 132  	return nil
	 133  }
	 134  
	 135  func appendUint64(b []byte, x uint64) []byte {
	 136  	a := [8]byte{
	 137  		byte(x >> 56),
	 138  		byte(x >> 48),
	 139  		byte(x >> 40),
	 140  		byte(x >> 32),
	 141  		byte(x >> 24),
	 142  		byte(x >> 16),
	 143  		byte(x >> 8),
	 144  		byte(x),
	 145  	}
	 146  	return append(b, a[:]...)
	 147  }
	 148  
	 149  func readUint64(b []byte) uint64 {
	 150  	_ = b[7]
	 151  	return uint64(b[7]) | uint64(b[6])<<8 | uint64(b[5])<<16 | uint64(b[4])<<24 |
	 152  		uint64(b[3])<<32 | uint64(b[2])<<40 | uint64(b[1])<<48 | uint64(b[0])<<56
	 153  }
	 154  
	 155  func update(crc uint64, tab *Table, p []byte) uint64 {
	 156  	buildSlicing8TablesOnce()
	 157  	crc = ^crc
	 158  	// Table comparison is somewhat expensive, so avoid it for small sizes
	 159  	for len(p) >= 64 {
	 160  		var helperTable *[8]Table
	 161  		if *tab == slicing8TableECMA[0] {
	 162  			helperTable = slicing8TableECMA
	 163  		} else if *tab == slicing8TableISO[0] {
	 164  			helperTable = slicing8TableISO
	 165  			// For smaller sizes creating extended table takes too much time
	 166  		} else if len(p) > 16384 {
	 167  			helperTable = makeSlicingBy8Table(tab)
	 168  		} else {
	 169  			break
	 170  		}
	 171  		// Update using slicing-by-8
	 172  		for len(p) > 8 {
	 173  			crc ^= uint64(p[0]) | uint64(p[1])<<8 | uint64(p[2])<<16 | uint64(p[3])<<24 |
	 174  				uint64(p[4])<<32 | uint64(p[5])<<40 | uint64(p[6])<<48 | uint64(p[7])<<56
	 175  			crc = helperTable[7][crc&0xff] ^
	 176  				helperTable[6][(crc>>8)&0xff] ^
	 177  				helperTable[5][(crc>>16)&0xff] ^
	 178  				helperTable[4][(crc>>24)&0xff] ^
	 179  				helperTable[3][(crc>>32)&0xff] ^
	 180  				helperTable[2][(crc>>40)&0xff] ^
	 181  				helperTable[1][(crc>>48)&0xff] ^
	 182  				helperTable[0][crc>>56]
	 183  			p = p[8:]
	 184  		}
	 185  	}
	 186  	// For reminders or small sizes
	 187  	for _, v := range p {
	 188  		crc = tab[byte(crc)^v] ^ (crc >> 8)
	 189  	}
	 190  	return ^crc
	 191  }
	 192  
	 193  // Update returns the result of adding the bytes in p to the crc.
	 194  func Update(crc uint64, tab *Table, p []byte) uint64 {
	 195  	return update(crc, tab, p)
	 196  }
	 197  
	 198  func (d *digest) Write(p []byte) (n int, err error) {
	 199  	d.crc = update(d.crc, d.tab, p)
	 200  	return len(p), nil
	 201  }
	 202  
	 203  func (d *digest) Sum64() uint64 { return d.crc }
	 204  
	 205  func (d *digest) Sum(in []byte) []byte {
	 206  	s := d.Sum64()
	 207  	return append(in, byte(s>>56), byte(s>>48), byte(s>>40), byte(s>>32), byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
	 208  }
	 209  
	 210  // Checksum returns the CRC-64 checksum of data
	 211  // using the polynomial represented by the Table.
	 212  func Checksum(data []byte, tab *Table) uint64 { return update(0, tab, data) }
	 213  
	 214  // tableSum returns the ISO checksum of table t.
	 215  func tableSum(t *Table) uint64 {
	 216  	var a [2048]byte
	 217  	b := a[:0]
	 218  	if t != nil {
	 219  		for _, x := range t {
	 220  			b = appendUint64(b, x)
	 221  		}
	 222  	}
	 223  	return Checksum(b, MakeTable(ISO))
	 224  }
	 225
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