github.com/MangoDowner/go-gm@v0.0.0-20180818020936-8baa2bd4408c/src/hash/crc64/crc64.go (about)

     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  var (
    28  	slicing8TableISO  = makeSlicingBy8Table(makeTable(ISO))
    29  	slicing8TableECMA = makeSlicingBy8Table(makeTable(ECMA))
    30  )
    31  
    32  // MakeTable returns a Table constructed from the specified polynomial.
    33  // The contents of this Table must not be modified.
    34  func MakeTable(poly uint64) *Table {
    35  	switch poly {
    36  	case ISO:
    37  		return &slicing8TableISO[0]
    38  	case ECMA:
    39  		return &slicing8TableECMA[0]
    40  	default:
    41  		return makeTable(poly)
    42  	}
    43  }
    44  
    45  func makeTable(poly uint64) *Table {
    46  	t := new(Table)
    47  	for i := 0; i < 256; i++ {
    48  		crc := uint64(i)
    49  		for j := 0; j < 8; j++ {
    50  			if crc&1 == 1 {
    51  				crc = (crc >> 1) ^ poly
    52  			} else {
    53  				crc >>= 1
    54  			}
    55  		}
    56  		t[i] = crc
    57  	}
    58  	return t
    59  }
    60  
    61  func makeSlicingBy8Table(t *Table) *[8]Table {
    62  	var helperTable [8]Table
    63  	helperTable[0] = *t
    64  	for i := 0; i < 256; i++ {
    65  		crc := t[i]
    66  		for j := 1; j < 8; j++ {
    67  			crc = t[crc&0xff] ^ (crc >> 8)
    68  			helperTable[j][i] = crc
    69  		}
    70  	}
    71  	return &helperTable
    72  }
    73  
    74  // digest represents the partial evaluation of a checksum.
    75  type digest struct {
    76  	crc uint64
    77  	tab *Table
    78  }
    79  
    80  // New creates a new hash.Hash64 computing the CRC-64 checksum
    81  // using the polynomial represented by the Table.
    82  // Its Sum method will lay the value out in big-endian byte order.
    83  func New(tab *Table) hash.Hash64 { return &digest{0, tab} }
    84  
    85  func (d *digest) Size() int { return Size }
    86  
    87  func (d *digest) BlockSize() int { return 1 }
    88  
    89  func (d *digest) Reset() { d.crc = 0 }
    90  
    91  func update(crc uint64, tab *Table, p []byte) uint64 {
    92  	crc = ^crc
    93  	// Table comparison is somewhat expensive, so avoid it for small sizes
    94  	for len(p) >= 64 {
    95  		var helperTable *[8]Table
    96  		if *tab == slicing8TableECMA[0] {
    97  			helperTable = slicing8TableECMA
    98  		} else if *tab == slicing8TableISO[0] {
    99  			helperTable = slicing8TableISO
   100  			// For smaller sizes creating extended table takes too much time
   101  		} else if len(p) > 16384 {
   102  			helperTable = makeSlicingBy8Table(tab)
   103  		} else {
   104  			break
   105  		}
   106  		// Update using slicing-by-8
   107  		for len(p) > 8 {
   108  			crc ^= uint64(p[0]) | uint64(p[1])<<8 | uint64(p[2])<<16 | uint64(p[3])<<24 |
   109  				uint64(p[4])<<32 | uint64(p[5])<<40 | uint64(p[6])<<48 | uint64(p[7])<<56
   110  			crc = helperTable[7][crc&0xff] ^
   111  				helperTable[6][(crc>>8)&0xff] ^
   112  				helperTable[5][(crc>>16)&0xff] ^
   113  				helperTable[4][(crc>>24)&0xff] ^
   114  				helperTable[3][(crc>>32)&0xff] ^
   115  				helperTable[2][(crc>>40)&0xff] ^
   116  				helperTable[1][(crc>>48)&0xff] ^
   117  				helperTable[0][crc>>56]
   118  			p = p[8:]
   119  		}
   120  	}
   121  	// For reminders or small sizes
   122  	for _, v := range p {
   123  		crc = tab[byte(crc)^v] ^ (crc >> 8)
   124  	}
   125  	return ^crc
   126  }
   127  
   128  // Update returns the result of adding the bytes in p to the crc.
   129  func Update(crc uint64, tab *Table, p []byte) uint64 {
   130  	return update(crc, tab, p)
   131  }
   132  
   133  func (d *digest) Write(p []byte) (n int, err error) {
   134  	d.crc = update(d.crc, d.tab, p)
   135  	return len(p), nil
   136  }
   137  
   138  func (d *digest) Sum64() uint64 { return d.crc }
   139  
   140  func (d *digest) Sum(in []byte) []byte {
   141  	s := d.Sum64()
   142  	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))
   143  }
   144  
   145  // Checksum returns the CRC-64 checksum of data
   146  // using the polynomial represented by the Table.
   147  func Checksum(data []byte, tab *Table) uint64 { return update(0, tab, data) }