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 https://en.wikipedia.org/wiki/Cyclic_redundancy_check for
     7  // information.
     8  package crc64
     9  
    10  import (
    11  	"errors"
    12  	"hash"
    13  	"internal/byteorder"
    14  	"sync"
    15  )
    16  
    17  // The size of a CRC-64 checksum in bytes.
    18  const Size = 8
    19  
    20  // Predefined polynomials.
    21  const (
    22  	// The ISO polynomial, defined in ISO 3309 and used in HDLC.
    23  	ISO = 0xD800000000000000
    24  
    25  	// The ECMA polynomial, defined in ECMA 182.
    26  	ECMA = 0xC96C5795D7870F42
    27  )
    28  
    29  // Table is a 256-word table representing the polynomial for efficient processing.
    30  type Table [256]uint64
    31  
    32  var (
    33  	slicing8TablesBuildOnce sync.Once
    34  	slicing8TableISO        *[8]Table
    35  	slicing8TableECMA       *[8]Table
    36  )
    37  
    38  func buildSlicing8TablesOnce() {
    39  	slicing8TablesBuildOnce.Do(buildSlicing8Tables)
    40  }
    41  
    42  func buildSlicing8Tables() {
    43  	slicing8TableISO = makeSlicingBy8Table(makeTable(ISO))
    44  	slicing8TableECMA = makeSlicingBy8Table(makeTable(ECMA))
    45  }
    46  
    47  // MakeTable returns a [Table] constructed from the specified polynomial.
    48  // The contents of this [Table] must not be modified.
    49  func MakeTable(poly uint64) *Table {
    50  	buildSlicing8TablesOnce()
    51  	switch poly {
    52  	case ISO:
    53  		return &slicing8TableISO[0]
    54  	case ECMA:
    55  		return &slicing8TableECMA[0]
    56  	default:
    57  		return makeTable(poly)
    58  	}
    59  }
    60  
    61  func makeTable(poly uint64) *Table {
    62  	t := new(Table)
    63  	for i := 0; i < 256; i++ {
    64  		crc := uint64(i)
    65  		for j := 0; j < 8; j++ {
    66  			if crc&1 == 1 {
    67  				crc = (crc >> 1) ^ poly
    68  			} else {
    69  				crc >>= 1
    70  			}
    71  		}
    72  		t[i] = crc
    73  	}
    74  	return t
    75  }
    76  
    77  func makeSlicingBy8Table(t *Table) *[8]Table {
    78  	var helperTable [8]Table
    79  	helperTable[0] = *t
    80  	for i := 0; i < 256; i++ {
    81  		crc := t[i]
    82  		for j := 1; j < 8; j++ {
    83  			crc = t[crc&0xff] ^ (crc >> 8)
    84  			helperTable[j][i] = crc
    85  		}
    86  	}
    87  	return &helperTable
    88  }
    89  
    90  // digest represents the partial evaluation of a checksum.
    91  type digest struct {
    92  	crc uint64
    93  	tab *Table
    94  }
    95  
    96  // New creates a new hash.Hash64 computing the CRC-64 checksum using the
    97  // polynomial represented by the [Table]. Its Sum method will lay the
    98  // value out in big-endian byte order. The returned Hash64 also
    99  // implements [encoding.BinaryMarshaler] and [encoding.BinaryUnmarshaler] to
   100  // marshal and unmarshal the internal state of the hash.
   101  func New(tab *Table) hash.Hash64 { return &digest{0, tab} }
   102  
   103  func (d *digest) Size() int { return Size }
   104  
   105  func (d *digest) BlockSize() int { return 1 }
   106  
   107  func (d *digest) Reset() { d.crc = 0 }
   108  
   109  const (
   110  	magic         = "crc\x02"
   111  	marshaledSize = len(magic) + 8 + 8
   112  )
   113  
   114  func (d *digest) MarshalBinary() ([]byte, error) {
   115  	b := make([]byte, 0, marshaledSize)
   116  	b = append(b, magic...)
   117  	b = byteorder.BeAppendUint64(b, tableSum(d.tab))
   118  	b = byteorder.BeAppendUint64(b, d.crc)
   119  	return b, nil
   120  }
   121  
   122  func (d *digest) UnmarshalBinary(b []byte) error {
   123  	if len(b) < len(magic) || string(b[:len(magic)]) != magic {
   124  		return errors.New("hash/crc64: invalid hash state identifier")
   125  	}
   126  	if len(b) != marshaledSize {
   127  		return errors.New("hash/crc64: invalid hash state size")
   128  	}
   129  	if tableSum(d.tab) != byteorder.BeUint64(b[4:]) {
   130  		return errors.New("hash/crc64: tables do not match")
   131  	}
   132  	d.crc = byteorder.BeUint64(b[12:])
   133  	return nil
   134  }
   135  
   136  func update(crc uint64, tab *Table, p []byte) uint64 {
   137  	buildSlicing8TablesOnce()
   138  	crc = ^crc
   139  	// Table comparison is somewhat expensive, so avoid it for small sizes
   140  	for len(p) >= 64 {
   141  		var helperTable *[8]Table
   142  		if *tab == slicing8TableECMA[0] {
   143  			helperTable = slicing8TableECMA
   144  		} else if *tab == slicing8TableISO[0] {
   145  			helperTable = slicing8TableISO
   146  			// For smaller sizes creating extended table takes too much time
   147  		} else if len(p) >= 2048 {
   148  			// According to the tests between various x86 and arm CPUs, 2k is a reasonable
   149  			// threshold for now. This may change in the future.
   150  			helperTable = makeSlicingBy8Table(tab)
   151  		} else {
   152  			break
   153  		}
   154  		// Update using slicing-by-8
   155  		for len(p) > 8 {
   156  			crc ^= byteorder.LeUint64(p)
   157  			crc = helperTable[7][crc&0xff] ^
   158  				helperTable[6][(crc>>8)&0xff] ^
   159  				helperTable[5][(crc>>16)&0xff] ^
   160  				helperTable[4][(crc>>24)&0xff] ^
   161  				helperTable[3][(crc>>32)&0xff] ^
   162  				helperTable[2][(crc>>40)&0xff] ^
   163  				helperTable[1][(crc>>48)&0xff] ^
   164  				helperTable[0][crc>>56]
   165  			p = p[8:]
   166  		}
   167  	}
   168  	// For reminders or small sizes
   169  	for _, v := range p {
   170  		crc = tab[byte(crc)^v] ^ (crc >> 8)
   171  	}
   172  	return ^crc
   173  }
   174  
   175  // Update returns the result of adding the bytes in p to the crc.
   176  func Update(crc uint64, tab *Table, p []byte) uint64 {
   177  	return update(crc, tab, p)
   178  }
   179  
   180  func (d *digest) Write(p []byte) (n int, err error) {
   181  	d.crc = update(d.crc, d.tab, p)
   182  	return len(p), nil
   183  }
   184  
   185  func (d *digest) Sum64() uint64 { return d.crc }
   186  
   187  func (d *digest) Sum(in []byte) []byte {
   188  	s := d.Sum64()
   189  	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))
   190  }
   191  
   192  // Checksum returns the CRC-64 checksum of data
   193  // using the polynomial represented by the [Table].
   194  func Checksum(data []byte, tab *Table) uint64 { return update(0, tab, data) }
   195  
   196  // tableSum returns the ISO checksum of table t.
   197  func tableSum(t *Table) uint64 {
   198  	var a [2048]byte
   199  	b := a[:0]
   200  	if t != nil {
   201  		for _, x := range t {
   202  			b = byteorder.BeAppendUint64(b, x)
   203  		}
   204  	}
   205  	return Checksum(b, MakeTable(ISO))
   206  }
   207
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