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Source file src/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	// 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) }
   148	

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