...
Run Format

Source file src/hash/crc64/crc64.go

Documentation: hash/crc64

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Package crc64 implements the 64-bit cyclic redundancy check, or CRC-64,
  // checksum. See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for
  // information.
  package crc64
  
  import "hash"
  
  // The size of a CRC-64 checksum in bytes.
  const Size = 8
  
  // Predefined polynomials.
  const (
  	// The ISO polynomial, defined in ISO 3309 and used in HDLC.
  	ISO = 0xD800000000000000
  
  	// The ECMA polynomial, defined in ECMA 182.
  	ECMA = 0xC96C5795D7870F42
  )
  
  // Table is a 256-word table representing the polynomial for efficient processing.
  type Table [256]uint64
  
  var (
  	slicing8TableISO  = makeSlicingBy8Table(makeTable(ISO))
  	slicing8TableECMA = makeSlicingBy8Table(makeTable(ECMA))
  )
  
  // MakeTable returns a Table constructed from the specified polynomial.
  // The contents of this Table must not be modified.
  func MakeTable(poly uint64) *Table {
  	switch poly {
  	case ISO:
  		return &slicing8TableISO[0]
  	case ECMA:
  		return &slicing8TableECMA[0]
  	default:
  		return makeTable(poly)
  	}
  }
  
  func makeTable(poly uint64) *Table {
  	t := new(Table)
  	for i := 0; i < 256; i++ {
  		crc := uint64(i)
  		for j := 0; j < 8; j++ {
  			if crc&1 == 1 {
  				crc = (crc >> 1) ^ poly
  			} else {
  				crc >>= 1
  			}
  		}
  		t[i] = crc
  	}
  	return t
  }
  
  func makeSlicingBy8Table(t *Table) *[8]Table {
  	var helperTable [8]Table
  	helperTable[0] = *t
  	for i := 0; i < 256; i++ {
  		crc := t[i]
  		for j := 1; j < 8; j++ {
  			crc = t[crc&0xff] ^ (crc >> 8)
  			helperTable[j][i] = crc
  		}
  	}
  	return &helperTable
  }
  
  // digest represents the partial evaluation of a checksum.
  type digest struct {
  	crc uint64
  	tab *Table
  }
  
  // New creates a new hash.Hash64 computing the CRC-64 checksum
  // using the polynomial represented by the Table.
  // Its Sum method will lay the value out in big-endian byte order.
  func New(tab *Table) hash.Hash64 { return &digest{0, tab} }
  
  func (d *digest) Size() int { return Size }
  
  func (d *digest) BlockSize() int { return 1 }
  
  func (d *digest) Reset() { d.crc = 0 }
  
  func update(crc uint64, tab *Table, p []byte) uint64 {
  	crc = ^crc
  	// Table comparison is somewhat expensive, so avoid it for small sizes
  	for len(p) >= 64 {
  		var helperTable *[8]Table
  		if *tab == slicing8TableECMA[0] {
  			helperTable = slicing8TableECMA
  		} else if *tab == slicing8TableISO[0] {
  			helperTable = slicing8TableISO
  			// For smaller sizes creating extended table takes too much time
  		} else if len(p) > 16384 {
  			helperTable = makeSlicingBy8Table(tab)
  		} else {
  			break
  		}
  		// Update using slicing-by-8
  		for len(p) > 8 {
  			crc ^= uint64(p[0]) | uint64(p[1])<<8 | uint64(p[2])<<16 | uint64(p[3])<<24 |
  				uint64(p[4])<<32 | uint64(p[5])<<40 | uint64(p[6])<<48 | uint64(p[7])<<56
  			crc = helperTable[7][crc&0xff] ^
  				helperTable[6][(crc>>8)&0xff] ^
  				helperTable[5][(crc>>16)&0xff] ^
  				helperTable[4][(crc>>24)&0xff] ^
  				helperTable[3][(crc>>32)&0xff] ^
  				helperTable[2][(crc>>40)&0xff] ^
  				helperTable[1][(crc>>48)&0xff] ^
  				helperTable[0][crc>>56]
  			p = p[8:]
  		}
  	}
  	// For reminders or small sizes
  	for _, v := range p {
  		crc = tab[byte(crc)^v] ^ (crc >> 8)
  	}
  	return ^crc
  }
  
  // Update returns the result of adding the bytes in p to the crc.
  func Update(crc uint64, tab *Table, p []byte) uint64 {
  	return update(crc, tab, p)
  }
  
  func (d *digest) Write(p []byte) (n int, err error) {
  	d.crc = update(d.crc, d.tab, p)
  	return len(p), nil
  }
  
  func (d *digest) Sum64() uint64 { return d.crc }
  
  func (d *digest) Sum(in []byte) []byte {
  	s := d.Sum64()
  	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))
  }
  
  // Checksum returns the CRC-64 checksum of data
  // using the polynomial represented by the Table.
  func Checksum(data []byte, tab *Table) uint64 { return update(0, tab, data) }
  

View as plain text