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Source file src/hash/crc32/crc32_generic.go

Documentation: hash/crc32

     1  // Copyright 2011 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  // This file contains CRC32 algorithms that are not specific to any architecture
     6  // and don't use hardware acceleration.
     7  //
     8  // The simple (and slow) CRC32 implementation only uses a 256*4 bytes table.
     9  //
    10  // The slicing-by-8 algorithm is a faster implementation that uses a bigger
    11  // table (8*256*4 bytes).
    12  
    13  package crc32
    14  
    15  // simpleMakeTable allocates and constructs a Table for the specified
    16  // polynomial. The table is suitable for use with the simple algorithm
    17  // (simpleUpdate).
    18  func simpleMakeTable(poly uint32) *Table {
    19  	t := new(Table)
    20  	simplePopulateTable(poly, t)
    21  	return t
    22  }
    23  
    24  // simplePopulateTable constructs a Table for the specified polynomial, suitable
    25  // for use with simpleUpdate.
    26  func simplePopulateTable(poly uint32, t *Table) {
    27  	for i := 0; i < 256; i++ {
    28  		crc := uint32(i)
    29  		for j := 0; j < 8; j++ {
    30  			if crc&1 == 1 {
    31  				crc = (crc >> 1) ^ poly
    32  			} else {
    33  				crc >>= 1
    34  			}
    35  		}
    36  		t[i] = crc
    37  	}
    38  }
    39  
    40  // simpleUpdate uses the simple algorithm to update the CRC, given a table that
    41  // was previously computed using simpleMakeTable.
    42  func simpleUpdate(crc uint32, tab *Table, p []byte) uint32 {
    43  	crc = ^crc
    44  	for _, v := range p {
    45  		crc = tab[byte(crc)^v] ^ (crc >> 8)
    46  	}
    47  	return ^crc
    48  }
    49  
    50  // Use slicing-by-8 when payload >= this value.
    51  const slicing8Cutoff = 16
    52  
    53  // slicing8Table is array of 8 Tables, used by the slicing-by-8 algorithm.
    54  type slicing8Table [8]Table
    55  
    56  // slicingMakeTable constructs a slicing8Table for the specified polynomial. The
    57  // table is suitable for use with the slicing-by-8 algorithm (slicingUpdate).
    58  func slicingMakeTable(poly uint32) *slicing8Table {
    59  	t := new(slicing8Table)
    60  	simplePopulateTable(poly, &t[0])
    61  	for i := 0; i < 256; i++ {
    62  		crc := t[0][i]
    63  		for j := 1; j < 8; j++ {
    64  			crc = t[0][crc&0xFF] ^ (crc >> 8)
    65  			t[j][i] = crc
    66  		}
    67  	}
    68  	return t
    69  }
    70  
    71  // slicingUpdate uses the slicing-by-8 algorithm to update the CRC, given a
    72  // table that was previously computed using slicingMakeTable.
    73  func slicingUpdate(crc uint32, tab *slicing8Table, p []byte) uint32 {
    74  	if len(p) >= slicing8Cutoff {
    75  		crc = ^crc
    76  		for len(p) > 8 {
    77  			crc ^= uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
    78  			crc = tab[0][p[7]] ^ tab[1][p[6]] ^ tab[2][p[5]] ^ tab[3][p[4]] ^
    79  				tab[4][crc>>24] ^ tab[5][(crc>>16)&0xFF] ^
    80  				tab[6][(crc>>8)&0xFF] ^ tab[7][crc&0xFF]
    81  			p = p[8:]
    82  		}
    83  		crc = ^crc
    84  	}
    85  	if len(p) == 0 {
    86  		return crc
    87  	}
    88  	return simpleUpdate(crc, &tab[0], p)
    89  }
    90  

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