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Source file src/hash/crc32/crc32.go

Documentation: hash/crc32

     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 crc32 implements the 32-bit cyclic redundancy check, or CRC-32,
     6  // checksum. See https://en.wikipedia.org/wiki/Cyclic_redundancy_check for
     7  // information.
     8  //
     9  // Polynomials are represented in LSB-first form also known as reversed representation.
    10  //
    11  // See https://en.wikipedia.org/wiki/Mathematics_of_cyclic_redundancy_checks#Reversed_representations_and_reciprocal_polynomials
    12  // for information.
    13  package crc32
    14  
    15  import (
    16  	"errors"
    17  	"hash"
    18  	"sync"
    19  )
    20  
    21  // The size of a CRC-32 checksum in bytes.
    22  const Size = 4
    23  
    24  // Predefined polynomials.
    25  const (
    26  	// IEEE is by far and away the most common CRC-32 polynomial.
    27  	// Used by ethernet (IEEE 802.3), v.42, fddi, gzip, zip, png, ...
    28  	IEEE = 0xedb88320
    29  
    30  	// Castagnoli's polynomial, used in iSCSI.
    31  	// Has better error detection characteristics than IEEE.
    32  	// https://dx.doi.org/10.1109/26.231911
    33  	Castagnoli = 0x82f63b78
    34  
    35  	// Koopman's polynomial.
    36  	// Also has better error detection characteristics than IEEE.
    37  	// https://dx.doi.org/10.1109/DSN.2002.1028931
    38  	Koopman = 0xeb31d82e
    39  )
    40  
    41  // Table is a 256-word table representing the polynomial for efficient processing.
    42  type Table [256]uint32
    43  
    44  // This file makes use of functions implemented in architecture-specific files.
    45  // The interface that they implement is as follows:
    46  //
    47  //    // archAvailableIEEE reports whether an architecture-specific CRC32-IEEE
    48  //    // algorithm is available.
    49  //    archAvailableIEEE() bool
    50  //
    51  //    // archInitIEEE initializes the architecture-specific CRC3-IEEE algorithm.
    52  //    // It can only be called if archAvailableIEEE() returns true.
    53  //    archInitIEEE()
    54  //
    55  //    // archUpdateIEEE updates the given CRC32-IEEE. It can only be called if
    56  //    // archInitIEEE() was previously called.
    57  //    archUpdateIEEE(crc uint32, p []byte) uint32
    58  //
    59  //    // archAvailableCastagnoli reports whether an architecture-specific
    60  //    // CRC32-C algorithm is available.
    61  //    archAvailableCastagnoli() bool
    62  //
    63  //    // archInitCastagnoli initializes the architecture-specific CRC32-C
    64  //    // algorithm. It can only be called if archAvailableCastagnoli() returns
    65  //    // true.
    66  //    archInitCastagnoli()
    67  //
    68  //    // archUpdateCastagnoli updates the given CRC32-C. It can only be called
    69  //    // if archInitCastagnoli() was previously called.
    70  //    archUpdateCastagnoli(crc uint32, p []byte) uint32
    71  
    72  // castagnoliTable points to a lazily initialized Table for the Castagnoli
    73  // polynomial. MakeTable will always return this value when asked to make a
    74  // Castagnoli table so we can compare against it to find when the caller is
    75  // using this polynomial.
    76  var castagnoliTable *Table
    77  var castagnoliTable8 *slicing8Table
    78  var castagnoliArchImpl bool
    79  var updateCastagnoli func(crc uint32, p []byte) uint32
    80  var castagnoliOnce sync.Once
    81  
    82  func castagnoliInit() {
    83  	castagnoliTable = simpleMakeTable(Castagnoli)
    84  	castagnoliArchImpl = archAvailableCastagnoli()
    85  
    86  	if castagnoliArchImpl {
    87  		archInitCastagnoli()
    88  		updateCastagnoli = archUpdateCastagnoli
    89  	} else {
    90  		// Initialize the slicing-by-8 table.
    91  		castagnoliTable8 = slicingMakeTable(Castagnoli)
    92  		updateCastagnoli = func(crc uint32, p []byte) uint32 {
    93  			return slicingUpdate(crc, castagnoliTable8, p)
    94  		}
    95  	}
    96  }
    97  
    98  // IEEETable is the table for the IEEE polynomial.
    99  var IEEETable = simpleMakeTable(IEEE)
   100  
   101  // ieeeTable8 is the slicing8Table for IEEE
   102  var ieeeTable8 *slicing8Table
   103  var ieeeArchImpl bool
   104  var updateIEEE func(crc uint32, p []byte) uint32
   105  var ieeeOnce sync.Once
   106  
   107  func ieeeInit() {
   108  	ieeeArchImpl = archAvailableIEEE()
   109  
   110  	if ieeeArchImpl {
   111  		archInitIEEE()
   112  		updateIEEE = archUpdateIEEE
   113  	} else {
   114  		// Initialize the slicing-by-8 table.
   115  		ieeeTable8 = slicingMakeTable(IEEE)
   116  		updateIEEE = func(crc uint32, p []byte) uint32 {
   117  			return slicingUpdate(crc, ieeeTable8, p)
   118  		}
   119  	}
   120  }
   121  
   122  // MakeTable returns a Table constructed from the specified polynomial.
   123  // The contents of this Table must not be modified.
   124  func MakeTable(poly uint32) *Table {
   125  	switch poly {
   126  	case IEEE:
   127  		ieeeOnce.Do(ieeeInit)
   128  		return IEEETable
   129  	case Castagnoli:
   130  		castagnoliOnce.Do(castagnoliInit)
   131  		return castagnoliTable
   132  	}
   133  	return simpleMakeTable(poly)
   134  }
   135  
   136  // digest represents the partial evaluation of a checksum.
   137  type digest struct {
   138  	crc uint32
   139  	tab *Table
   140  }
   141  
   142  // New creates a new hash.Hash32 computing the CRC-32 checksum using the
   143  // polynomial represented by the Table. Its Sum method will lay the
   144  // value out in big-endian byte order. The returned Hash32 also
   145  // implements encoding.BinaryMarshaler and encoding.BinaryUnmarshaler to
   146  // marshal and unmarshal the internal state of the hash.
   147  func New(tab *Table) hash.Hash32 {
   148  	if tab == IEEETable {
   149  		ieeeOnce.Do(ieeeInit)
   150  	}
   151  	return &digest{0, tab}
   152  }
   153  
   154  // NewIEEE creates a new hash.Hash32 computing the CRC-32 checksum using
   155  // the IEEE polynomial. Its Sum method will lay the value out in
   156  // big-endian byte order. The returned Hash32 also implements
   157  // encoding.BinaryMarshaler and encoding.BinaryUnmarshaler to marshal
   158  // and unmarshal the internal state of the hash.
   159  func NewIEEE() hash.Hash32 { return New(IEEETable) }
   160  
   161  func (d *digest) Size() int { return Size }
   162  
   163  func (d *digest) BlockSize() int { return 1 }
   164  
   165  func (d *digest) Reset() { d.crc = 0 }
   166  
   167  const (
   168  	magic         = "crc\x01"
   169  	marshaledSize = len(magic) + 4 + 4
   170  )
   171  
   172  func (d *digest) MarshalBinary() ([]byte, error) {
   173  	b := make([]byte, 0, marshaledSize)
   174  	b = append(b, magic...)
   175  	b = appendUint32(b, tableSum(d.tab))
   176  	b = appendUint32(b, d.crc)
   177  	return b, nil
   178  }
   179  
   180  func (d *digest) UnmarshalBinary(b []byte) error {
   181  	if len(b) < len(magic) || string(b[:len(magic)]) != magic {
   182  		return errors.New("hash/crc32: invalid hash state identifier")
   183  	}
   184  	if len(b) != marshaledSize {
   185  		return errors.New("hash/crc32: invalid hash state size")
   186  	}
   187  	if tableSum(d.tab) != readUint32(b[4:]) {
   188  		return errors.New("hash/crc32: tables do not match")
   189  	}
   190  	d.crc = readUint32(b[8:])
   191  	return nil
   192  }
   193  
   194  func appendUint32(b []byte, x uint32) []byte {
   195  	a := [4]byte{
   196  		byte(x >> 24),
   197  		byte(x >> 16),
   198  		byte(x >> 8),
   199  		byte(x),
   200  	}
   201  	return append(b, a[:]...)
   202  }
   203  
   204  func readUint32(b []byte) uint32 {
   205  	_ = b[3]
   206  	return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
   207  }
   208  
   209  // Update returns the result of adding the bytes in p to the crc.
   210  func Update(crc uint32, tab *Table, p []byte) uint32 {
   211  	switch tab {
   212  	case castagnoliTable:
   213  		return updateCastagnoli(crc, p)
   214  	case IEEETable:
   215  		// Unfortunately, because IEEETable is exported, IEEE may be used without a
   216  		// call to MakeTable. We have to make sure it gets initialized in that case.
   217  		ieeeOnce.Do(ieeeInit)
   218  		return updateIEEE(crc, p)
   219  	default:
   220  		return simpleUpdate(crc, tab, p)
   221  	}
   222  }
   223  
   224  func (d *digest) Write(p []byte) (n int, err error) {
   225  	switch d.tab {
   226  	case castagnoliTable:
   227  		d.crc = updateCastagnoli(d.crc, p)
   228  	case IEEETable:
   229  		// We only create digest objects through New() which takes care of
   230  		// initialization in this case.
   231  		d.crc = updateIEEE(d.crc, p)
   232  	default:
   233  		d.crc = simpleUpdate(d.crc, d.tab, p)
   234  	}
   235  	return len(p), nil
   236  }
   237  
   238  func (d *digest) Sum32() uint32 { return d.crc }
   239  
   240  func (d *digest) Sum(in []byte) []byte {
   241  	s := d.Sum32()
   242  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
   243  }
   244  
   245  // Checksum returns the CRC-32 checksum of data
   246  // using the polynomial represented by the Table.
   247  func Checksum(data []byte, tab *Table) uint32 { return Update(0, tab, data) }
   248  
   249  // ChecksumIEEE returns the CRC-32 checksum of data
   250  // using the IEEE polynomial.
   251  func ChecksumIEEE(data []byte) uint32 {
   252  	ieeeOnce.Do(ieeeInit)
   253  	return updateIEEE(0, data)
   254  }
   255  
   256  // tableSum returns the IEEE checksum of table t.
   257  func tableSum(t *Table) uint32 {
   258  	var a [1024]byte
   259  	b := a[:0]
   260  	if t != nil {
   261  		for _, x := range t {
   262  			b = appendUint32(b, x)
   263  		}
   264  	}
   265  	return ChecksumIEEE(b)
   266  }
   267  

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