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  	"internal/byteorder"
    19  	"sync"
    20  	"sync/atomic"
    21  )
    22  
    23  // The size of a CRC-32 checksum in bytes.
    24  const Size = 4
    25  
    26  // Predefined polynomials.
    27  const (
    28  	// IEEE is by far and away the most common CRC-32 polynomial.
    29  	// Used by ethernet (IEEE 802.3), v.42, fddi, gzip, zip, png, ...
    30  	IEEE = 0xedb88320
    31  
    32  	// Castagnoli's polynomial, used in iSCSI.
    33  	// Has better error detection characteristics than IEEE.
    34  	// https://dx.doi.org/10.1109/26.231911
    35  	Castagnoli = 0x82f63b78
    36  
    37  	// Koopman's polynomial.
    38  	// Also has better error detection characteristics than IEEE.
    39  	// https://dx.doi.org/10.1109/DSN.2002.1028931
    40  	Koopman = 0xeb31d82e
    41  )
    42  
    43  // Table is a 256-word table representing the polynomial for efficient processing.
    44  type Table [256]uint32
    45  
    46  // This file makes use of functions implemented in architecture-specific files.
    47  // The interface that they implement is as follows:
    48  //
    49  //    // archAvailableIEEE reports whether an architecture-specific CRC32-IEEE
    50  //    // algorithm is available.
    51  //    archAvailableIEEE() bool
    52  //
    53  //    // archInitIEEE initializes the architecture-specific CRC3-IEEE algorithm.
    54  //    // It can only be called if archAvailableIEEE() returns true.
    55  //    archInitIEEE()
    56  //
    57  //    // archUpdateIEEE updates the given CRC32-IEEE. It can only be called if
    58  //    // archInitIEEE() was previously called.
    59  //    archUpdateIEEE(crc uint32, p []byte) uint32
    60  //
    61  //    // archAvailableCastagnoli reports whether an architecture-specific
    62  //    // CRC32-C algorithm is available.
    63  //    archAvailableCastagnoli() bool
    64  //
    65  //    // archInitCastagnoli initializes the architecture-specific CRC32-C
    66  //    // algorithm. It can only be called if archAvailableCastagnoli() returns
    67  //    // true.
    68  //    archInitCastagnoli()
    69  //
    70  //    // archUpdateCastagnoli updates the given CRC32-C. It can only be called
    71  //    // if archInitCastagnoli() was previously called.
    72  //    archUpdateCastagnoli(crc uint32, p []byte) uint32
    73  
    74  // castagnoliTable points to a lazily initialized Table for the Castagnoli
    75  // polynomial. MakeTable will always return this value when asked to make a
    76  // Castagnoli table so we can compare against it to find when the caller is
    77  // using this polynomial.
    78  var castagnoliTable *Table
    79  var castagnoliTable8 *slicing8Table
    80  var updateCastagnoli func(crc uint32, p []byte) uint32
    81  var castagnoliOnce sync.Once
    82  var haveCastagnoli atomic.Bool
    83  
    84  func castagnoliInit() {
    85  	castagnoliTable = simpleMakeTable(Castagnoli)
    86  
    87  	if archAvailableCastagnoli() {
    88  		archInitCastagnoli()
    89  		updateCastagnoli = archUpdateCastagnoli
    90  	} else {
    91  		// Initialize the slicing-by-8 table.
    92  		castagnoliTable8 = slicingMakeTable(Castagnoli)
    93  		updateCastagnoli = func(crc uint32, p []byte) uint32 {
    94  			return slicingUpdate(crc, castagnoliTable8, p)
    95  		}
    96  	}
    97  
    98  	haveCastagnoli.Store(true)
    99  }
   100  
   101  // IEEETable is the table for the [IEEE] polynomial.
   102  var IEEETable = simpleMakeTable(IEEE)
   103  
   104  // ieeeTable8 is the slicing8Table for IEEE
   105  var ieeeTable8 *slicing8Table
   106  var updateIEEE func(crc uint32, p []byte) uint32
   107  var ieeeOnce sync.Once
   108  
   109  func ieeeInit() {
   110  	if archAvailableIEEE() {
   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  	default:
   133  		return simpleMakeTable(poly)
   134  	}
   135  }
   136  
   137  // digest represents the partial evaluation of a checksum.
   138  type digest struct {
   139  	crc uint32
   140  	tab *Table
   141  }
   142  
   143  // New creates a new [hash.Hash32] computing the CRC-32 checksum using the
   144  // polynomial represented by the [Table]. Its Sum method will lay the
   145  // value out in big-endian byte order. The returned Hash32 also
   146  // implements [encoding.BinaryMarshaler] and [encoding.BinaryUnmarshaler] to
   147  // marshal and unmarshal the internal state of the hash.
   148  func New(tab *Table) hash.Hash32 {
   149  	if tab == IEEETable {
   150  		ieeeOnce.Do(ieeeInit)
   151  	}
   152  	return &digest{0, tab}
   153  }
   154  
   155  // NewIEEE creates a new [hash.Hash32] computing the CRC-32 checksum using
   156  // the [IEEE] polynomial. Its Sum method will lay the value out in
   157  // big-endian byte order. The returned Hash32 also implements
   158  // [encoding.BinaryMarshaler] and [encoding.BinaryUnmarshaler] to marshal
   159  // and unmarshal the internal state of the hash.
   160  func NewIEEE() hash.Hash32 { return New(IEEETable) }
   161  
   162  func (d *digest) Size() int { return Size }
   163  
   164  func (d *digest) BlockSize() int { return 1 }
   165  
   166  func (d *digest) Reset() { d.crc = 0 }
   167  
   168  const (
   169  	magic         = "crc\x01"
   170  	marshaledSize = len(magic) + 4 + 4
   171  )
   172  
   173  func (d *digest) MarshalBinary() ([]byte, error) {
   174  	b := make([]byte, 0, marshaledSize)
   175  	b = append(b, magic...)
   176  	b = byteorder.BeAppendUint32(b, tableSum(d.tab))
   177  	b = byteorder.BeAppendUint32(b, d.crc)
   178  	return b, nil
   179  }
   180  
   181  func (d *digest) UnmarshalBinary(b []byte) error {
   182  	if len(b) < len(magic) || string(b[:len(magic)]) != magic {
   183  		return errors.New("hash/crc32: invalid hash state identifier")
   184  	}
   185  	if len(b) != marshaledSize {
   186  		return errors.New("hash/crc32: invalid hash state size")
   187  	}
   188  	if tableSum(d.tab) != byteorder.BeUint32(b[4:]) {
   189  		return errors.New("hash/crc32: tables do not match")
   190  	}
   191  	d.crc = byteorder.BeUint32(b[8:])
   192  	return nil
   193  }
   194  
   195  func update(crc uint32, tab *Table, p []byte, checkInitIEEE bool) uint32 {
   196  	switch {
   197  	case haveCastagnoli.Load() && tab == castagnoliTable:
   198  		return updateCastagnoli(crc, p)
   199  	case tab == IEEETable:
   200  		if checkInitIEEE {
   201  			ieeeOnce.Do(ieeeInit)
   202  		}
   203  		return updateIEEE(crc, p)
   204  	default:
   205  		return simpleUpdate(crc, tab, p)
   206  	}
   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  	// Unfortunately, because IEEETable is exported, IEEE may be used without a
   212  	// call to MakeTable. We have to make sure it gets initialized in that case.
   213  	return update(crc, tab, p, true)
   214  }
   215  
   216  func (d *digest) Write(p []byte) (n int, err error) {
   217  	// We only create digest objects through New() which takes care of
   218  	// initialization in this case.
   219  	d.crc = update(d.crc, d.tab, p, false)
   220  	return len(p), nil
   221  }
   222  
   223  func (d *digest) Sum32() uint32 { return d.crc }
   224  
   225  func (d *digest) Sum(in []byte) []byte {
   226  	s := d.Sum32()
   227  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
   228  }
   229  
   230  // Checksum returns the CRC-32 checksum of data
   231  // using the polynomial represented by the [Table].
   232  func Checksum(data []byte, tab *Table) uint32 { return Update(0, tab, data) }
   233  
   234  // ChecksumIEEE returns the CRC-32 checksum of data
   235  // using the [IEEE] polynomial.
   236  func ChecksumIEEE(data []byte) uint32 {
   237  	ieeeOnce.Do(ieeeInit)
   238  	return updateIEEE(0, data)
   239  }
   240  
   241  // tableSum returns the IEEE checksum of table t.
   242  func tableSum(t *Table) uint32 {
   243  	var a [1024]byte
   244  	b := a[:0]
   245  	if t != nil {
   246  		for _, x := range t {
   247  			b = byteorder.BeAppendUint32(b, x)
   248  		}
   249  	}
   250  	return ChecksumIEEE(b)
   251  }
   252
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