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  	"sync/atomic"
    20  )
    21  
    22  // The size of a CRC-32 checksum in bytes.
    23  const Size = 4
    24  
    25  // Predefined polynomials.
    26  const (
    27  	// IEEE is by far and away the most common CRC-32 polynomial.
    28  	// Used by ethernet (IEEE 802.3), v.42, fddi, gzip, zip, png, ...
    29  	IEEE = 0xedb88320
    30  
    31  	// Castagnoli's polynomial, used in iSCSI.
    32  	// Has better error detection characteristics than IEEE.
    33  	// https://dx.doi.org/10.1109/26.231911
    34  	Castagnoli = 0x82f63b78
    35  
    36  	// Koopman's polynomial.
    37  	// Also has better error detection characteristics than IEEE.
    38  	// https://dx.doi.org/10.1109/DSN.2002.1028931
    39  	Koopman = 0xeb31d82e
    40  )
    41  
    42  // Table is a 256-word table representing the polynomial for efficient processing.
    43  type Table [256]uint32
    44  
    45  // This file makes use of functions implemented in architecture-specific files.
    46  // The interface that they implement is as follows:
    47  //
    48  //    // archAvailableIEEE reports whether an architecture-specific CRC32-IEEE
    49  //    // algorithm is available.
    50  //    archAvailableIEEE() bool
    51  //
    52  //    // archInitIEEE initializes the architecture-specific CRC3-IEEE algorithm.
    53  //    // It can only be called if archAvailableIEEE() returns true.
    54  //    archInitIEEE()
    55  //
    56  //    // archUpdateIEEE updates the given CRC32-IEEE. It can only be called if
    57  //    // archInitIEEE() was previously called.
    58  //    archUpdateIEEE(crc uint32, p []byte) uint32
    59  //
    60  //    // archAvailableCastagnoli reports whether an architecture-specific
    61  //    // CRC32-C algorithm is available.
    62  //    archAvailableCastagnoli() bool
    63  //
    64  //    // archInitCastagnoli initializes the architecture-specific CRC32-C
    65  //    // algorithm. It can only be called if archAvailableCastagnoli() returns
    66  //    // true.
    67  //    archInitCastagnoli()
    68  //
    69  //    // archUpdateCastagnoli updates the given CRC32-C. It can only be called
    70  //    // if archInitCastagnoli() was previously called.
    71  //    archUpdateCastagnoli(crc uint32, p []byte) uint32
    72  
    73  // castagnoliTable points to a lazily initialized Table for the Castagnoli
    74  // polynomial. MakeTable will always return this value when asked to make a
    75  // Castagnoli table so we can compare against it to find when the caller is
    76  // using this polynomial.
    77  var castagnoliTable *Table
    78  var castagnoliTable8 *slicing8Table
    79  var updateCastagnoli func(crc uint32, p []byte) uint32
    80  var castagnoliOnce sync.Once
    81  var haveCastagnoli atomic.Bool
    82  
    83  func castagnoliInit() {
    84  	castagnoliTable = simpleMakeTable(Castagnoli)
    85  
    86  	if archAvailableCastagnoli() {
    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  	haveCastagnoli.Store(true)
    98  }
    99  
   100  // IEEETable is the table for the [IEEE] polynomial.
   101  var IEEETable = simpleMakeTable(IEEE)
   102  
   103  // ieeeTable8 is the slicing8Table for IEEE
   104  var ieeeTable8 *slicing8Table
   105  var updateIEEE func(crc uint32, p []byte) uint32
   106  var ieeeOnce sync.Once
   107  
   108  func ieeeInit() {
   109  	if archAvailableIEEE() {
   110  		archInitIEEE()
   111  		updateIEEE = archUpdateIEEE
   112  	} else {
   113  		// Initialize the slicing-by-8 table.
   114  		ieeeTable8 = slicingMakeTable(IEEE)
   115  		updateIEEE = func(crc uint32, p []byte) uint32 {
   116  			return slicingUpdate(crc, ieeeTable8, p)
   117  		}
   118  	}
   119  }
   120  
   121  // MakeTable returns a [Table] constructed from the specified polynomial.
   122  // The contents of this [Table] must not be modified.
   123  func MakeTable(poly uint32) *Table {
   124  	switch poly {
   125  	case IEEE:
   126  		ieeeOnce.Do(ieeeInit)
   127  		return IEEETable
   128  	case Castagnoli:
   129  		castagnoliOnce.Do(castagnoliInit)
   130  		return castagnoliTable
   131  	default:
   132  		return simpleMakeTable(poly)
   133  	}
   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  // appendUint32 is semantically the same as [binary.BigEndian.AppendUint32]
   195  // We copied this function because we can not import "encoding/binary" here.
   196  func appendUint32(b []byte, x uint32) []byte {
   197  	return append(b,
   198  		byte(x>>24),
   199  		byte(x>>16),
   200  		byte(x>>8),
   201  		byte(x),
   202  	)
   203  }
   204  
   205  // readUint32 is semantically the same as [binary.BigEndian.Uint32]
   206  // We copied this function because we can not import "encoding/binary" here.
   207  func readUint32(b []byte) uint32 {
   208  	_ = b[3]
   209  	return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
   210  }
   211  
   212  func update(crc uint32, tab *Table, p []byte, checkInitIEEE bool) uint32 {
   213  	switch {
   214  	case haveCastagnoli.Load() && tab == castagnoliTable:
   215  		return updateCastagnoli(crc, p)
   216  	case tab == IEEETable:
   217  		if checkInitIEEE {
   218  			ieeeOnce.Do(ieeeInit)
   219  		}
   220  		return updateIEEE(crc, p)
   221  	default:
   222  		return simpleUpdate(crc, tab, p)
   223  	}
   224  }
   225  
   226  // Update returns the result of adding the bytes in p to the crc.
   227  func Update(crc uint32, tab *Table, p []byte) uint32 {
   228  	// Unfortunately, because IEEETable is exported, IEEE may be used without a
   229  	// call to MakeTable. We have to make sure it gets initialized in that case.
   230  	return update(crc, tab, p, true)
   231  }
   232  
   233  func (d *digest) Write(p []byte) (n int, err error) {
   234  	// We only create digest objects through New() which takes care of
   235  	// initialization in this case.
   236  	d.crc = update(d.crc, d.tab, p, false)
   237  	return len(p), nil
   238  }
   239  
   240  func (d *digest) Sum32() uint32 { return d.crc }
   241  
   242  func (d *digest) Sum(in []byte) []byte {
   243  	s := d.Sum32()
   244  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
   245  }
   246  
   247  // Checksum returns the CRC-32 checksum of data
   248  // using the polynomial represented by the [Table].
   249  func Checksum(data []byte, tab *Table) uint32 { return Update(0, tab, data) }
   250  
   251  // ChecksumIEEE returns the CRC-32 checksum of data
   252  // using the [IEEE] polynomial.
   253  func ChecksumIEEE(data []byte) uint32 {
   254  	ieeeOnce.Do(ieeeInit)
   255  	return updateIEEE(0, data)
   256  }
   257  
   258  // tableSum returns the IEEE checksum of table t.
   259  func tableSum(t *Table) uint32 {
   260  	var a [1024]byte
   261  	b := a[:0]
   262  	if t != nil {
   263  		for _, x := range t {
   264  			b = appendUint32(b, x)
   265  		}
   266  	}
   267  	return ChecksumIEEE(b)
   268  }
   269
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