adler32.go

Documentation: hash/adler32

     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 adler32 implements the Adler-32 checksum.
     6  //
     7  // It is defined in RFC 1950:
     8  //
     9  //	Adler-32 is composed of two sums accumulated per byte: s1 is
    10  //	the sum of all bytes, s2 is the sum of all s1 values. Both sums
    11  //	are done modulo 65521. s1 is initialized to 1, s2 to zero.  The
    12  //	Adler-32 checksum is stored as s2*65536 + s1 in most-
    13  //	significant-byte first (network) order.
    14  package adler32
    15  
    16  import (
    17  	"errors"
    18  	"hash"
    19  )
    20  
    21  const (
    22  	// mod is the largest prime that is less than 65536.
    23  	mod = 65521
    24  	// nmax is the largest n such that
    25  	// 255 * n * (n+1) / 2 + (n+1) * (mod-1) <= 2^32-1.
    26  	// It is mentioned in RFC 1950 (search for "5552").
    27  	nmax = 5552
    28  )
    29  
    30  // The size of an Adler-32 checksum in bytes.
    31  const Size = 4
    32  
    33  // digest represents the partial evaluation of a checksum.
    34  // The low 16 bits are s1, the high 16 bits are s2.
    35  type digest uint32
    36  
    37  func (d *digest) Reset() { *d = 1 }
    38  
    39  // New returns a new hash.Hash32 computing the Adler-32 checksum. Its
    40  // Sum method will lay the value out in big-endian byte order. The
    41  // returned Hash32 also implements [encoding.BinaryMarshaler] and
    42  // [encoding.BinaryUnmarshaler] to marshal and unmarshal the internal
    43  // state of the hash.
    44  func New() hash.Hash32 {
    45  	d := new(digest)
    46  	d.Reset()
    47  	return d
    48  }
    49  
    50  func (d *digest) Size() int { return Size }
    51  
    52  func (d *digest) BlockSize() int { return 4 }
    53  
    54  const (
    55  	magic         = "adl\x01"
    56  	marshaledSize = len(magic) + 4
    57  )
    58  
    59  func (d *digest) MarshalBinary() ([]byte, error) {
    60  	b := make([]byte, 0, marshaledSize)
    61  	b = append(b, magic...)
    62  	b = appendUint32(b, uint32(*d))
    63  	return b, nil
    64  }
    65  
    66  func (d *digest) UnmarshalBinary(b []byte) error {
    67  	if len(b) < len(magic) || string(b[:len(magic)]) != magic {
    68  		return errors.New("hash/adler32: invalid hash state identifier")
    69  	}
    70  	if len(b) != marshaledSize {
    71  		return errors.New("hash/adler32: invalid hash state size")
    72  	}
    73  	*d = digest(readUint32(b[len(magic):]))
    74  	return nil
    75  }
    76  
    77  // appendUint32 is semantically the same as [binary.BigEndian.AppendUint32]
    78  // We copied this function because we can not import "encoding/binary" here.
    79  func appendUint32(b []byte, x uint32) []byte {
    80  	return append(b,
    81  		byte(x>>24),
    82  		byte(x>>16),
    83  		byte(x>>8),
    84  		byte(x),
    85  	)
    86  }
    87  
    88  // readUint32 is semantically the same as [binary.BigEndian.Uint32]
    89  // We copied this function because we can not import "encoding/binary" here.
    90  func readUint32(b []byte) uint32 {
    91  	_ = b[3]
    92  	return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
    93  }
    94  
    95  // Add p to the running checksum d.
    96  func update(d digest, p []byte) digest {
    97  	s1, s2 := uint32(d&0xffff), uint32(d>>16)
    98  	for len(p) > 0 {
    99  		var q []byte
   100  		if len(p) > nmax {
   101  			p, q = p[:nmax], p[nmax:]
   102  		}
   103  		for len(p) >= 4 {
   104  			s1 += uint32(p[0])
   105  			s2 += s1
   106  			s1 += uint32(p[1])
   107  			s2 += s1
   108  			s1 += uint32(p[2])
   109  			s2 += s1
   110  			s1 += uint32(p[3])
   111  			s2 += s1
   112  			p = p[4:]
   113  		}
   114  		for _, x := range p {
   115  			s1 += uint32(x)
   116  			s2 += s1
   117  		}
   118  		s1 %= mod
   119  		s2 %= mod
   120  		p = q
   121  	}
   122  	return digest(s2<<16 | s1)
   123  }
   124  
   125  func (d *digest) Write(p []byte) (nn int, err error) {
   126  	*d = update(*d, p)
   127  	return len(p), nil
   128  }
   129  
   130  func (d *digest) Sum32() uint32 { return uint32(*d) }
   131  
   132  func (d *digest) Sum(in []byte) []byte {
   133  	s := uint32(*d)
   134  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
   135  }
   136  
   137  // Checksum returns the Adler-32 checksum of data.
   138  func Checksum(data []byte) uint32 { return uint32(update(1, data)) }
   139
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