adler32.go

     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  	"internal/byteorder"
    20  )
    21  
    22  const (
    23  	// mod is the largest prime that is less than 65536.
    24  	mod = 65521
    25  	// nmax is the largest n such that
    26  	// 255 * n * (n+1) / 2 + (n+1) * (mod-1) <= 2^32-1.
    27  	// It is mentioned in RFC 1950 (search for "5552").
    28  	nmax = 5552
    29  )
    30  
    31  // The size of an Adler-32 checksum in bytes.
    32  const Size = 4
    33  
    34  // digest represents the partial evaluation of a checksum.
    35  // The low 16 bits are s1, the high 16 bits are s2.
    36  type digest uint32
    37  
    38  func (d *digest) Reset() { *d = 1 }
    39  
    40  // New returns a new hash.Hash32 computing the Adler-32 checksum. Its
    41  // Sum method will lay the value out in big-endian byte order. The
    42  // returned Hash32 also implements [encoding.BinaryMarshaler] and
    43  // [encoding.BinaryUnmarshaler] to marshal and unmarshal the internal
    44  // state of the hash.
    45  func New() hash.Hash32 {
    46  	d := new(digest)
    47  	d.Reset()
    48  	return d
    49  }
    50  
    51  func (d *digest) Size() int { return Size }
    52  
    53  func (d *digest) BlockSize() int { return 4 }
    54  
    55  const (
    56  	magic         = "adl\x01"
    57  	marshaledSize = len(magic) + 4
    58  )
    59  
    60  func (d *digest) MarshalBinary() ([]byte, error) {
    61  	b := make([]byte, 0, marshaledSize)
    62  	b = append(b, magic...)
    63  	b = byteorder.BeAppendUint32(b, uint32(*d))
    64  	return b, nil
    65  }
    66  
    67  func (d *digest) UnmarshalBinary(b []byte) error {
    68  	if len(b) < len(magic) || string(b[:len(magic)]) != magic {
    69  		return errors.New("hash/adler32: invalid hash state identifier")
    70  	}
    71  	if len(b) != marshaledSize {
    72  		return errors.New("hash/adler32: invalid hash state size")
    73  	}
    74  	*d = digest(byteorder.BeUint32(b[len(magic):]))
    75  	return nil
    76  }
    77  
    78  // Add p to the running checksum d.
    79  func update(d digest, p []byte) digest {
    80  	s1, s2 := uint32(d&0xffff), uint32(d>>16)
    81  	for len(p) > 0 {
    82  		var q []byte
    83  		if len(p) > nmax {
    84  			p, q = p[:nmax], p[nmax:]
    85  		}
    86  		for len(p) >= 4 {
    87  			s1 += uint32(p[0])
    88  			s2 += s1
    89  			s1 += uint32(p[1])
    90  			s2 += s1
    91  			s1 += uint32(p[2])
    92  			s2 += s1
    93  			s1 += uint32(p[3])
    94  			s2 += s1
    95  			p = p[4:]
    96  		}
    97  		for _, x := range p {
    98  			s1 += uint32(x)
    99  			s2 += s1
   100  		}
   101  		s1 %= mod
   102  		s2 %= mod
   103  		p = q
   104  	}
   105  	return digest(s2<<16 | s1)
   106  }
   107  
   108  func (d *digest) Write(p []byte) (nn int, err error) {
   109  	*d = update(*d, p)
   110  	return len(p), nil
   111  }
   112  
   113  func (d *digest) Sum32() uint32 { return uint32(*d) }
   114  
   115  func (d *digest) Sum(in []byte) []byte {
   116  	s := uint32(*d)
   117  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
   118  }
   119  
   120  // Checksum returns the Adler-32 checksum of data.
   121  func Checksum(data []byte) uint32 { return uint32(update(1, data)) }
   122
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