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  //	Adler-32 is composed of two sums accumulated per byte: s1 is
     9  //	the sum of all bytes, s2 is the sum of all s1 values. Both sums
    10  //	are done modulo 65521. s1 is initialized to 1, s2 to zero.  The
    11  //	Adler-32 checksum is stored as s2*65536 + s1 in most-
    12  //	significant-byte first (network) order.
    13  package adler32
    14  
    15  import (
    16  	"errors"
    17  	"hash"
    18  )
    19  
    20  const (
    21  	// mod is the largest prime that is less than 65536.
    22  	mod = 65521
    23  	// nmax is the largest n such that
    24  	// 255 * n * (n+1) / 2 + (n+1) * (mod-1) <= 2^32-1.
    25  	// It is mentioned in RFC 1950 (search for "5552").
    26  	nmax = 5552
    27  )
    28  
    29  // The size of an Adler-32 checksum in bytes.
    30  const Size = 4
    31  
    32  // digest represents the partial evaluation of a checksum.
    33  // The low 16 bits are s1, the high 16 bits are s2.
    34  type digest uint32
    35  
    36  func (d *digest) Reset() { *d = 1 }
    37  
    38  // New returns a new hash.Hash32 computing the Adler-32 checksum. Its
    39  // Sum method will lay the value out in big-endian byte order. The
    40  // returned Hash32 also implements encoding.BinaryMarshaler and
    41  // encoding.BinaryUnmarshaler to marshal and unmarshal the internal
    42  // state of the hash.
    43  func New() hash.Hash32 {
    44  	d := new(digest)
    45  	d.Reset()
    46  	return d
    47  }
    48  
    49  func (d *digest) Size() int { return Size }
    50  
    51  func (d *digest) BlockSize() int { return 4 }
    52  
    53  const (
    54  	magic         = "adl\x01"
    55  	marshaledSize = len(magic) + 4
    56  )
    57  
    58  func (d *digest) MarshalBinary() ([]byte, error) {
    59  	b := make([]byte, 0, marshaledSize)
    60  	b = append(b, magic...)
    61  	b = appendUint32(b, uint32(*d))
    62  	return b, nil
    63  }
    64  
    65  func (d *digest) UnmarshalBinary(b []byte) error {
    66  	if len(b) < len(magic) || string(b[:len(magic)]) != magic {
    67  		return errors.New("hash/adler32: invalid hash state identifier")
    68  	}
    69  	if len(b) != marshaledSize {
    70  		return errors.New("hash/adler32: invalid hash state size")
    71  	}
    72  	*d = digest(readUint32(b[len(magic):]))
    73  	return nil
    74  }
    75  
    76  func appendUint32(b []byte, x uint32) []byte {
    77  	a := [4]byte{
    78  		byte(x >> 24),
    79  		byte(x >> 16),
    80  		byte(x >> 8),
    81  		byte(x),
    82  	}
    83  	return append(b, a[:]...)
    84  }
    85  
    86  func readUint32(b []byte) uint32 {
    87  	_ = b[3]
    88  	return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
    89  }
    90  
    91  // Add p to the running checksum d.
    92  func update(d digest, p []byte) digest {
    93  	s1, s2 := uint32(d&0xffff), uint32(d>>16)
    94  	for len(p) > 0 {
    95  		var q []byte
    96  		if len(p) > nmax {
    97  			p, q = p[:nmax], p[nmax:]
    98  		}
    99  		for len(p) >= 4 {
   100  			s1 += uint32(p[0])
   101  			s2 += s1
   102  			s1 += uint32(p[1])
   103  			s2 += s1
   104  			s1 += uint32(p[2])
   105  			s2 += s1
   106  			s1 += uint32(p[3])
   107  			s2 += s1
   108  			p = p[4:]
   109  		}
   110  		for _, x := range p {
   111  			s1 += uint32(x)
   112  			s2 += s1
   113  		}
   114  		s1 %= mod
   115  		s2 %= mod
   116  		p = q
   117  	}
   118  	return digest(s2<<16 | s1)
   119  }
   120  
   121  func (d *digest) Write(p []byte) (nn int, err error) {
   122  	*d = update(*d, p)
   123  	return len(p), nil
   124  }
   125  
   126  func (d *digest) Sum32() uint32 { return uint32(*d) }
   127  
   128  func (d *digest) Sum(in []byte) []byte {
   129  	s := uint32(*d)
   130  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
   131  }
   132  
   133  // Checksum returns the Adler-32 checksum of data.
   134  func Checksum(data []byte) uint32 { return uint32(update(1, data)) }
   135
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