github.com/varialus/godfly@v0.0.0-20130904042352-1934f9f095ab/src/pkg/hash/adler32/adler32.go (about)

     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 "hash"
    16  
    17  const (
    18  	// mod is the largest prime that is less than 65536.
    19  	mod = 65521
    20  	// nmax is the largest n such that
    21  	// 255 * n * (n+1) / 2 + (n+1) * (mod-1) <= 2^32-1.
    22  	// It is mentioned in RFC 1950 (search for "5552").
    23  	nmax = 5552
    24  )
    25  
    26  // The size of an Adler-32 checksum in bytes.
    27  const Size = 4
    28  
    29  // digest represents the partial evaluation of a checksum.
    30  // The low 16 bits are s1, the high 16 bits are s2.
    31  type digest uint32
    32  
    33  func (d *digest) Reset() { *d = 1 }
    34  
    35  // New returns a new hash.Hash32 computing the Adler-32 checksum.
    36  func New() hash.Hash32 {
    37  	d := new(digest)
    38  	d.Reset()
    39  	return d
    40  }
    41  
    42  func (d *digest) Size() int { return Size }
    43  
    44  func (d *digest) BlockSize() int { return 1 }
    45  
    46  // Add p to the running checksum d.
    47  func update(d digest, p []byte) digest {
    48  	s1, s2 := uint32(d&0xffff), uint32(d>>16)
    49  	for len(p) > 0 {
    50  		var q []byte
    51  		if len(p) > nmax {
    52  			p, q = p[:nmax], p[nmax:]
    53  		}
    54  		for _, x := range p {
    55  			s1 += uint32(x)
    56  			s2 += s1
    57  		}
    58  		s1 %= mod
    59  		s2 %= mod
    60  		p = q
    61  	}
    62  	return digest(s2<<16 | s1)
    63  }
    64  
    65  func (d *digest) Write(p []byte) (nn int, err error) {
    66  	*d = update(*d, p)
    67  	return len(p), nil
    68  }
    69  
    70  func (d *digest) Sum32() uint32 { return uint32(*d) }
    71  
    72  func (d *digest) Sum(in []byte) []byte {
    73  	s := uint32(*d)
    74  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
    75  }
    76  
    77  // Checksum returns the Adler-32 checksum of data.
    78  func Checksum(data []byte) uint32 { return uint32(update(1, data)) }