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Source file src/hash/adler32/adler32.go

Documentation: hash/adler32

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Package adler32 implements the Adler-32 checksum.
  //
  // It is defined in RFC 1950:
  //	Adler-32 is composed of two sums accumulated per byte: s1 is
  //	the sum of all bytes, s2 is the sum of all s1 values. Both sums
  //	are done modulo 65521. s1 is initialized to 1, s2 to zero.  The
  //	Adler-32 checksum is stored as s2*65536 + s1 in most-
  //	significant-byte first (network) order.
  package adler32
  
  import "hash"
  
  const (
  	// mod is the largest prime that is less than 65536.
  	mod = 65521
  	// nmax is the largest n such that
  	// 255 * n * (n+1) / 2 + (n+1) * (mod-1) <= 2^32-1.
  	// It is mentioned in RFC 1950 (search for "5552").
  	nmax = 5552
  )
  
  // The size of an Adler-32 checksum in bytes.
  const Size = 4
  
  // digest represents the partial evaluation of a checksum.
  // The low 16 bits are s1, the high 16 bits are s2.
  type digest uint32
  
  func (d *digest) Reset() { *d = 1 }
  
  // New returns a new hash.Hash32 computing the Adler-32 checksum.
  // Its Sum method will lay the value out in big-endian byte order.
  func New() hash.Hash32 {
  	d := new(digest)
  	d.Reset()
  	return d
  }
  
  func (d *digest) Size() int { return Size }
  
  func (d *digest) BlockSize() int { return 4 }
  
  // Add p to the running checksum d.
  func update(d digest, p []byte) digest {
  	s1, s2 := uint32(d&0xffff), uint32(d>>16)
  	for len(p) > 0 {
  		var q []byte
  		if len(p) > nmax {
  			p, q = p[:nmax], p[nmax:]
  		}
  		for len(p) >= 4 {
  			s1 += uint32(p[0])
  			s2 += s1
  			s1 += uint32(p[1])
  			s2 += s1
  			s1 += uint32(p[2])
  			s2 += s1
  			s1 += uint32(p[3])
  			s2 += s1
  			p = p[4:]
  		}
  		for _, x := range p {
  			s1 += uint32(x)
  			s2 += s1
  		}
  		s1 %= mod
  		s2 %= mod
  		p = q
  	}
  	return digest(s2<<16 | s1)
  }
  
  func (d *digest) Write(p []byte) (nn int, err error) {
  	*d = update(*d, p)
  	return len(p), nil
  }
  
  func (d *digest) Sum32() uint32 { return uint32(*d) }
  
  func (d *digest) Sum(in []byte) []byte {
  	s := uint32(*d)
  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
  }
  
  // Checksum returns the Adler-32 checksum of data.
  func Checksum(data []byte) uint32 { return uint32(update(1, data)) }
  

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