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 // Its Sum method will lay the value out in big-endian byte order. 37 func New() hash.Hash32 { 38 d := new(digest) 39 d.Reset() 40 return d 41 } 42 43 func (d *digest) Size() int { return Size } 44 45 func (d *digest) BlockSize() int { return 4 } 46 47 // Add p to the running checksum d. 48 func update(d digest, p []byte) digest { 49 s1, s2 := uint32(d&0xffff), uint32(d>>16) 50 for len(p) > 0 { 51 var q []byte 52 if len(p) > nmax { 53 p, q = p[:nmax], p[nmax:] 54 } 55 for len(p) >= 4 { 56 s1 += uint32(p[0]) 57 s2 += s1 58 s1 += uint32(p[1]) 59 s2 += s1 60 s1 += uint32(p[2]) 61 s2 += s1 62 s1 += uint32(p[3]) 63 s2 += s1 64 p = p[4:] 65 } 66 for _, x := range p { 67 s1 += uint32(x) 68 s2 += s1 69 } 70 s1 %= mod 71 s2 %= mod 72 p = q 73 } 74 return digest(s2<<16 | s1) 75 } 76 77 func (d *digest) Write(p []byte) (nn int, err error) { 78 *d = update(*d, p) 79 return len(p), nil 80 } 81 82 func (d *digest) Sum32() uint32 { return uint32(*d) } 83 84 func (d *digest) Sum(in []byte) []byte { 85 s := uint32(*d) 86 return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s)) 87 } 88 89 // Checksum returns the Adler-32 checksum of data. 90 func Checksum(data []byte) uint32 { return uint32(update(1, data)) } 91
