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.
13package adler32
14
15import "hash"
16
17const (
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.
27const 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.
31type digest uint32
32
33func (d *digest) Reset() { *d = 1 }
34
35// New returns a new hash.Hash32 computing the Adler-32 checksum.
36func New() hash.Hash32 {
37	d := new(digest)
38	d.Reset()
39	return d
40}
41
42func (d *digest) Size() int { return Size }
43
44func (d *digest) BlockSize() int { return 1 }
45
46// Add p to the running checksum d.
47func 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
65func (d *digest) Write(p []byte) (nn int, err error) {
66	*d = update(*d, p)
67	return len(p), nil
68}
69
70func (d *digest) Sum32() uint32 { return uint32(*d) }
71
72func (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.
78func Checksum(data []byte) uint32 { return uint32(update(1, data)) }
79