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1 // Copyright 2009 The Go Authors. All rights reserved. | 1 // Copyright 2009 The Go Authors. All rights reserved. |
2 // Use of this source code is governed by a BSD-style | 2 // Use of this source code is governed by a BSD-style |
3 // license that can be found in the LICENSE file. | 3 // license that can be found in the LICENSE file. |
4 | 4 |
5 // Package adler32 implements the Adler-32 checksum. | 5 // Package adler32 implements the Adler-32 checksum. |
6 // Defined in RFC 1950: | 6 // |
| 7 // It is defined in RFC 1950: |
7 // Adler-32 is composed of two sums accumulated per byte: s1 is | 8 // Adler-32 is composed of two sums accumulated per byte: s1 is |
8 // the sum of all bytes, s2 is the sum of all s1 values. Both sums | 9 // the sum of all bytes, s2 is the sum of all s1 values. Both sums |
9 // are done modulo 65521. s1 is initialized to 1, s2 to zero. The | 10 // are done modulo 65521. s1 is initialized to 1, s2 to zero. The |
10 // Adler-32 checksum is stored as s2*65536 + s1 in most- | 11 // Adler-32 checksum is stored as s2*65536 + s1 in most- |
11 // significant-byte first (network) order. | 12 // significant-byte first (network) order. |
12 package adler32 | 13 package adler32 |
13 | 14 |
14 import "hash" | 15 import "hash" |
15 | 16 |
16 const ( | 17 const ( |
| 18 // mod is the largest prime that is less than 65536. |
17 mod = 65521 | 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 |
18 ) | 24 ) |
19 | 25 |
20 // The size of an Adler-32 checksum in bytes. | 26 // The size of an Adler-32 checksum in bytes. |
21 const Size = 4 | 27 const Size = 4 |
22 | 28 |
23 // digest represents the partial evaluation of a checksum. | 29 // digest represents the partial evaluation of a checksum. |
24 type digest struct { | 30 // The low 16 bits are s1, the high 16 bits are s2. |
25 » // invariant: (a < mod && b < mod) || a <= b | 31 type digest uint32 |
26 » // invariant: a + b + 255 <= 0xffffffff | |
27 » a, b uint32 | |
28 } | |
29 | 32 |
30 func (d *digest) Reset() { d.a, d.b = 1, 0 } | 33 func (d *digest) Reset() { *d = 1 } |
31 | 34 |
32 // New returns a new hash.Hash32 computing the Adler-32 checksum. | 35 // New returns a new hash.Hash32 computing the Adler-32 checksum. |
33 func New() hash.Hash32 { | 36 func New() hash.Hash32 { |
34 d := new(digest) | 37 d := new(digest) |
35 d.Reset() | 38 d.Reset() |
36 return d | 39 return d |
37 } | 40 } |
38 | 41 |
39 func (d *digest) Size() int { return Size } | 42 func (d *digest) Size() int { return Size } |
40 | 43 |
41 func (d *digest) BlockSize() int { return 1 } | 44 func (d *digest) BlockSize() int { return 1 } |
42 | 45 |
43 // Add p to the running checksum a, b. | 46 // Add p to the running checksum d. |
44 func update(a, b uint32, p []byte) (aa, bb uint32) { | 47 func update(d digest, p []byte) digest { |
45 » for _, pi := range p { | 48 » s1, s2 := uint32(d&0xffff), uint32(d>>16) |
46 » » a += uint32(pi) | 49 » for len(p) > 0 { |
47 » » b += a | 50 » » var q []byte |
48 » » // invariant: a <= b | 51 » » if len(p) > nmax { |
49 » » if b > (0xffffffff-255)/2 { | 52 » » » p, q = p[:nmax], p[nmax:] |
50 » » » a %= mod | |
51 » » » b %= mod | |
52 » » » // invariant: a < mod && b < mod | |
53 » » } else { | |
54 » » » // invariant: a + b + 255 <= 2 * b + 255 <= 0xffffffff | |
55 } | 53 } |
| 54 for _, x := range p { |
| 55 s1 += uint32(x) |
| 56 s2 += s1 |
| 57 } |
| 58 s1 %= mod |
| 59 s2 %= mod |
| 60 p = q |
56 } | 61 } |
57 » return a, b | 62 » return digest(s2<<16 | s1) |
58 } | |
59 | |
60 // Return the 32-bit checksum corresponding to a, b. | |
61 func finish(a, b uint32) uint32 { | |
62 » if b >= mod { | |
63 » » a %= mod | |
64 » » b %= mod | |
65 » } | |
66 » return b<<16 | a | |
67 } | 63 } |
68 | 64 |
69 func (d *digest) Write(p []byte) (nn int, err error) { | 65 func (d *digest) Write(p []byte) (nn int, err error) { |
70 » d.a, d.b = update(d.a, d.b, p) | 66 » *d = update(*d, p) |
71 return len(p), nil | 67 return len(p), nil |
72 } | 68 } |
73 | 69 |
74 func (d *digest) Sum32() uint32 { return finish(d.a, d.b) } | 70 func (d *digest) Sum32() uint32 { return uint32(*d) } |
75 | 71 |
76 func (d *digest) Sum(in []byte) []byte { | 72 func (d *digest) Sum(in []byte) []byte { |
77 » s := d.Sum32() | 73 » s := uint32(*d) |
78 return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s)) | 74 return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s)) |
79 } | 75 } |
80 | 76 |
81 // Checksum returns the Adler-32 checksum of data. | 77 // Checksum returns the Adler-32 checksum of data. |
82 func Checksum(data []byte) uint32 { return finish(update(1, 0, data)) } | 78 func Checksum(data []byte) uint32 { return uint32(update(1, data)) } |
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