1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
|
// Adler32.cs - Computes Adler32 data checksum of a data stream
// Copyright (C) 2001 Mike Krueger
//
// This file was translated from java, it was part of the GNU Classpath
// Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License
// as published by the Free Software Foundation; either version 2
// of the License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
//
// Linking this library statically or dynamically with other modules is
// making a combined work based on this library. Thus, the terms and
// conditions of the GNU General Public License cover the whole
// combination.
//
// As a special exception, the copyright holders of this library give you
// permission to link this library with independent modules to produce an
// executable, regardless of the license terms of these independent
// modules, and to copy and distribute the resulting executable under
// terms of your choice, provided that you also meet, for each linked
// independent module, the terms and conditions of the license of that
// module. An independent module is a module which is not derived from
// or based on this library. If you modify this library, you may extend
// this exception to your version of the library, but you are not
// obligated to do so. If you do not wish to do so, delete this
// exception statement from your version.
using System;
namespace Plupload.PngEncoder {
/// <summary>
/// Computes Adler32 checksum for a stream of data. An Adler32
/// checksum is not as reliable as a CRC32 checksum, but a lot faster to
/// compute.
///
/// The specification for Adler32 may be found in RFC 1950.
/// ZLIB Compressed Data Format Specification version 3.3)
///
///
/// From that document:
///
/// "ADLER32 (Adler-32 checksum)
/// This contains a checksum value of the uncompressed data
/// (excluding any dictionary data) computed according to Adler-32
/// algorithm. This algorithm is a 32-bit extension and improvement
/// of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
/// standard.
///
/// 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."
///
/// "8.2. The Adler-32 algorithm
///
/// The Adler-32 algorithm is much faster than the CRC32 algorithm yet
/// still provides an extremely low probability of undetected errors.
///
/// The modulo on unsigned long accumulators can be delayed for 5552
/// bytes, so the modulo operation time is negligible. If the bytes
/// are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
/// and order sensitive, unlike the first sum, which is just a
/// checksum. That 65521 is prime is important to avoid a possible
/// large class of two-byte errors that leave the check unchanged.
/// (The Fletcher checksum uses 255, which is not prime and which also
/// makes the Fletcher check insensitive to single byte changes 0 -
/// 255.)
///
/// The sum s1 is initialized to 1 instead of zero to make the length
/// of the sequence part of s2, so that the length does not have to be
/// checked separately. (Any sequence of zeroes has a Fletcher
/// checksum of zero.)"
/// </summary>
/// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.InflaterInputStream"/>
/// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.DeflaterOutputStream"/>
public sealed class Adler32 : IChecksum {
/// <summary>
/// largest prime smaller than 65536
/// </summary>
const uint BASE = 65521;
/// <summary>
/// Returns the Adler32 data checksum computed so far.
/// </summary>
public long Value {
get {
return checksum;
}
}
/// <summary>
/// Creates a new instance of the Adler32 class.
/// The checksum starts off with a value of 1.
/// </summary>
public Adler32() {
Reset();
}
/// <summary>
/// Resets the Adler32 checksum to the initial value.
/// </summary>
public void Reset() {
checksum = 1;
}
/// <summary>
/// Updates the checksum with a byte value.
/// </summary>
/// <param name="value">
/// The data value to add. The high byte of the int is ignored.
/// </param>
public void Update(int value) {
// We could make a length 1 byte array and call update again, but I
// would rather not have that overhead
uint s1 = checksum & 0xFFFF;
uint s2 = checksum >> 16;
s1 = (s1 + ((uint) value & 0xFF)) % BASE;
s2 = (s1 + s2) % BASE;
checksum = (s2 << 16) + s1;
}
/// <summary>
/// Updates the checksum with an array of bytes.
/// </summary>
/// <param name="buffer">
/// The source of the data to update with.
/// </param>
public void Update(byte[] buffer) {
if (buffer == null) {
throw new ArgumentNullException("buffer");
}
Update(buffer, 0, buffer.Length);
}
/// <summary>
/// Updates the checksum with the bytes taken from the array.
/// </summary>
/// <param name="buffer">
/// an array of bytes
/// </param>
/// <param name="offset">
/// the start of the data used for this update
/// </param>
/// <param name="count">
/// the number of bytes to use for this update
/// </param>
public void Update(byte[] buffer, int offset, int count) {
if (buffer == null) {
throw new ArgumentNullException("buffer");
}
if (offset < 0) {
throw new ArgumentOutOfRangeException("offset");
}
if (count < 0) {
throw new ArgumentOutOfRangeException("count");
}
if (offset >= buffer.Length) {
throw new ArgumentOutOfRangeException("offset");
}
if (offset + count > buffer.Length) {
throw new ArgumentOutOfRangeException("count");
}
//(By Per Bothner)
uint s1 = checksum & 0xFFFF;
uint s2 = checksum >> 16;
while (count > 0) {
// We can defer the modulo operation:
// s1 maximally grows from 65521 to 65521 + 255 * 3800
// s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
int n = 3800;
if (n > count) {
n = count;
}
count -= n;
while (--n >= 0) {
s1 = s1 + (uint) (buffer[offset++] & 0xff);
s2 = s2 + s1;
}
s1 %= BASE;
s2 %= BASE;
}
checksum = (s2 << 16) | s1;
}
#region Instance Fields
uint checksum;
#endregion
}
}
|
