1 // Adler32.cs - Computes Adler32 data checksum of a data stream
2 // Copyright (C) 2001 Mike Krueger
4 // This file was translated from java, it was part of the GNU Classpath
5 // Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
7 // This program is free software; you can redistribute it and/or
8 // modify it under the terms of the GNU General Public License
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39 using ICSharpCode.SharpZipLib.Silverlight.Zip.Compression.Streams;
41 namespace ICSharpCode.SharpZipLib.Silverlight.Checksums
44 /// Computes Adler32 checksum for a stream of data. An Adler32
45 /// checksum is not as reliable as a CRC32 checksum, but a lot faster to
48 /// The specification for Adler32 may be found in RFC 1950.
49 /// ZLIB Compressed Data Format Specification version 3.3)
52 /// From that document:
54 /// "ADLER32 (Adler-32 checksum)
55 /// This contains a checksum value of the uncompressed data
56 /// (excluding any dictionary data) computed according to Adler-32
57 /// algorithm. This algorithm is a 32-bit extension and improvement
58 /// of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
61 /// Adler-32 is composed of two sums accumulated per byte: s1 is
62 /// the sum of all bytes, s2 is the sum of all s1 values. Both sums
63 /// are done modulo 65521. s1 is initialized to 1, s2 to zero. The
64 /// Adler-32 checksum is stored as s2*65536 + s1 in most-
65 /// significant-byte first (network) order."
67 /// "8.2. The Adler-32 algorithm
69 /// The Adler-32 algorithm is much faster than the CRC32 algorithm yet
70 /// still provides an extremely low probability of undetected errors.
72 /// The modulo on unsigned long accumulators can be delayed for 5552
73 /// bytes, so the modulo operation time is negligible. If the bytes
74 /// are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
75 /// and order sensitive, unlike the first sum, which is just a
76 /// checksum. That 65521 is prime is important to avoid a possible
77 /// large class of two-byte errors that leave the check unchanged.
78 /// (The Fletcher checksum uses 255, which is not prime and which also
79 /// makes the Fletcher check insensitive to single byte changes 0 -
82 /// The sum s1 is initialized to 1 instead of zero to make the length
83 /// of the sequence part of s2, so that the length does not have to be
84 /// checked separately. (Any sequence of zeroes has a Fletcher
85 /// checksum of zero.)"
87 /// <see cref="InflaterInputStream"/>
88 /// <see cref="DeflaterOutputStream"/>
89 public sealed class Adler32 : IChecksum
92 /// largest prime smaller than 65536
94 const uint BASE = 65521;
97 /// Returns the Adler32 data checksum computed so far.
106 /// Creates a new instance of the Adler32 class.
107 /// The checksum starts off with a value of 1.
115 /// Resets the Adler32 checksum to the initial value.
123 /// Updates the checksum with a byte value.
125 /// <param name="value">
126 /// The data value to add. The high byte of the int is ignored.
128 public void Update(int value)
130 // We could make a length 1 byte array and call update again, but I
131 // would rather not have that overhead
132 uint s1 = checksum & 0xFFFF;
133 uint s2 = checksum >> 16;
135 s1 = (s1 + ((uint)value & 0xFF)) % BASE;
136 s2 = (s1 + s2) % BASE;
138 checksum = (s2 << 16) + s1;
142 /// Updates the checksum with an array of bytes.
144 /// <param name="buffer">
145 /// The source of the data to update with.
147 public void Update(byte[] buffer)
149 if ( buffer == null ) {
150 throw new ArgumentNullException("buffer");
153 Update(buffer, 0, buffer.Length);
157 /// Updates the checksum with the bytes taken from the array.
159 /// <param name="buffer">
160 /// an array of bytes
162 /// <param name="offset">
163 /// the start of the data used for this update
165 /// <param name="count">
166 /// the number of bytes to use for this update
168 public void Update(byte[] buffer, int offset, int count)
170 if (buffer == null) {
171 throw new ArgumentNullException("buffer");
175 throw new ArgumentOutOfRangeException("offset", "cannot be negative");
180 throw new ArgumentOutOfRangeException("count", "cannot be negative");
183 if (offset >= buffer.Length)
185 throw new ArgumentOutOfRangeException("offset", "not a valid index into buffer");
188 if (offset + count > buffer.Length)
190 throw new ArgumentOutOfRangeException("count", "exceeds buffer size");
194 var s1 = checksum & 0xFFFF;
195 var s2 = checksum >> 16;
198 // We can defer the modulo operation:
199 // s1 maximally grows from 65521 to 65521 + 255 * 3800
200 // s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
207 s1 = s1 + (uint)(buffer[offset++] & 0xff);
214 checksum = (s2 << 16) | s1;
217 #region Instance Fields