// Adler32.cs - Computes Adler32 data checksum of a data stream
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// Copyright (C) 2001 Mike Krueger
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//
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// This file was translated from java, it was part of the GNU Classpath
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// Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
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//
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// This program is free software; you can redistribute it and/or
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// modify it under the terms of the GNU General Public License
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// as published by the Free Software Foundation; either version 2
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// of the License, or (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program; if not, write to the Free Software
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// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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//
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// Linking this library statically or dynamically with other modules is
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// making a combined work based on this library. Thus, the terms and
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// conditions of the GNU General Public License cover the whole
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// combination.
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//
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// As a special exception, the copyright holders of this library give you
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// permission to link this library with independent modules to produce an
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// executable, regardless of the license terms of these independent
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// modules, and to copy and distribute the resulting executable under
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// terms of your choice, provided that you also meet, for each linked
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// independent module, the terms and conditions of the license of that
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// module. An independent module is a module which is not derived from
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// or based on this library. If you modify this library, you may extend
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// this exception to your version of the library, but you are not
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// obligated to do so. If you do not wish to do so, delete this
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// exception statement from your version.
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using System;
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namespace ICSharpCode.SharpZipLib.Checksums
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{
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/// <summary>
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/// Computes Adler32 checksum for a stream of data. An Adler32
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/// checksum is not as reliable as a CRC32 checksum, but a lot faster to
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/// compute.
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///
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/// The specification for Adler32 may be found in RFC 1950.
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/// ZLIB Compressed Data Format Specification version 3.3)
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///
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///
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/// From that document:
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///
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/// "ADLER32 (Adler-32 checksum)
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/// This contains a checksum value of the uncompressed data
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/// (excluding any dictionary data) computed according to Adler-32
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/// algorithm. This algorithm is a 32-bit extension and improvement
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/// of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
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/// standard.
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///
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/// Adler-32 is composed of two sums accumulated per byte: s1 is
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/// the sum of all bytes, s2 is the sum of all s1 values. Both sums
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/// are done modulo 65521. s1 is initialized to 1, s2 to zero. The
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/// Adler-32 checksum is stored as s2*65536 + s1 in most-
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/// significant-byte first (network) order."
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///
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/// "8.2. The Adler-32 algorithm
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///
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/// The Adler-32 algorithm is much faster than the CRC32 algorithm yet
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/// still provides an extremely low probability of undetected errors.
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///
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/// The modulo on unsigned long accumulators can be delayed for 5552
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/// bytes, so the modulo operation time is negligible. If the bytes
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/// are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
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/// and order sensitive, unlike the first sum, which is just a
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/// checksum. That 65521 is prime is important to avoid a possible
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/// large class of two-byte errors that leave the check unchanged.
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/// (The Fletcher checksum uses 255, which is not prime and which also
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/// makes the Fletcher check insensitive to single byte changes 0 -
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/// 255.)
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///
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/// The sum s1 is initialized to 1 instead of zero to make the length
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/// of the sequence part of s2, so that the length does not have to be
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/// checked separately. (Any sequence of zeroes has a Fletcher
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/// checksum of zero.)"
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/// </summary>
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/// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.InflaterInputStream"/>
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/// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.DeflaterOutputStream"/>
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public sealed class Adler32 : IChecksum
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{
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/// <summary>
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/// largest prime smaller than 65536
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/// </summary>
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const uint BASE = 65521;
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/// <summary>
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/// Returns the Adler32 data checksum computed so far.
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/// </summary>
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public long Value {
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get {
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return checksum;
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}
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}
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/// <summary>
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/// Creates a new instance of the Adler32 class.
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/// The checksum starts off with a value of 1.
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/// </summary>
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public Adler32()
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{
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Reset();
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}
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/// <summary>
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/// Resets the Adler32 checksum to the initial value.
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/// </summary>
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public void Reset()
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{
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checksum = 1;
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}
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/// <summary>
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/// Updates the checksum with a byte value.
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/// </summary>
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/// <param name="value">
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/// The data value to add. The high byte of the int is ignored.
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/// </param>
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public void Update(int value)
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{
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// We could make a length 1 byte array and call update again, but I
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// would rather not have that overhead
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uint s1 = checksum & 0xFFFF;
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uint s2 = checksum >> 16;
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s1 = (s1 + ((uint)value & 0xFF)) % BASE;
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s2 = (s1 + s2) % BASE;
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checksum = (s2 << 16) + s1;
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}
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/// <summary>
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/// Updates the checksum with an array of bytes.
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/// </summary>
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/// <param name="buffer">
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/// The source of the data to update with.
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/// </param>
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public void Update(byte[] buffer)
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{
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if ( buffer == null ) {
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throw new ArgumentNullException("buffer");
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}
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Update(buffer, 0, buffer.Length);
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}
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/// <summary>
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/// Updates the checksum with the bytes taken from the array.
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/// </summary>
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/// <param name="buffer">
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/// an array of bytes
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/// </param>
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/// <param name="offset">
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/// the start of the data used for this update
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/// </param>
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/// <param name="count">
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/// the number of bytes to use for this update
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/// </param>
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public void Update(byte[] buffer, int offset, int count)
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{
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if (buffer == null) {
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throw new ArgumentNullException("buffer");
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}
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if (offset < 0) {
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#if NETCF_1_0
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throw new ArgumentOutOfRangeException("offset");
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#else
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throw new ArgumentOutOfRangeException("offset", "cannot be negative");
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#endif
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}
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if ( count < 0 )
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{
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#if NETCF_1_0
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throw new ArgumentOutOfRangeException("count");
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#else
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throw new ArgumentOutOfRangeException("count", "cannot be negative");
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#endif
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}
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if (offset >= buffer.Length)
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{
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#if NETCF_1_0
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throw new ArgumentOutOfRangeException("offset");
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#else
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throw new ArgumentOutOfRangeException("offset", "not a valid index into buffer");
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#endif
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}
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if (offset + count > buffer.Length)
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{
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#if NETCF_1_0
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throw new ArgumentOutOfRangeException("count");
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#else
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throw new ArgumentOutOfRangeException("count", "exceeds buffer size");
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#endif
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}
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//(By Per Bothner)
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uint s1 = checksum & 0xFFFF;
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uint s2 = checksum >> 16;
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while (count > 0) {
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// We can defer the modulo operation:
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// s1 maximally grows from 65521 to 65521 + 255 * 3800
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// s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
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int n = 3800;
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if (n > count) {
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n = count;
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}
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count -= n;
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while (--n >= 0) {
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s1 = s1 + (uint)(buffer[offset++] & 0xff);
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s2 = s2 + s1;
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}
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s1 %= BASE;
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s2 %= BASE;
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}
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checksum = (s2 << 16) | s1;
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}
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#region Instance Fields
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uint checksum;
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#endregion
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}
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}
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