kenneth.skovhede@gmail.com
b0a359fd27
git-svn-id: https://duplicati.googlecode.com/svn/trunk@770 59da171f-624f-0410-aa54-27559c288bec
104 lines
3.9 KiB
C#
104 lines
3.9 KiB
C#
#region Disclaimer / License
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// Copyright (C) 2011, Kenneth Skovhede
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// http://www.hexad.dk, opensource@hexad.dk
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//
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// This library is free software; you can redistribute it and/or
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// modify it under the terms of the GNU Lesser General Public
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// License as published by the Free Software Foundation; either
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// version 2.1 of the License, or (at your option) any later version.
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//
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// This library 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 GNU
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// Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public
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// License along with this library; if not, write to the Free Software
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// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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//
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#endregion
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using System;
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using System.Collections.Generic;
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using System.Text;
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namespace Duplicati.Library.SharpRSync
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{
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/// <summary>
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/// This is an implementation of the Adler-32 rolling checksum.
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/// This implementation is converted to C# from C code in librsync.
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/// </summary>
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public class Adler32Checksum
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{
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/// <summary>
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/// The charated offset used in the checksum
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/// </summary>
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private const ushort CHAR_OFFSET = 31;
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/// <summary>
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/// Calculates the Adler32 checksum for the given data
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/// </summary>
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/// <param name="data">The data to calculate the checksum for</param>
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/// <param name="offset">The offset into the data to start reading</param>
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/// <param name="count">The number of bytes to process</param>
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/// <returns>The adler32 checksum for the data</returns>
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public static uint Calculate(byte[] data, int offset, int count)
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{
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int i;
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ushort s1 = 0;
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ushort s2 = 0;
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int leftoverdata = (count % 4);
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long rounds = count - leftoverdata;
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//First do an optimized processing in chunks of 4 bytes
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for (i = 0; i < rounds; i += 4)
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{
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s2 += (ushort)(
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4 * (s1 + data[offset + 0]) +
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3 * data[offset + 1] +
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2 * data[offset + 2] +
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data[offset + 3] +
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10 * CHAR_OFFSET);
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s1 += (ushort)(
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data[offset + 0] +
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data[offset + 1] +
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data[offset + 2] +
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data[offset + 3] +
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4 * CHAR_OFFSET);
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offset += 4;
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}
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//Do a single step update of the remaining bytes
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for (i = 0; i < leftoverdata; i++)
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{
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s1 += (ushort)(data[offset + i] + CHAR_OFFSET);
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s2 += s1;
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}
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return (uint)((s1 & 0xffff) + (s2 << 16));
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}
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/// <summary>
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/// Updates an adler32 cheksum by excluding a byte, and including another
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/// </summary>
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/// <param name="out_byte">The byte that is no longer in the checksum</param>
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/// <param name="in_byte">The byte that is added to the checksum</param>
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/// <param name="checksum">The checksum including the out_byte</param>
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/// <param name="bytecount">The number of bytes the checksum contains</param>
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/// <returns>An updated checksum</returns>
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public static uint Roll(byte out_byte, byte in_byte, uint checksum, long bytecount)
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{
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ushort s1 = (ushort)(checksum & 0xffff);
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ushort s2 = (ushort)(checksum >> 16);
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s1 += (ushort)(in_byte - out_byte);
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s2 += (ushort)(s1 - (bytecount * (out_byte + CHAR_OFFSET)));
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return (uint)((s1 & 0xffff) + (s2 << 16));
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}
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}
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}
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