text: // 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 ICSharpCode.SharpZipLib.Checksums 
  {
  	
  	/// <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) {
  #if NETCF_1_0
  				throw new ArgumentOutOfRangeException("offset");
  #else
  				throw new ArgumentOutOfRangeException("offset", "cannot be negative");
  #endif				
  			}
  
  			if ( count < 0 ) 
  			{
  #if NETCF_1_0
  				throw new ArgumentOutOfRangeException("count");
  #else
  				throw new ArgumentOutOfRangeException("count", "cannot be negative");
  #endif				
  			}
  
  			if (offset >= buffer.Length) 
  			{
  #if NETCF_1_0
  				throw new ArgumentOutOfRangeException("offset");
  #else
  				throw new ArgumentOutOfRangeException("offset", "not a valid index into buffer");
  #endif				
  			}
  			
  			if (offset + count > buffer.Length) 
  			{
  #if NETCF_1_0
  				throw new ArgumentOutOfRangeException("count");
  #else
  				throw new ArgumentOutOfRangeException("count", "exceeds buffer size");
  #endif				
  			}
  
  			//(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
  	}
  }
  