// 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
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// independent module, the terms and conditions of the license of that
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// 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
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// 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
	}
}