// 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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using System;
using ICSharpCode.SharpZipLib.Silverlight.Zip.Compression.Streams;

namespace ICSharpCode.SharpZipLib.Silverlight.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="InflaterInputStream"/>
    /// <see cref="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) {
                throw new ArgumentOutOfRangeException("offset", "cannot be negative");	
            }

            if ( count < 0 ) 
            {
                throw new ArgumentOutOfRangeException("count", "cannot be negative");			
            }

            if (offset >= buffer.Length) 
            {
                throw new ArgumentOutOfRangeException("offset", "not a valid index into buffer");			
            }
			
            if (offset + count > buffer.Length) 
            {
                throw new ArgumentOutOfRangeException("count", "exceeds buffer size");			
            }

            //(By Per Bothner)
            var s1 = checksum & 0xFFFF;
            var 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
                var 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
    }
}