This class is part of #ziplib.

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.)"
Copyright (C) 2001 Mike Krueger

    public sealed class Adler32
    {
        /// <summary>
        /// largest prime smaller than 65536
        /// </summary>
        private static readonly uint BASE = 65521;

        private uint checksum;

        /// <summary>

        /// Returns the Adler32 data checksum computed so far.
        /// </summary>
        public long Value
        {
            get { return checksum; }
        }

        /// <summary>

        /// Creates a new instance of the <code>Adler32</code> 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; //Initialize to 1
        }

        /// <summary>
        /// Updates the checksum with the byte b.
        /// </summary>
        /// <param name="bval">

        /// the data value to add. The high byte of the int is ignored.
        /// </param>
        public void Update(int bval)
        {
            //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)bval & 0xFF)) % BASE;
            s2 = (s1 + s2) % BASE;

            checksum = (s2 </// <summary>

        /// Updates the checksum with the bytes taken from the array.
        /// </summary>
        /// <param name="buffer">
        /// buffer an array of bytes
        /// </param>
        public void Update(byte[] buffer)
        {
            Update(buffer, 0, buffer.Length);
        }

        /// <summary>

        /// Updates the checksum with the bytes taken from the array.
        /// </summary>
        /// <param name="buf">
        /// an array of bytes
        /// </param>
        /// <param name="off">

        /// the start of the data used for this update
        /// </param>
        /// <param name="len">
        /// the number of bytes to use for this update
        /// </param>
        public void Update(byte[] buf, int off, int len)
        {
            if (buf == null)
            {
                throw new ArgumentNullException("buf");
            }

            if (off  buf.Length)
            {
                throw new ArgumentOutOfRangeException();
            }

            //(By Per Bothner)

            uint s1 = checksum & 0xFFFF;
            uint s2 = checksum >> 16;

            while (len > 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 > len)
                {
                    n = len;
                }
                len -= n;
                while (--n >= 0)
                {
                    s1 = s1 + (uint)(buf[off++] & 0xFF);
                    s2 = s2 + s1;
                }
                s1 %= BASE;
                s2 %= BASE;
            }

            checksum = (s2 << 16) | s1;
        }
    }