namespace ImageProcessor.Formats
{
    using System;

    /// <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 this.checksum;
            }
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Adler32"/> class.
        /// Creates a new instance of the Adler32 class.
        /// The checksum starts off with a value of 1.
        /// </summary>
        public Adler32()
        {
            this.Reset();
        }

        /// <summary>
        /// Resets the Adler32 checksum to the initial value.
        /// </summary>
        public void Reset()
        {
            this.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 = this.checksum & 0xFFFF;
            uint s2 = this.checksum >> 16;

            s1 = (s1 + ((uint)value & 0xFF)) % BASE;
            s2 = (s1 + s2) % BASE;

            this.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");
            }

            this.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)
            uint s1 = this.checksum & 0xFFFF;
            uint s2 = this.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;
            }

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

        #region Instance Fields
        uint checksum;
        #endregion
    }
}