CRC.cs

/* Copyright(c) 2016 UltimaLive
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

#region References

using Server;

#endregion

namespace UltimaLive;

public class CRC
{
    //CRC caching
    //[map][block]
    public static ushort[][] MapCRCs;

    public static void InvalidateBlockCRC(int map, int block)
    {
        MapCRCs[map][block] = ushort.MaxValue;
    }

    public static void Configure()
    {
        EventSink.WorldLoad += OnLoad;
    }

    public static void OnLoad()
    {
        MapCRCs = new ushort[256][];

        //We need CRCs for every block in every map.
        foreach (var kvp in MapRegistry.Definitions)
        {
            var blocks = Map.Maps[kvp.Key].Tiles.BlockWidth * Map.Maps[kvp.Key].Tiles.BlockHeight;
            MapCRCs[kvp.Key] = new ushort[blocks];

            for (var j = 0; j < blocks; j++)
            {
                MapCRCs[kvp.Key][j] = ushort.MaxValue;
            }
        }
    }

    /* Thank you http://en.wikipedia.org/wiki/Fletcher%27s_checksum
     * Each sum is computed modulo 255 and thus remains less than
     * 0xFF at all times. This implementation will thus never
     * produce the checksum results 0x00FF, 0xFF00 or 0xFFFF.
    /**/
    public static ushort Fletcher16(byte[] data)
    {
        ushort sum1 = 0;
        ushort sum2 = 0;
        int index;
        for (index = 0; index < data.Length; ++index)
        {
            sum1 = (ushort)(sum1 + data[index]);
            if (sum1 >= 255)
            {
                sum1 -= 255;
            }
            sum2 = (ushort)(sum2 + sum1);
            if (sum2 >= 255)
            {
                sum2 -= 255;
            }
        }

        return (ushort)((sum2 << 8) | sum1);
    }
}