Aaru/Aaru.Compression/cuetools.net/CUETools.Codecs/CRC/CRC16CCITT.cs

namespace CUETools.Codecs;

public enum InitialCrcValue
{
    Zeros,
    NonZero1 = 0xffff,
    NonZero2 = 0x1D0F
}

public class Crc16Ccitt
{
    const    ushort   poly = 4129;
    readonly ushort   initialValue;
    readonly ushort[] table = new ushort[256];

    public Crc16Ccitt(InitialCrcValue initialValue)
    {
        this.initialValue = (ushort)initialValue;
        ushort temp, a;

        for(var i = 0; i < table.Length; i++)
        {
            temp = 0;
            a    = (ushort)(i << 8);

            for(var j = 0; j < 8; j++)
            {
                if(((temp                     ^ a) & 0x8000) != 0)
                    temp = (ushort)(temp << 1 ^ poly);
                else
                    temp <<= 1;

                a <<= 1;
            }

            table[i] = temp;
        }
    }

    public ushort ComputeChecksum(byte[] bytes, int pos, int count)
    {
        ushort crc                                 = initialValue;
        for(int i = pos; i < pos + count; i++) crc = (ushort)(crc << 8 ^ table[crc >> 8 ^ 0xff & bytes[i]]);

        return crc;
    }

    public byte[] ComputeChecksumBytes(byte[] bytes, int pos, int count)
    {
        ushort crc = ComputeChecksum(bytes, pos, count);

        return [(byte)(crc >> 8), (byte)(crc & 0x00ff)];
    }
}