using System;
using System.Text;

public static class Crc16IbmSdlc
{
    private const ushort PolyReversed = 0x8408; // Reflected 0x1021
    private const ushort Init = 0xFFFF;
    private const ushort XorOut = 0xFFFF;

    private static readonly ushort[] Table = new ushort[256];

    static Crc16IbmSdlc()
    {
        for (ushort i = 0; i < 256; i++)
        {
            ushort value = i;
            for (byte j = 0; j < 8; j++)
            {
                if ((value & 1) != 0)
                    value = (ushort)((value >> 1) ^ PolyReversed);
                else
                    value >>= 1;
            }
            Table[i] = value;
        }
    }

    public static ushort ComputeChecksum(ReadOnlySpan<byte> data)
    {
        ushort crc = Init;
        foreach (byte b in data)
        {
            crc = (ushort)((crc >> 8) ^ Table[(crc ^ b) & 0xFF]);
        }
        return (ushort)(crc ^ XorOut);
    }
}