
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class CRC
{
    /* Table of CRCs of all 8-bit messages. */
    ulong[] crc_table = new ulong[256];

    /* Flag: has the table been computed? Initially false. */
    bool crc_table_computed = false;

    /* Make the table for a fast CRC. */
    void make_crc_table()
    {
        ulong c;
        int n, k;

        for (n = 0; n < 256; n++)
        {
            c = (ulong)n;
            for (k = 0; k < 8; k++)
            {
                if ((c & 1) > 0)
                    c = 0xedb88320L ^ (c >> 1);
                else
                    c = c >> 1;
            }
            crc_table[n] = c;
        }
        crc_table_computed = true;
    }

    /* Update a running CRC with the bytes buf[0..len-1]--the CRC
  should be initialized to all 1's, and the transmitted value
  is the 1's complement of the final running CRC (see the
  crc() routine below)). */

    ulong update_crc(ulong crc, byte[] buf, int len)
    {
        ulong c = crc;
        int n;

        if (!crc_table_computed)
            make_crc_table();
        for (n = 0; n < len; n++)
        {
            c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
        }
        return c;
    }

    public ulong CalculateCRC(byte[] buf)
    {
        return CalculateCRC(buf, buf.Length);
    }

    /* Return the CRC of the bytes buf[0..len-1]. */
    public ulong CalculateCRC(byte[] buf, int len)
    {
        return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
    }
}
