#define POLY 0x8408
/*
//                                      16   12   5
// this is the CCITT CRC 16 polynomial X  + X  + X  + 1.
// This works out to be 0x1021, but the way the algorithm works
// lets us use 0x8408 (the reverse of the bit pattern).  The high
// bit is always assumed to be set, thus we only use 16 bits to
// represent the 17 bit value.
*/

UINT16 CCITT_CRC16(char *data_p, UINT16 length)
{
    unsigned char i;
    unsigned int data;
    unsigned int crc = 0xffff;

    if (length == 0)
        return (~crc);

    do
    {
        for (i=0, data=(unsigned int)0xff & *data_p++;i < 8; 
                i++, data >>= 1)
        {
            if ((crc & 0x0001) ^ (data & 0x0001))
                crc = (crc >> 1) ^ POLY;
            else  crc >>= 1;
        }
    } while (--length);

    crc = ~crc;
    data = crc;
    crc = (crc << 8) | ((data >> 8) & 0xff);

    return (crc);
}