/* CRC-CCITT :                                                            */
/*                                                                        */
/* crc polynomial = 11021H                                                */

#define CRC_INIT   0xFFFF  /* all bits are 1, in case of leading zeroes   */
                /* Complement CRC before sending in case of extra zeroes. */
#define CRC_MAGIC  0x1D0F  /* The received CRC must match this number.    */

unsigned short crccitt(unsigned short crc,unsigned char data);

/* This function is used to generate the crc table. */
static unsigned short crccittx(
  unsigned short crc,         /* old crc value      */
  unsigned char data          /* data to be crc'd   */
)
{
  crc = ((crc << 8) + (crc >> 8)) ^ ((int)data);
  crc = crc ^ ((crc & 0x00ff) >> 4);
  crc = crc ^ ((crc & 0x00ff) << 12) ^ ((crc & 0x00ff) << 5);
  return crc;
}

/* This crc routine uses a table lookup. */
unsigned short crccitt(
  unsigned short crc,      /* old crc value     */
  unsigned char data       /* data to be crc'd  */
)
{
  static int inited = 0;
  static unsigned short crctbl[256];

  if(!inited){
    int i;

    for (i = 0; i < 256; i++)
      crctbl[i] = crccittx(0,(unsigned char)i);

    inited = 1;
  }

  crc = ((crc >> 8) ^ data) + (crc << 8);
  crc = (crc & 0xff00) ^ crctbl[(crc & 0x00ff)];
  return crc;
}