proxmark3/common/crc16.c

WORD update_crc16( WORD crc, BYTE c ) {
	WORD i, v, tcrc = 0;

	v = (crc ^ c) & 0xff;
  for (i = 0; i < 8; i++) {
      tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;
      v >>= 1;
  }

  return ((crc >> 8) ^ tcrc)&0xffff;
}