gistfile1.c

short passcode(const char *callsign) {
  uint8_t i, len;
  short hash = KEY;
  char no_ssid[10], *p1 = no_ssid, *ptr = no_ssid;
  
  // remove SSID
  while((*callsign != '-') && (*callsign != '\0')) *p1++ = toupper((int) (*callsign++));
  *p1 = '\0';
           
  i = 0;
  len = strlen(no_ssid);
  while(i < len) {
    hash ^= (unsigned char) (*ptr++) << 8; // high byte
    hash ^= (*ptr++);                      // lower byte
    i+=2;
  }
    
  return (short) (hash & 0x7fff); // force positive value
}