class ShipBell(threading.Thread):
    SECONDS_PER_MINUTE = 60
    MINUTES_PER_HALF_HOUR = 30
    MAX_DOUBLE_STRIKES = 4

    def run(self):
        while True:
            current_time = time.localtime()
            minutes = current_time.tm_min
            hours = current_time.tm_hour
            # Strike bell at every half or full hour.
            if (minutes % ShipBell.MINUTES_PER_HALF_HOUR) == 0:
                double_strikes, single_strikes = self.compute_strikes(hours, minutes)
                self.strike_bell(double_strikes, single_strikes)
            time.sleep(self.compute_sleep_time(minutes))

    def compute_strikes(self, hours, minutes):
        # Single strike only on half hour.
        single_strikes = 1 if minutes == ShipBell.MINUTES_PER_HALF_HOUR else 0
        double_strikes = hours % ShipBell.MAX_DOUBLE_STRIKES
        # Correction for four double-strikes at 00:00, 04:00, 08:00 ...
        if double_strikes == 0:
            double_strikes = ShipBell.MAX_DOUBLE_STRIKES
        return (double_strikes, single_strikes)

    def compute_sleep_time(self, minutes):
        # Remaining minutes till half or full hour.
        delta_minutes = ShipBell.MINUTES_PER_HALF_HOUR - minutes % ShipBell.MINUTES_PER_HALF_HOUR
        # If enough time is left, sleep through half of it.
        if delta_minutes >= 2:
            return delta_minutes / 2.0 * ShipBell.SECONDS_PER_MINUTE
        # During the last minute, check every second.
        return 1.0

    def strike_bell(self, double_strikes, single_strikes):
        # Just a simulation -- print to screen instead of playing sounds.
        print(time.asctime() + ": " + "PING-PING  " * double_strikes + "PING" * single_strikes)