"""Simple clock implementation which supports basic functions like
addition and subtracting minutes. Time overflows are supported.
"""

# number of minutes of a day
day_minutes = 24 * 60


class Clock():
    def __init__(self, hours, minutes):
        # handle internal state with minutes. For output:
        # hours:minutes are calculated on the fly
        self.minutes = hours * 60 + minutes

    def add(self, minutes):
        self.minutes = (self.minutes + (minutes % day_minutes)) % day_minutes

    def subtract(self, minutes):
        normalized_minutes = minutes % day_minutes
        if self.minutes - normalized_minutes > 0:
            self.minutes = self.minutes - normalized_minutes
        else:
            difference = normalized_minutes - self.minutes
            self.minutes = day_minutes - difference

    def __str__(self):
        h = self.minutes // 60
        m = self.minutes % 60
        # format time like DD:DD, pads single digits with zero, e.g. 08:05
        return "{0:02d}:{1:02d}".format(h, m)


def test():
    c = Clock(8, 0)
    assert str(c) == "08:00"

    c.add(20)
    assert str(c) == "08:20"

    c.subtract(30)
    assert str(c) == "07:50"

    c = Clock(23, 30)
    c.add(60)
    assert str(c) == "00:30"

    c.subtract(40)
    assert str(c) == "23:50"

    c.add(25 * 60)
    assert str(c) == "00:50"

    c.subtract(25 * 60)
    assert str(c) == "23:50"