The Light Bulb Problem

result.json

{
    "off": 60,
    "on": 90,
    "size": 150
}

solution.py

import json


class LightBulb(object):
    on = None

    def __init__(self, on=False):
        self.on = on

    def turn_on(self):
        self.on = True

    def turn_off(self):
        self.on = False

    def flip_switch(self):
        self.on = not self.on


class Bus(object):
    light_bulbs = []

    def __init__(self, size):
        for i in range(size):
            self.light_bulbs.append(LightBulb(on=True))

    def analyze(self):
        on_count = 0
        off_count = 0
        for lb in self.light_bulbs:
            if lb.on:
                on_count += 1
            else:
                off_count += 1

        return {
            'size': len(self.light_bulbs),
            'on': on_count,
            'off': off_count
        }

    def flip_if_index_is_a_multiple_of(self, divisor):
        for index, light_bulb in enumerate(self.light_bulbs):
            human_readable_index = index + 1
            if human_readable_index % divisor == 0:
                light_bulb.flip_switch()


def main():
    bus = Bus(size=150)
    bus.flip_if_index_is_a_multiple_of(divisor=3)
    bus.flip_if_index_is_a_multiple_of(divisor=5)
    analysis = bus.analyze()
    with open('result.json', 'w+') as file:
        file.write(json.dumps(analysis, indent=4, sort_keys=True))


if __name__ == '__main__':
    main()

the-light-bulb-problem.md

The Light Bulb Problem

There are 150 light bulbs each with a flip switch, indexed from 1 to 150. Initially, they are all on.

First, you look at each of the light bulbs sequentially, and flip the switch if its index number is divisible by 3.

Then, you iterate through all the light bulbs again, but this time you only flip the switch if the index number is divisible by 5.

How many light bulbs are on now?