zerosleeps

Since 2010

Advent of Code 2020 day 23

Thursday 24 December 2020

Advent of Code 2020 day 23. Ooh I liked this one because it taught me something new: linked lists!

I took the “obvious” approach for part one, but almost nothing of that version is shown below as I first re-factored it, then stuck part_two_answer on top of it.

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import unittest

class Game():
    def __init__(self, starting_circle, part=1):
        starting_circle = [int(char) for char in starting_circle]
        self.max_cup = max(starting_circle)
        self.min_cup = min(starting_circle)

        if part == 2:
            starting_circle += [ i for i in range(self.max_cup + 1, 1_000_000 + 1)]
            self.max_cup = 1_000_000

        self.circle = {
            cup: starting_circle[(i + 1) % len(starting_circle)]
            for i, cup in enumerate(starting_circle)
        }

        self.length = len(self.circle)
        self.current_cup = starting_circle[0]

    def move(self):
        crab = [
            self.circle[self.current_cup],
            self.circle[self.circle[self.current_cup]],
            self.circle[self.circle[self.circle[self.current_cup]]]
        ]

        destination = self.current_cup - 1

        if destination < self.min_cup:
            destination = self.max_cup

        while destination in crab:
            destination -= 1
            if destination < self.min_cup:
                destination = self.max_cup

        destination_original_link = self.circle[destination]
        chain_end_original_link = self.circle[crab[-1]]

        self.circle[destination] = crab[0]
        self.circle[crab[-1]] = destination_original_link
        self.circle[self.current_cup] = chain_end_original_link
        self.current_cup = chain_end_original_link

    def part_one_answer(self):
        cup = 1
        result = ""
        for i in range(self.length - 1):
            result += str(self.circle[cup])
            cup = self.circle[cup]
        return result

    def part_two_answer(self):
        return self.circle[1] * self.circle[self.circle[1]]

def part_one(input):
    game = Game(input)
    for _ in range(100):
        game.move()
    return game.part_one_answer()

def part_two(input):
    game = Game(input, 2)
    for _ in range(10_000_000):
        game.move()
    return game.part_two_answer()

class TestPartOne(unittest.TestCase):
    def test_part_one_example(self):
        self.assertEqual(part_one("389125467"), '67384529')

class TestPartTwo(unittest.TestCase):
    def test_part_two_example(self):
        self.assertEqual(part_two("389125467"), 149245887792)

if __name__ == '__main__':

    print(f"Part one: {part_one(INPUT)}")
    print(f"Part two: {part_two(INPUT)}")