diff --git a/2020/d23/ex2/ex2.py b/2020/d23/ex2/ex2.py new file mode 100755 index 0000000..8993dcb --- /dev/null +++ b/2020/d23/ex2/ex2.py @@ -0,0 +1,55 @@ +#!/usr/bin/env python + +import itertools +import sys +from typing import List + + +def solve(circle: List[int]) -> int: + def to_links(circle: List[int]) -> List[int]: + links = [-1 for __ in range(len(circle) + 1)] + cyclic = itertools.cycle(circle) + next(cyclic) # Advance it by one + for prev, cur in zip(circle, cyclic): + links[prev] = cur + return links + + def step(links: List[int], current: int) -> int: + cup0 = links[current] + cup1 = links[cup0] + cup2 = links[cup1] + + links[current] = links[cup2] # Remove 3-tuple from the linked-list + + # Find destination + dest = (current - 1) if current > 1 else (len(links) - 1) + while dest in (cup0, cup1, cup2): + dest = (dest - 1) if dest > 1 else (len(links) - 1) + + # Update our links + links[cup2], links[dest] = links[dest], cup0 + + return links[current] # What's the next value in the cycle ? + + def to_answer(links: List[int]) -> int: + next = links[1] + return next * links[next] + + current = circle[0] + circle += [i + 1 for i in range(max(circle), 1000000)] + links = to_links(circle) + + for __ in range(10000000): + current = step(links, current) + + return to_answer(links) + + +def main() -> None: + input = [line.strip() for line in sys.stdin.readlines()] + assert len(input) == 1 + print(solve([int(c) for c in input[0]])) + + +if __name__ == "__main__": + main()