advent-of-code/2020/d23/ex1/ex1.py

59 lines
1.5 KiB
Python
Executable file

#!/usr/bin/env python
import itertools
import sys
from typing import List, Tuple
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]
res = 0
while next != 1:
res = res * 10 + next
next = links[next]
return res
current = circle[0]
links = to_links(circle)
for __ in range(100):
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()