advent-of-code/2024/d18/ex2/ex2.py

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2024-12-18 06:25:04 +01:00
#!/usr/bin/env python
import heapq
import sys
from typing import Iterator, NamedTuple
class Point(NamedTuple):
x: int
y: int
def neighbours(self) -> Iterator["Point"]:
for dx, dy in (
(-1, 0),
(1, 0),
(0, -1),
(0, 1),
):
yield Point(self.x + dx, self.y + dy)
DIMS = Point(70, 70)
def solve(input: str) -> str:
def parse(input: list[str]) -> list[Point]:
return [Point(*map(int, line.split(","))) for line in input]
def djikstra(start: Point, end: Point, blocks: set[Point]) -> int | None:
# Priority queue of (distance, point)
queue = [(0, start)]
seen: set[Point] = set()
while len(queue) > 0:
cost, p = heapq.heappop(queue)
if p == end:
return cost
# We must have seen p with a smaller distance before
if p in seen:
continue
# First time encountering p, must be the smallest distance to it
seen.add(p)
# Add all neighbours to be visited
for n in p.neighbours():
if p in blocks:
continue
if not 0 <= p.x <= DIMS.x:
continue
if not 0 <= p.y <= DIMS.y:
continue
heapq.heappush(queue, (cost + 1, n))
return None
def bisect_cutoff(start: Point, end: Point, blocks: list[Point]) -> Point:
# Cutting off the path is monotonic: once cut-off, it's never uncut
low, high = 0, len(blocks)
while low < high:
mid = low + (high - low) // 2
if djikstra(start, end, set(blocks[: mid + 1])) is None:
high = mid
else:
low = mid + 1
return blocks[low]
coords = parse(input.splitlines())
byte = bisect_cutoff(Point(0, 0), DIMS, coords)
return f"{byte.x},{byte.y}"
def main() -> None:
input = sys.stdin.read()
print(solve(input))
if __name__ == "__main__":
main()