105 lines
2.6 KiB
Python
Executable file
105 lines
2.6 KiB
Python
Executable file
#!/usr/bin/env python
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import copy
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import sys
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from collections.abc import Iterator
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from typing import NamedTuple, Optional
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class Point(NamedTuple):
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x: int
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y: int
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def neighbours(self) -> Iterator["Point"]:
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for dx, dy in (
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(-1, 0),
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(1, 0),
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(0, -1),
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(0, 1),
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):
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yield Point(self.x + dx, self.y + dy)
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Trails = set[Point]
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Graph = dict[Point, dict[Point, int]]
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def solve(input: list[str]) -> int:
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def parse(input: list[str]) -> Trails:
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res: Trails = set()
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for x, line in enumerate(input):
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for y, c in enumerate(line):
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if c == "#":
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continue
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res.add(Point(x, y))
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return res
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def to_graph(trails: Trails) -> Graph:
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graph: Graph = {p: {} for p in trails}
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for p in trails:
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for n in p.neighbours():
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if n not in trails:
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continue
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graph[p][n] = 1
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return graph
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# Remove every node which has exactly two neighbours (i.e: straight lines)
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def condense_graph(graph: Graph) -> Graph:
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graph = copy.deepcopy(graph)
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nodes = list(graph.keys())
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for n in nodes:
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if len(graph[n]) != 2:
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continue
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(n1, d1), (n2, d2) = graph[n].items()
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del graph[n1][n]
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del graph[n2][n]
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del graph[n]
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graph[n1][n2] = d1 + d2
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graph[n2][n1] = d1 + d2
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return graph
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def explore(graph: Graph, start: Point, end: Point) -> int:
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def recurse(start: Point, seen: set[Point]) -> Optional[int]:
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if start == end:
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return 0
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if start not in graph:
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return None
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next_step = (
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(dist, recurse(n, seen | {n}))
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for n, dist in graph[start].items()
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if n not in seen
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)
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distances = [
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(dist + steps) for dist, steps in next_step if steps is not None
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]
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if not distances:
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return None
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return max(distances)
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res = recurse(start, {start})
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assert res is not None # Sanity check
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return res
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trails = parse(input)
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graph = to_graph(trails)
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graph = condense_graph(graph)
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start, dest = Point(0, 1), Point(len(input) - 1, len(input[0]) - 2)
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assert start in graph # Sanity check
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assert dest in graph # Sanity check
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return explore(graph, start, dest)
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def main() -> None:
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input = sys.stdin.read().splitlines()
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print(solve(input))
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if __name__ == "__main__":
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main()
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