advent-of-code/2022/d12/ex1/ex1.py

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2022-12-12 16:14:46 +01:00
#!/usr/bin/env python
import dataclasses
import heapq
import sys
from collections.abc import Iterator
from typing import NamedTuple
class Point(NamedTuple):
x: int
y: int
@dataclasses.dataclass
class HeightMap:
heights: list[list[int]]
start: Point
end: Point
def reachable_neighbours(self, p: Point) -> Iterator[Point]:
reachable_height = self.heights[p.x][p.y] + 1
for (dx, dy) in ((-1, 0), (1, 0), (0, -1), (0, 1)):
x, y = p.x + dx, p.y + dy
if x < 0 or x >= len(self.heights):
continue
if y < 0 or y >= len(self.heights[0]):
continue
if self.heights[x][y] > reachable_height:
continue
yield Point(x, y)
def solve(input: list[str]) -> int:
def to_height_map(input: list[str]) -> HeightMap:
start, end = None, None
heights = [[0 for _ in range(len(input[0]))] for _ in range(len(input))]
for i, line in enumerate(input):
for j, c in enumerate(line):
height = ord(c)
if c == "S":
height = ord("a")
start = Point(i, j)
if c == "E":
height = ord("z")
end = Point(i, j)
heights[i][j] = height - ord("a")
assert start is not None # Sanity check
assert end is not None # Sanity check
return HeightMap(heights, start, end)
def djikstra(map: HeightMap) -> int:
# Priority queue of (distance, point)
queue = [(0, map.start)]
seen: set[Point] = set()
while len(queue) > 0:
dist, p = heapq.heappop(queue)
if p == map.end:
return dist
# 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 map.reachable_neighbours(p):
heapq.heappush(queue, (dist + 1, n))
assert False # Sanity check
map = to_height_map(input)
return djikstra(map)
def main() -> None:
input = sys.stdin.read().splitlines()
print(solve(input))
if __name__ == "__main__":
main()