2021: d15: ex1: add solution

2021/d15/ex1/ex1.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python
+
+import heapq
+import itertools
+import sys
+from typing import Iterator, List, NamedTuple, Set, Tuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+def solve(input: List[str]) -> int:
+    levels = [[int(c) for c in line] for line in input]
+
+    def neighbours(p: Point) -> Iterator[Point]:
+        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(levels):
+                continue
+            if y < 0 or y >= len(levels[0]):
+                continue
+            yield Point(x, y)
+
+    def djikstra(start: Point, end: Point) -> int:
+        # Priority queue of (distance, point)
+        queue = [(0, start)]
+        seen: Set[Point] = set()
+
+        while len(queue) > 0:
+            dist, p = heapq.heappop(queue)
+            if p == 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 neighbours(p):
+                heapq.heappush(queue, (dist + levels[n.x][n.y], n))
+
+        assert False  # Sanity check
+
+    return djikstra(Point(0, 0), Point(len(levels) - 1, len(levels[0]) - 1))
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()