2023: d17: ex2: add solution

2023/d17/ex2/ex2.py

@@ -0,0 +1,109 @@
+#!/usr/bin/env python
+
+import functools
+import heapq
+import sys
+from collections.abc import Iterator
+from enum import Enum
+from types import NotImplementedType
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+@functools.total_ordering
+class Direction(Enum):
+    NORTH = Point(-1, 0)
+    SOUTH = Point(1, 0)
+    EAST = Point(0, 1)
+    WEST = Point(0, -1)
+
+    def apply(self, pos: Point) -> Point:
+        dx, dy = self.value
+        return Point(pos.x + dx, pos.y + dy)
+
+    def __le__(self, other: object) -> bool | NotImplementedType:
+        if not isinstance(other, Direction):
+            return NotImplemented
+        return self.value <= other.value
+
+
+def solve(input: list[str]) -> int:
+    def parse(input: list[str]) -> dict[Point, int]:
+        res: dict[Point, int] = {}
+
+        for x, line in enumerate(input):
+            for y, c in enumerate(line):
+                res[Point(x, y)] = int(c)
+
+        return res
+
+    def possible_directions(
+        pos: Point, dir: Direction, in_a_row: int, map: dict[Point, int]
+    ) -> Iterator[tuple[Direction, int]]:
+        if in_a_row < 10:
+            yield dir, in_a_row + 1
+        DIRECTIONS = {
+            Direction.NORTH: (Direction.EAST, Direction.WEST),
+            Direction.SOUTH: (Direction.EAST, Direction.WEST),
+            Direction.WEST: (Direction.NORTH, Direction.SOUTH),
+            Direction.EAST: (Direction.NORTH, Direction.SOUTH),
+        }
+        if in_a_row >= 4:
+            for new_dir in DIRECTIONS[dir]:
+                dx, dy = new_dir.value
+                # Crucible must be able to travel 4 blocks in that direction
+                if Point(pos.x + 4 * dx, pos.y + 4 * dy) not in map:
+                    continue
+                yield new_dir, 1
+
+    def minimal_path(map: dict[Point, int], start: Point, end: Point) -> int:
+        class PathNode(NamedTuple):
+            pos: Point
+            dir: Direction
+            in_a_row: int
+
+        QueueNode = tuple[int, PathNode]
+
+        # Start with arbitrary all directions with *0* in a row, to get correct neighbours
+        queue: list[QueueNode] = [
+            (0, PathNode(start, Direction.SOUTH, 0)),
+            (0, PathNode(start, Direction.EAST, 0)),
+        ]
+        seen: set[PathNode] = set()
+
+        while queue:
+            dist, node = heapq.heappop(queue)
+            if node.pos == end:
+                return dist
+            # If we've already seen that exact node before, don't look at it again
+            if node in seen:
+                continue
+            # First time encountering those node conditions, record it
+            seen.add(node)
+            for dir, in_a_row in possible_directions(*node, map):
+                new_pos = dir.apply(node.pos)
+                if new_pos not in map:
+                    continue
+                new_dist = dist + map[new_pos]
+                new_node = PathNode(new_pos, dir, in_a_row)
+                heapq.heappush(queue, (new_dist, new_node))
+
+        assert False  # Sanity check
+
+    map = parse(input)
+    start = Point(0, 0)
+    end = Point(len(input) - 1, len(input[0]) - 1)
+    return minimal_path(map, start, end)
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()