2024: d15: ex2: add solution

2024/d15/ex2/ex2.py

@@ -0,0 +1,150 @@
+#!/usr/bin/env python
+
+import copy
+import enum
+import sys
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+class Direction(enum.StrEnum):
+    UP = "^"
+    RIGHT = ">"
+    DOWN = "v"
+    LEFT = "<"
+
+    def step(self, p: Point) -> Point:
+        dx: int
+        dy: int
+
+        match self:
+            case Direction.UP:
+                dx, dy = -1, 0
+            case Direction.RIGHT:
+                dx, dy = 0, 1
+            case Direction.DOWN:
+                dx, dy = 1, 0
+            case Direction.LEFT:
+                dx, dy = 0, -1
+
+        return Point(p.x + dx, p.y + dy)
+
+
+class Object(enum.StrEnum):
+    BOX = "O"
+    WALL = "#"
+
+
+# Maze always contains the left part of the object
+Maze = dict[Point, Object]
+# WideMaze maps left and right side of an object to its (left, right) tuple
+WideMaze = dict[Point, tuple[Point, Point]]
+
+
+def solve(input: str) -> int:
+    def parse_maze(input: list[str]) -> tuple[Point, Maze]:
+        robot: Point | None = None
+        maze: Maze = {}
+        for x, line in enumerate(input):
+            for y, c in enumerate(line):
+                if c == ".":
+                    continue
+                if c == "@":
+                    robot = Point(x, y * 2)
+                    continue
+                maze[Point(x, y * 2)] = Object(c)
+
+        assert robot is not None  # Sanity check
+        return robot, maze
+
+    def parse_directions(input: str) -> list[Direction]:
+        return [Direction(c) for c in input if c in Direction]
+
+    def parse(input: str) -> tuple[Point, Maze, list[Direction]]:
+        maze_input, directions_input = input.split("\n\n")
+        robot, maze = parse_maze(maze_input.splitlines())
+        directions = parse_directions(directions_input)
+        return robot, maze, directions
+
+    def step(robot: Point, maze: Maze, d: Direction) -> tuple[Point, Maze]:
+        def widen_maze() -> WideMaze:
+            res: WideMaze = {}
+            for p in maze.keys():
+                right_p = Point(p.x, p.y + 1)
+                res[p] = (p, right_p)
+                res[right_p] = (p, right_p)
+            return res
+
+        def boxes_along(wide_maze: WideMaze) -> set[Point] | None:
+            def helper(current: Point) -> set[Point] | None:
+                # Return empty set if we hit the air
+                if current not in wide_maze:
+                    return set()
+
+                # Query both sides of the object
+                left, right = wide_maze[current]
+
+                # Return None if we hit a wall
+                if maze[left] == Object.WALL:
+                    return None
+                assert maze.get(right) is None  # Sanity check
+
+                # Try to move both sides of the box recursively
+                res_left: set[Point] = set()
+                res_right: set[Point] = set()
+                # Only check next_left if not moving right
+                if (next_left := d.step(left)) != right:
+                    if (try_left := helper(next_left)) is None:
+                        return None
+                    res_left = try_left
+                # And only check next_right if not moving left
+                if (next_right := d.step(right)) != left:
+                    if (try_right := helper(next_right)) is None:
+                        return None
+                    res_right = try_right
+
+                # Both sides succeeded, return the set of boxes to move
+                return {left} | res_left | res_right
+
+            return helper(d.step(robot))
+
+        def move_boxes(boxes: set[Point]) -> Maze:
+            new_maze = copy.copy(maze)
+            # Do the move in two steps to avoid overwriting any values during movement
+            changes = {d.step(box): maze[box] for box in boxes}
+            for box in boxes:
+                new_maze.pop(box)
+            for box, obj in changes.items():
+                new_maze[box] = obj
+            return new_maze
+
+        new_robot = d.step(robot)
+        # If we hit a wall, abort the step
+        if maze.get(new_robot) == Object.WALL:
+            return robot, maze
+        # If a box hits a wall, abort the step
+        if (boxes := boxes_along(widen_maze())) is None:
+            return robot, maze
+        # Otherwise move everything along the direction
+        return new_robot, move_boxes(boxes)
+
+    def compute_coordinates(maze: Maze) -> int:
+        return sum(100 * p.x + p.y for p, obj in maze.items() if obj == Object.BOX)
+
+    robot, maze, directions = parse(input)
+    for d in directions:
+        robot, maze = step(robot, maze, d)
+    return compute_coordinates(maze)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()