Add solution to 2024-12-15

Author: fuglede

2024/day15/solutions.py

@@ -0,0 +1,46 @@
+with open("input") as f:
+    ls, moves = f.read().strip().split("\n\n")
+
+dirs = {"^": -1, "v": 1, "<": -1j, ">": 1j}
+moves = list(map(dirs.__getitem__, moves.replace("\n", "")))
+board = {i + 1j * j: x for i, l in enumerate(ls.split("\n")) for j, x in enumerate(l)}
+
+
+def solve(part2):
+    walls = {z for z, x in board.items() if x == "#"}
+    boxes = {z for z, x in board.items() if x == "O"}
+    robot = next(z for z, x in board.items() if x == "@")
+    if part2:
+        walls = {w.real + 2 * w.imag * 1j for w in walls}
+        walls |= {z + 1j for z in walls}
+        # For part 2, only track the left side of the boxes
+        boxes = {b.real + 2 * b.imag * 1j for b in boxes}
+        robot = robot.real + 2 * robot.imag * 1j
+    for dz in moves:
+        to_move = set()
+        to_check = [robot + dz]
+        while to_check:
+            z = to_check.pop()
+            is_right_side = part2 and z - 1j in boxes
+            if z in boxes or is_right_side:
+                to_move.add(z)
+                to_check.append(z + dz)
+                if part2 and dz.real:
+                    other = z - 1j if is_right_side else z + 1j
+                    to_move.add(other)
+                    to_check.append(other + dz)
+            elif z in walls:
+                break
+        else:
+            to_move = to_move & boxes
+            boxes -= to_move
+            boxes |= {w + dz for w in to_move}
+            robot += dz
+    return sum(z.real * 100 + z.imag for z in boxes)
+
+
+# Part 1
+print(solve(False))
+
+# Part 2
+print(solve(True))