Add solution to 2024-12-15

Author: fuglede

2024/day15/solutions.py

@@ -0,0 +1,49 @@
+with open("input") as f:
+    ls, moves = f.read().strip().split("\n\n")
+
+dirs = {"^": -1, "v": 1, "<": -1j, ">": 1j}
+moves = [dirs[x] for x in moves.replace("\n", "")]
+
+
+def solve(part2):
+    walls = set()
+    boxes = set()
+
+    for i, l in enumerate(ls.split("\n")):
+        for j, x in enumerate(l):
+            z = i + j * (2j if part2 else 1j)
+            match x:
+                case "#":
+                    walls |= {z, z + 1j} if part2 else {z}
+                case "O":
+                    boxes.add(z)
+                case "@":
+                    robot = z
+
+    for dz in moves:
+        to_move = set()
+        to_check = [robot + dz]
+        while to_check:
+            z = to_check.pop()
+            is_right_side = part2 and (left := z - 1j) in boxes
+            if z in boxes or is_right_side:
+                to_move.add(left if is_right_side else z)
+                to_check.append(z + dz)
+                if part2 and dz.real:
+                    other = left if is_right_side else z + 1j
+                    to_check.append(other + dz)
+            elif z in walls:
+                break
+        else:
+            boxes -= to_move
+            boxes |= {w + dz for w in to_move}
+            robot += dz
+    tot = sum(boxes)
+    return tot.real * 100 + tot.imag
+
+
+# Part 1
+print(solve(False))
+
+# Part 2
+print(solve(True))