[2024/6] p2 solved (slow - takes 5m30s)

Author: StarlitGhost

.aoc_tiles/tiles/2024/06.png

@@ -7,28 +7,93 @@ def move(pos: tuple, dir_: Dir, grid: Grid) -> tuple[tuple, Dir]:
     next_pos = pos + Vec2(dir_)
 
     # out of bounds or open, return next position
-    if not grid.in_bounds(next_pos) or not grid[next_pos] == '#':
+    if not grid.in_bounds(next_pos) or not grid[next_pos] in '#O':
         return next_pos.as_tuple(), dir_
 
     # obstacle, rotate right 90 degrees
     else:
         return pos, dir_.turn_right()
 
 
-def main():
-    grid = Grid(aoc.read_lines())
-
-    pos = grid.find('^')
-    dir_ = Dir.UP
-
+def simulate(pos: tuple, dir_: Dir, grid: Grid) -> tuple[set, list, bool]:
     visited = set()
     visited.add(pos)
+    path = []
+    path.append((pos, dir_))
+    loops = False
+
     while grid.in_bounds(pos):
         pos, dir_ = move(pos, dir_, grid)
         visited.add(pos)
+        if path[-1][0] == pos:
+            path.append((pos, dir_))
+            path.append((pos, None))
+        elif (pos, dir_) in path:
+            loops = True
+            break
+        else:
+            path.append((pos, dir_))
+
+    return visited, path, loops
+
+
+def print_path(grid: Grid, path: list, start_pos: tuple):
+    dir_chars = {
+        Dir.UP: '|',
+        Dir.DOWN: '|',
+        Dir.LEFT: '-',
+        Dir.RIGHT: '-',
+        None: '+',
+    }
+    print(grid.render_with_overlays([{pos: dir_chars[dir_] for pos, dir_ in path},
+                                     {start_pos: '^'}]))
+
+
+def main():
+    grid = Grid(aoc.read_lines())
+
+    start_pos = grid.find('^')
+    dir_ = Dir.UP
+
+    visited, path, _ = simulate(start_pos, dir_, grid)
+
+    # print(grid.render_with_overlays([{pos: 'X' for pos in visited}]))
+    # print("p1:", len(visited) - 1)
+
+    # print_path(grid, path, start_pos)
+
+    loop_obstacles = set()
+    tried_obstacles = set()
+    path = [p for p in path if p[1] is not None]
+
+    # try placing an obstacle at every position along our initial path
+    for i, pair in enumerate(path):
+        pos = pair[0]
+
+        # skip the start position, out-of-bounds, and previously tried obstacles
+        if pos == start_pos or not grid.in_bounds(pos) or pos in tried_obstacles:
+            continue
+
+        # place the obstacle
+        grid[pos] = 'O'
+        tried_obstacles.add(pos)
+
+        # start pathing from just before the newly placed obstacle
+        new_start_pos, new_dir = path[i-1] if path[i-1][0] != pos else path[i-2]
+
+        new_visited, new_path, loops = simulate(new_start_pos, new_dir, grid)
+
+        # track obstacles that cause loops
+        if loops:
+            loop_obstacles.add(pos)
+            # print_path(grid, new_path, new_start_pos)
+            print(f"{i}/{len(path)} {len(loop_obstacles)}")
+
+        # remove the current obstacle so we can try the next one
+        grid[pos] = '.'
 
-    print(grid.render_with_overlays([{pos: 'X' for pos in visited}]))
     print("p1:", len(visited) - 1)
+    print("p2:", len(loop_obstacles))
 
 
 if __name__ == "__main__":

2024/6/script.py

@@ -3,10 +3,10 @@ My solutions to the yearly Advents of Code
 
 <!-- AOC TILES BEGIN -->
 <h1 align="center">
-  Advent of Code - 172/462 ⭐
+  Advent of Code - 173/462 ⭐
 </h1>
 <h1 align="center">
-  2024 - 11 ⭐ - Python
+  2024 - 12 ⭐ - Python
 </h1>
 <a href="2024/1/script.py">
   <img src=".aoc_tiles/tiles/2024/01.png" width="161px">