Advent of Code 2023 Day 10

After a few days’ delay, I’ve finally solved this vexing puzzle. You are given a large grid strewn with characters - -, |, J, F, 7, and L - that represent pieces of pipe that point in different directions. The pipe contains a large loop that starts at a given point. Part 1 simply asks for the size of the loop.

Part 2 “reveals” that you actually want to find the area enclosed by the pipe loop, including disconnected pipe pieces lying around. Just one problem: it’s possible to squeeze between pipes, (e.g., two parallel lines of pipe) meaning an area completely surrounded by pipe tiles might be accessible.

I struggled to deal with this before finding the obvious solution: convert each grid square into nine, so squeezing between pipes can be directly modeled. Then use flood-fill to find all coordinates enclosed by the grid, convert them back to the original coordinates by integer-dividing by 3, then count them.

After more debugging than I would like, I got it right:

def flood_fill(grid, xmin, xmax, ymin, ymax, get_neighbors, traversed):
    exposed = set()
    enclosed = set()
    for coord, el in grid.items():
        if (el and downscale(el, 3) not in traversed) or (coord in enclosed or coord in exposed):
            continue
        queue = deque()
        queue.append(coord)
        visited = set()
        open = False

        while queue:
            new = queue.pop()
            visited.add(new)
            open = open or (
                new in exposed
                or (
                    new.real == xmin
                    or new.real == xmax
                    or new.imag == ymin
                    or new.imag == ymax
                )
            )
            new_neighbors = get_neighbors(new)

            for neighbor in new_neighbors:
                # if neighbor in exposed:
                #     open = True
                if not ((grid[neighbor] and downscale(neighbor, 3) in traversed) or neighbor in visited):
                    queue.append(neighbor)
        if open:
            exposed.update(visited)
        else:
            enclosed.update(visited)
    return enclosed