deeply silly day

aoc2023_day10.py

pipe_types = {
    "-": [(-1, 0), (1, 0)],
    "|": [(0, -1), (0, 1)],
    "7": [(-1, 0), (0, 1)],
    "F": [(-1, 0), (0, -1)],
    "J": [(1, 0), (0, 1)],
    "L": [(1, 0), (0, -1)],
}

def nbrs(pos):
    x, y = pos
    yield x, y + 1
    yield x, y - 1
    yield x + 1, y
    yield x - 1, y

def make_terrain(tmap):
    connections = {}
    start = None
    for i, line in enumerate(tmap):
        for j, char in enumerate(line):
            pos = i, j
            if char == "S":
                start = pos
                continue
            conns = connections.setdefault(pos, set(nbrs(pos)))
            for dx, dy in pipe_types.get(char, ()):
                new_pos = i + dx, j + dy
                conns.discard(new_pos)
                connections.setdefault(new_pos, set(nbrs(new_pos))).discard(pos)
    return connections, start, (i + 1, j + 1)

def floodfill(conns, start):
    queue = [(start, 0)]
    ix = 0
    out = {}
    while ix < len(queue):
        pos, dist = queue[ix]
        ix += 1
        if pos in out: continue
        out[pos] = dist
        queue += [(nbr, dist + 1) for nbr in conns.get(pos, ())]
    return out

def scan(dims, terrain, boundary):
    h, w = dims
    count = 0
    for i in range(h):
        low, high = False, False
        for j in range(w):
            pos = i, j
            if pos in boundary:
                nbrs = terrain[pos]
                low ^= (i - 1, j) in nbrs
                high ^= (i + 1, j) in nbrs
            elif low and high:
                count += 1
    return count

with open("day10.in", "r") as fh:
    tmap = list(map(str.strip, fh))
    terrain, start, dims = make_terrain(tmap)

boundary = floodfill(terrain, start)

print(max(boundary.values()))
print(scan(dims, terrain, boundary))