day18b.py

from collections import deque

faces = {}

offsets = [(0, 0, 0.5), (0, 0.5, 0), (0.5, 0, 0), (0, 0, -0.5), (0, -0.5, 0), (-0.5, 0, 0)]

mx = my = mz = float("inf")
Mx = My = Mz = -float("inf")

droplet = set()

for line in open('input_day18.txt'):
    x, y, z = cell = tuple(map(int, line.split(",")))
    droplet.add(cell)
    
    mx = min(mx, x)
    my = min(my, y)
    mz = min(mz, z)
    
    Mx = max(Mx, x)
    My = max(My, y)
    Mz = max(Mz, z)
    
    for dx, dy, dz in offsets:
        k = (x + dx, y + dy, z + dz)
        if k not in faces:
            faces[k] = 0
        faces[k] += 1

mx -= 1
my -= 1
mz -= 1

Mx += 1
My += 1
Mz += 1

q = deque([(mx, my, mz)])
air = {(mx, my, mz)}

while q:
    x, y, z = q.popleft()

    for dx, dy, dz in offsets:
        nx, ny, nz = k = (x + dx * 2, y + dy * 2, z + dz * 2)
        
        if not (mx <= nx <= Mx and my <= ny <= My and mz <= nz <= Mz):
            continue
    
        if k in droplet or k in air:
            continue
    
        air.add(k)
        q.append(k)

free = set()

for x, y, z in air:
    for dx, dy, dz in offsets:
        free.add((x + dx, y + dy, z + dz))

print(len(set(faces) & free))