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import Control.Lens
import Control.Monad
import Data.List
import Linear.V2
import Linear.V3
import Linear.V4
import Linear.Vector
main :: IO ()
main = do
input <- lines <$> readFile "input"
let a = toVec input
_ = a :: [V4 Int]
-- _ = a :: [V3 Int]
print $ day17 a
day17 = length . flip (!!) 6 . iterate step
toVec input =
[ zero & _xy .~ V2 x y
| (y, line) <- zip [0 ..] input
, (x, c) <- zip [0 ..] line
, c == '#'
]
step state =
(nub .
concatMap fst .
filter f . ap zip (map length) . group . sort . updateNeighbours)
state
where
f (x, n) = n == 3 || n == 2 && head x `elem` state
updateNeighbours = concatMap f
where
f coord =
[ coord + v
| v <-
filter (/= zero) $
nub $ map sum $ subsequences (ap (++) (map negate) basis)
]