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module Main (main) where
import Control.Monad (when)
import Data.List (foldl')
import Data.Map qualified as M
import System.Exit (die)
exampleExpectedOutput = 848
type Coord = (Int, Int, Int, Int)
type Input = M.Map Coord Bool
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
return . M.fromList . fst $ foldl' process ([], (0, 0, 0, 0)) input
where
process :: ([(Coord, Bool)], Coord) -> Char -> ([(Coord, Bool)], Coord)
process (acc, (x, y, z, w)) c = case c of
'.' -> (acc, (x+1, y, z, w))
'#' -> (((x, y, z, w), True):acc, (x+1, y, z, w))
'\n' -> (acc, (0, y+1, z, w))
r :: [Int]
r = [-1..1]
complete :: Input -> Input
complete input = M.foldrWithKey process M.empty input
where
process :: Coord -> Bool -> Input -> Input
process (x, y, z, w) False acc = acc
process (x, y, z, w) True acc = M.unions [M.singleton (x, y, z, w) True, acc, M.fromList [((x+a, y+b, z+c, w+d), False) | a<-r, b<-r, c<-r, d<-r]]
step :: Int -> Input -> Input
step 0 input = input
step n input = step (n-1) (M.foldrWithKey evolve M.empty input')
where
input' = complete input
evolve :: Coord -> Bool -> Input -> Input
evolve (x, y, z, w) s acc = case s of
False -> M.insert (x, y, z, w) (actives == 3) acc
True -> M.insert (x, y, z, w) (actives == 3 || actives == 4) acc
where
actives = length . filter id $ [lookup (x+a, y+b, z+c, w+d) | a<-r, b<-r, c<-r, d<-r]
lookup :: Coord -> Bool
lookup c = case M.lookup c input' of
Just True -> True
otherwise -> False
compute :: Input -> Int
compute input = M.size . M.filter id $ step 6 input
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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