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