54 lines
2.1 KiB
Haskell
54 lines
2.1 KiB
Haskell
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module Main (main) where
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import Control.Monad (when)
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import Data.Array qualified as A
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import Data.List (foldl')
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import Data.Maybe (catMaybes)
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import System.Exit (die)
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exampleExpectedOutput = 37
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type Seating = A.Array (Int, Int) (Maybe Bool)
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inputToArrayAssociations :: String -> [((Int, Int), Maybe Bool)]
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inputToArrayAssociations = snd . foldl' nextSeat ((0, 0), [])
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where
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nextSeat :: ((Int, Int), [((Int, Int), Maybe Bool)]) -> Char -> ((Int, Int), [((Int, Int), Maybe Bool)])
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nextSeat ((x, y), acc) '\n' = ((0, y+1), acc)
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nextSeat ((x, y), acc) '.' = ((x+1, y), acc ++ [((x, y), Nothing)])
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nextSeat ((x, y), acc) 'L' = ((x+1, y), acc ++ [((x, y), Just False)])
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parseInput :: String -> IO Seating
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parseInput filename = do
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input <- readFile filename
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let ls = lines input
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height = (length ls) - 1
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width = (length (ls !! 0)) - 1
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return $ A.array ((0,0), (width, height)) (inputToArrayAssociations input)
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compute :: Seating -> Int
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compute seating
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| seating == seating' = length . filter id . catMaybes $ A.elems seating
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| otherwise = compute seating'
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where
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(width, height) = snd $ A.bounds seating
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seating' :: Seating
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seating' = seating A.// (foldl' next [] (A.assocs seating))
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next :: [((Int, Int), Maybe Bool)] -> ((Int, Int), Maybe Bool) -> [((Int, Int), Maybe Bool)]
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next acc ((x, y), Just False) = ((x, y), Just $ around (x, y) == []) : acc
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next acc ((x, y), Just True) = ((x, y), Just $ length (around (x,y)) < 4) : acc
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next acc _ = acc
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around :: (Int, Int) -> [Bool]
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around (x, y) = filter id . catMaybes $ map lookup [(x-1, y-1), (x, y-1), (x+1, y-1), (x-1, y), (x+1, y), (x-1, y+1), (x, y+1), (x+1, y+1)]
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lookup :: (Int, Int) -> Maybe Bool
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lookup (x, y)
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| x < 0 || y < 0 || x > width || y > height = Nothing
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| otherwise = seating A.! (x, y)
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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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