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