54 lines
2 KiB
Haskell
54 lines
2 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.Maybe (catMaybes, mapMaybe)
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import qualified Data.Map as M
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import System.Exit (die)
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exampleExpectedOutput = 26
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type Seating = M.Map (Int, Int) (Maybe Bool)
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inputToList :: String -> [((Int, Int), Maybe Bool)]
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inputToList = 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), ((x, y), Nothing) : acc)
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nextSeat ((x, y), acc) 'L' = ((x+1, y), ((x, y), Just False) : acc)
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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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return $ M.fromList (inputToList input)
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compute :: Seating -> Int
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compute seating
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| seating == seating' = length . filter id . catMaybes $ map snd (M.toList seating)
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| otherwise = compute seating'
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where
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seating' :: Seating
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seating' = M.mapWithKey next seating
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next :: (Int, Int) -> Maybe Bool -> Maybe Bool
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next _ Nothing = Nothing
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next (x, y) (Just False) = Just $ around (x, y) == []
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next (x, y) (Just True) = Just $ length (around (x,y)) < 5
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around :: (Int, Int) -> [Bool]
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around (x, y) = filter id . catMaybes $ mapMaybe lookup [(-1, -1), (0, -1), (1, -1), (-1, 0), (1, 0), (-1, 1), (0, 1), (1, 1)]
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where
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lookup :: (Int, Int) -> Maybe (Maybe Bool)
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lookup (dx, dy) = lookup' (1, dx, dy)
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lookup' :: (Int, Int, Int) -> Maybe (Maybe Bool)
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lookup' (i, dx, dy) = case M.lookup (x + i * dx, y + i * dy) seating of
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Just (Just a) -> Just $ Just a
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Just Nothing -> lookup' (i+1, dx, dy)
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Nothing -> Nothing
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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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