54 lines
1.5 KiB
Haskell
54 lines
1.5 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators
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module Main (main) where
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import Control.Monad (void, when)
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import Data.List (elemIndex)
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import Data.Maybe (catMaybes, fromJust)
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import Data.Void (Void)
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import Text.Megaparsec
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import Text.Megaparsec.Char
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import System.Exit (die)
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exampleExpectedOutput = 295
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type Input = (Int, [Maybe Int])
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type Parser = Parsec Void String
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parseShuttle :: Parser (Maybe Int)
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parseShuttle = do
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num <- (Just . read <$> some digitChar) <|> (char 'x' *> return Nothing)
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void . optional $ char ','
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return $ num
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parseOps :: Parser Input
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parseOps = do
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time <- some digitChar
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void $ char '\n'
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shuttles <- some parseShuttle
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void $ char '\n'
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void $ eof
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return $ ((read time), shuttles)
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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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case runParser parseOps filename input of
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Left bundle -> die $ errorBundlePretty bundle
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Right ops -> return ops
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compute :: Input -> Int
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compute (time, shuttles) = departure * busId
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where
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busId = allShuttles !! (fromJust $ elemIndex departure nextDepartures)
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departure = minimum nextDepartures
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nextDepartures = map (\a -> (-time) `mod` a) allShuttles
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allShuttles = catMaybes shuttles
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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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