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