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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import qualified Data.List as L
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 11
type Pair = (Int, Int)
type Input = [Pair]
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some digitChar <* optional hspace
parsePair :: Parser Pair
parsePair = (,) <$> parseNumber
<*> parseNumber
parseInput' :: Parser Input
parseInput' = some (parsePair <* eol) <* eof
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> error $ errorBundlePretty bundle
Right input' -> return input'
compute :: Input -> Int
compute input = sum compute'
where
(l, r) = unzip input
compute' = fmap abs $ zipWith (-) (L.sort l) (L.sort r)
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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