module Main where

import System.Environment (getArgs)
import System.IO (Handle, hIsEOF, hGetLine, withFile, IOMode(ReadMode))
import Text.Read (readMaybe)

import Data.List (sort)
import Data.Char (isDigit, isSpace)

mapSnd :: (b -> c) -> (a, b) -> (a, c)
mapSnd f (a, b) = (a, f b)

parseLine :: String -> IO (Int, Int)
parseLine line = let (l, (_, r)) = (mapSnd (span isSpace). span isDigit) line
                 in
                    case readMaybe l >>= \l' -> readMaybe r
                                     >>= \r' -> return (l', r') of
                        Just a -> return a
                        Nothing -> fail "Input file is incorrectly formatted"

readLists :: Handle -> IO ([Int], [Int])
readLists h = do
                  shouldStop <- hIsEOF h
                  if shouldStop
                      then return ([], [])
                      else do
                          line <- hGetLine h
                          (l, r) <- parseLine line
                          (ls, rs) <- readLists h
                          return (l:ls, r:rs)

computeDistance :: [Int] -> [Int] -> Int
computeDistance l r = sum (zipWith (curry (abs . uncurry (-))) l r)

main :: IO ()
main = do
    args <- getArgs
    (fp:_) <- return args
    (l, r) <- withFile fp ReadMode readLists

    putStrLn ("Solution (Part 1) : " ++ show (computeDistance (sort l) (sort r)))