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module Main where
import Control.Monad ((>=>))
import Data.Char (ord)
import System.Environment (getArgs)
import System.Exit (exitFailure)
main :: IO ()
main = do
testISBN <- getArgs >>= maybe (die usage) return . safeHead
either die print $ calculateCheckDigit <$> parseISBN testISBN
where usage = "Pass ISBN as first argument."
parseISBN :: String -> Either String [Int]
parseISBN =
maybeToEither "Contains invalid ISBN characters." . mapM digitToInt
. filter (/= '-')
>=> checkLength
where
checkLength x | length x == 12 = return x
| otherwise = Left "Need 12 digits in ISBN."
calculateCheckDigit :: [Int] -> Int
calculateCheckDigit isbn12 =
let weightedSums = sum $ zipWith (*) isbn12 weights
in (10 - weightedSums `mod` 10) `mod` 10
where
weights = [1, 3] ++ weights
digitToInt :: Char -> Maybe Int
digitToInt c =
let digit = ord c - ord '0'
in if 0 <= digit && digit <= 9
then return digit
else Nothing
safeHead :: [a] -> Maybe a
safeHead [] = Nothing
safeHead (x : _) = Just x
maybeToEither :: a -> Maybe b -> Either a b
maybeToEither a = maybe (Left a) Right
die :: String -> IO a
die msg = putStrLn msg >> exitFailure
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