-- module XPrintable where
import Numeric
import Data.Char
import System.Environment

main :: IO ()
main = do
  args <- getArgs
  case args of
    ["encode"] -> interact encode
    ["decode"] -> interact decode
    _          -> error "Usage: x-printable [encode|decode]"

encode, decode :: String -> String

encode = concatMap (f . ord)
  where f x | x > 255      = error "impossible"
            | x > 128      = 'x' : showHex x []
            | x == ord 'x' = "xx"
            | otherwise    = [chr x]

decode = f
  where f [] = []
        f ('x' : i : j : xs)
          | isHexDigit i && isHexDigit j =
                case readHex [i, j] of
                  [(n, [])] -> chr n : f xs
                  _ -> 'x' : i : j : f xs
        f ('x' : 'x' : xs) = 'x' : f xs
        f (x : xs) = x : f xs

-- decode_encode_prop x = x == decode (encode x)
-- quickCheckWith stdArgs{maxSuccess=1000,maxSize=1000} decode_encode_prop