import System.Console.GetOpt
import System.IO
import System.Exit
import System.Environment
import Data.List
import Data.Char
import Control.Monad
import Text.Printf

main = do
    (args, files) <- getArgs >>= parse
    when (Unbuffered `elem` args) $ hSetBuffering stdout NoBuffering
    mapM_ (cat args) files

withFile s f = putStr . unlines . f . lines =<< open s
  where
    open f = if f == "-" then getContents else readFile f

cat [] f = withFile f id
cat as f = withFile f (newline . number . visible as)
  where
    number  s    = if Blanks `elem` as then numberSome s else ifset Number numberAll s
    newline s    = ifset Dollar (map (++"$")) s
    visible as s = foldl' (flip render) s as
    ifset a f    = if a `elem` as then f else id

render Squeeze   = map head. groupBy (\x y -> all (all isSpace) [x,y])
render Tabs      = map $ concatMap (\c -> if c == '\t' then "^I" else [c])
render Invisible = map $ concatMap visible
  where
    visible c | c == '\t' || isPrint c = [c]
              | otherwise              = init . tail . show $ c
render _ = id

numberLine      = printf "%6d  %s"
numberAll s     = zipWith numberLine [(1 :: Integer)..] s
numberSome s    = reverse . snd $ foldl' draw (1,[]) s
  where
    draw (n,acc) s
            | all isSpace s = (n,   s : acc)
            | otherwise     = (n+1, numberLine n s : acc)

data Flag
    = Blanks                -- -b
    | Dollar                -- -e 
    | Squeeze               -- -s
    | Tabs                  -- -t
    | Unbuffered            -- -u
    | Invisible             -- -v
    | Number                -- -n
    | Help                  -- --help
    deriving (Eq,Ord,Enum,Show,Bounded)

flags =
   [Option ['b'] []       (NoArg Blanks)
        "Implies the -n option but doesn't count blank lines."
   ,Option ['e'] []       (NoArg Dollar)
        "Implies the -v option and also prints a dollar sign (`$') at the end of each line."
   ,Option ['n'] []       (NoArg Number)
        "Number the output lines, starting at 1."
   ,Option ['s'] []       (NoArg Squeeze)
        "Squeeze multiple adjacent empty lines, causing the output to be single spaced."
   ,Option ['t'] []       (NoArg Tabs)
        "Implies the -v option and also prints tab characters as `^I'."
   ,Option ['u'] []       (NoArg Unbuffered)
        "The output is guaranteed to be unbuffered (see setbuf(3))."
   ,Option ['v'] []       (NoArg Invisible)
        "Displays non-printing characters so they are visible."
   ,Option []    ["help"] (NoArg Help)
        "Print this help message"
   ]

parse argv = case getOpt Permute flags argv of
    (args,fs,[]) -> do
        let files = if null fs then ["-"] else fs
        if Help `elem` args
            then do hPutStrLn stderr (usageInfo header flags)
                    exitWith ExitSuccess
            else return (nub (concatMap set args), files)

    (_,_,errs)      -> do
        hPutStrLn stderr (concat errs ++ usageInfo header flags)
        exitWith (ExitFailure 1)

    where header = "Usage: cat [-benstuv] [file ...]"

          set Dollar = [Dollar, Invisible]
          set Tabs   = [Tabs,   Invisible]
          set f      = [f]