    module System.Console.Haskeline.Backend.DumbTerm where

    import System.Console.Haskeline.Backend.Posix
    import System.Console.Haskeline.Backend.Posix.Encoder (putEncodedStr)
    import System.Console.Haskeline.Backend.WCWidth
    import System.Console.Haskeline.Term
    import System.Console.Haskeline.LineState
    import System.Console.Haskeline.Monads as Monads

    import System.IO
    import Control.Monad(liftM)

    -- TODO:
    ---- Put "<" and ">" at end of term if scrolls off.
    ---- Have a margin at the ends

    data Window = Window {pos :: Int -- ^ # of visible chars to left of cursor
                    }

    initWindow :: Window
    initWindow = Window {pos=0}

    newtype DumbTerm m a = DumbTerm {unDumbTerm :: StateT Window (PosixT m) a}
                    deriving (Monad, MonadIO, MonadException,
                              MonadState Window,
                              MonadReader Handles, MonadReader Encoder)

    type DumbTermM a = forall m . (MonadIO m, MonadReader Layout m) => DumbTerm m a

    instance MonadTrans DumbTerm where
        lift = DumbTerm . lift . lift . lift

    evalDumb :: (MonadReader Layout m, CommandMonad m) => EvalTerm (PosixT m)
    evalDumb = EvalTerm (evalStateT' initWindow . unDumbTerm) (DumbTerm . lift)

    runDumbTerm :: Handles -> MaybeT IO RunTerm
    runDumbTerm h = liftIO $ posixRunTerm h (posixLayouts h) [] id evalDumb

    instance (MonadException m, MonadReader Layout m) => Term (DumbTerm m) where
        reposition _ s = refitLine s
        drawLineDiff = drawLineDiff'

        printLines = mapM_ (printText . (++ crlf))
        moveToNextLine _ = printText crlf
        clearLayout = clearLayoutD
        ringBell True = printText "\a"
        ringBell False = return ()

    printText :: MonadIO m => String -> DumbTerm m ()
    printText str = do
        h <- liftM ehOut ask
        encode <- ask
        liftIO $ putEncodedStr encode h str
        liftIO $ hFlush h

    -- Things we can assume a dumb terminal knows how to do
    cr,crlf :: String
    crlf = "\r\n"
    cr = "\r"

    backs,spaces :: Int -> String
    backs n = replicate n '\b'
    spaces n = replicate n ' '

    clearLayoutD :: DumbTermM ()
    clearLayoutD = do
        w <- maxWidth
        printText (cr ++ spaces w ++ cr)

    -- Don't want to print in the last column, as that may wrap to the next line.
    maxWidth :: DumbTermM Int
    maxWidth = asks (\lay -> width lay - 1)

    drawLineDiff' :: LineChars -> LineChars -> DumbTermM ()
    drawLineDiff' (xs1,ys1) (xs2,ys2) = do
        Window {pos=p} <- get
        w <- maxWidth
        let (xs1',xs2') = matchInit xs1 xs2
        let (xw1, xw2) = (gsWidth xs1', gsWidth xs2')
        let newP = p + xw2 - xw1
        let (ys2', yw2) = takeWidth (w-newP) ys2
        if xw1 > p  || newP >= w
            then refitLine (xs2,ys2)
            else do -- we haven't moved outside the margins
                put Window {pos=newP}
                case (xs1',xs2') of
                    ([],[]) | ys1 == ys2    -> return () -- no change
                    (_,[]) | xs1' ++ ys1 == ys2 -> -- moved left
                        printText $ backs xw1
                    ([],_) | ys1 == xs2' ++ ys2 -> -- moved right
                        printText (graphemesToString xs2')
                    _ -> let extraLength = xw1 + snd (takeWidth (w-p) ys1)
                                            - xw2 - yw2
                         in printText $ backs xw1
                            ++ graphemesToString (xs2' ++ ys2') ++ clearDeadText extraLength
                            ++ backs yw2

    refitLine :: ([Grapheme],[Grapheme]) -> DumbTermM ()
    refitLine (xs,ys) = do
        w <- maxWidth
        let (xs',p) = dropFrames w xs
        put Window {pos=p}
        let (ys',k) = takeWidth (w - p) ys
        printText $ cr ++ graphemesToString (xs' ++ ys')
            ++ spaces (w-k-p)
            ++ backs (w-p)
      where
        -- returns the width of the returned characters.
        dropFrames w zs = case splitAtWidth w zs of
                            (_,[],l) -> (zs,l)
                            (_,zs',_) -> dropFrames w zs'

    clearDeadText :: Int -> String
    clearDeadText n | n > 0 = spaces n ++ backs n
                    | otherwise = ""