    {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, DeriveDataTypeable, PatternGuards #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Layout.LimitWindows
    -- Copyright   :  (c) 2009 Adam Vogt
    --                (c) 2009 Max Rabkin -- wrote limitSelect
    -- License     :  BSD-style (see xmonad/LICENSE)
    --
    -- Maintainer  :  vogt.adam@gmail.com
    -- Stability   :  unstable
    -- Portability :  unportable
    --
    -- A layout modifier that limits the number of windows that can be shown.
    -- See "XMonad.Layout.Minimize" for manually setting hidden windows.
    --
    -----------------------------------------------------------------------------

    module XMonad.Layout.LimitWindows (
        -- * Usage
        -- $usage

        -- * Layout Modifiers
        limitWindows,limitSlice,limitSelect,

        -- * Change the number of windows
        increaseLimit,decreaseLimit,setLimit
        ) where

    import XMonad.Layout.LayoutModifier
    import XMonad
    import qualified XMonad.StackSet as W
    import XMonad.Layout (IncMasterN (..))
    import Control.Monad((<=<),guard)
    import Control.Applicative((<$>))
    import Data.Maybe(fromJust)

    -- $usage
    -- To use this module, add the following import to @~\/.xmonad\/xmonad.hs@:
    --
    -- > import XMonad.Layout.LimitWindows
    --
    -- > myLayout = limitWindows 6 $ Tall 1 0.03 0.5 ||| Full ||| RandomOtherLayout...
    -- > main = xmonad defaultConfig { layoutHook = myLayout }
    --
    -- You may also be interested in dynamically changing the number dynamically,
    -- by binding keys to the 'increaseLimit', 'decreaseLimit', or 'setLimit'
    -- actions.
    --
    -- For detailed instructions on editing your key bindings, see
    -- "XMonad.Doc.Extending#Editing_key_bindings".
    --
    -- See also 'XMonad.Layout.BoringWindows.boringAuto' for keybindings that skip
    -- the hidden windows.

    increaseLimit :: X ()
    increaseLimit = sendMessage $ LimitChange succ

    decreaseLimit :: X ()
    decreaseLimit = sendMessage . LimitChange $ max 1 . pred

    setLimit :: Int -> X ()
    setLimit tgt = sendMessage . LimitChange $ const tgt

    -- | Only display the first @n@ windows.
    limitWindows :: Int -> l a -> ModifiedLayout LimitWindows l a
    limitWindows n = ModifiedLayout (LimitWindows FirstN n)

    -- | Only display @n@ windows around the focused window. This makes sense with
    -- layouts that arrange windows linearily, like 'XMonad.Layout.Layout.Accordion'.
    limitSlice :: Int -> l a -> ModifiedLayout LimitWindows l a
    limitSlice n = ModifiedLayout (LimitWindows Slice n)

    -- | Only display the first @m@ windows and @r@ others.
    -- The @IncMasterN@ message will change @m@, as well as passing it onto the
    -- underlying layout.
    limitSelect :: Int -> Int -> l a -> ModifiedLayout Selection l a
    limitSelect m r = ModifiedLayout Sel{ nMaster=m, start=m, nRest=r }

    data LimitWindows a = LimitWindows SliceStyle Int deriving (Read,Show)

    data SliceStyle = FirstN | Slice deriving (Read,Show)

    data LimitChange = LimitChange { unLC :: (Int -> Int) } deriving (Typeable)

    instance Message LimitChange

    instance LayoutModifier LimitWindows a where
         pureMess (LimitWindows s n) =
            fmap (LimitWindows s) . pos <=< (`app` n) . unLC <=< fromMessage
          where pos x   = guard (x>=1)     >> return x
                app f x = guard (f x /= x) >>  return (f x)

         modifyLayout (LimitWindows style n) ws r =
            runLayout ws { W.stack = f n <$> W.stack ws } r
          where f = case style of
                        FirstN -> firstN
                        Slice -> slice

    firstN ::  Int -> W.Stack a -> W.Stack a
    firstN n st = upfocus $ fromJust $ W.differentiate $ take (max 1 n) $ W.integrate st
        where upfocus = foldr (.) id $ replicate (length (W.up st)) W.focusDown'

    -- | A non-wrapping, fixed-size slice of a stack around the focused element
    slice ::  Int -> W.Stack t -> W.Stack t
    slice n (W.Stack f u d) =
            W.Stack f (take (nu + unusedD) u)
                      (take (nd + unusedU) d)
        where unusedD = max 0 $ nd - length d
              unusedU = max 0 $ nu - length u
              nd = div (n - 1) 2
              nu = uncurry (+) $ divMod (n - 1) 2

    data Selection a = Sel { nMaster :: Int, start :: Int, nRest :: Int }
        deriving (Read, Show, Eq)

    instance LayoutModifier Selection a where
        modifyLayout s w r =
            runLayout (w { W.stack = updateAndSelect s <$> W.stack w }) r

        pureModifier sel _ stk wins = (wins, update sel <$> stk)

        pureMess sel m
            | Just f <- unLC <$> fromMessage m =
                Just $ sel { nRest = max 0 (f (nMaster sel + nRest sel) - nMaster sel) }
            | Just (IncMasterN n) <- fromMessage m =
                Just $ sel { nMaster = max 0 (nMaster sel + n) }
            | otherwise =
                Nothing

    select :: Selection l -> W.Stack a -> W.Stack a
    select s stk
        | lups < nMaster s
            = stk { W.down=take (nMaster s - lups - 1) downs ++
                        (take (nRest s) . drop (start s - lups - 1) $ downs) }
        | otherwise
            = stk { W.up=reverse (take (nMaster s) ups ++ drop (start s) ups),
                    W.down=take ((nRest s) - (lups - start s) - 1) downs }
        where
            downs = W.down stk
            ups = reverse $ W.up stk
            lups = length ups

    updateStart :: Selection l -> W.Stack a -> Int
    updateStart s stk
        | lups < nMaster s  -- the focussed window is in the master pane
            = start s `min` (lups + ldown - (nRest s) + 1) `max` nMaster s
        | otherwise
            = start s `min` lups
                      `max` (lups - (nRest s) + 1)
                      `min` (lups + ldown - (nRest s) + 1)
                      `max` nMaster s
        where
            lups = length $ W.up stk
            ldown = length $ W.down stk

    update :: Selection l -> W.Stack a -> Selection a
    update sel stk = sel { start=updateStart sel stk }

    updateAndSelect :: Selection l -> W.Stack a -> W.Stack a
    updateAndSelect sel stk = select (update sel stk) stk