    {-# LANGUAGE TypeSynonymInstances, DeriveDataTypeable, MultiParamTypeClasses, FlexibleContexts, FlexibleInstances, PatternGuards #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Layout.ComboP
    -- Copyright   :  (c) Konstantin Sobolev <konstantin.sobolev@gmail.com>
    -- License     :  BSD-style (see LICENSE)
    --
    -- Maintainer  :  Konstantin Sobolev <konstantin.sobolev@gmail.com>
    -- Stability   :  unstable
    -- Portability :  unportable
    --
    -- A layout that combines multiple layouts and allows to specify where to put
    -- new windows.
    --
    -----------------------------------------------------------------------------

    module XMonad.Layout.ComboP (
                                 -- * Usage
                                 -- $usage
                                 combineTwoP,
                                 CombineTwoP,
                                 SwapWindow(..),
                                 Property(..)
                                ) where

    import Data.List ( delete, intersect, (\\) )
    import Data.Maybe ( isJust )
    import Control.Monad
    import XMonad hiding (focus)
    import XMonad.StackSet ( integrate, Workspace (..), Stack(..) )
    import XMonad.Layout.WindowNavigation
    import XMonad.Util.WindowProperties
    import qualified XMonad.StackSet as W

    -- $usage
    -- You can use this module with the following in your @~\/.xmonad\/xmonad.hs@:
    --
    -- > import XMonad.Layout.ComboP
    --
    -- and add something like
    --
    -- > combineTwoP (TwoPane 0.03 0.5) (tabbed shrinkText defaultTConf) (tabbed shrinkText defaultTConf) (ClassName "Firefox")
    --
    -- to your layouts. This way all windows with class = \"Firefox\" will always go
    -- to the left pane, all others - to the right.
    --
    -- For more detailed instructions on editing the layoutHook see:
    --
    -- "XMonad.Doc.Extending#Editing_the_layout_hook"
    --
    -- 'combineTwoP' is a simple layout combinator based on 'combineTwo' from Combo, with
    -- addition of a 'Property' which tells where to put new windows. Windows mathing
    -- the property will go into the first part, all others will go into the second
    -- part. It supports @Move@ messages as 'combineTwo' does, but it also introduces
    -- 'SwapWindow' message which sends focused window to the other part. It is
    -- required because @Move@ commands don't work when one of the parts is empty.
    -- To use it, import \"XMonad.Layout.WindowNavigation\", and add the following key
    -- bindings (or something similar):
    --
    -- >    , ((modm .|. controlMask .|. shiftMask, xK_Right), sendMessage $ Move R)
    -- >    , ((modm .|. controlMask .|. shiftMask, xK_Left ), sendMessage $ Move L)
    -- >    , ((modm .|. controlMask .|. shiftMask, xK_Up   ), sendMessage $ Move U)
    -- >    , ((modm .|. controlMask .|. shiftMask, xK_Down ), sendMessage $ Move D)
    -- >    , ((modm .|. controlMask .|. shiftMask, xK_s    ), sendMessage $ SwapWindow)
    --
    -- For detailed instruction on editing the key binding see
    -- "XMonad.Doc.Extending#Editing_key_bindings".

    data SwapWindow =  SwapWindow        -- ^ Swap window between panes
                     | SwapWindowN Int   -- ^ Swap window between panes in the N-th nested ComboP. @SwapWindowN 0@ equals to SwapWindow
                     deriving (Read, Show, Typeable)
    instance Message SwapWindow

    data CombineTwoP l l1 l2 a = C2P [a] [a] [a] l (l1 a) (l2 a) Property
                                    deriving (Read, Show)

    combineTwoP :: (LayoutClass super(), LayoutClass l1 Window, LayoutClass l2 Window) =>
                    super () -> l1 Window -> l2 Window -> Property -> CombineTwoP (super ()) l1 l2 Window
    combineTwoP = C2P [] [] []

    instance (LayoutClass l (), LayoutClass l1 Window, LayoutClass l2 Window) =>
        LayoutClass (CombineTwoP (l ()) l1 l2) Window where
        doLayout (C2P f w1 w2 super l1 l2 prop) rinput s =
            let origws = W.integrate s           -- passed in windows
                w1c = origws `intersect` w1      -- current windows in the first pane
                w2c = origws `intersect` w2      -- current windows in the second pane
                new = origws \\ (w1c ++ w2c)     -- new windows
                superstack = Just Stack { focus=(), up=[], down=[()] }
                f' = focus s:delete (focus s) f  -- list of focused windows, contains 2 elements at most
            in do
                matching <- (hasProperty prop) `filterM` new  -- new windows matching predecate
                let w1' = w1c ++ matching                     -- updated first pane windows
                    w2' = w2c ++ (new \\ matching)            -- updated second pane windows
                    s1 = differentiate f' w1'                 -- first pane stack
                    s2 = differentiate f' w2'                 -- second pane stack
                ([((),r1),((),r2)], msuper') <- runLayout (Workspace "" super superstack) rinput
                (wrs1, ml1') <- runLayout (Workspace "" l1 s1) r1
                (wrs2, ml2') <- runLayout (Workspace "" l2 s2) r2
                return  (wrs1++wrs2, Just $ C2P f' w1' w2' (maybe super id msuper')
                    (maybe l1 id ml1') (maybe l2 id ml2') prop)

        handleMessage us@(C2P f ws1 ws2 super l1 l2 prop) m
            | Just SwapWindow      <- fromMessage m = swap us
            | Just (SwapWindowN 0) <- fromMessage m = swap us
            | Just (SwapWindowN n) <- fromMessage m = forwardToFocused us $ SomeMessage $ SwapWindowN $ n-1

            | Just (MoveWindowToWindow w1 w2) <- fromMessage m,
              w1 `elem` ws1,
              w2 `elem` ws2 = return $ Just $ C2P f (delete w1 ws1) (w1:ws2) super l1 l2 prop

            | Just (MoveWindowToWindow w1 w2) <- fromMessage m,
              w1 `elem` ws2,
              w2 `elem` ws1 = return $ Just $ C2P f (w1:ws1) (delete w1 ws2) super l1 l2 prop

            | otherwise = do ml1' <- handleMessage l1 m
                             ml2' <- handleMessage l2 m
                             msuper' <- handleMessage super m
                             if isJust msuper' || isJust ml1' || isJust ml2'
                                then return $ Just $ C2P f ws1 ws2
                                                     (maybe super id msuper')
                                                     (maybe l1 id ml1')
                                                     (maybe l2 id ml2') prop
                                else return Nothing

        description (C2P _ _ _ super l1 l2 prop) = "combining " ++ description l1 ++ " and "++
                                    description l2 ++ " with " ++ description super ++ " using "++ (show prop)

    -- send focused window to the other pane. Does nothing if we don't
    -- own the focused window
    swap :: (LayoutClass s a, LayoutClass l1 Window, LayoutClass l2 Window) =>
            CombineTwoP (s a) l1 l2 Window -> X (Maybe (CombineTwoP (s a) l1 l2 Window))
    swap (C2P f ws1 ws2 super l1 l2 prop) = do
        mst <- gets (W.stack . W.workspace . W.current . windowset)
        let (ws1', ws2') = case mst of
                Nothing -> (ws1, ws2)
                Just st -> if foc `elem` ws1
                               then (foc `delete` ws1, foc:ws2)
                               else if foc `elem` ws2
                                   then (foc:ws1, foc `delete` ws2)
                                   else (ws1, ws2)
                           where foc = W.focus st
        if (ws1,ws2) == (ws1',ws2')
            then return Nothing
            else return $ Just $ C2P f ws1' ws2' super l1 l2 prop

    -- forwards the message to the sublayout which contains the focused window
    forwardToFocused :: (LayoutClass l1 Window, LayoutClass l2 Window, LayoutClass s a) =>
                        CombineTwoP (s a) l1 l2 Window -> SomeMessage -> X (Maybe (CombineTwoP (s a) l1 l2 Window))
    forwardToFocused (C2P f ws1 ws2 super l1 l2 prop) m = do
        ml1 <- forwardIfFocused l1 ws1 m
        ml2 <- forwardIfFocused l2 ws2 m
        ms <- if isJust ml1 || isJust ml2
                then return Nothing
                else handleMessage super m
        if isJust ml1 || isJust ml2 || isJust ms
            then return $ Just $ C2P f ws1 ws2 (maybe super id ms) (maybe l1 id ml1) (maybe l2 id ml2) prop
            else return Nothing

    -- forwards message m to layout l if focused window is among w
    forwardIfFocused :: (LayoutClass l Window) => l Window -> [Window] -> SomeMessage -> X (Maybe (l Window))
    forwardIfFocused l w m = do
        mst <- gets (W.stack . W.workspace . W.current . windowset)
        maybe (return Nothing) send mst where
        send st = if (W.focus st) `elem` w
                    then handleMessage l m
                    else return Nothing

    -- code from CombineTwo
    -- given two sets of zs and xs takes the first z from zs that also belongs to xs
    -- and turns xs into a stack with z being current element. Acts as
    -- StackSet.differentiate if zs and xs don't intersect
    differentiate :: Eq q => [q] -> [q] -> Maybe (Stack q)
    differentiate (z:zs) xs | z `elem` xs = Just $ Stack { focus=z
                                                         , up = reverse $ takeWhile (/=z) xs
                                                         , down = tail $ dropWhile (/=z) xs }
                            | otherwise = differentiate zs xs
    differentiate [] xs = W.differentiate xs

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