    {-# LANGUAGE GeneralizedNewtypeDeriving #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module       : XMonad.Actions.PhysicalScreens
    -- Copyright    : (c) Nelson Elhage <nelhage@mit.edu>
    -- License      : BSD
    --
    -- Maintainer   : Nelson Elhage <nelhage@mit.edu>
    -- Stability    : unstable
    -- Portability  : unportable
    --
    -- Manipulate screens ordered by physical location instead of ID
    -----------------------------------------------------------------------------

    module XMonad.Actions.PhysicalScreens (
                                            -- * Usage
                                            -- $usage
                                            PhysicalScreen(..)
                                          , getScreen
                                          , viewScreen
                                          , sendToScreen
                                          , onNextNeighbour
                                          , onPrevNeighbour
                                          ) where

    import XMonad
    import qualified XMonad.StackSet as W

    import Data.List (sortBy,findIndex)
    import Data.Function (on)

    {- $usage

    This module allows you name Xinerama screens from XMonad using their
    physical location relative to each other (as reported by Xinerama),
    rather than their @ScreenID@ s, which are arbitrarily determined by
    your X server and graphics hardware.

    Screens are ordered by the upper-left-most corner, from top-to-bottom
    and then left-to-right.

    Example usage in your @~\/.xmonad\/xmonad.hs@ file:

    > import XMonad.Actions.PhysicalScreens

    > , ((modMask, xK_a), onPrevNeighbour W.view)
    > , ((modMask, xK_o), onNextNeighbour W.view)
    > , ((modMask .|. shiftMask, xK_a), onPrevNeighbour W.shift)
    > , ((modMask .|. shiftMask, xK_o), onNextNeighbour W.shift)

    > --
    > -- mod-{w,e,r}, Switch to physical/Xinerama screens 1, 2, or 3
    > -- mod-shift-{w,e,r}, Move client to screen 1, 2, or 3
    > --
    > [((modm .|. mask, key), f sc)
    >     | (key, sc) <- zip [xK_w, xK_e, xK_r] [0..]
    >     , (f, mask) <- [(viewScreen, 0), (sendToScreen, shiftMask)]]

    For detailed instructions on editing your key bindings, see
    "XMonad.Doc.Extending#Editing_key_bindings".
     -}

    -- | The type of the index of a screen by location
    newtype PhysicalScreen = P Int deriving (Eq,Ord,Show,Read,Enum,Num,Integral,Real)

    -- | Translate a physical screen index to a "ScreenId"
    getScreen :: PhysicalScreen -> X (Maybe ScreenId)
    getScreen (P i) = do w <- gets windowset
                         let screens = W.current w : W.visible w
                         if i<0 || i >= length screens
                          then return Nothing
                          else let ss = sortBy (cmpScreen `on` (screenRect . W.screenDetail)) screens
                               in return $ Just $ W.screen $ ss !! i

    -- | Switch to a given physical screen
    viewScreen :: PhysicalScreen -> X ()
    viewScreen p = do i <- getScreen p
                      whenJust i $ \s -> do
                      w <- screenWorkspace s
                      whenJust w $ windows . W.view

    -- | Send the active window to a given physical screen
    sendToScreen :: PhysicalScreen -> X ()
    sendToScreen p = do i <- getScreen p
                        whenJust i $ \s -> do
                          w <- screenWorkspace s
                          whenJust w $ windows . W.shift

    -- | Compare two screens by their top-left corners, ordering
    -- | top-to-bottom and then left-to-right.
    cmpScreen :: Rectangle -> Rectangle -> Ordering
    cmpScreen (Rectangle x1 y1 _ _) (Rectangle x2 y2 _ _) = compare (y1,x1) (y2,x2)

    -- | Get ScreenId for neighbours of the current screen based on position offset.
    getNeighbour :: Int -> X ScreenId
    getNeighbour d = do w <- gets windowset
                        let ss = map W.screen $ sortBy (cmpScreen `on` (screenRect . W.screenDetail)) $ W.current w : W.visible w
                            curPos = maybe 0 id $ findIndex (== W.screen (W.current w)) ss
                            pos = (curPos + d) `mod` length ss
                        return $ ss !! pos

    neighbourWindows :: Int -> (WorkspaceId -> WindowSet -> WindowSet) -> X ()
    neighbourWindows d f = do s <- getNeighbour d
                              w <- screenWorkspace s
                              whenJust w $ windows . f

    -- | Apply operation on a WindowSet with the WorkspaceId of the next screen in the physical order as parameter.
    onNextNeighbour :: (WorkspaceId -> WindowSet -> WindowSet) -> X ()
    onNextNeighbour = neighbourWindows 1

    -- | Apply operation on a WindowSet with the WorkspaceId of the previous screen in the physical order as parameter.
    onPrevNeighbour :: (WorkspaceId -> WindowSet -> WindowSet) -> X ()
    onPrevNeighbour = neighbourWindows (-1)