    {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Layout.ThreeColumns
    -- Copyright   :  (c) Kai Grossjohann <kai@emptydomain.de>
    -- License     :  BSD3-style (see LICENSE)
    --
    -- Maintainer  :  ?
    -- Stability   :  unstable
    -- Portability :  unportable
    --
    -- A layout similar to tall but with three columns. With 2560x1600 pixels this
    -- layout can be used for a huge main window and up to six reasonable sized
    -- slave windows.
    -----------------------------------------------------------------------------

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

                                  -- * Screenshots
                                  -- $screenshot
                                  ThreeCol(..)
                                 ) where

    import XMonad
    import qualified XMonad.StackSet as W

    import Data.Ratio

    import Control.Monad

    -- $usage
    -- You can use this module with the following in your @~\/.xmonad\/xmonad.hs@:
    --
    -- > import XMonad.Layout.ThreeColumns
    --
    -- Then edit your @layoutHook@ by adding the ThreeCol layout:
    --
    -- > myLayout = ThreeCol 1 (3/100) (1/2) ||| ThreeColMid 1 (3/100) (1/2) ||| etc..
    -- > main = xmonad defaultConfig { layoutHook = myLayout }
    --
    -- The first argument specifies how many windows initially appear in the main
    -- window. The second argument argument specifies the amount to resize while
    -- resizing and the third argument specifies the initial size of the columns.
    -- A positive size designates the fraction of the screen that the main window
    -- should occupy, but if the size is negative the absolute value designates the
    -- fraction a slave column should occupy. If both slave columns are visible,
    -- they always occupy the same amount of space.
    --
    -- The ThreeColMid variant places the main window between the slave columns.
    --
    -- For more detailed instructions on editing the layoutHook see:
    --
    -- "XMonad.Doc.Extending#Editing_the_layout_hook"

    -- $screenshot
    -- <<http://server.c-otto.de/xmonad/ThreeColumnsMiddle.png>>

    -- | Arguments are nmaster, delta, fraction
    data ThreeCol a = ThreeColMid { threeColNMaster :: !Int, threeColDelta :: !Rational, threeColFrac :: !Rational}
                    | ThreeCol    { threeColNMaster :: !Int, threeColDelta :: !Rational, threeColFrac :: !Rational}
        deriving (Show,Read)

    instance LayoutClass ThreeCol a where
        pureLayout (ThreeCol n _ f) r    = doL False n f r
        pureLayout (ThreeColMid n _ f) r = doL True n f r
        handleMessage l m =
            return $ msum [fmap resize     (fromMessage m)
                          ,fmap incmastern (fromMessage m)]
                where resize Shrink = l { threeColFrac = max (-0.5) $ f-d }
                      resize Expand = l { threeColFrac = min 1 $ f+d }
                      incmastern (IncMasterN x) = l { threeColNMaster = max 0 (n+x) }
                      n = threeColNMaster l
                      d = threeColDelta l
                      f = threeColFrac l
        description _ = "ThreeCol"

    doL :: Bool-> Int-> Rational-> Rectangle-> W.Stack a-> [(a, Rectangle)]
    doL m n f r = ap zip (tile3 m f r n . length) . W.integrate

    -- | tile3.  Compute window positions using 3 panes
    tile3 :: Bool -> Rational -> Rectangle -> Int -> Int -> [Rectangle]
    tile3 middle f r nmaster n
        | n <= nmaster || nmaster == 0 = splitVertically n r
        | n <= nmaster+1 = splitVertically nmaster s1 ++ splitVertically (n-nmaster) s2
        | otherwise = splitVertically nmaster r1 ++ splitVertically nslave1 r2 ++ splitVertically nslave2 r3
            where (r1, r2, r3) = split3HorizontallyBy middle (if f<0 then 1+2*f else f) r
                  (s1, s2) = splitHorizontallyBy (if f<0 then 1+f else f) r
                  nslave = (n - nmaster)
                  nslave1 = ceiling (nslave % 2)
                  nslave2 = (n - nmaster - nslave1)

    split3HorizontallyBy :: Bool -> Rational -> Rectangle -> (Rectangle, Rectangle, Rectangle)
    split3HorizontallyBy middle f (Rectangle sx sy sw sh) =
        if middle
        then ( Rectangle (sx + fromIntegral r3w) sy r1w sh
             , Rectangle (sx + fromIntegral r3w + fromIntegral r1w) sy r2w sh
             , Rectangle sx sy r3w sh )
        else ( Rectangle sx sy r1w sh
             , Rectangle (sx + fromIntegral r1w) sy r2w sh
             , Rectangle (sx + fromIntegral r1w + fromIntegral r2w) sy r3w sh )
            where r1w = ceiling $ fromIntegral sw * f
                  r2w = ceiling ( (sw - r1w) % 2 )
                  r3w = sw - r1w - r2w