Three column layout

This layout is similar to tall, but has three columns.  The first column
is the master column.

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+-----------------------------------------------------------------------------
+-- |
+-- Module      :  XMonadContrib.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.
+--
+-----------------------------------------------------------------------------
+
+module XMonadContrib.ThreeColumns (
+                              -- * Usage
+                              -- $usage
+                              threeCol
+                             ) where
+
+import XMonad
+import qualified StackSet as W
+import Operations ( Resize(..), IncMasterN(..), splitVertically, tall )
+
+import Data.Ratio
+
+--import Control.Monad.State
+import Control.Monad.Reader
+
+import Graphics.X11.Xlib
+
+-- $usage
+--
+-- You can use this module with the following in your Config.hs file:
+--
+-- > import XMonadContrib.ThreeColumns
+--
+--  and add, to the list of layouts:
+--
+-- > threeCol
+
+threeCol :: Int -> Rational -> Rational -> Layout a
+threeCol nmaster delta frac =
+    Layout { doLayout     = \r -> return . (\x->(x,Nothing)) .
+                                  ap zip (tile3 frac r nmaster . length) . W.integrate
+           , modifyLayout = \m -> return $ msum [fmap resize     (fromMessage m)
+                                                ,fmap incmastern (fromMessage m)] }
+
+    where resize Shrink = tall nmaster delta (max 0 $ frac-delta)
+          resize Expand = tall nmaster delta (min 1 $ frac+delta)
+          incmastern (IncMasterN d) = tall (max 0 (nmaster+d)) delta frac
+
+-- | tile3.  Compute window positions using 3 panes
+tile3 :: Rational -> Rectangle -> Int -> Int -> [Rectangle]
+tile3 f r nmaster n = if n <= nmaster || nmaster == 0
+    then splitVertically n r
+    else splitVertically nmaster r1 ++ splitVertically nmid r2 ++ splitVertically nright r3
+  where (r1, r2, r3) = split3HorizontallyBy f r
+        nslave = (n - nmaster)
+        nmid = floor (nslave % 2)
+        nright = (n - nmaster - nmid)
+
+split3HorizontallyBy :: Rational -> Rectangle -> (Rectangle, Rectangle, Rectangle)
+split3HorizontallyBy f (Rectangle sx sy sw sh) =
+    ( Rectangle sx sy leftw sh
+    , Rectangle (sx + fromIntegral leftw) sy midw sh
+    , Rectangle (sx + fromIntegral leftw + fromIntegral midw) sy rightw sh )
+  where leftw = floor $ fromIntegral sw * (2/3) * f
+        midw = floor ( (sw - leftw) % 2 )
+        rightw = sw - leftw - midw