    {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Layout.Grid
    -- Copyright   :  (c) Lukas Mai
    -- License     :  BSD-style (see LICENSE)
    --
    -- Maintainer  :  <l.mai@web.de>
    -- Stability   :  unstable
    -- Portability :  unportable
    --
    -- A simple layout that attempts to put all windows in a square grid.
    --
    -----------------------------------------------------------------------------

    module XMonad.Layout.Grid (
        -- * Usage
        -- $usage
        Grid(..), arrange, defaultRatio
    ) where

    import XMonad
    import XMonad.StackSet

    -- $usage
    -- You can use this module with the following in your @~\/.xmonad\/xmonad.hs@:
    --
    -- > import XMonad.Layout.Grid
    --
    -- Then edit your @layoutHook@ by adding the Grid layout:
    --
    -- > myLayout = Grid ||| Full ||| etc..
    -- > main = xmonad defaultConfig { layoutHook = myLayout }
    --
    -- You can also specify an aspect ratio for Grid to strive for with the
    -- GridRatio constructor.  For example, if you want Grid to try to make a grid
    -- four windows wide and three windows tall, you could use
    --
    -- > myLayout = GridRatio (4/3) ||| etc.
    --
    -- For more detailed instructions on editing the layoutHook see:
    --
    -- "XMonad.Doc.Extending#Editing_the_layout_hook"

    data Grid a = Grid | GridRatio Double deriving (Read, Show)

    defaultRatio :: Double
    defaultRatio = 16/9

    instance LayoutClass Grid a where
        pureLayout Grid          r = pureLayout (GridRatio defaultRatio) r
        pureLayout (GridRatio d) r = arrange d r . integrate

    arrange :: Double -> Rectangle -> [a] -> [(a, Rectangle)]
    arrange aspectRatio (Rectangle rx ry rw rh) st = zip st rectangles
        where
        nwins = length st
        ncols = max 1 . round . sqrt $ fromIntegral nwins * fromIntegral rw / (fromIntegral rh * aspectRatio)
        mincs = max 1 $ nwins `div` ncols
        extrs = nwins - ncols * mincs
        chop :: Int -> Dimension -> [(Position, Dimension)]
        chop n m = ((0, m - k * fromIntegral (pred n)) :) . map (flip (,) k) . tail . reverse . take n . tail . iterate (subtract k') $ m'
            where
            k :: Dimension
            k = m `div` fromIntegral n
            m' = fromIntegral m
            k' :: Position
            k' = fromIntegral k
        xcoords = chop ncols rw
        ycoords = chop mincs rh
        ycoords' = chop (succ mincs) rh
        (xbase, xext) = splitAt (ncols - extrs) xcoords
        rectangles = combine ycoords xbase ++ combine ycoords' xext
            where
            combine ys xs = [Rectangle (rx + x) (ry + y) w h | (x, w) <- xs, (y, h) <- ys]