{- Written by Antti-Juhani Kaijanaho.
   You may treat this file as if it were in the public domain.
-}
module Main where

import Graphics.SOE.Gtk

main =
  runGraphics $ do
    w <- openWindowEx "Bouncing Ball" Nothing
                      (Just (300, 300)) drawBufferedGraphic
                      (Just 20) --20ms timer
    loop w 300 100 5 5


loop w x y xd yd = do
  setGraphic w $ withColor Yellow $
    ellipse (x-5,y-5) (x+5,y+5)
  (xmax, ymax) <- getWindowSize w
  let x' = x + xd + 5
      y' = y + yd + 5
      xd' | x' >= xmax || x' < 0 = -xd
          | otherwise = xd
      yd' | y' >= ymax || y' < 0 = -yd
          | otherwise = yd
      x'' = x + xd'
      y'' = y + yd'
      x''' | x'' + 5 > xmax = xmax `div` 2
           | otherwise = x''
      y''' | y'' + 5 > ymax = ymax `div` 2
           | otherwise = y''

  let nextEvent = do
       e <- maybeGetWindowEvent w
       case e of

         -- stop when the user closes the window
         Just Closed -> return ()

         -- ignore mouse and other button events
         Just _      -> nextEvent

         -- if there's no events pending, wait for the timer
         -- then go round again
         Nothing     -> do getWindowTick w
                           loop w x''' y''' xd' yd'
  nextEvent