% from Lucasfilm Ltd.

bouncedemo = proc ()
    nbounce = 20
    bwidth: int := int$parse(xdemo_default("bounce", "BorderWidth"))
       except when not_found, overflow, bad_format: bwidth := 2 end
    back: x_pixmap := x_display$white()
    bdr: x_pixmap := x_display$black()
    plane: int := 1
    ballpix: int := BlackPixel
    if x_display$cells() > 2
       then begin
	    r, g, b: int := x_parse_color(xdemo_default("bounce", "Border"))
	    bdr := x_pixmap$tile(x_display$alloc_color(r, g, b))
	    end except when not_found: end
	    pixs: pixellist
	    pixs, plane := x_display$alloc_cells(1, 1, false)
	    back := x_pixmap$tile(pixs[1])
	    r, g, b: int := x_parse_color(xdemo_default("bounce", "Background"))
	       except when not_found:
			   r, g, b := x_display$query_color(WhitePixel)
		      end
	    x_display$store_color(pixs[1], r, g, b)
	    ballpix := pixs[1] + plane
	    random_color(ballpix)
       end
    w: x_window, wid0, hgt0: int := x_cons("bounce", back, bdr,
					   xdemo_geometry(), "=400x400+1+1",
					   40, 40, bwidth)
    w.name := "bounce"
    w.input := UnmapWindow
    x_window$map(w)
    w.input := ExposeWindow + UnmapWindow
    ev: event := x_input$empty_event()
    nobit: x_bitmap := x_bitmap$none()
    while true do
	x_window$clear(w)
	sx, sy, width, height, bw, ms, wk: int, iw: x_window := x_window$query(w)
	if width <= 30  cor  height <= 30
	   then x_window$destroy(w)
		return
	   end
	width := width - nbounce
	height := height - nbounce - 1
	x: int := 0
	y: int := 0
	vx: int := 4
	vy: int := 0
	wait: bool := false
	while ~x_input$pending() do
	    x_window$pix_fill(w, 0, nobit, x, y, nbounce, nbounce,
			      GXinvert, plane)
	    wait := ~wait
	    if wait
	       then x_window$query_mouse(w) end
	    x := x + vx
	    if x >= width
	       then x := 2 * width - x - 2
		    vx := -vx
	     elseif x < 0
	       then x := -x
		    vx := -vx
	     end
	    vy := vy + 1
	    y := y + vy
	    if y >= height
	       then y := 2 * height - y
		    if vy < nbounce
		       then vy := 1 - vy
		       else vy := vy / nbounce - vy
		       end
		    if vy = 0
		       then x := 0
			    y := 0
			    vx := 4
			    vy := 0
			    if ballpix ~= BlackPixel
			       then random_color(ballpix) end
		       end
	       end
	    end
	x_input$deq(ev)
	if ev.kind = UnmapWindow
	   then x_input$deq(ev) end
	end
    end bouncedemo