% from Lucasfilm Ltd.

linesdemo = proc ()
    slack = 150
    bwidth: int := int$parse(xdemo_default("lines", "BorderWidth"))
       except when not_found, overflow, bad_format: bwidth := 2 end
    back: x_pixmap := x_display$white()
    bdr: x_pixmap := x_display$black()
    plane: int := 1
    backpix: int := WhitePixel
    linepix: int := BlackPixel
    if x_display$cells() > 2
       then begin
	    r, g, b: int := x_parse_color(xdemo_default("lines", "Border"))
	    bdr := x_pixmap$tile(x_display$alloc_color(r, g, b))
	    end except when not_found: end
	    pixs: pixellist
	    pixs, plane := x_display$alloc_cells(1, 1, false)
	    backpix := pixs[1]
	    back := x_pixmap$tile(backpix)
	    r, g, b: int := x_parse_color(xdemo_default("lines", "Background"))
	       except when not_found:
			   r, g, b := x_display$query_color(WhitePixel)
		      end
	    x_display$store_color(backpix, r, g, b)
	    linepix := pixs[1] + plane
	    random_color(linepix)
       end
    w: x_window, wid0, hgt0: int := x_cons("lines", back, bdr,
					   xdemo_geometry(), "=400x400+1+1",
					   40, 40, bwidth)
    w.name := "lines"
    w.input := UnmapWindow
    x_window$map(w)
    w.input := ExposeWindow + UnmapWindow
    ev: event := x_input$empty_event()
    while true do
	x_window$clear(w)
	sx, sy, width, height, bw, ms, wk: int, iw: x_window := x_window$query(w)
	if width <= slack  cor  height <= slack
	   then x_window$destroy(w)
		return
	   end
	x: int := random$next(width)
	y: int := height - random$next(slack) - 1
	x1: int := random$next(width)
	y1: int := height - random$next(slack) - 1
	dx: int := random$next(3) + 1
	dy: int := random$next(3) + 1
	dx1: int := -1 - random$next(3)
	dy1: int := -1 - random$next(3)
	while ~x_input$pending() do
	    x := x + dx
	    if x < 0  cor  x >= width
	       then x := x - 2 * dx
		    dx := -dx
		    if linepix ~= BlackPixel
		       then random_color(linepix) end
	       end
	    y := y + dy
	    if y < height - slack  cor  y >= height
	       then y := y - 2 * dy
		    dy := -dy
	       end
	    x1 := x1 + dx1
	    if x1 < 0  cor  x1 >= width
	       then x1 := x1 - 2 * dx1
		    dx1 := -dx1
	       end
	    y1 := y1 + dy1
	    if y1 < height - slack  cor  y1 >= height
	       then y1 := y1 - 2 * dy1
		    dy1 := -dy1
		    if linepix ~= BlackPixel
		       then random_color(linepix) end
	       end
	    x_window$line(w, 0, 1, 1, x, y, x1, y1, GXinvert, plane)
	    x_window$move_area(w, 0, 1, width, height, 0, 0)
	    x_window$pix_set(w, backpix, 0, height - 1, width, 1)
	    end
	x_input$deq(ev)
	if ev.kind = UnmapWindow
	   then x_input$deq(ev) end
	end
    end linesdemo