martin attractor

This pattern generator, discovered by Barry Martin, was nicknamed 'Hopalong' when Scientific American introduced it in their September '86 issue.

Clicking the window adjusts the viewport position; there is also an alternate version with color and animation.

Also, for a certain Rubyist friend, I wrote another lazily-evaluated, colored and animated implementation in Ruby.

	import Control.Arrow ((***))
	import Graphics.UI.SDL as SDL
	(xres, yres) = (850, 850)
	(a,b,c) = (5, 1, 20)
	main = withInit [InitVideo] $ do
	w <- setVideoMode xres yres 32 [NoFrame]
	enableEvent SDLMouseMotion False
	setCaption "Hopalong Pattern" "Barry Martin"
	render w p ps
	loop w p ps
	where
	p = (xres `div` 2, yres `div` 2)
	ps = take 7000000 points
	loop w p ps = do
	delay 32
	e <- pollEvent
	case e of
	KeyUp (Keysym SDLK_ESCAPE _ _) -> return ()
	MouseButtonUp x y _ -> click $ f x y
	_ -> loop w p ps
	where
	click p = render w p ps >> loop w p ps
	f x y = (fromIntegral x, fromIntegral y)
	render w (x,y) ps = do
	s <- createRGBSurface [SWSurface] 1 1 32 0 0 0 0
	fillRect w (Just $ Rect 0 0 xres yres) (Pixel 0)
	mapM_ (draw s 0xFFFFFF . rect . center) ps
	SDL.flip w
	where
	center = ((xres - x) +) *** (+ (yres - y))
	rect (x,y) = Just $ Rect x y 1 1
	draw s c p = do fillRect s Nothing $ Pixel c
	blitSurface s Nothing w p
	points = map (round *** round) $ iterate f (0,0)
	where
	f (x,y) = (g x y, a - x)
	g x y = y - (signum x * (sqrt . abs $ b*x - c))

view raw barry-martin-hopalong.hs hosted with ❤ by GitHub