Heighway Dragon - byorgey

heighway.hs

{- Heighway dragon. See 
http://en.wikipedia.org/wiki/Dragon_curve -}

module Main where
import Graphics.Rendering.Diagrams
import Control.Monad.State
import Data.Maybe

dragonStr :: Int -> String
dragonStr 0 = "FX" 
dragonStr n = concatMap rules $ dragonStr (n-1) 
  where rules 'X' = "X+YF+"
        rules 'Y' = "-FX-Y"
        rules c = [c]

strToPath :: String -> Path
strToPath s = pathFromVectors . catMaybes $ evalState c (0,-1) 
  where c = mapM exec s
        exec 'F' = Just `fmap` get
        exec '-' = modify left >> return Nothing
        exec '+' = modify right >> return Nothing 
        exec _ = return Nothing left (x,y) = (-y,x) right (x,y) = (y,-x)

dragon :: Int -> Diagram
dragon = lc red . curved 0.8 . strToPath . dragonStr

main = renderAs PNG "dragon.png" (Width 300) (dragon 12)