module Main where
import Graphics.Rendering.Diagrams
import Data.Colour.SRGB.Linear

second f (x,y) = (x,f y)

select :: [a] -> [(a,[a])]
select []     = []
select (x:xs) = (x,xs) : map (second (x:)) (select xs)

permutations :: [a] -> [[a]]
permutations [] = [[]]
permutations xs = do (e, rest) <- select xs
                     map (e:) $ permutations rest

dia :: Diagram
dia = arrange $ map colorsToStrip (permutations [red, yellow, blue, green'])
  where green' = rgb 0 1 0

arrange :: [Diagram] -> Diagram
arrange = hsep 10 . map (vsep 5) . groups
  where groups = takeWhile (not . null) . map (take 6) . iterate (drop 6)

colorsToStrip :: Color c => [c] -> Diagram
colorsToStrip = hcat . map (\c -> fc c $ rect 10 10)

main = renderAs PNG "permutations.png" (Width 300) dia