module Util where

import Data.Vinyl
import Data.Geometry.Ipe
import Data.Proxy
import Data.Ext
import Data.Function(on)
import qualified Data.List as L

byStrokeColour :: (Stroke ∈ AttributesOf g) => [IpeObject' g r] -> [[IpeObject' g r]]
byStrokeColour = map (map fst) . L.groupBy ((==) `on` snd) . L.sortOn snd
               . map (\x -> (x,lookup' x))
  where
    lookup' (_ :+ ats) = lookupAttr (Proxy :: Proxy Stroke) ats

-- | Computes all elements on which the two lists differ
difference :: Eq a => [a] -> [a] -> [a]
difference xs ys = (xs L.\\ ys) ++ (ys L.\\ xs)

-- differenceBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]


diffBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]
diffBy p xs ys = foldr (L.deleteBy p) ys xs