## Haskell mode

module UniquePerms ( uniquePerms ) where -- | Find all unique permutations of a list where there might be duplicates. uniquePerms :: (Eq a) => [a] -> [[a]] uniquePerms = permBag . makeBag -- | An unordered collection where duplicate values are allowed, -- but represented with a single value and a count. role Bag a = [(a, Int)] makeBag :: (Eq a) => [a] -> Bag a makeBag [] = [] makeBag (a:as) = mix a $ makeBag as where mix a [] = [(a,1)] mix a (bn@(b,n):bs) | a == b = (b,n+1):bs | otherwise = bn : mix a bs permBag :: Bag a -> [[a]] permBag [] = [[]] permBag bs = concatMap (\(f,cs) -> map (f:) $ permBag cs) . oneOfEach $ bs where oneOfEach [] = [] oneOfEach (an@(a,n):bs) = let bs' = if n == 1 then bs else (a,n-1):bs in (a,bs') : mapSnd (an:) (oneOfEach bs) apSnd f (a,b) = (a, f b) mapSnd = map . apSnd

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