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| <h2>Haskell mode</h2> | |
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| 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. | |
| type 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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