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<h1>CodeMirror: Haskell mode</h1> | ||||
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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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