16 lines
428 B
Haskell
16 lines
428 B
Haskell
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sPermutations :: [a] -> [([a], Int)]
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sPermutations = flip zip (cycle [-1, 1]) . foldr aux [[]]
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where
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aux x items = do
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(f, item) <- zip (repeat id) items
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f (insertEv x item)
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insertEv x [] = [[x]]
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insertEv x l@(y:ys) = (x : l) : ((y :) <$> insertEv x ys)
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main :: IO ()
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main = do
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putStrLn "3 items:"
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mapM_ print $ sPermutations [1 .. 3]
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putStrLn "\n4 items:"
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mapM_ print $ sPermutations [1 .. 4]
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