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ahammar/Shuffle.hs

Created February 17, 2014 23:20
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Fisher-Yates shuffle
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Shuffle.hs
{-# LANGUAGE BangPatterns #-}
import System.Random
shuffle :: [a] -> Int -> IO [a]
shuffle xs 1 = return xs
shuffle !xs !k = do
i <- getStdRandom (randomR (0 , k - 1))
shuffle (sswap i k xs) (k - 1)
sswap :: Int -> Int -> [a] -> [a]
sswap !i !j !xs = take i xs ++ [xs !! j] ++ (take (j - i - 1) $ drop (i + 1) xs) ++ [xs !! i] ++ drop (j + 1) xs
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