new files

Task/Balanced-brackets/Haskell/balanced-brackets-2.hs

@@ -0,0 +1,20 @@
+module VShuffle (shuffle) where
+
+import Data.List (mapAccumL)
+import System.Random
+import Control.Monad.ST
+import qualified Data.Vector as V
+import qualified Data.Vector.Generic.Mutable as M
+
+-- Generate a list of array index pairs, each corresponding to a swap.
+pairs :: (Enum a, Random a, RandomGen g) => a -> a -> g -> [(a, a)]
+pairs l u r = snd $ mapAccumL step r [l..pred u]
+    where step r i = let (j, r') = randomR (i, u) r in (r', (i, j))
+
+-- Return a random permutation of the list.  We use the algorithm described in
+-- http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle#The_modern_algorithm.
+shuffle :: (RandomGen g) => [a] -> g -> [a]
+shuffle xs r = V.toList . runST $ do
+                 v <- V.unsafeThaw $ V.fromList xs
+                 mapM_ (uncurry $ M.swap v) $ pairs 0 (M.length v - 1) r
+                 V.unsafeFreeze v