```{- |
Copyright    : 2010 Aristid Breitkreuz
Stability    : experimental
Portability  : portable

-}

(
-- * Random functions
-- ** Shuffling
shuffle
, shuffleSeq
-- ** Choice
, choiceExtract
, choiceExtractSeq
, choice
, choiceSeq
)
where

import qualified Data.Sequence as Seq
import Data.Sequence ((><), ViewL((:<)))

extract :: [a] -> Int -> Maybe (a, [a])
extract s i | null r    = Nothing
| otherwise = Just (b, a ++ c)
where (a, r) = splitAt i s
(b : c) = r

extractSeq :: Seq.Seq a -> Int -> Maybe (a, Seq.Seq a)
extractSeq s i | Seq.null r = Nothing
| otherwise  = Just (b, a >< c)
where (a, r) = Seq.splitAt i s
(b :< c) = Seq.viewl r

-- | Shuffle a list randomly. The method is based on Oleg Kiselyov's
-- /perfect shuffle/ <http://okmij.org/ftp/Haskell/perfect-shuffle.txt>,
-- but much simpler because it uses existing data structures. The efficiency
-- of both methods should be comparable.
--
-- Complexity: O(n * log n)
shuffle :: (MonadRandom m) => [a] -> m [a]
shuffle = shuffleSeq . Seq.fromList

-- | Shuffle a sequence randomly. This is being used by 'shuffle',
-- so it logically uses the same method.
--
-- Complexity: O(n * log n)
shuffleSeq :: (MonadRandom m, MonadPlus f) => Seq.Seq a -> m (f a)
shuffleSeq = unfoldrM' choiceExtractSeq

-- | Randomly choose and extract an element from a list.
--
-- Complexity: O(n)
choiceExtract :: (MonadRandom m) => [a] -> m (Maybe (a, [a]))
choiceExtract [] = return Nothing
choiceExtract xs = extract xs `liftM` getRandomR (0, length xs - 1)

-- | Randomly choose and extract an element from a sequence.
--
-- Complexity: O(log n)
choiceExtractSeq :: (MonadRandom m) => Seq.Seq a -> m (Maybe (a, Seq.Seq a))
choiceExtractSeq s | Seq.null s = return Nothing
| otherwise  = extractSeq s `liftM` getRandomR (0, Seq.length s - 1)

-- | Select a random element from a list.
--
-- Complexity: O(n).
choice :: (MonadRandom m) => [a] -> m a
choice [] = error "Control.Monad.Random.Extras.choice: empty list"
choice xs = (xs !!) `liftM` getRandomR (0, length xs - 1)

-- | Select a random element from a sequence.
--
-- Complexity: O(log n).
choiceSeq :: (MonadRandom m) => Seq.Seq a -> m a
choiceSeq s | Seq.null s = error "Control.Monad.Random.Extras.choiceSeq: empty sequence"
| otherwise  = Seq.index s `liftM` getRandomR (0, Seq.length s - 1)
```