```{- |
Stability    : experimental
Portability  : portable

-}

(
-- * Random functions
-- ** Shuffling
shuffle
, shuffleSeq
-- ** Sampling
, sample
, sampleSeq
-- ** Choice
, choiceExtract
, choiceExtractSeq
, choice
, safeChoice
, iterativeChoice
, choiceSeq
, safeChoiceSeq
, choiceArray
-- ** Choices
, choices
, safeChoices
, choicesArray
)
where

import System.Random (Random)
import Data.List (mapAccumL, foldl')
import Data.Maybe (fromJust)
import qualified Data.Sequence as Seq
import Data.Sequence ((><), ViewL((:<)))
import qualified Data.Array.IArray as Arr
import qualified Data.Array
import Data.Array.IArray ((!))

(.:) :: (c -> c') -> (a -> b -> c) -> (a -> b -> c')
(.:) = (.).(.)

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

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

getRandomR' :: (MonadRandom m, Random a) => a -> a -> m a
getRandomR' = curry getRandomR

getRandomRNums :: (MonadRandom m, Random a, Num a) => [a] -> m [a]
getRandomRNums = mapM (getRandomR' 0)

backsaw :: Int -> [Int]
backsaw n = [n - 1, n - 2 .. 0]

-- Shuffling

-- | Shuffle a list randomly. The method is based on Oleg Kiselyov's
-- but much simpler because it uses existing data structures. The efficiency
-- of both methods should be comparable.
--
-- /Complexity:/ O(n * log n), where /n/ is the length of the input list.
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), where /n/ is the length of the input sequence.
shuffleSeq :: (MonadRandom m) => Seq.Seq a -> m [a]
shuffleSeq s = do
samples <- getRandomRNums . backsaw \$ Seq.length s
return (shuffleSeq' s samples)

shuffleSeq' :: Seq.Seq a -> [Int] -> [a]
shuffleSeq' = snd .: mapAccumL (fromJust .: extractSeq)

-- Sampling

-- | Take a random sample from a list.
--
-- /Complexity:/ O(n + m * log n), where /n/ is the length of the input list
-- and /m/ is the sample size.
sample :: (MonadRandom m) => Int -> [a] -> m [a]
sample m = sampleSeq m . Seq.fromList

-- | Take a random sample from a sequence.
--
-- /Complexity:/ O(m * log n), where /n/ is the length of the input sequence
-- and /m/ is the sample size.
sampleSeq :: (MonadRandom m) => Int -> Seq.Seq a -> m [a]
sampleSeq m s = do
samples <- getRandomRNums . take m . backsaw \$ Seq.length s
return (shuffleSeq' s samples)

-- Choice

-- | Randomly choose and extract an element from a list.
--
-- /Complexity:/ O(n), where /n/ is the length of the input list.
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), where /n/ is the length of the input sequence.
choiceExtractSeq :: (MonadRandom m) => Seq.Seq a -> m (Maybe (Seq.Seq a, 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.
--
-- /Partial function:/ This function is only defined on non-empty lists.
--
-- /Complexity:/ O(n), where /n/ is the length of the input list.
choice :: (MonadRandom m) => [a] -> m a
choice [] = error "Control.Monad.Random.Extras.choice: empty list"
choice xs = (xs !!) `liftM` getRandomR (0, length xs - 1)

-- | Safely select a random element from a list.
--
-- /Complexity:/ O(n), where /n/ is the length of the input list.
safeChoice :: (MonadRandom m) => [a] -> Maybe (m a)
safeChoice [] = Nothing
safeChoice xs = Just \$ choice xs

-- | Select a random element from a list, traversing the list only once.
--
-- /Partial function:/ This function is only defined on non-empty lists
--                     with a length below ('maxBound' + 1 :: Int).
--
-- /Complexity:/ O(n), where /n/ is the length of the input list.
iterativeChoice :: MonadRandom m => [a] -> m a
iterativeChoice xs = fst `liftM` foldl' stepM (return start) xs
where stepM x y = x >>= step y
step offered (old, n) = do
i <- getRandomR (0, n)
let new | i == 0    = offered
| otherwise = old
return \$! new `seq` (new, n + 1)
start = (err, 0 :: Int)
err = error "Control.Monad.Random.Extras.iterativeChoice: empty list"

-- | Select a random element from a sequence.
--
-- /Partial function:/ This function is only defined on non-empty sequences.
--
-- /Complexity:/ O(log n), where /n/ is the length of the input sequence.
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)

-- | Safely select a random element from a sequence.
--
-- /Complexity:/ O(log n), where /n/ is the length of the input sequence.
safeChoiceSeq :: (MonadRandom m) => Seq.Seq a -> Maybe (m a)
safeChoiceSeq s | Seq.null s = Nothing
| otherwise  = Just \$ choiceSeq s

-- | Select a random element from an array.
--
-- /Complexity:/ O(1).
choiceArray :: (MonadRandom m, Arr.IArray arr a, Arr.Ix i, Random i) => arr i a -> m a
choiceArray v = (v !) `liftM` getRandomR (Arr.bounds v)

-- Choices

-- | A stream of random elements from a list.
--
-- /Partial function:/ This function is only defined on non-empty lists.
--
-- /Complexity:/ O(n) base and O(1) per element.
choices :: (MonadRandom m) => [a] -> m [a]
choices [] = error "Control.Monad.Random.Extras.choices: empty list"
choices xs = choicesArray \$ Data.Array.listArray (1, length xs) xs

-- | Safely get a stream of random elements from a list.
--
-- /Complexity:/ O(n) base and O(1) per element, where /n/ is the length of
-- the input list.
safeChoices :: (MonadRandom m) => [a] -> Maybe (m [a])
safeChoices [] = Nothing
safeChoices xs = Just \$ choices xs

-- | A stream of random elements from an array.
--
-- /Complexity:/ O(1) per element.
choicesArray :: (MonadRandom m, Arr.IArray arr a, Arr.Ix i, Random i) => arr i a -> m [a]
choicesArray v = map (v !) `liftM` getRandomRs (Arr.bounds v)
```