module Shuffle
( shuffle
) where

import Data.List
import System.Random

----------------------------------------------------------------------
-- Shuffle a list so that the elements of the list are randomly     --
-- perumated.                                                       --
----------------------------------------------------------------------

----------------------------- Constants ------------------------------

-- The seed used to generate random numbers.
seed :: Int
seed = 20

----------------------------------------------------------------------

-- Shuffle a list of values.
shuffle :: [a] -> [a]
shuffle l = map (l !!) $ generateIndices $ length l

-- Generate indices to map the elements of a list over so that they 
-- are randomly shuffled.
generateIndices :: Int -> [Int]
generateIndices size = take size uniqueList
    where randomList = randomRs (0, size - 1) randomGen
          uniqueList = nub randomList

-- Generate a random generator
randomGen :: StdGen
randomGen = mkStdGen seed