randomstr4.hs

module RandomStr4  where

-- uses `random` package

import           Data.List     as L
import           Data.Text     as T
import           System.Random (RandomGen, newStdGen, randomRIO, randomRs)

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

newToken :: IO Text
newToken = do
    g <- newStdGen
    newToken' g

newToken' :: RandomGen g => g -> IO Text
newToken' g = fmap T.pack $ scramble $ L.concat [p1, p2, p3]
    where
        p1 = makeStr 4 ('A', 'Z')
        p2 = makeStr 4 ('a', 'z')
        p3 = makeStr 4 ('0', '9')

        makeStr :: Int -> (Char, Char) -> String
        makeStr len range = L.take len $ randomRs range g

        -- TODO: benchmark this.
        scramble :: String -> IO String
        scramble [] = return []
        scramble xs = do
            n <- randomRIO (0, L.length xs - 1)
            let e = xs !! n
            result <- scramble (L.delete e xs)
            return (e:result)

allUnique :: IO Bool
allUnique = do
    xs <- mapM (const newToken) [1..1000]
    return $ xs == nub xs

main :: IO ()
main = allUnique >>= print

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