import Control.Monad
import System.Random
import Text.Printf
import VShuffle

-- Return whether a string contains balanced brackets.  Nothing indicates a
-- balanced string, while (Just i) means an imbalance was found at, or just
-- after, the i'th bracket.  We assume the string contains only brackets.
isBalanced :: String -> Maybe Int
isBalanced = bal (-1) 0
  where
    bal :: Int -> Int -> String -> Maybe Int
    bal _ 0 [] = Nothing
    bal i _ [] = Just i
    bal i (-1) _ = Just i
    bal i n ('[':bs) = bal (i + 1) (n + 1) bs
    bal i n (']':bs) = bal (i + 1) (n - 1) bs

-- Print a string, indicating whether it contains balanced brackets.  If not,
-- indicate the bracket at which the imbalance was found.
check :: String -> IO ()
check s = maybe (good s) (bad s) (isBalanced s)
  where
    good = printf "Good \"%s\"\n"
    bad s n = printf "Bad  \"%s\"\n%*s^\n" s (n + 6) " "

main :: IO ()
main = do
  let bs = cycle "[]"
  rs <- replicateM 10 newStdGen
  zipWithM_ (\n r -> check $ shuffle (take n bs) r) [0,2 ..] rs