30 lines
1.1 KiB
Haskell
30 lines
1.1 KiB
Haskell
import Control.Monad
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import System.Random
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import Text.Printf
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import VShuffle
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-- Return whether a string contains balanced brackets. Nothing indicates a
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-- balanced string, while (Just i) means an imbalance was found at, or just
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-- after, the i'th bracket. We assume the string contains only brackets.
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isBalanced :: String -> Maybe Int
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isBalanced = bal (-1) 0
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where bal :: Int -> Int -> String -> Maybe Int
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bal _ 0 [] = Nothing
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bal i _ [] = Just i
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bal i (-1) _ = Just i
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bal i n ('[':bs) = bal (i+1) (n+1) bs
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bal i n (']':bs) = bal (i+1) (n-1) bs
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-- Print a string, indicating whether it contains balanced brackets. If not,
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-- indicate the bracket at which the imbalance was found.
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check :: String -> IO ()
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check s = maybe (good s) (bad s) (isBalanced s)
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where good s = printf "Good \"%s\"\n" s
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bad s n = printf "Bad \"%s\"\n%*s^\n" s (n+6) " "
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main :: IO ()
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main = do
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let bs = cycle "[]"
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rs <- replicateM 10 newStdGen
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zipWithM_ (\n r -> check $ shuffle (take n bs) r) [0,2..] rs
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