module Exercise04 where

{-WETT-}

xmlLight :: String -> Bool
xmlLight s = null (walkOutside s)

walkOutside :: String -> [String] -- check next element outside of brackets
walkOutside "" = [] -- empty String is legal
walkOutside (x : xs)
  | x == '<' = walkInside xs ""
  | x == '>' = [""]
  | otherwise = walkOutside xs

walkInside :: String -> String -> [String] -- check next element inside of brackets
walkInside "" _ = [""] -- illegal to finish inside brackets
walkInside (x : xs) acc
  | x == '>' = removePair acc (walkOutside xs)
  | otherwise = walkInside xs (acc ++ [x])

removePair :: String -> [String] -> [String]
removePair s acc
  | s /= "" && last s == ' ' = removePair (init s) acc -- remove trailing whitespace
  | acc /= [] && '/' : s == head acc && not (any (`elem` s) ['>', '<', ' ', '/']) = tail acc -- remove valid pair of ids
  | otherwise = s : acc

{-TTEW-}