module Exercise04 where
import Data.List (elemIndex)
import Data.Maybe (isNothing, fromMaybe, fromJust)

{-WETT-}
xmlLight :: String -> Bool
xmlLight = aux []
    where
    aux tags string
        | '<' `notElem` string = notElem '>' string && null tags
        | isNothing mb || fromJust ((>) <$> startPosition <*> elemIndex '>' string) = False
        | closingTag = not(null tags) && head tags == tagId && aux (tail tags) (fromJust rest)
        | otherwise = aux (tagId:tags) $fromJust rest
        where
            startPosition = elemIndex '<' string
            tagLength = succ <$> (elemIndex '>' . flip drop string =<< startPosition)
            rest = flip drop string <$> ((+) <$> startPosition <*> tagLength)
            (tagId, closingTag) = fromJust mb
            mb = getTagId =<< flip take <$> (flip drop string <$> startPosition) <*> tagLength

            getTagId t = if all (==' ') $take (length t - end - 1)$drop end t then Just (drop (if closing then 2 else 1) $take end t, closing) else Nothing
                where
                    end = fromMaybe (length t - 1) $elemIndex ' ' t
                    closing = t !! 1 == '/'

{-TTEW-}