module Exercise04 where

{-WETT-}
xmlLight :: String -> Bool
xmlLight = xmlHelper [] 

xmlHelper :: [String] -> String -> Bool
xmlHelper stack "" = null stack
xmlHelper stack ('<':chs)
    | null chs          = False
    | head chs == '/'   = not ( null stack || null tag) && init tag == head stack && xmlHelper (tail stack) rest 
    | otherwise         = tag /= "" && xmlHelper (tag:stack) rest
    where (tag, rest) = xmlTagger chs ""
xmlHelper stack (c:chs) = c `notElem` ">" && xmlHelper stack chs

xmlTaggerErr :: (String, String)
xmlTaggerErr = ("", "")

xmlTagger :: String -> String -> (String, String)
xmlTagger "" _ = xmlTaggerErr
xmlTagger (c:chs) tag
    | c == '>'                                     = (last $ words tag, chs)
    | c `elem` "<" || (c /= ' ' && ' ' `elem` tag) = xmlTaggerErr
    | otherwise                                    = xmlTagger chs (c:tag)
{-TTEW-}