module Exercise04 where

import Data.List ()

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

-- Testing whether space is not the infix & prefix;
-- if '</' comes, the tagID must be the same as the first ID in the list
isValidTag :: String -> [String] -> Bool
isValidTag [] (b:_) = null b
isValidTag _ [] = False
isValidTag [a] (b:_) = a `notElem` "<>" && null b
isValidTag (c:d:cs) r@(xs:xss)
    | c == '>' = False
    | c /= '<' = isValidTag (d:cs) r
    | d `notElem` "</ >" = all (`notElem` "</") tag && noWSAndClosed && isValidTag (tail rf) ((d:tagID):r)
    | d == '/' = not (null xs) && noWSAndClosed && xs == tagID && isValidTag (tail rf) xss
    | otherwise = False
    where noWSAndClosed = length (words (d:tag)) == 1 && rf /= "" && head rf == '>'
          tag = takeWhile (/= '>') cs -- '>' is not included
          tagID = takeWhile (/= ' ') tag
          rf = dropWhile (/= '>') cs

{-TTEW-}