module Exercise04 where

{-WETT-}

xmlLight :: String -> Bool
xmlLight = check [] . chop

chop (char1 : char2 : text)
  | char1 == '>' || char2 == '<' = [char1] : c : cs
  | otherwise = (char1 : c) : cs
  where
    c : cs = chop $ char2 : text
chop text = [text]

check tags (bracket : brackets)
  | elem '<' bracket && elem '>' bracket = isValidTag $ init $ tail bracket
  | elem '<' bracket || elem '>' bracket = False
  | otherwise = check tags brackets
  where
    isValidTag (h : tag)
      | elem '/' cleaned || elem ' ' cleaned = False
      | h /= '/' = check (cleaned : tags) brackets
      | null tags = False
      | otherwise = cleaned == head tags && tail tags `check` brackets
      where
        cleaned = reverse $ dropWhile (== ' ') $ reverse $ if h == '/' then tag else h : tag
    isValidTag _ = False
check tags _ = null tags

{-TTEW-}