module Exercise04 where

import Data.List as List
import Data.Char as Char

{-WETT-}
xmlLight :: String -> Bool
xmlLight xml = xmlLightHelp (dropWhile (\c -> c /= '<' && c /= '>') xml) [] False -- todo: trim text before and after

xmlLightHelp :: String -> [String] -> Bool -> Bool
xmlLightHelp [] [] False = True
xmlLightHelp [] _ _ = False
xmlLightHelp (x:xs) acc lastWasOpen
    | x == '<' = 
        let tag = takeWhile (/= '>') xs
            left = drop (length tag + 1) xs
            close = "/" == take 1 tag
            invalid0 = length xs == length tag
            invalid1 = ' ' `elem` strip tag
            invalid2 = '<' `elem` tag
            invalid3 = '/' `elem` drop 1 tag
        in not (invalid0 || invalid1 || invalid2 || invalid3) &&
            if close
                then 
                    (not (null acc) && '/' : head acc == strip tag) && xmlLightHelp left (tail acc) False
                else xmlLightHelp left (strip tag : acc) True

    | x == '>' = False
    | otherwise = xmlLightHelp xs acc lastWasOpen

strip :: String -> String
strip = dropWhileEnd isSpace

{-TTEW-}