module Exercise04 where

import Data.Char (isSpace)
import Data.List (dropWhileEnd)

{-WETT-}
xmlLight :: String -> Bool
xmlLight = checkTags [] . getTags

getTags :: String -> [String]
getTags [] = []
getTags (x:xs)
    | x == '<' = getTagsOpened xs
    | x == '>' = pure " "
    | otherwise = getTags xs

getTagsOpened :: String -> [String]
getTagsOpened [] = pure " "
getTagsOpened s@(x:xs)
    | x == '>'      = getTags xs
    | otherwise     = dropWhileEnd isSpace tag : getTagsOpened rest
        where (tag, rest) = break (=='>') s

checkTags :: [String] -> [String] -> Bool
checkTags [] [] = True
checkTags os (t:ts)
    | any isSpace t     = False                     -- tag containes spaces
    | head t /= '/'     = checkTags (t:os) ts       -- newly opened tag
    | otherwise         = not (null os)             
                        && tail t == head os        -- check if tag properly closed opened tag
                        && checkTags (tail os) ts
checkTags _ _ = False
{-TTEW-}