module Exercise04 where

import qualified Data.Text as T
import qualified Data.List as L

import Data.Char

import Data.Maybe
{-WETT-}
xmlLight :: String -> Bool
xmlLight s =  
    isNothing (T.find checkBracket $L.head split) --edge case 
    && all isJust tags_parsed --any invalid tags
    && stackValidation (catMaybes tags_parsed) []
    where split = T.split (== '<') $T.pack s
          tags_parsed = map tagHelper $ tail split


checkBracket :: Char -> Bool
checkBracket c = c == '<' || c =='>' 

--Bool True if tag is opened, Nothing if invalid tag, strips the tag, 
tagHelper :: T.Text -> Maybe (Bool, T.Text)
tagHelper tag = if valid then Just (opening, tag_stripped) else Nothing
    where tag_without_rest =  (/= '>') `T.dropWhileEnd` tag
          tag_init = T.init tag_without_rest
          tag_stripped = if opening then T.stripEnd tag_init else T.tail $T.stripEnd tag_init
          valid = not (T.null tag_without_rest) && isNothing (T.find isSpace tag_stripped)
          opening = T.head tag /= '/'

stackValidation :: [(Bool, T.Text)] -> [T.Text] -> Bool
stackValidation [] stack = null stack
stackValidation (t:tags) stack = if fst t --if opening
                                    then stackValidation tags $snd t:stack --recursive call with tag on stack
                                    else not (null stack) &&  head stack == snd t && stackValidation tags (tail stack) --validate stack, recursive call with tag removed from stack

{-TTEW-}