{-# LANGUAGE OverloadedStrings #-}

module Exercise04 where

import Data.Text (count, dropAround, isInfixOf, isPrefixOf, null, pack, splitOn, stripEnd, tail, unpack)
import Prelude hiding (null, tail)

{-WETT-}
xmlLight :: String -> Bool
xmlLight str = null tag && pure "<" `notElem` scanl processTag [""] (tags ++ pure "/>")
  where
    tag : tags = splitOn "<" $ dropAround (`notElem` unpack "<>" {-unpack so the ambiguity between String and Text is resolved-} ) $ pack str -- Actually exhaustive, splitOn always produces at least one result (or error)
    processTag (lastOpenedTag : openedTags) currentTag -- "<" marks invalid tag. This can't occur in an actual tag since I split on it.
      | null currentTag || count ">" currentTag /= 1 = pure "<" -- Empty Tag (counted as invalid)
      | isPrefixOf "/" currentTag' && tail currentTag' == lastOpenedTag = openedTags -- Matched closing tag
      | isPrefixOf "/" currentTag' || isInfixOf " " currentTag' = pure "<" -- Non matched closing tag or new opening tag with space
      | otherwise = currentTag' : lastOpenedTag : openedTags -- New opening tag
      where
        currentTag' = stripEnd $ head $ splitOn ">" currentTag

{-TTEW-}