module Exercise04 where

import Data.Char (isSpace)

{-WETT-}
-- Extracts all tags from the XML string
getTags :: String -> [String]
getTags xs = helper xs "" False
    where
        {-
            Input:  String: XML input
                    String: Accumulated tag
                    Bool: Are we in a tag?
            Output:
                    [String]: Parsed tags
        -}
        helper :: String -> String -> Bool -> [String]
        helper "" _ b = [" " | b]
        helper (x:ys) tag b
            | x == '<' && not b                         = helper ys "" True
            | (x == '<' && b) || (x == '>' && not b)    = [" "]
            | x == '>' && b                             = reverse (dropWhile isSpace tag) : getTags ys
            | otherwise                                 = helper ys ([x | b] ++ tag) b

-- Checks whether syntax and semantics are valid.
check :: [String] -> Bool
check xs = all (notElem ' ') xs && helper xs []
    where
        helper :: [String] -> [String] -> Bool
        helper [] st                    = null st                           -- Falls es keine Tokens mehr zu bearbeiten gibt
        helper (('/':_):_) []           = False                             -- Schließender Tag obwohl es keinen offenen Tag gab
        helper (('/':t):ys) (s:st)      = s == t && helper ys st            -- Schließender Tag 
        helper (t:ys) st                = helper ys (t:st)                  -- Offener Tag

xmlLight :: String -> Bool
xmlLight = check . getTags
{-TTEW-}

-- Zum Testen von XML Dateien ohne neuzukompilieren
main :: IO ()
main = do
    xml <- readFile "invalid.xml"
    print $ xmlLight xml