Dimakl / YALEP

    // on change of grammar should change it too

    private static final String ALLOWED_TOKENS = "()!&|^ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    protected static void expressionIsValid(String rawExpression) throws WrongExpressionFormatException {

        rawExpression = rawExpression.replaceAll(" ","");

        String expression = getLogicalExpressionAndValidateBasics(rawExpression);

        checkSyntaxErrors(expression);

        checkInvalidTokens(expression);

    }

    private static void checkSyntaxErrors(String expression) throws WrongExpressionFormatException {

        LogicalExpressionLexer lexer = new LogicalExpressionLexer(CharStreams.fromString(expression));

        lexer.removeErrorListeners();

        CommonTokenStream tokens = new CommonTokenStream(lexer);

        LogicalExpressionParser parser = new LogicalExpressionParser(tokens);

        parser.removeErrorListeners();

        parser.addErrorListener(new LogicalExpressionErrorListener());

        try {

            parser.eval();

        } catch (RuntimeException e) {

            throw new WrongExpressionFormatException(e.getMessage());

        }

        if (parser.getNumberOfSyntaxErrors() != 0) {

            throw new WrongExpressionFormatException("Expression has syntax errors");

        }

    }

    private static void checkInvalidTokens(String expression) throws WrongExpressionFormatException {

        Set<Character> tokenSet = new HashSet<>();

        for (Character c : ALLOWED_TOKENS.toCharArray()) {

            tokenSet.add(c);

        }

        for (Character c : expression.toCharArray()) {

            if (!tokenSet.contains(c)) {

                throw new WrongExpressionFormatException("Expression contains wrong tokens");

            }

        }

    }

    private static String getLogicalExpressionAndValidateBasics(String rawExpression) throws WrongExpressionFormatException {

        int equalSignPos = rawExpression.indexOf("=");

        if (equalSignPos == -1) {

            throw new WrongExpressionFormatException("Expression doesn't contain '='");

        }

        String answerString = rawExpression.substring(equalSignPos + 1);

        if (!(answerString.equals("1") || answerString.equals("0"))) {

            throw new WrongExpressionFormatException("Wrong answer format");

        }

        return  rawExpression.substring(0, equalSignPos);