parse.java

    /**
     * Parse function for each non-terminal with error recovery.
     * TODO: add error recovery!
     *
     * @param symbol the non-terminal to be parsed.
     * @return the parsed value of `symbol` if parsing succeeded, otherwise `null`.
     */
    private SemValue parse(int symbol, Set<Integer> follow) {
        Pair<Integer, List<Integer>> result = query(symbol, lookahead); // get production by lookahead symbol

        follow.addAll(followSet(symbol));

        if (result == null){
            error();

            while (!follow.contains(lookahead) &&
                (result = query(symbol, lookahead)) == null){
                lookahead = lex();
            }

            if (follow.contains(lookahead)){
                return null;
            }
        }

        int actionId = result.getKey();
        List<Integer> right = result.getValue();

        int length = right.size();
        SemValue[] params = new SemValue[length + 1];

        for (int i = 0; i < length; i++) { // parse right-hand side symbols one by one
            int term = right.get(i);
            params[i + 1] = isNonTerminal(term)
                    ? parse(term, follow) // for non terminals: recursively parse it
                    : matchToken(term) // for terminals: match token
                    ;
        }


        for (int i = 0; i < length; i++) {
            if(params[i + 1] == null)
            {
                return null;
            }
        }
        params[0] = new SemValue(); // initialize return value
        act(actionId, params); // do user-defined action
        
        return params[0];            
    }