public class Parser {
  static Token nextToken;
  static Lexer l;

  // expr
  // Parses strings in the language generated by the rule:
  //   <expr> -> <term> {(+ | -) <term>}
  public static void expr() {
    System.out.println("Enter <expr>");
    term();
    while ((nextToken.getTokenID() == TokenTypes.ADD_OP) || (nextToken.getTokenID() == TokenTypes.SUB_OP)) {
      nextToken = l.lex();
      term();
    }
    System.out.println("Exit <expr>");
  }

  // term
  // Parses strings in the language generated by the rule:
  //   <term> -> <factor> {(* | /) <factor>}
  public static void term() {
    System.out.println("Enter <term>");
    factor();
    while ((nextToken.getTokenID() == TokenTypes.MULT_OP) || (nextToken.getTokenID() == TokenTypes.DIV_OP)) {
      nextToken = l.lex();
      factor();
    }
    System.out.println("Exit <term>");
  }

  // factor
  // Parses strings in the language generated by the rule:
  //   <factor> -> id | int_constant | ( <expr> )
  public static void factor() {
    System.out.println("Enter <factor>");
    if ((nextToken.getTokenID() == TokenTypes.IDENT) || (nextToken.getTokenID() == TokenTypes.INT_LIT))
      nextToken = l.lex();
    else { // if the RHS is ( <expr> ), pass over (, call expr, check for )
      if (nextToken.getTokenID() == TokenTypes.LPAREN) {
        nextToken = l.lex();
        expr();
        if (nextToken.getTokenID() == TokenTypes.RPAREN)
          nextToken = l.lex();
        else {
          error("Expecting RPAREN");
          System.exit(-1);
        }
      }
      else {
        error("Expecting LPAREN"); 
        System.exit(-1);
      }
    }
    System.out.println("Exit <factor>");
  }

  public static void error(String s) {
    System.out.println("SYNTAX ERROR: "+s);
  }

  public static void main(String args[]) {
    l = new Lexer(args[0]);
    nextToken = l.lex();
    expr();
  }

}