import java.util.Scanner;
import java.util.Stack;
import java.util.regex.Pattern;

/**
* Postfix lab.
* @author cs416
* @version 2
*/
public class PostfixCalculator {

    private final Pattern integerPattern = Pattern.compile("-?\\d+");

    /**
     * Evaluates the postfix expression and returns the answer as an int.
     *
     * This function throws an IllegalStateException for the following
     * conditions:
     * - an invalid operator is encountered
     * - There are too few operands on the stack to perform the operation
     * - trying to divide by zero
     * - There are too manu operands left on the stack after processing each
     *    token of the expression (stack size != 1 after processing)
     *
     * @param expression The postfix expression to evaluate
     * @return int
     */
    public int evaluate(String expression) {
	throw new UnsupportedOperationException("TO BE IMPLEMENTED");
    }

    /**
     * Returns true if the token parameters matches the regex
     * for an integer.
     * @param token the token to validate
     * @return boolean
     */
    private boolean isInteger(String token) {
        if (token == null || token.isEmpty()) {
            return false;
        }

        return integerPattern.matcher(token).matches();
    }


    /**
     * If the token parameter is a valid operator return true, otherwise return false.
     * @param token The token to validate
     * @return boolean
     */
    public boolean isValidOperator(String token) {
	throw new UnsupportedOperationException("TO BE IMPLEMENTED");
    }


    /**
    * The main function.
    * @param args The commandline arguments.
    */
    public static void main(String[] args) {
        PostfixCalculator pcalc = new PostfixCalculator();

        System.out.println("Enter postfix expressions until EOF ( or Ctrl-D)");

        Scanner s = new Scanner(System.in);

        while (s.hasNextLine()) {
            String exp = s.nextLine();
            try {
                System.out.println(pcalc.evaluate(exp));
            } catch (IllegalStateException e) {
                System.err.println(e);
            }
        }

        s.close();
    }
}