/*
 * RPN calculator supporting addition, subtraction and multiplication
 * of anything that implements the CalculatorOperand interface.
 * Author Leo Reyzin with minor mods by Wayne Snyder
 *
 */

public class RPNCalculator<T extends CalculatorOperand<T>> {

  EvalStack<T> stack; // the stack holding the operands

  public RPNCalculator () {
    stack = new EvalStack<T>(); 
  }

  /**
   * Pushes the operand on the calculator stack
   */
  public void operand (T value) {
    stack.push(value);
  }

  /**
   * Performs an operation on the two topmost elements of the stack
   * If t1 is topmost and t2 is second topmost, then t1 and t2 will be removed
   * from the stack and (t2 op t1) will be placed on top of the stack.
   * Does not modify the stack if it contains fewer than two elements.
   * @param op operation to be performed
   */

  public void operation (String op) {
    T op1 = stack.pop();
    if (op1 == null)
      return;
    T op2 = stack.pop();
    if (op2 == null) {
      stack.push(op1);
      return;
    }

    if(op.equals("+"))
      op2.addTo(op1);           // instead of adding (which only works for numbers) apply generic operation defined by T
    else if(op.equals("-"))
      op2.subtractFrom(op1);
    else if(op.equals("*"))
      op2.multiplyBy(op1);
    
    stack.push (op2);
  }



 /**
  * Prints the calculator stack
  */

 public void print() {
  if (stack.isEmpty())
   System.out.println("Empty stack");
  else 
   System.out.println("Stack:\n" + stack.toString());
 }
 
}