/**
 * 
 */
package de.fhswf.in.inf.java1.aufgabe11;

import java.util.Stack;

/**
 * A class that calculates in UPN.
 *
 * @author $Author: $
 * @version $Revision: $, $Date: $ UTC
 */
public class UPN
{

   private Stack<Double> upnStack = new Stack<>();

   /**
    * Empty constructor.
    *
    */
   public UPN()
   {
   }

   /**
    * Reads a string and puts it on the UPN upnStack.
    *
    * @param upnString
    *           A space delimited string in UPN notation.
    */
   public final void calculate(String upnString)
   {
      if (upnString == null)
      {
         throw new IllegalArgumentException("UpnString can't be null.");
      }
      if (upnString.isEmpty())
      {
         throw new IllegalArgumentException("UpnString can't be empty.");
      }

      for (String string : upnString.split("\\s+"))
      {

         if (!string.isEmpty())
         {
            Double[] operands = null;

            switch (string)
            {
               case "+":
                  operands = fetch();
                  upnStack.add(operands[0] + operands[1]);
                  break;
               case "-":
                  operands = fetch();
                  upnStack.add(operands[0] - operands[1]);
                  break;
               case "*":
                  operands = fetch();
                  upnStack.add(operands[0] * operands[1]);
                  break;
               case "/":
                  divisorZero();
                  operands = fetch();
                  upnStack.add(operands[0] / operands[1]);
                  break;
               case "%":
                  divisorZero();
                  operands = fetch();
                  upnStack.add(operands[0] % operands[1]);
                  break;
               default:
                  try
                  {
                     upnStack.add(Double.valueOf(string));
                  }
                  catch (NumberFormatException e)
                  {
                     throw new IllegalArgumentException("String: \"" + string
                           + "\" is not a valid"
                           + " operator nor a valid operand.");
                  }
            }
         }
      }
   }

   /**
    * Get the result of a calculation done before.
    *
    * @return The top most value on the UPN stack.
    */
   public final double getResult()
   {
      if (upnStack.empty())
      {
         throw new IllegalArgumentException("UPN stack is empty.");
      }
      return upnStack.peek();
   }

   /**
    * Checks if Divisor will be zero.
    *
    */
   private void divisorZero()
   {
      if (upnStack.peek() == 0.0)
      {
         throw new IllegalArgumentException("Divisor can't be 0.");
      }
   }

   /**
    * Checks if there are two values on the stack and returns them.
    *
    * @return The two top most doubles from the stack.
    */
   private Double[] fetch()
   {
      if (upnStack.size() < 2)
      {
         throw new IllegalArgumentException(
               "Two operands are needed for an operation.");
      }

      Double[] ret = new Double[2];

      ret[1] = upnStack.pop();
      ret[0] = upnStack.pop();

      return ret;
   }

}