package com.iluwatar;

import java.util.Stack;

public class App 
{
	
	/**
	 * 
	 * Expressions can be evaluated using prefix, infix or postfix notations
	 * This sample uses postfix, where operator comes after the operands
	 * 
	 */
    public static void main( String[] args )
    {
    	String tokenString = "4 3 2 - 1 + *";
		Stack<Expression> stack = new Stack<>();

		String[] tokenList = tokenString.split(" ");
		for (String s : tokenList) {
			if (isOperator(s)) {
				Expression rightExpression = stack.pop();
				Expression leftExpression = stack.pop();
				System.out.println(String.format("popped from stack left: %d right: %d", 
						leftExpression.interpret(), rightExpression.interpret()));
				Expression operator = getOperatorInstance(s, leftExpression,
						rightExpression);
				System.out.println(String.format("operator: %s", operator));
				int result = operator.interpret();
				NumberExpression resultExpression = new NumberExpression(result);
				stack.push(resultExpression);
				System.out.println(String.format("push result to stack: %d", resultExpression.interpret()));
			} else {
				Expression i = new NumberExpression(s);
				stack.push(i);
				System.out.println(String.format("push to stack: %d", i.interpret()));
			}
		}
		System.out.println(String.format("result: %d", stack.pop().interpret()));    	
    }
    
    public static boolean isOperator(String s) {
		if (s.equals("+") || s.equals("-") || s.equals("*"))
			return true;
		else
			return false;
	}

	public static Expression getOperatorInstance(String s, Expression left,
			Expression right) {
		switch (s) {
		case "+":
			return new PlusExpression(left, right);
		case "-":
			return new MinusExpression(left, right);
		case "*":
			return new MultiplyExpression(left, right);
		}
		return null;
	}	
}