class BinaryOperator
  def operate(stack)
    y = stack.pop
    x = stack.pop
    stack.push binary_operate(x, y)
  end
end

class Addition < BinaryOperator
  protected
  def binary_operate(x, y)
    x + y
  end
end

class Subtraction < BinaryOperator
  protected
  def binary_operate(x, y)
    x - y
  end
end

class Multiplication < BinaryOperator
  protected
  def binary_operate(x, y)
    x * y
  end
end

class Division < BinaryOperator
  protected
  def binary_operate(x, y)
    x / y
  end
end

class Factorial
  def operate(stack)
    stack.push factorial(stack.pop)
  end

  private
  def factorial(i)
    (2..i).inject { |x,y| x * y }
  end
end

class OperatorFactory
  def self.create_operator(token)
    case token
      when '+' then Addition.new
      when '-' then Subtraction.new
      when '*' then Multiplication.new
      when '/' then Division.new
      when '!' then Factorial.new
    end
  end
end

class Calculator
  def evaluate(*tokens)
    stack = []
    tokens.each do |token|
      if token =~ /\d+/ then
        stack.push token.to_i
      else
        op = OperatorFactory.create_operator token
        op.operate(stack)
      end
    end
    stack.pop
  end
end

c = Calculator.new
puts c.evaluate('2', '3', '+', '!')