net and gross

# [ Fixnum, Float ].each{ |klass| klass.send :include, Extensions::NetAndGross }
# 
# Examples:
# 1.net # => 1
# 1.gross # => 1.19
# 1.gross.gross # => 1.19
# 1.gross.gross.net # => 1.0
#

module Extensions
  module NetAndGross

    def net
      return self if net?
      (self / net_to_gross_factor).net!
    end

    def gross
      return self if gross?
      (self * net_to_gross_factor).gross!
    end

    def gross?
      !net?
    end

    def net?
      @is_net = @is_net.nil? ? true : @is_net
    end

    def net!
      @is_net = true
      self
    end

    def gross!
      @is_net = false
      self
    end

    private

    def net_to_gross_factor
      1 + Configuration.sales_tax.to_f / 100
    end

  end
end