def empty_space
      pack_all if @needs_packing
      max = @sizes.each.max
      space = 0

      @sizes.each { |size| space += max - size }

      space
    end
def empty_space
      pack_all if @needs_packing
      max = @sizes.each.max
      space = 0

      @sizes.each { |size| space += max - size }

      space
    end
Total empty space left over by uneven packing.