stop-watch.rb

class StopWatch
  def start
    @start_time ||= Time.now
    @stop_time = nil
    self
  end

  def stop
    @stop_time = Time.now
    self
  end

  def reset
    @start_time = nil
    @stop_time = nil
    self
  end

  def elapsed_millis
    raise "not started." unless @start_time

    if @stop_time
      ((@stop_time - @start_time) * 1000).to_i
    else
      ((Time.now - @start_time) * 1000).to_i
    end
  end
end