code: private double computePhi(DescriptiveStatistics samples, long lastHeartbeat, long currentTime) {
      long size = samples.getN();
      long t = currentTime - lastHeartbeat;
      return (size > 0)
          ? phiFactor * t / samples.getMean()
          : 100;
    }
docs: Computes the phi value from the given samples.
  <p>
  The original phi value in Hayashibara's paper is calculated based on a normal distribution.
  Here, we calculate it based on an exponential distribution.
  
  @param samples       the samples from which to compute phi
  @param lastHeartbeat the last heartbeat
  @param currentTime   the current time
  @return phi