问如何计算Apache Spark中的百分数？

  /**
   * compute percentile from an unsorted Spark RDD
   * @param data: input data set of Long integers
   * @param tile: percentile to compute (eg. 85 percentile)
   * @return value of input data at the specified percentile
   */
  def computePercentile(data: RDD[Long], tile: Double): Double = {
    // NIST method; data to be sorted in ascending order
    val r = data.sortBy(x => x)
    val c = r.count()
    if (c == 1) r.first()
    else {
      val n = (tile / 100d) * (c + 1d)
      val k = math.floor(n).toLong
      val d = n - k
      if (k <= 0) r.first()
      else {
        val index = r.zipWithIndex().map(_.swap)
        val last = c
        if (k >= c) {
          index.lookup(last - 1).head
        } else {
          index.lookup(k - 1).head + d * (index.lookup(k).head - index.lookup(k - 1).head)
        }
      }
    }
  }

public static double[] getPercentiles(JavaRDD<Double> rdd, double[] percentiles, long rddSize, int numPartitions) {
    double[] values = new double[percentiles.length];

    JavaRDD<Double> sorted = rdd.sortBy((Double d) -> d, true, numPartitions);
    JavaPairRDD<Long, Double> indexed = sorted.zipWithIndex().mapToPair((Tuple2<Double, Long> t) -> t.swap());

    for (int i = 0; i < percentiles.length; i++) {
        double percentile = percentiles[i];
        long id = (long) (rddSize * percentile);
        values[i] = indexed.lookup(id).get(0);
    }

    return values;
}