content: /**
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.aurora.scheduler.sla;
  
  import java.util.List;
  
  import com.google.common.math.Quantiles;
  
  /**
   * Utility methods for the SLA calculations.
   */
  final class SlaUtil {
  
    private SlaUtil() {
      // Utility class.
    }
  
    /**
     * Reports the percentile value from the continuous distribution described by a given list of
     * samples.
     *
     * Example: [30, 60, 70, 90], the 50 percentile is 65.0 (i.e. larger values cover 50% of the PDF
     * (Probability Density Function)).
     * Example: [30, 60, 70, 90], the 75 percentile is 52.5 (i.e. larger values cover 75% of the PDF).
     * Example: [30, 60, 70, 90], the 90 percentile is 39.0 (i.e. larger values cover 85% of the PDF).
     * Example: [30, 60, 70, 90], the 99 percentile is 30.9 (i.e. larger values cover 95% of the PDF).
     *
     * @param list List to calculate percentile for.
     * @param percentile Percentile value to apply.
     * @return Element at the given percentile.
     */
    static Number percentile(List<Long> list, double percentile) {
      if (list.isEmpty()) {
        return 0.0;
      }
  
      // index should be a full integer. use quantile scale to allow reporting of percentile values
      // such as p99.9.
      double percentileCopy = percentile;
      int quantileScale = 100;
      while ((percentileCopy - Math.floor(percentileCopy)) > 0) {
        quantileScale *= 10;
        percentileCopy *= 10;
      }
  
      return Quantiles.scale(quantileScale).index((int) Math.floor(quantileScale - percentileCopy))
          .compute(list);
    }
  }
  