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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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   */
  package java.util;
  
  import java.util.function.DoubleConsumer;
  import java.util.stream.Collector;
  
  /**
   * A state object for collecting statistics such as count, min, max, sum, and
   * average.
   *
   * <p>This class is designed to work with (though does not require)
   * {@linkplain java.util.stream streams}. For example, you can compute
   * summary statistics on a stream of doubles with:
   * <pre> {@code
   * DoubleSummaryStatistics stats = doubleStream.collect(DoubleSummaryStatistics::new,
   *                                                      DoubleSummaryStatistics::accept,
   *                                                      DoubleSummaryStatistics::combine);
   * }</pre>
   *
   * <p>{@code DoubleSummaryStatistics} can be used as a
   * {@linkplain java.util.stream.Stream#collect(Collector) reduction}
   * target for a {@linkplain java.util.stream.Stream stream}. For example:
   *
   * <pre> {@code
   * DoubleSummaryStatistics stats = people.stream()
   *     .collect(Collectors.summarizingDouble(Person::getWeight));
   *}</pre>
   *
   * This computes, in a single pass, the count of people, as well as the minimum,
   * maximum, sum, and average of their weights.
   *
   * @implNote This implementation is not thread safe. However, it is safe to use
   * {@link java.util.stream.Collectors#summarizingDouble(java.util.function.ToDoubleFunction)
   * Collectors.toDoubleStatistics()} on a parallel stream, because the parallel
   * implementation of {@link java.util.stream.Stream#collect Stream.collect()}
   * provides the necessary partitioning, isolation, and merging of results for
   * safe and efficient parallel execution.
   * @since 1.8
   */
  public class DoubleSummaryStatistics implements DoubleConsumer {
      private long count;
      private double sum;
      private double sumCompensation; // Low order bits of sum
      private double simpleSum; // Used to compute right sum for non-finite inputs
      private double min = Double.POSITIVE_INFINITY;
      private double max = Double.NEGATIVE_INFINITY;
  
      /**
       * Construct an empty instance with zero count, zero sum,
       * {@code Double.POSITIVE_INFINITY} min, {@code Double.NEGATIVE_INFINITY}
       * max and zero average.
       */
      public DoubleSummaryStatistics() { }
  
      /**
       * Records another value into the summary information.
       *
       * @param value the input value
       */
      @Override
      public void accept(double value) {
          ++count;
          simpleSum += value;
          sumWithCompensation(value);
          min = Math.min(min, value);
          max = Math.max(max, value);
      }
  
      /**
       * Combines the state of another {@code DoubleSummaryStatistics} into this
       * one.
       *
       * @param other another {@code DoubleSummaryStatistics}
       * @throws NullPointerException if {@code other} is null
       */
      public void combine(DoubleSummaryStatistics other) {
          count += other.count;
          simpleSum += other.simpleSum;
          sumWithCompensation(other.sum);
          sumWithCompensation(other.sumCompensation);
          min = Math.min(min, other.min);
          max = Math.max(max, other.max);
      }
  
      /**
       * Incorporate a new double value using Kahan summation /
       * compensated summation.
       */
      private void sumWithCompensation(double value) {
          double tmp = value - sumCompensation;
          double velvel = sum + tmp; // Little wolf of rounding error
          sumCompensation = (velvel - sum) - tmp;
          sum = velvel;
      }
  
      /**
       * Return the count of values recorded.
       *
       * @return the count of values
       */
      public final long getCount() {
          return count;
      }
  
      /**
       * Returns the sum of values recorded, or zero if no values have been
       * recorded.
       *
       * If any recorded value is a NaN or the sum is at any point a NaN
       * then the sum will be NaN.
       *
       * <p> The value of a floating-point sum is a function both of the
       * input values as well as the order of addition operations. The
       * order of addition operations of this method is intentionally
       * not defined to allow for implementation flexibility to improve
       * the speed and accuracy of the computed result.
       *
       * In particular, this method may be implemented using compensated
       * summation or other technique to reduce the error bound in the
       * numerical sum compared to a simple summation of {@code double}
       * values.
       *
       * @apiNote Values sorted by increasing absolute magnitude tend to yield
       * more accurate results.
       *
       * @return the sum of values, or zero if none
       */
      public final double getSum() {
          // Better error bounds to add both terms as the final sum
          double tmp =  sum + sumCompensation;
          if (Double.isNaN(tmp) && Double.isInfinite(simpleSum))
              // If the compensated sum is spuriously NaN from
              // accumulating one or more same-signed infinite values,
              // return the correctly-signed infinity stored in
              // simpleSum.
              return simpleSum;
          else
              return tmp;
      }
  
      /**
       * Returns the minimum recorded value, {@code Double.NaN} if any recorded
       * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
       * recorded. Unlike the numerical comparison operators, this method
       * considers negative zero to be strictly smaller than positive zero.
       *
       * @return the minimum recorded value, {@code Double.NaN} if any recorded
       * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
       * recorded
       */
      public final double getMin() {
          return min;
      }
  
      /**
       * Returns the maximum recorded value, {@code Double.NaN} if any recorded
       * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
       * recorded. Unlike the numerical comparison operators, this method
       * considers negative zero to be strictly smaller than positive zero.
       *
       * @return the maximum recorded value, {@code Double.NaN} if any recorded
       * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
       * recorded
       */
      public final double getMax() {
          return max;
      }
  
      /**
       * Returns the arithmetic mean of values recorded, or zero if no
       * values have been recorded.
       *
       * If any recorded value is a NaN or the sum is at any point a NaN
       * then the average will be code NaN.
       *
       * <p>The average returned can vary depending upon the order in
       * which values are recorded.
       *
       * This method may be implemented using compensated summation or
       * other technique to reduce the error bound in the {@link #getSum
       * numerical sum} used to compute the average.
       *
       * @apiNote Values sorted by increasing absolute magnitude tend to yield
       * more accurate results.
       *
       * @return the arithmetic mean of values, or zero if none
       */
      public final double getAverage() {
          return getCount() > 0 ? getSum() / getCount() : 0.0d;
      }
  
      /**
       * {@inheritDoc}
       *
       * Returns a non-empty string representation of this object suitable for
       * debugging. The exact presentation format is unspecified and may vary
       * between implementations and versions.
       */
      @Override
      public String toString() {
          return String.format(
              "%s{count=%d, sum=%f, min=%f, average=%f, max=%f}",
              this.getClass().getSimpleName(),
              getCount(),
              getSum(),
              getMin(),
              getAverage(),
              getMax());
      }
  }
  