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 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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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());
 }
}