1 /*
   2  * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.util;
  26 
  27 import java.util.function.DoubleConsumer;
  28 import java.util.stream.Collector;
  29 
  30 /**
  31  * A state object for collecting statistics such as count, min, max, sum, and
  32  * average.
  33  *
  34  * <p>This class is designed to work with (though does not require)
  35  * {@linkplain java.util.stream streams}. For example, you can compute
  36  * summary statistics on a stream of doubles with:
  37  * <pre> {@code
  38  * DoubleSummaryStatistics stats = doubleStream.collect(DoubleSummaryStatistics::new,
  39  *                                                      DoubleSummaryStatistics::accept,
  40  *                                                      DoubleSummaryStatistics::combine);
  41  * }</pre>
  42  *
  43  * <p>{@code DoubleSummaryStatistics} can be used as a
  44  * {@linkplain java.util.stream.Stream#collect(Collector) reduction}
  45  * target for a {@linkplain java.util.stream.Stream stream}. For example:
  46  *
  47  * <pre> {@code
  48  * DoubleSummaryStatistics stats = people.stream()
  49  *     .collect(Collectors.summarizingDouble(Person::getWeight));
  50  *}</pre>
  51  *
  52  * This computes, in a single pass, the count of people, as well as the minimum,
  53  * maximum, sum, and average of their weights.
  54  *
  55  * @implNote This implementation is not thread safe. However, it is safe to use
  56  * {@link java.util.stream.Collectors#summarizingDouble(java.util.function.ToDoubleFunction)
  57  * Collectors.toDoubleStatistics()} on a parallel stream, because the parallel
  58  * implementation of {@link java.util.stream.Stream#collect Stream.collect()}
  59  * provides the necessary partitioning, isolation, and merging of results for
  60  * safe and efficient parallel execution.
  61  * @since 1.8
  62  */
  63 public class DoubleSummaryStatistics implements DoubleConsumer {
  64     private long count;
  65     private double sum;
  66     private double sumCompensation; // Low order bits of sum
  67     private double min = Double.POSITIVE_INFINITY;
  68     private double max = Double.NEGATIVE_INFINITY;
  69 
  70     /**
  71      * Construct an empty instance with zero count, zero sum,
  72      * {@code Double.POSITIVE_INFINITY} min, {@code Double.NEGATIVE_INFINITY}
  73      * max and zero average.
  74      */
  75     public DoubleSummaryStatistics() { }
  76 
  77     /**
  78      * Records another value into the summary information.
  79      *
  80      * @param value the input value
  81      */
  82     @Override
  83     public void accept(double value) {
  84         ++count;
  85         sumWithCompensation(value);
  86         min = Math.min(min, value);
  87         max = Math.max(max, value);
  88     }
  89 
  90     /**
  91      * Combines the state of another {@code DoubleSummaryStatistics} into this
  92      * one.
  93      *
  94      * @param other another {@code DoubleSummaryStatistics}
  95      * @throws NullPointerException if {@code other} is null
  96      */
  97     public void combine(DoubleSummaryStatistics other) {
  98         count += other.count;
  99         sumWithCompensation(other.sum);
 100         sumWithCompensation(other.sumCompensation);
 101         min = Math.min(min, other.min);
 102         max = Math.max(max, other.max);
 103     }
 104 
 105     /**
 106      * Incorporate a new double value using Kahan summation /
 107      * compensated summation.
 108      */
 109     private void sumWithCompensation(double value) {
 110         double tmp = value - sumCompensation;
 111         double velvel = sum + tmp; // Little wolf of rounding error
 112         sumCompensation = (velvel - sum) - tmp;
 113         sum = velvel;
 114     }
 115 
 116     /**
 117      * Return the count of values recorded.
 118      *
 119      * @return the count of values
 120      */
 121     public final long getCount() {
 122         return count;
 123     }
 124 
 125     /**
 126      * Returns the sum of values recorded, or zero if no values have been
 127      * recorded.
 128      *
 129      * If any recorded value is a NaN or the sum is at any point a NaN
 130      * then the sum will be NaN.
 131      *
 132      * <p> The value of a floating-point sum is a function both of the
 133      * input values as well as the order of addition operations. The
 134      * order of addition operations of this method is intentionally
 135      * not defined to allow for implementation flexibility to improve
 136      * the speed and accuracy of the computed result.
 137      *
 138      * In particular, this method may be implemented using compensated
 139      * summation or other technique to reduce the error bound in the
 140      * numerical sum compared to a simple summation of {@code double}
 141      * values.
 142      *
 143      * @apiNote Values sorted by increasing absolute magnitude tend to yield
 144      * more accurate results.
 145      *
 146      * @return the sum of values, or zero if none
 147      */
 148     public final double getSum() {
 149         return sum;
 150     }
 151 
 152     /**
 153      * Returns the minimum recorded value, {@code Double.NaN} if any recorded
 154      * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
 155      * recorded. Unlike the numerical comparison operators, this method
 156      * considers negative zero to be strictly smaller than positive zero.
 157      *
 158      * @return the minimum recorded value, {@code Double.NaN} if any recorded
 159      * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
 160      * recorded
 161      */
 162     public final double getMin() {
 163         return min;
 164     }
 165 
 166     /**
 167      * Returns the maximum recorded value, {@code Double.NaN} if any recorded
 168      * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
 169      * recorded. Unlike the numerical comparison operators, this method
 170      * considers negative zero to be strictly smaller than positive zero.
 171      *
 172      * @return the maximum recorded value, {@code Double.NaN} if any recorded
 173      * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
 174      * recorded
 175      */
 176     public final double getMax() {
 177         return max;
 178     }
 179 
 180     /**
 181      * Returns the arithmetic mean of values recorded, or zero if no
 182      * values have been recorded.
 183      *
 184      * If any recorded value is a NaN or the sum is at any point a NaN
 185      * then the average will be code NaN.
 186      *
 187      * <p>The average returned can vary depending upon the order in
 188      * which values are recorded.
 189      *
 190      * This method may be implemented using compensated summation or
 191      * other technique to reduce the error bound in the {@link #getSum
 192      * numerical sum} used to compute the average.
 193      *
 194      * @apiNote Values sorted by increasing absolute magnitude tend to yield
 195      * more accurate results.
 196      *
 197      * @return the arithmetic mean of values, or zero if none
 198      */
 199     public final double getAverage() {
 200         return getCount() > 0 ? getSum() / getCount() : 0.0d;
 201     }
 202 
 203     /**
 204      * {@inheritDoc}
 205      *
 206      * Returns a non-empty string representation of this object suitable for
 207      * debugging. The exact presentation format is unspecified and may vary
 208      * between implementations and versions.
 209      */
 210     @Override
 211     public String toString() {
 212         return String.format(
 213             "%s{count=%d, sum=%f, min=%f, average=%f, max=%f}",
 214             this.getClass().getSimpleName(),
 215             getCount(),
 216             getSum(),
 217             getMin(),
 218             getAverage(),
 219             getMax());
 220     }
 221 }