001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.statistics.descriptive;
018
019/**
020 * Returns the maximum of the available values. Uses {@link Math#max(double, double) Math.max} as an
021 * underlying function to compute the {@code maximum}.
022 *
023 * <ul>
024 *   <li>The result is {@link Double#NEGATIVE_INFINITY negative infinity} if no values are added.
025 *   <li>The result is {@code NaN} if any of the values is {@code NaN}.
026 *   <li>The value {@code -0.0} is considered strictly smaller than {@code 0.0}.
027 * </ul>
028 *
029 * <p>This class is designed to work with (though does not require)
030 * {@linkplain java.util.stream streams}.
031 *
032 * <p><strong>This implementation is not thread safe.</strong>
033 * If multiple threads access an instance of this class concurrently,
034 * and at least one of the threads invokes the {@link java.util.function.DoubleConsumer#accept(double) accept} or
035 * {@link StatisticAccumulator#combine(StatisticResult) combine} method, it must be synchronized externally.
036 *
037 * <p>However, it is safe to use {@link java.util.function.DoubleConsumer#accept(double) accept}
038 * and {@link StatisticAccumulator#combine(StatisticResult) combine}
039 * as {@code accumulator} and {@code combiner} functions of
040 * {@link java.util.stream.Collector Collector} on a parallel stream,
041 * because the parallel implementation of {@link java.util.stream.Stream#collect Stream.collect()}
042 * provides the necessary partitioning, isolation, and merging of results for
043 * safe and efficient parallel execution.
044 *
045 * @since 1.1
046 * @see Math#max(double, double)
047 */
048public final class Max implements DoubleStatistic, StatisticAccumulator<Max> {
049
050    /** Current maximum. */
051    private double maximum = Double.NEGATIVE_INFINITY;
052
053    /**
054     * Create an instance.
055     */
056    private Max() {
057        // No-op
058    }
059
060    /**
061     * Creates an instance.
062     *
063     * <p>The initial result is {@link Double#NEGATIVE_INFINITY negative infinity}.
064     *
065     * @return {@code Max} instance.
066     */
067    public static Max create() {
068        return new Max();
069    }
070
071    /**
072     * Returns an instance populated using the input {@code values}.
073     *
074     * <p>The result is {@code NaN} if any of the values is {@code NaN}.
075     *
076     * <p>When the input is an empty array, the result is
077     * {@link Double#NEGATIVE_INFINITY negative infinity}.
078     *
079     * @param values Values.
080     * @return {@code Max} instance.
081     */
082    public static Max of(double... values) {
083        return Statistics.add(new Max(), values);
084    }
085
086    /**
087     * Updates the state of the statistic to reflect the addition of {@code value}.
088     *
089     * @param value Value.
090     */
091    @Override
092    public void accept(double value) {
093        maximum = Math.max(maximum, value);
094    }
095
096    /**
097     * Gets the maximum of all input values.
098     *
099     * <p>When no values have been added, the result is
100     * {@link Double#NEGATIVE_INFINITY negative infinity}.
101     *
102     * @return maximum of all values.
103     */
104    @Override
105    public double getAsDouble() {
106        return maximum;
107    }
108
109    @Override
110    public Max combine(Max other) {
111        accept(other.getAsDouble());
112        return this;
113    }
114}