1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 package org.apache.commons.statistics.descriptive; 18 19 /** 20 * Returns the sum of the squares of the available values. Uses the following definition: 21 * 22 * <p>\[ \sum_{i=1}^n x_i^2 \] 23 * 24 * <p>where \( n \) is the number of samples. 25 * 26 * <ul> 27 * <li>The result is zero if no values are observed. 28 * <li>The result is {@code NaN} if any of the values is {@code NaN}. 29 * <li>The result is {@code +infinity} if any of the values is {@code infinity}, 30 * or the sum overflows. 31 * </ul> 32 * 33 * <p>This class is designed to work with (though does not require) 34 * {@linkplain java.util.stream streams}. 35 * 36 * <p><strong>This instance is not thread safe.</strong> 37 * If multiple threads access an instance of this class concurrently, 38 * and at least one of the threads invokes the {@link java.util.function.DoubleConsumer#accept(double) accept} or 39 * {@link StatisticAccumulator#combine(StatisticResult) combine} method, it must be synchronized externally. 40 * 41 * <p>However, it is safe to use {@link java.util.function.DoubleConsumer#accept(double) accept} 42 * and {@link StatisticAccumulator#combine(StatisticResult) combine} 43 * as {@code accumulator} and {@code combiner} functions of 44 * {@link java.util.stream.Collector Collector} on a parallel stream, 45 * because the parallel instance of {@link java.util.stream.Stream#collect Stream.collect()} 46 * provides the necessary partitioning, isolation, and merging of results for 47 * safe and efficient parallel execution. 48 * 49 * @since 1.1 50 */ 51 public final class SumOfSquares implements DoubleStatistic, StatisticAccumulator<SumOfSquares> { 52 53 /** Sum of squares of all values. */ 54 private double ss; 55 56 /** 57 * Create an instance. 58 */ 59 private SumOfSquares() { 60 // No-op 61 } 62 63 /** 64 * Creates an instance. 65 * 66 * <p>The initial result is zero. 67 * 68 * @return {@code SumOfSquares} instance. 69 */ 70 public static SumOfSquares create() { 71 return new SumOfSquares(); 72 } 73 74 /** 75 * Returns an instance populated using the input {@code values}. 76 * 77 * <p>The result is {@code NaN} if any of the values is {@code NaN}. 78 * 79 * <p>When the input is an empty array, the result is zero. 80 * 81 * @param values Values. 82 * @return {@code SumOfSquares} instance. 83 */ 84 public static SumOfSquares of(double... values) { 85 return Statistics.add(new SumOfSquares(), values); 86 } 87 88 /** 89 * Returns an instance populated using the specified range of {@code values}. 90 * 91 * <p>The result is {@code NaN} if any of the values is {@code NaN}. 92 * 93 * <p>When the range is empty, the result is zero. 94 * 95 * @param values Values. 96 * @param from Inclusive start of the range. 97 * @param to Exclusive end of the range. 98 * @return {@code SumOfSquares} instance. 99 * @throws IndexOutOfBoundsException if the sub-range is out of bounds 100 * @since 1.2 101 */ 102 public static SumOfSquares ofRange(double[] values, int from, int to) { 103 Statistics.checkFromToIndex(from, to, values.length); 104 return createFromRange(values, from, to); 105 } 106 107 /** 108 * Create an instance using the specified range of {@code values}. 109 * 110 * <p>Warning: No range checks are performed. 111 * 112 * @param values Values. 113 * @param from Inclusive start of the range. 114 * @param to Exclusive end of the range. 115 * @return {@code SumOfSquares} instance. 116 */ 117 static SumOfSquares createFromRange(double[] values, int from, int to) { 118 return Statistics.add(new SumOfSquares(), values, from, to); 119 } 120 121 /** 122 * Updates the state of the statistic to reflect the addition of {@code value}. 123 * 124 * @param value Value. 125 */ 126 @Override 127 public void accept(double value) { 128 ss += value * value; 129 } 130 131 /** 132 * Gets the sum of squares of all input values. 133 * 134 * <p>When no values have been added, the result is zero. 135 * 136 * @return sum of squares of all values. 137 */ 138 @Override 139 public double getAsDouble() { 140 return ss; 141 } 142 143 @Override 144 public SumOfSquares combine(SumOfSquares other) { 145 ss += other.ss; 146 return this; 147 } 148 }