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 * or the product at any point is a {@code NaN}.
79 *
80 * <p>When the input is an empty array, the result is zero.
81 *
82 * @param values Values.
83 * @return {@code SumOfSquares} instance.
84 */
85 public static SumOfSquares of(double... values) {
86 return Statistics.add(new SumOfSquares(), values);
87 }
88
89 /**
90 * Updates the state of the statistic to reflect the addition of {@code value}.
91 *
92 * @param value Value.
93 */
94 @Override
95 public void accept(double value) {
96 ss += value * value;
97 }
98
99 /**
100 * Gets the sum of squares of all input values.
101 *
102 * <p>When no values have been added, the result is zero.
103 *
104 * @return sum of squares of all values.
105 */
106 @Override
107 public double getAsDouble() {
108 return ss;
109 }
110
111 @Override
112 public SumOfSquares combine(SumOfSquares other) {
113 ss += other.ss;
114 return this;
115 }
116 }