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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    *      https://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  
18  /*
19   * This is not the original file distributed by the Apache Software Foundation
20   * It has been modified by the Hipparchus project
21   */
22  
23  package org.hipparchus.random;
24  
25  import org.hipparchus.util.FastMath;
26  
27  
28  /**
29   * Generate random vectors isotropically located on the surface of a sphere.
30   */
31  public class UnitSphereRandomVectorGenerator
32      implements RandomVectorGenerator {
33  
34      /** RNG used for generating the individual components of the vectors. */
35      private final RandomGenerator rand;
36      /** Space dimension. */
37      private final int dimension;
38  
39      /** Simple constructor.
40       * @param dimension Space dimension.
41       * @param rand RNG for the individual components of the vectors.
42       */
43      public UnitSphereRandomVectorGenerator(final int dimension,
44                                             final RandomGenerator rand) {
45          this.dimension = dimension;
46          this.rand = rand;
47      }
48  
49      /**
50       * Create an object that will use a default RNG ({@link MersenneTwister}),
51       * in order to generate the individual components.
52       *
53       * @param dimension Space dimension.
54       */
55      public UnitSphereRandomVectorGenerator(final int dimension) {
56          this(dimension, new MersenneTwister());
57      }
58  
59      /** {@inheritDoc} */
60      @Override
61      public double[] nextVector() {
62          final double[] v = new double[dimension];
63  
64          // See http://mathworld.wolfram.com/SpherePointPicking.html for example.
65          // Pick a point by choosing a standard Gaussian for each element, and then
66          // normalizing to unit length.
67          double normSq = 0;
68          for (int i = 0; i < dimension; i++) {
69              final double comp = rand.nextGaussian();
70              v[i] = comp;
71              normSq += comp * comp;
72          }
73  
74          final double f = 1 / FastMath.sqrt(normSq);
75          for (int i = 0; i < dimension; i++) {
76              v[i] *= f;
77          }
78  
79          return v;
80      }
81  }