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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    *      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.rng.sampling.distribution;
18  
19  import org.apache.commons.rng.UniformRandomProvider;
20  
21  /**
22   * <a href="https://en.wikipedia.org/wiki/Box%E2%80%93Muller_transform">
23   * Box-Muller algorithm</a> for sampling from Gaussian distribution with
24   * mean 0 and standard deviation 1.
25   *
26   * <p>Sampling uses {@link UniformRandomProvider#nextDouble()}.</p>
27   *
28   * @since 1.1
29   */
30  public class BoxMullerNormalizedGaussianSampler
31      implements NormalizedGaussianSampler, SharedStateContinuousSampler {
32      /** Next gaussian. */
33      private double nextGaussian = Double.NaN;
34      /** Underlying source of randomness. */
35      private final UniformRandomProvider rng;
36  
37      /**
38       * @param rng Generator of uniformly distributed random numbers.
39       */
40      public BoxMullerNormalizedGaussianSampler(UniformRandomProvider rng) {
41          this.rng = rng;
42      }
43  
44      /** {@inheritDoc} */
45      @Override
46      public double sample() {
47          double random;
48          if (Double.isNaN(nextGaussian)) {
49              // Generate a pair of Gaussian numbers.
50  
51              final double x = rng.nextDouble();
52              final double y = rng.nextDouble();
53              final double alpha = 2 * Math.PI * x;
54              final double r = Math.sqrt(-2 * Math.log(y));
55  
56              // Return the first element of the generated pair.
57              random = r * Math.cos(alpha);
58  
59              // Keep second element of the pair for next invocation.
60              nextGaussian = r * Math.sin(alpha);
61          } else {
62              // Use the second element of the pair (generated at the
63              // previous invocation).
64              random = nextGaussian;
65  
66              // Both elements of the pair have been used.
67              nextGaussian = Double.NaN;
68          }
69  
70          return random;
71      }
72  
73      /** {@inheritDoc} */
74      @Override
75      public String toString() {
76          return "Box-Muller normalized Gaussian deviate [" + rng.toString() + "]";
77      }
78  
79      /**
80       * {@inheritDoc}
81       *
82       * @since 1.3
83       */
84      @Override
85      public SharedStateContinuousSampler withUniformRandomProvider(UniformRandomProvider rng) {
86          return new BoxMullerNormalizedGaussianSampler(rng);
87      }
88  
89      /**
90       * Create a new normalised Gaussian sampler.
91       *
92       * @param <S> Sampler type.
93       * @param rng Generator of uniformly distributed random numbers.
94       * @return the sampler
95       * @since 1.3
96       */
97      @SuppressWarnings("unchecked")
98      public static <S extends NormalizedGaussianSampler & SharedStateContinuousSampler> S
99              of(UniformRandomProvider rng) {
100         return (S) new BoxMullerNormalizedGaussianSampler(rng);
101     }
102 }