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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.simple;
18  
19  import java.util.EnumMap;
20  
21  import org.apache.commons.rng.UniformRandomProvider;
22  
23  /**
24   * This class provides a thread-local {@link UniformRandomProvider}.
25   *
26   * <p>The {@link UniformRandomProvider} is created once-per-thread using the default
27   * construction method {@link RandomSource#create()}.
28   *
29   * <p>Example:</p>
30   * <pre><code>
31   * import org.apache.commons.rng.simple.RandomSource;
32   * import org.apache.commons.rng.simple.ThreadLocalRandomSource;
33   * import org.apache.commons.rng.sampling.distribution.PoissonSampler;
34   *
35   * // Access a thread-safe random number generator
36   * UniformRandomProvider rng = ThreadLocalRandomSource.current(RandomSource.SPLIT_MIX_64);
37   *
38   * // One-time Poisson sample
39   * double mean = 12.3;
40   * int counts = PoissonSampler.of(rng, mean).sample();
41   * </code></pre>
42   *
43   * <p>Note if the {@link RandomSource} requires additional arguments then it is not
44   * supported. The same can be achieved using:</p>
45   *
46   * <pre><code>
47   * import org.apache.commons.rng.simple.RandomSource;
48   * import org.apache.commons.rng.sampling.distribution.PoissonSampler;
49   *
50   * // Provide a thread-safe random number generator with data arguments
51   * private static ThreadLocal&lt;UniformRandomProvider&gt; rng =
52   *     new ThreadLocal&lt;UniformRandomProvider&gt;() {
53   *         &#64;Override
54   *         protected UniformRandomProvider initialValue() {
55   *             return RandomSource.TWO_CMRES_SELECT.create(null, 3, 4);
56   *         }
57   *     };
58   *
59   * // One-time Poisson sample using a thread-safe random number generator
60   * double mean = 12.3;
61   * int counts = PoissonSampler.of(rng.get(), mean).sample();
62   * </code></pre>
63   *
64   * @since 1.3
65   */
66  public final class ThreadLocalRandomSource {
67      /**
68       * A map containing the {@link ThreadLocal} instance for each {@link RandomSource}.
69       *
70       * <p>This should only be modified to create new instances in a synchronized block.
71       */
72      private static final EnumMap<RandomSource, ThreadLocal<UniformRandomProvider>> SOURCES =
73          new EnumMap<>(RandomSource.class);
74  
75      /** No public construction. */
76      private ThreadLocalRandomSource() {}
77  
78      /**
79       * Extend the {@link ThreadLocal} to allow creation of the desired {@link RandomSource}.
80       */
81      private static class ThreadLocalRng extends ThreadLocal<UniformRandomProvider> {
82          /** The source. */
83          private final RandomSource source;
84  
85          /**
86           * Create a new instance.
87           *
88           * @param source the source
89           */
90          ThreadLocalRng(RandomSource source) {
91              this.source = source;
92          }
93  
94          @Override
95          protected UniformRandomProvider initialValue() {
96              // Create with the default seed generation method
97              return source.create();
98          }
99      }
100 
101     /**
102      * Returns the current thread's copy of the given {@code source}. If there is no
103      * value for the current thread, it is first initialized to the value returned
104      * by {@link RandomSource#create()}.
105      *
106      * <p>Note if the {@code source} requires additional arguments then it is not
107      * supported.
108      *
109      * @param source the source
110      * @return the current thread's value of the {@code source}.
111      * @throws IllegalArgumentException if the source is null or the source requires arguments
112      */
113     public static UniformRandomProvider current(RandomSource source) {
114         ThreadLocal<UniformRandomProvider> rng = SOURCES.get(source);
115         // Implement double-checked locking:
116         // https://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java
117         if (rng == null) {
118             // Do the checks on the source here since it is an edge case
119             // and the EnumMap handles null (returning null).
120             if (source == null) {
121                 throw new IllegalArgumentException("Random source is null");
122             }
123 
124             synchronized (SOURCES) {
125                 rng = SOURCES.computeIfAbsent(source, ThreadLocalRng::new);
126             }
127         }
128         return rng.get();
129     }
130 }