View Javadoc
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  
18  package org.apache.commons.rng.sampling.distribution;
19  
20  import org.apache.commons.rng.UniformRandomProvider;
21  
22  /**
23   * Discrete uniform distribution sampler.
24   */
25  public class DiscreteUniformSampler
26      extends SamplerBase
27      implements DiscreteSampler {
28      /** Lower bound. */
29      private final int lower;
30      /** Upper bound. */
31      private final int upper;
32      /** Underlying source of randomness. */
33      private final UniformRandomProvider rng;
34  
35      /**
36       * @param rng Generator of uniformly distributed random numbers.
37       * @param lower Lower bound (inclusive) of the distribution.
38       * @param upper Upper bound (inclusive) of the distribution.
39       * @throws IllegalArgumentException if {@code lower > upper}.
40       */
41      public DiscreteUniformSampler(UniformRandomProvider rng,
42                                    int lower,
43                                    int upper) {
44          super(null);
45          this.rng = rng;
46          if (lower > upper) {
47              throw new IllegalArgumentException(lower  + " > " + upper);
48          }
49  
50          this.lower = lower;
51          this.upper = upper;
52      }
53  
54      /** {@inheritDoc} */
55      @Override
56      public int sample() {
57          final int max = (upper - lower) + 1;
58          if (max <= 0) {
59              // The range is too wide to fit in a positive int (larger
60              // than 2^31); as it covers more than half the integer range,
61              // we use a simple rejection method.
62              while (true) {
63                  final int r = rng.nextInt();
64                  if (r >= lower &&
65                      r <= upper) {
66                      return r;
67                  }
68              }
69          } else {
70              // We can shift the range and directly generate a positive int.
71              return lower + rng.nextInt(max);
72          }
73      }
74  
75      /** {@inheritDoc} */
76      @Override
77      public String toString() {
78          return "Uniform deviate [" + rng.toString() + "]";
79      }
80  }