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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  
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   * @since 1.0
26   */
27  public class DiscreteUniformSampler
28      extends SamplerBase
29      implements DiscreteSampler {
30      /** Lower bound. */
31      private final int lower;
32      /** Upper bound. */
33      private final int upper;
34      /** Underlying source of randomness. */
35      private final UniformRandomProvider rng;
36  
37      /**
38       * @param rng Generator of uniformly distributed random numbers.
39       * @param lower Lower bound (inclusive) of the distribution.
40       * @param upper Upper bound (inclusive) of the distribution.
41       * @throws IllegalArgumentException if {@code lower > upper}.
42       */
43      public DiscreteUniformSampler(UniformRandomProvider rng,
44                                    int lower,
45                                    int upper) {
46          super(null);
47          this.rng = rng;
48          if (lower > upper) {
49              throw new IllegalArgumentException(lower  + " > " + upper);
50          }
51  
52          this.lower = lower;
53          this.upper = upper;
54      }
55  
56      /** {@inheritDoc} */
57      @Override
58      public int sample() {
59          final int max = (upper - lower) + 1;
60          if (max <= 0) {
61              // The range is too wide to fit in a positive int (larger
62              // than 2^31); as it covers more than half the integer range,
63              // we use a simple rejection method.
64              while (true) {
65                  final int r = rng.nextInt();
66                  if (r >= lower &&
67                      r <= upper) {
68                      return r;
69                  }
70              }
71          } else {
72              // We can shift the range and directly generate a positive int.
73              return lower + rng.nextInt(max);
74          }
75      }
76  
77      /** {@inheritDoc} */
78      @Override
79      public String toString() {
80          return "Uniform deviate [" + rng.toString() + "]";
81      }
82  }