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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one
3    * or more contributor license agreements.  See the NOTICE file
4    * distributed with this work for additional information
5    * regarding copyright ownership.  The ASF licenses this file
6    * to you under the Apache License, Version 2.0 (the
7    * "License"); you may not use this file except in compliance
8    * with the License.  You may obtain a copy of the License at
9    *
10   *   http://www.apache.org/licenses/LICENSE-2.0
11   *
12   * Unless required by applicable law or agreed to in writing,
13   * software distributed under the License is distributed on an
14   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
15   * KIND, either express or implied.  See the License for the
16   * specific language governing permissions and limitations
17   * under the License.
18   */
19  package org.apache.maven.plugin.surefire.booterclient;
20  
21  import java.util.Queue;
22  import java.util.concurrent.ConcurrentLinkedQueue;
23  import java.util.concurrent.atomic.AtomicInteger;
24  
25  /**
26   * A bucket from which fork numbers can be drawn. Any drawn number needs to be returned to the bucket, in order to keep
27   * the range of provided values delivered as small as possible.
28   *
29   * @author Andreas Gudian
30   */
31  public final class ForkNumberBucket {
32      private static final ForkNumberBucket INSTANCE = new ForkNumberBucket();
33  
34      private final Queue<Integer> qFree = new ConcurrentLinkedQueue<>();
35  
36      private final AtomicInteger highWaterMark = new AtomicInteger(1);
37  
38      /**
39       * Non-public constructor
40       */
41      private ForkNumberBucket() {}
42  
43      /**
44       * @return a fork number that is not currently in use. The value must be returned to the bucket using
45       *         {@link #returnNumber(int)}.
46       */
47      public static int drawNumber() {
48          return getInstance().drawNumberInternal();
49      }
50  
51      /**
52       * @param number the number to return to the bucket so that it can be reused.
53       */
54      public static void returnNumber(int number) {
55          getInstance().returnNumberInternal(number);
56      }
57  
58      /**
59       * @return a singleton instance
60       */
61      private static ForkNumberBucket getInstance() {
62          return INSTANCE;
63      }
64  
65      /**
66       * @return a fork number that is not currently in use. The value must be returned to the bucket using
67       *         {@link #returnNumber(int)}.
68       */
69      private int drawNumberInternal() {
70          Integer nextFree = qFree.poll();
71          return nextFree == null ? highWaterMark.getAndIncrement() : nextFree;
72      }
73  
74      /**
75       * @return the highest number that has been drawn
76       */
77      private int getHighestDrawnNumber() {
78          return highWaterMark.get() - 1;
79      }
80  
81      /**
82       * @param number the number to return to the bucket so that it can be reused.
83       */
84      private void returnNumberInternal(int number) {
85          qFree.add(number);
86      }
87  }