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BlockingElementsSet xref
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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, software
13   * distributed under the License is distributed on an "AS IS" BASIS,
14   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15   * See the License for the specific language governing permissions and
16   * limitations under the License.
17   */
18  
19  package org.apache.giraph.utils;
20  
21  import org.apache.hadoop.util.Progressable;
22  
23  import com.google.common.base.Preconditions;
24  
25  import java.util.ArrayList;
26  import java.util.Collections;
27  import java.util.List;
28  import java.util.concurrent.Semaphore;
29  
30  /**
31   * Keeps a set of elements, and allows for waiting on certain number of
32   * elements to become available. Assumes that at any point no more elements
33   * than we'll be asking for will be added to the set. Reusable.
34   *
35   * @param <T> Element type
36   */
37  public class BlockingElementsSet<T> {
38    /** Semaphore to keep track of element count */
39    private final Semaphore semaphore = new Semaphore(0);
40    /** Elements */
41    private final List<T> elements =
42        Collections.synchronizedList(new ArrayList<T>());
43  
44    /**
45     * Put an element in the set
46     *
47     * @param element Element to put
48     */
49    public void offer(T element) {
50      elements.add(element);
51      semaphore.release();
52    }
53  
54    /**
55     * Get one element when it becomes available,
56     * reporting progress while waiting
57     *
58     * @param progressable Progressable to report progress
59     * @return Element acquired
60     */
61    public T getElement(Progressable progressable) {
62      return getElements(1, progressable).get(0);
63    }
64  
65    /**
66     * Get desired number of elements when they become available,
67     * reporting progress while waiting
68     *
69     * @param elementCount How many elements to wait for
70     * @param progressable Progressable to report progress
71     * @return List of elements acquired
72     */
73    public List<T> getElements(int elementCount, Progressable progressable) {
74      ProgressableUtils.awaitSemaphorePermits(
75          semaphore, elementCount, progressable);
76      Preconditions.checkState(elements.size() == elementCount);
77      List<T> ret = new ArrayList<>(elements);
78      elements.clear();
79      return ret;
80    }
81  }