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Aggregator 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.aggregators;
20  
21  import org.apache.hadoop.io.Writable;
22  
23  /**
24   * Interface for Aggregator.  Allows aggregate operations for all vertices
25   * in a given superstep.
26   *
27   * @param <A> Aggregated value
28   */
29  public interface Aggregator<A extends Writable> {
30    /**
31     * Add a new value.
32     * Needs to be commutative and associative
33     *
34     * @param value Value to be aggregated.
35     */
36    void aggregate(A value);
37  
38    /**
39     * Return new aggregated value which is neutral to aggregate operation.
40     * Must be changeable without affecting internals of Aggregator
41     *
42     * @return Neutral value
43     */
44    A createInitialValue();
45  
46    /**
47     * Return current aggregated value.
48     * Needs to be initialized if aggregate or setAggregatedValue
49     * have not been called before.
50     *
51     * @return Aggregated
52     */
53    A getAggregatedValue();
54  
55    /**
56     * Set aggregated value.
57     * Can be used for initialization or reset.
58     *
59     * @param value Value to be set.
60     */
61    void setAggregatedValue(A value);
62  
63    /**
64     * Reset the value of aggregator to neutral value
65     */
66    void reset();
67  }