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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  package org.apache.giraph.reducers;
19  
20  import org.apache.hadoop.io.Writable;
21  
22  /**
23   * Reduce operations defining how to reduce single values
24   * passed on workers, into partial values on workers, and then
25   * into a single global reduced value.
26   *
27   * Object should be thread safe. Most frequently it should be
28   * immutable object, so that functions can execute concurrently.
29   * Rarely when object is mutable
30   * ({@link org.apache.giraph.master.AggregatorReduceOperation}),
31   * i.e. stores reusable object inside, accesses should be synchronized.
32   *
33   * @param <S> Single value type, objects passed on workers
34   * @param <R> Reduced value type
35   */
36  public interface ReduceOperation<S, R extends Writable> extends Writable {
37    /**
38     * Return new reduced value which is neutral to reduce operation.
39     *
40     * @return Neutral value
41     */
42    R createInitialValue();
43    /**
44     * Add a new value.
45     * Needs to be commutative and associative
46     *
47     * Commonly, returned value should be same as curValue argument.
48     *
49     * @param curValue Partial value into which to reduce and store the result
50     * @param valueToReduce Single value to be reduced
51     * @return reduced value
52     */
53    R reduce(R curValue, S valueToReduce);
54    /**
55     * Add partially reduced value to current partially reduced value.
56     *
57     * Commonly, returned value should be same as curValue argument.
58     *
59     * @param curValue Partial value into which to reduce and store the result
60     * @param valueToReduce Partial value to be reduced
61     * @return reduced value
62     */
63    R reduceMerge(R curValue, R valueToReduce);
64  }