/*

  * Licensed to the Apache Software Foundation (ASF) under one

  * or more contributor license agreements.  See the NOTICE file

  * distributed with this work for additional information

  * regarding copyright ownership.  The ASF licenses this file

  * to you under the Apache License, Version 2.0 (the

  * "License"); you may not use this file except in compliance

  * with the License.  You may obtain a copy of the License at

  *

  *     http://www.apache.org/licenses/LICENSE-2.0

  *

  * Unless required by applicable law or agreed to in writing, software

  * distributed under the License is distributed on an "AS IS" BASIS,

  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

  * See the License for the specific language governing permissions and

  * limitations under the License.

  */

 package org.apache.giraph.combiner;

 import org.apache.hadoop.io.Writable;

 import org.apache.hadoop.io.WritableComparable;

 /**

  * Abstract class to extend for combining messages sent to the same vertex.

  * MessageCombiner for applications where each two messages for one

  * vertex can be combined into one.

  *

  * @param <I> Vertex id

  * @param <M> Message data

  */

 public interface MessageCombiner<I extends WritableComparable,

     M extends Writable> {

   /**

    * Combine messageToCombine with originalMessage, by modifying

    * originalMessage.  Note that the messageToCombine object

    * may be reused by the infrastructure, therefore, you cannot directly

    * use it or any objects from it in original message

    *

    * @param vertexIndex Index of the vertex getting these messages

    * @param originalMessage The first message which we want to combine;

    *                        put the result of combining in this message

    * @param messageToCombine The second message which we want to combine

    *                         (object may be reused - do not reference it or its

    *                         member objects)

    */

   void combine(I vertexIndex, M originalMessage,

       M messageToCombine);

   /**

    * Get the initial message. When combined with any other message M,

    * the result should be M.

    *

    * @return Initial message

    */

   M createInitialMessage();

 }