flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/ReduceNeighborsFunction.java

/*
 * 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.flink.graph;

import org.apache.flink.api.common.functions.Function;

import java.io.Serializable;

/**
 * Interface to be implemented by the function applied to a vertex neighborhood in the {@link
 * Graph#reduceOnNeighbors(ReduceNeighborsFunction, EdgeDirection)} method.
 *
 * @param <VV> the vertex value type
 */
public interface ReduceNeighborsFunction<VV> extends Function, Serializable {

    /**
     * It combines two neighboring vertex values into one new value of the same type. For each
     * vertex, this function is consecutively called, until only a single value for each vertex
     * remains.
     *
     * @param firstNeighborValue the first neighboring vertex value to combine
     * @param secondNeighborValue the second neighboring vertex value to combine
     * @return the combined value of both input values
     */
    VV reduceNeighbors(VV firstNeighborValue, VV secondNeighborValue);
}