/*
 * 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.library;

import org.apache.flink.api.java.DataSet;
import org.apache.flink.graph.Graph;
import org.apache.flink.graph.GraphAlgorithm;
import org.apache.flink.graph.Vertex;
import org.apache.flink.graph.spargel.MessageIterator;
import org.apache.flink.graph.spargel.MessagingFunction;
import org.apache.flink.graph.spargel.VertexUpdateFunction;
import org.apache.flink.graph.utils.NullValueEdgeMapper;
import org.apache.flink.types.NullValue;

/**
 * A vertex-centric implementation of the Weakly Connected Components algorithm.
 *
 * This implementation assumes that the vertex values of the input Graph are initialized with Long component IDs.
 * The vertices propagate their current component ID in iterations.
 * Upon receiving component IDs from its neighbors, a vertex adopts a new component ID if its value
 * is lower than its current component ID.
 *
 * The algorithm converges when vertices no longer update their component ID value
 * or when the maximum number of iterations has been reached.
 * 
 * The result is a DataSet of vertices, where the vertex value corresponds to the assigned component ID.
 * 
 * @see org.apache.flink.graph.library.GSAConnectedComponents
 */
@SuppressWarnings("serial")
public class ConnectedComponents<K, EV> implements GraphAlgorithm<K, Long, EV, DataSet<Vertex<K, Long>>> {

	private Integer maxIterations;

	/**
	 * Creates an instance of the Connected Components algorithm.
	 * The algorithm computes weakly connected components
	 * and converges when no vertex updates its component ID
	 * or when the maximum number of iterations has been reached.
	 * 
	 * @param maxIterations The maximum number of iterations to run.
	 */
	public ConnectedComponents(Integer maxIterations) {
		this.maxIterations = maxIterations;
	}

	@Override
	public DataSet<Vertex<K, Long>> run(Graph<K, Long, EV> graph) throws Exception {

		Graph<K, Long, NullValue> undirectedGraph = graph.mapEdges(new NullValueEdgeMapper<K, EV>())
				.getUndirected();

		// initialize vertex values and run the Vertex Centric Iteration
		return undirectedGraph.runVertexCentricIteration(
				new CCUpdater<K>(), new CCMessenger<K>(), maxIterations)
				.getVertices();
	}

	/**
	 * Updates the value of a vertex by picking the minimum neighbor ID out of all the incoming messages.
	 */
	public static final class CCUpdater<K> extends VertexUpdateFunction<K, Long, Long> {

		@Override
		public void updateVertex(Vertex<K, Long> vertex, MessageIterator<Long> messages) throws Exception {
			long min = Long.MAX_VALUE;

			for (long msg : messages) {
				min = Math.min(min, msg);
			}

			// update vertex value, if new minimum
			if (min < vertex.getValue()) {
				setNewVertexValue(min);
			}
		}
	}

	/**
	 * Distributes the minimum ID associated with a given vertex among all the target vertices.
	 */
	public static final class CCMessenger<K> extends MessagingFunction<K, Long, Long, NullValue> {

		@Override
		public void sendMessages(Vertex<K, Long> vertex) throws Exception {
			// send current minimum to neighbors
			sendMessageToAllNeighbors(vertex.getValue());
		}
	}
}