/*
 * 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.common.functions.MapFunction;
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.gsa.ApplyFunction;
import org.apache.flink.graph.gsa.GatherFunction;
import org.apache.flink.graph.gsa.Neighbor;
import org.apache.flink.graph.gsa.SumFunction;

/**
 * This is an implementation of the Single Source Shortest Paths algorithm, using a gather-sum-apply iteration
 */
public class GSASingleSourceShortestPaths<K, VV> implements
	GraphAlgorithm<K, VV, Double, DataSet<Vertex<K, Double>>> {

	private final K srcVertexId;
	private final Integer maxIterations;

	/**
	 * Creates an instance of the GSA SingleSourceShortestPaths algorithm.
	 * 
	 * @param srcVertexId The ID of the source vertex.
	 * @param maxIterations The maximum number of iterations to run.
	 */
	public GSASingleSourceShortestPaths(K srcVertexId, Integer maxIterations) {
		this.srcVertexId = srcVertexId;
		this.maxIterations = maxIterations;
	}

	@Override
	public DataSet<Vertex<K, Double>> run(Graph<K, VV, Double> input) {

		return input.mapVertices(new InitVerticesMapper<K, VV>(srcVertexId))
				.runGatherSumApplyIteration(new CalculateDistances(), new ChooseMinDistance(),
						new UpdateDistance<K>(), maxIterations)
						.getVertices();
	}

	@SuppressWarnings("serial")
	public static final class InitVerticesMapper<K, VV> implements MapFunction<Vertex<K, VV>, Double> {

		private K srcVertexId;

		public InitVerticesMapper(K srcId) {
			this.srcVertexId = srcId;
		}

		public Double map(Vertex<K, VV> value) {
			if (value.f0.equals(srcVertexId)) {
				return 0.0;
			} else {
				return Double.MAX_VALUE;
			}
		}
	}

	// --------------------------------------------------------------------------------------------
	//  Single Source Shortest Path UDFs
	// --------------------------------------------------------------------------------------------

	@SuppressWarnings("serial")
	private static final class CalculateDistances extends GatherFunction<Double, Double, Double> {

		public Double gather(Neighbor<Double, Double> neighbor) {
			return neighbor.getNeighborValue() + neighbor.getEdgeValue();
		}
	}

	@SuppressWarnings("serial")
	private static final class ChooseMinDistance extends SumFunction<Double, Double, Double> {

		public Double sum(Double newValue, Double currentValue) {
			return Math.min(newValue, currentValue);
		}
	}

	@SuppressWarnings("serial")
	private static final class UpdateDistance<K> extends ApplyFunction<K, Double, Double> {

		public void apply(Double newDistance, Double oldDistance) {
			if (newDistance < oldDistance) {
				setResult(newDistance);
			}
		}
	}
}