/*
 * 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.generator;

import org.apache.flink.api.common.functions.FlatMapFunction;
import org.apache.flink.api.java.DataSet;
import org.apache.flink.api.java.ExecutionEnvironment;
import org.apache.flink.api.java.functions.FunctionAnnotation.ForwardedFields;
import org.apache.flink.api.java.tuple.Tuple2;
import org.apache.flink.graph.Edge;
import org.apache.flink.graph.Graph;
import org.apache.flink.graph.Vertex;
import org.apache.flink.types.LongValue;
import org.apache.flink.types.NullValue;
import org.apache.flink.util.Collector;
import org.apache.flink.util.LongValueSequenceIterator;
import org.apache.flink.util.Preconditions;

import java.util.ArrayList;
import java.util.List;

/**
 * @see <a href="http://mathworld.wolfram.com/GridGraph.html">Grid Graph at Wolfram MathWorld</a>
 */
public class GridGraph
extends AbstractGraphGenerator<LongValue, NullValue, NullValue> {

	// Required to create the DataSource
	private final ExecutionEnvironment env;

	// Required configuration
	private List<Tuple2<Long, Boolean>> dimensions = new ArrayList<>();

	private long vertexCount = 1;

	/**
	 * An undirected {@code Graph} connecting vertices in a regular tiling in
	 * one or more dimensions and where the endpoints are optionally connected.
	 *
	 * @param env the Flink execution environment
	 */
	public GridGraph(ExecutionEnvironment env) {
		this.env = env;
	}

	/**
	 * Required configuration for each dimension of the graph.
	 *
	 * @param size number of vertices; dimensions of size 1 are prohibited due to having no effect
	 *             on the generated graph
	 * @param wrapEndpoints whether to connect first and last vertices; this has no effect on
	 *                      dimensions of size 2
	 * @return this
	 */
	public GridGraph addDimension(long size, boolean wrapEndpoints) {
		Preconditions.checkArgument(size >= 2, "Dimension size must be at least 2");

		vertexCount *= size;

		// prevent duplicate edges
		if (size == 2) {
			wrapEndpoints = false;
		}

		dimensions.add(new Tuple2<>(size, wrapEndpoints));

		return this;
	}

	@Override
	public Graph<LongValue, NullValue, NullValue> generate() {
		Preconditions.checkState(!dimensions.isEmpty(), "No dimensions added to GridGraph");

		// Vertices
		DataSet<Vertex<LongValue, NullValue>> vertices = GraphGeneratorUtils.vertexSequence(env, parallelism, vertexCount);

		// Edges
		LongValueSequenceIterator iterator = new LongValueSequenceIterator(0, this.vertexCount - 1);

		DataSet<Edge<LongValue, NullValue>> edges = env
			.fromParallelCollection(iterator, LongValue.class)
				.setParallelism(parallelism)
				.name("Edge iterators")
			.flatMap(new LinkVertexToNeighbors(vertexCount, dimensions))
				.setParallelism(parallelism)
				.name("Grid graph edges");

		// Graph
		return Graph.fromDataSet(vertices, edges, env);
	}

	@ForwardedFields("*->f0")
	private static class LinkVertexToNeighbors
	implements FlatMapFunction<LongValue, Edge<LongValue, NullValue>> {

		private long vertexCount;

		private List<Tuple2<Long, Boolean>> dimensions;

		private LongValue target = new LongValue();

		private Edge<LongValue, NullValue> edge = new Edge<>(null, target, NullValue.getInstance());

		public LinkVertexToNeighbors(long vertexCount, List<Tuple2<Long, Boolean>> dimensions) {
			this.vertexCount = vertexCount;
			this.dimensions = dimensions;
		}

		@Override
		public void flatMap(LongValue source, Collector<Edge<LongValue, NullValue>> out)
				throws Exception {
			edge.f0 = source;
			long val = source.getValue();

			// the distance between neighbors in a given iteration
			long increment = vertexCount;

			// the value in the remaining dimensions
			long remainder = val;

			for (Tuple2<Long, Boolean> dimension : dimensions) {
				increment /= dimension.f0;

				// the index within this dimension
				long index = remainder / increment;

				if (index > 0) {
					target.setValue(val - increment);
					out.collect(edge);
				} else if (dimension.f1) {
					target.setValue(val + increment * (dimension.f0 - 1));
					out.collect(edge);
				}

				if (index < dimension.f0 - 1) {
					target.setValue(val + increment);
					out.collect(edge);
				} else if (dimension.f1) {
					target.setValue(val - increment * (dimension.f0 - 1));
					out.collect(edge);
				}

				remainder %= increment;
			}
		}
	}
}