/*
 * This file is part of Gradoop.
 *
 * Gradoop is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * Gradoop is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Gradoop. If not, see <http://www.gnu.org/licenses/>.
 */

package org.gradoop.flink.algorithms.labelpropagation;

import org.apache.flink.api.java.DataSet;
import org.apache.flink.graph.Graph;
import org.apache.flink.types.NullValue;
import org.gradoop.flink.algorithms.labelpropagation.functions.EdgeToGellyEdgeMapper;
import org.gradoop.flink.algorithms.labelpropagation.functions.LPVertexJoin;
import org.gradoop.flink.algorithms.labelpropagation.functions.VertexToGellyVertexMapper;
import org.gradoop.common.model.impl.id.GradoopId;
import org.gradoop.common.model.impl.pojo.Vertex;
import org.gradoop.common.model.impl.properties.PropertyValue;
import org.gradoop.flink.model.api.operators.UnaryGraphToGraphOperator;
import org.gradoop.flink.model.impl.LogicalGraph;
import org.gradoop.flink.model.impl.functions.epgm.Id;

import static com.google.common.base.Preconditions.checkNotNull;

/**
 * Wraps {@link LabelPropagation} into the EPGM model.
 *
 * During vertex centric iteration (Label Propagation Algorithm):
 *
 * In each super step each vertex will adopt the value sent by the majority of
 * their neighbors or the smallest one if there is just one neighbor. If
 * multiple labels occur with the same frequency, the minimum of them will be
 * selected as new label. If a vertex changes its value in a super step, the new
 * value will be propagated to the neighbours.
 *
 * The computation will terminate if no new values are assigned.
 */
public abstract class LabelPropagation implements UnaryGraphToGraphOperator {

  /**
   * Counter to define maximum number of iterations for the algorithm
   */
  private final int maxIterations;

  /**
   * Property key to access the label value which will be propagated
   */
  private final String propertyKey;

  /**
   * Constructor
   *
   * @param maxIterations Counter to define maximal iteration for the algorithm
   * @param propertyKey   Property key to access the label value
   */
  protected LabelPropagation(int maxIterations, String propertyKey) {
    this.maxIterations = maxIterations;
    this.propertyKey = checkNotNull(propertyKey);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public LogicalGraph execute(LogicalGraph logicalGraph) {
    // prepare vertex set for Gelly vertex centric iteration
    DataSet<org.apache.flink.graph.Vertex<GradoopId, PropertyValue>> vertices =
      logicalGraph.getVertices()
        .map(new VertexToGellyVertexMapper(propertyKey));

    // prepare edge set for Gelly vertex centric iteration
    DataSet<org.apache.flink.graph.Edge<GradoopId, NullValue>> edges =
      logicalGraph.getEdges().map(new EdgeToGellyEdgeMapper());

    // create Gelly graph
    Graph<GradoopId, PropertyValue, NullValue> gellyGraph = Graph.fromDataSet(
      vertices, edges, logicalGraph.getConfig().getExecutionEnvironment());

    DataSet<Vertex> labeledVertices = executeInternal(gellyGraph)
      .join(logicalGraph.getVertices())
      .where(0).equalTo(new Id<Vertex>())
      .with(new LPVertexJoin(propertyKey));

    // return labeled graph
    return LogicalGraph.fromDataSets(
      labeledVertices, logicalGraph.getEdges(), logicalGraph.getConfig());
  }

  /**
   * Executes the label propagation and returns the updated vertex dataset.
   *
   * @param gellyGraph gelly graph with initialized vertices
   * @return updated vertex set
   */
  protected abstract
  DataSet<org.apache.flink.graph.Vertex<GradoopId, PropertyValue>>
  executeInternal(Graph<GradoopId, PropertyValue, NullValue> gellyGraph);

  /**
   * Returns the maximum number of iterations the algorithm is executed.
   *
   * @return maximum number of iterations
   */
  protected int getMaxIterations() {
    return maxIterations;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public String getName() {
    return LabelPropagation.class.getName();
  }
}