/*

  * 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.giraph.examples.scc;

 import static org.apache.giraph.examples.scc.SccPhaseMasterCompute.PHASE;

 import static org.apache.giraph.examples.scc.SccPhaseMasterCompute.NEW_MAXIMUM;

 import static org.apache.giraph.examples.scc.SccPhaseMasterCompute.CONVERGED;

 import java.io.IOException;

 import org.apache.giraph.Algorithm;

 import org.apache.giraph.examples.scc.SccPhaseMasterCompute.Phases;

 import org.apache.giraph.graph.BasicComputation;

 import org.apache.giraph.graph.Vertex;

 import org.apache.hadoop.io.BooleanWritable;

 import org.apache.hadoop.io.IntWritable;

 import org.apache.hadoop.io.LongWritable;

 import org.apache.hadoop.io.NullWritable;

 /**

  * Finds strongly connected components of the graph.

  */

 @Algorithm(name = "Strongly Connected Components",

            description = "Finds strongly connected components of the graph")

 public class SccComputation extends

     BasicComputation<LongWritable, SccVertexValue, NullWritable, LongWritable> {

   /**

    * Current phase of the computation as defined in SccPhaseMasterCompute

    */

   private Phases currPhase;

   /**

    * Reusable object to encapsulate message value, in order to avoid

    * creating a new instance every time a message is sent.

    */

   private LongWritable messageValue = new LongWritable();

   /**

    * Reusable object to encapsulate a parent vertex id.

    */

   private LongWritable parentId = new LongWritable();

   @Override

   public void preSuperstep() {

     IntWritable phaseInt = getAggregatedValue(PHASE);

     currPhase = SccPhaseMasterCompute.getPhase(phaseInt);

   }

   @Override

   public void compute(

       Vertex<LongWritable, SccVertexValue, NullWritable> vertex,

       Iterable<LongWritable> messages) throws IOException {

     SccVertexValue vertexValue = vertex.getValue();

     if (!vertexValue.isActive()) {

       vertex.voteToHalt();

       return;

     }

     switch (currPhase) {

     case TRANSPOSE :

       vertexValue.clearParents();

       sendMessageToAllEdges(vertex, vertex.getId());

       break;

     case TRIMMING :

       trim(vertex, messages);

       break;

     case FORWARD_TRAVERSAL :

       forwardTraversal(vertex, messages);

       break;

     case BACKWARD_TRAVERSAL_START :

       backwardTraversalStart(vertex);

       break;

     case BACKWARD_TRAVERSAL_REST :

       backwardTraversalRest(vertex, messages);

       break;

     default :

       break;

     }

   }

   /**

    * Creates list of parents based on the received ids and halts the vertices

    * that don't have any parent or outgoing edge, hence, they can't be

    * part of an SCC.

    * @param vertex Current vertex.

    * @param messages Received ids from the Transpose phase.

    */

   private void trim(Vertex<LongWritable, SccVertexValue, NullWritable> vertex,

                     Iterable<LongWritable> messages) {

     SccVertexValue vertexValue = vertex.getValue();

     // Keep the ids of the parent nodes to allow for backwards traversal

     for (LongWritable parent : messages) {

       vertexValue.addParent(parent.get());

     }

     // If this node doesn't have any parents or outgoing edges,

     // it can't be part of an SCC

     vertexValue.set(vertex.getId().get());

     if (vertex.getNumEdges() == 0 || vertexValue.getParents() == null) {

       vertexValue.deactivate();

     } else {

       messageValue.set(vertexValue.get());

       sendMessageToAllEdges(vertex, messageValue);

     }

   }

   /**

    * Traverse the graph through outgoing edges and keep the maximum vertex

    * value.

    * If a new maximum value is found, propagate it until convergence.

    * @param vertex Current vertex.

    * @param messages Received values from neighbor vertices.

    */

   private void forwardTraversal(

       Vertex<LongWritable, SccVertexValue, NullWritable> vertex,

       Iterable<LongWritable> messages) {

     SccVertexValue vertexValue = vertex.getValue();

     boolean changed = setMaxValue(vertexValue, messages);

     if (changed) {

       messageValue.set(vertexValue.get());

       sendMessageToAllEdges(vertex, messageValue);

       aggregate(NEW_MAXIMUM, new BooleanWritable(true));

     }

   }

   /**

    * Traverse the transposed graph and keep the maximum vertex value.

    * @param vertex Current vertex.

    */

   private void backwardTraversalStart(

       Vertex<LongWritable, SccVertexValue, NullWritable> vertex) {

     SccVertexValue vertexValue = vertex.getValue();

     if (vertexValue.get() == vertex.getId().get()) {

       messageValue.set(vertexValue.get());

       sendMessageToAllParents(vertex, messageValue);

     }

   }

   /**

    * Traverse the transposed graph and keep the maximum vertex value.

    * @param vertex Current vertex.

    * @param messages Received values from children vertices.

    */

   private void backwardTraversalRest(

       Vertex<LongWritable, SccVertexValue, NullWritable> vertex,

       Iterable<LongWritable> messages) {

     SccVertexValue vertexValue = vertex.getValue();

     for (LongWritable m : messages) {

       if (vertexValue.get() == m.get()) {

         sendMessageToAllParents(vertex, m);

         aggregate(CONVERGED, new BooleanWritable(true));

         vertexValue.deactivate();

         vertex.voteToHalt();

         break;

       }

     }

   }

   /**

    * Compares the messages values with the current vertex value and finds

    * the maximum.

    * If the maximum value is different from the vertex value, makes it the

    * new vertex value and returns true, otherwise, returns false.

    * @param vertexValue Current vertex value.

    * @param messages Messages containing neighbors' vertex values.

    * @return True if a new maximum was found, otherwise, returns false.

    */

   private boolean setMaxValue(SccVertexValue vertexValue,

                               Iterable<LongWritable> messages) {

     boolean changed = false;

     for (LongWritable m : messages) {

       if (vertexValue.get() < m.get()) {

         vertexValue.set(m.get());

         changed = true;

       }

     }

     return changed;

   }

   /**

    * Send message to all parents.

    * @param vertex Current vertex.

    * @param message Message to be sent.

    */

   private void sendMessageToAllParents(

       Vertex<LongWritable, SccVertexValue, NullWritable> vertex,

       LongWritable message) {

     for (Long id : vertex.getValue().getParents()) {

       parentId.set(id);

       sendMessage(parentId, message);

     }

   }

 }