unitweightedtopologicalnumbering.java

Java数据结构开发包

/*
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package jdsl.graph.algo;

import jdsl.graph.api.*;


/**
  * This algorithm class computes the optimal unit-weighted topological 
  * numbering for a given DAG.
  *
  * Each Vertex is assigned a non-unique order-number.
  * The number of a given Vertex is equal to 1 plus the 
  * highest value of any of its predecessors' numbers.
  *
  * The order-number of a Vertex may be retrieved using the
  * number(Vertex) method.
  *
 * @author Natasha Gelfand (ng)
 * @author Lucy Perry (lep)
  *
  * @version JDSL 2.1.1 
  * @see AbstractTopologicalSort
  *

  */


public class UnitWeightedTopologicalNumbering extends AbstractTopologicalSort {


  /**
    * Constructor
    */
  public UnitWeightedTopologicalNumbering(){
    super();
  }




  /**
    * The meat of the algorithm.
    * This method is called from the superclass's execute(.) method.
    * If the Graph is acyclic, then the algorithm labels each
    * Vertex with a non-unique order-number.
    * If the Graph contains cycles, then some Vertices will not be visited,
    * and the is_cyclic_ boolean will be set to true.
    *
    * Takes O(V+E) time, where 
    * V = number of Vertices in the Graph, and
    * E = number of Edges,
    * because the algorithm traverses all the outgoing
    * edges of each visited Vertex exactly once.
    */
  protected final void sort(){
    is_cyclic_ = false;

    Object NUM_EXAMINED = new Object();
    Integer ZERO = new Integer(0);  // we use these a lot; might as well
    Integer NEG1 = new Integer(-1); // only have one instance

    VertexIterator verts = graph_.vertices();
    int numVerts = graph_.numVertices();
    
    Vertex v = null;

    //this loop takes care of initializing all the vertices
    while (verts.hasNext()){
      v = verts.nextVertex();

      // Initially, vertices have never been examined
      // (NUM_EXAMINED = the number of times the Vertex has been examined.)
      v.set(NUM_EXAMINED, ZERO);

      // Sources get a numbering of 0 and are inserted into the queue.
      // All others get the special value -1, which is less than the
      // smallest actual number.
      if (graph_.degree(v, EdgeDirection.IN) == 0){
	v.set(NUMBER_KEY_, ZERO);
	queue_.insertLast(v);
      }
      else{
	v.set(NUMBER_KEY_, NEG1);
      }
    }

    int numVisited = 0;//for use in determining if it has cycles

    //This loop sets the NUMBER_KEY_ of Vertices
    while (!queue_.isEmpty()){
      numVisited = numVisited+1;
      v = (Vertex)queue_.removeFirst();
      int v_number = _number(v);
      
      VertexIterator vi = graph_.adjacentVertices(v, EdgeDirection.OUT);
      while (vi.hasNext()){
	Vertex w = vi.nextVertex();
	int w_number = _number(w);
	//The NUMBER_KEY_ of a Vertex depends on the NUMBER_KEY_ of its predecessor
	if ((v_number+1) > w_number){
	  w.set(NUMBER_KEY_, new Integer(v_number+1));
	}
	int num_ex = ((Integer)w.get(NUM_EXAMINED)).intValue();
	w.set(NUM_EXAMINED, new Integer(num_ex+1));
	if (num_ex+1 == graph_.degree(w, EdgeDirection.IN)){
	  queue_.insertLast(w);
	}
      }
    }
    
    //clean up some decorations
    verts.reset();
    while(verts.hasNext()){
      Vertex a = verts.nextVertex();
      a.destroy(NUM_EXAMINED);
    }

    //if some vertices were never visited, then the graph has cycles
    if (numVisited < numVerts){
      is_cyclic_ = true;
    }
    
  }


  /*
    Private helper method which does the same thing as superclass's
    number(v) method, but which can be used internally by the algorithm
    without checking for is_cyclic_ (meaning that this method is 
    slightly faster.
  */
  private int _number(Vertex v){
    return ((Integer)v.get(NUMBER_KEY_)).intValue();
  }



}