/**
  * Partition a congruence class.
  *
  * @param partition the class to partition
  */
 private void partitionClass(OPT_GVCongruenceClass partition) {
   // store a reference to the first node in c, which will serve
   // as a representative for this class
   Iterator i = partition.iterator();
   OPT_ValueGraphVertex first = (OPT_ValueGraphVertex) i.next();
   ArrayList newClasses = new ArrayList();
   // now check each other node in c, to see if it matches the
   // representative
   ArrayList toRemove = new ArrayList();
   for (; i.hasNext(); ) {
     OPT_ValueGraphVertex v = (OPT_ValueGraphVertex) i.next();
     if (!checkCongruence(first, v)) {
       // NOT CONGRUENT!!  split the partition.  first check if
       // v fits in any other newly created congruence classes
       int index = findCongruenceMatch(newClasses, v);
       if (index > -1) {
         // MATCH FOUND!! place v in newClasses[index]
         OPT_GVCongruenceClass match = (OPT_GVCongruenceClass) B.get(index);
         match.addVertex(v);
         v.setValueNumber(match.getValueNumber());
       } else {
         // NO MATCH FOUND!! create a new congruence class
         // find the appropriate label for the new congruence class
         // and create a new congruence class with this label
         OPT_GVCongruenceClass c = createCongruenceClass(v);
         newClasses.add(c);
         c.addVertex(v);
         v.setValueNumber(c.getValueNumber());
       }
       // mark v as to be removed from partition
       // (Can't remove it yet while iterating over the set);
       toRemove.add(v);
     }
   }
   // remove necessary vertices
   for (Iterator it = toRemove.iterator(); it.hasNext(); ) {
     OPT_ValueGraphVertex v = (OPT_ValueGraphVertex) it.next();
     partition.removeVertex(v);
   }
   // if needed place the original partition back on the work list
   if ((newClasses.size() > 0) && (partition.size() > 1)) {
     workList.push(partition);
   }
   // place any new congruence classes with size > 1 on the worklist
   // also place any classes which might indirectly be affected
   for (int j = 0; j < newClasses.size(); j++) {
     OPT_GVCongruenceClass c = (OPT_GVCongruenceClass) newClasses.get(j);
     if (c.size() > 1) workList.push(c);
     addDependentClassesToWorklist(c);
   }
 }
  /** Merge the congruence classes containing vertices v1 and v2.; */
  public void mergeClasses(OPT_ValueGraphVertex v1, OPT_ValueGraphVertex v2) {
    if (DEBUG) {
      System.out.println("@@@@ mergeClasses called with v1 = " + v1 + " ; v2 = " + v2);
    }

    int val1 = v1.getValueNumber();
    int val2 = v2.getValueNumber();
    if (val1 == val2) return;

    OPT_GVCongruenceClass class1 = (OPT_GVCongruenceClass) B.get(val1);

    while (true) {
      OPT_GVCongruenceClass class2 = (OPT_GVCongruenceClass) B.get(val2);
      Iterator i = class2.iterator();
      if (!i.hasNext()) break;
      OPT_ValueGraphVertex v = (OPT_ValueGraphVertex) i.next();
      if (DEBUG)
        System.out.println("@@@@ moving vertex " + v + " from class " + val2 + " to class " + val1);
      class1.addVertex(v);
      class2.removeVertex(v);
      v.setValueNumber(val1);
    }

    // Null out entry for val2
    B.set(val2, null);
  }
 /**
  * Initialize the congruence classes, assuming that all nodes with the same label are congruent.
  */
 private void initialize() {
   // store a map from label -> congruenceClass
   HashMap labelMap = new HashMap(10);
   for (Enumeration e = valueGraph.enumerateVertices(); e.hasMoreElements(); ) {
     OPT_ValueGraphVertex v = (OPT_ValueGraphVertex) e.nextElement();
     Object label = v.getLabel();
     OPT_GVCongruenceClass c = findOrCreateCongruenceClass(label, labelMap);
     // add this node to the congruence class
     c.addVertex(v);
     // set the value number for the node
     v.setValueNumber(c.getValueNumber());
   }
 }