void PriorityQueue::percolateDown( unsigned short hole )
        {
            unsigned short start, child;
            HeapVertex tmp = array[ hole ];

            for ( ; (hole * 4 - 2) <= currentSize; hole = child ) {
              start = child = hole * 4 - 2;

              if (child != currentSize && array[child + 1].distance < array[child].distance )
                child++;
              if (start + 1 < currentSize && array[start + 2].distance < array[child].distance )
               child = start + 2;
              if (start + 2 < currentSize  && array[start + 3].distance < array[child].distance)
                child = start + 3;

              if (child != hole && array[child].distance < tmp.distance)
                array[ hole ] = array[ child ];
              else
                break;
            }
            array[ hole ] = tmp;
        }