// Textbook fragment 13.16
  /** The actual execution of Dijkstra's algorithm.
    * @param v source vertex.
    */
  protected void dijkstraVisit (Vertex<V> v) {
    // store all the vertices in priority queue Q
    for (Vertex<V> u: graph.vertices()) {
      int u_dist;
      if (u==v)
        u_dist = 0;
      else
        u_dist = INFINITE;
      Entry<Integer, Vertex<V>> u_entry = Q.insert(u_dist, u);  // autoboxing
      u.put(ENTRY, u_entry);
    }
    // grow the cloud, one vertex at a time
    while (!Q.isEmpty()) {
      // remove from Q and insert into cloud a vertex with minimum distance
      Entry<Integer, Vertex<V>> u_entry = Q.min();
      Vertex<V> u = u_entry.getValue();
      int u_dist = u_entry.getKey();
      Q.remove(u_entry); // remove u from the priority queue
      u.put(DIST,u_dist); // the distance of u is final
      u.remove(ENTRY); // remove the entry decoration of u
      if (u_dist == INFINITE)
        continue; // unreachable vertices are not processed
      // examine all the neighbors of u and update their distances
      for (Edge<E> e: graph.incidentEdges(u)) {
        Vertex<V> z = graph.opposite(u,e);
        Entry<Integer, Vertex<V>> z_entry
                                  = (Entry<Integer, Vertex<V>>) z.get(ENTRY);
        if (z_entry != null) { // check that z is in Q, i.e., not in the cloud
          int e_weight = (Integer) e.get(WEIGHT);
          int z_dist = z_entry.getKey();
          if ( u_dist + e_weight < z_dist ) // relaxation of edge e = (u,z)
            Q.replaceKey(z_entry, u_dist + e_weight);
        }
      }
    }
  }