sclass ConnectedInstances {
  static ExpiringMap2<S, Bool> recentlyConnected_internal = new ExpiringMap2(5000, r updateValue);
  static new MultiSet<S> activeConnections;
  static Map<S, Bool> recentlyConnected = synchroMap(recentlyConnected);
  Var<Int> connected2 = new(0); // filtered count

  *() {
    // TODO (maybe): add countdown to ExpiringMap instead
    doEvery(1000, r {
      synchronized(recentlyConnectedComputers) {
        recenctlyConnectedComputers_internal.clean();
      }
    });
  }

  void updateValue {
    Set<S> set1 = keys(activeConnections);
    Set<S> set2 = keys(recentlyConnected);
    int n = l(mergeSets(set1, set2));
    connected2.set(n);
    print("Updated value to " + n + " (" + l(set1) + "/" + l(set2) + ")");
  }
  
  void gotConnection(S computerID) {
    if (computerID != null) {
      activeConnections.add(computerID);
      print("Have active connections from " + computerID + ": " + activeConnections.get(computerID));
      updateValue();
    }
  }
  
  void lostConnection(S computerID) {
    if (computerID != null) {
      activeConnections.remove(computerID);
      print("Disconnected. Have active connections from " + computerID + ": " + activeConnections.get(computerID));
      recentlyConnectedComputers.put(computerID, true);
      updateValue();
    }
  }
  
  int value() { ret connected2!; }

  void waitForChange(int value, int timeout) {
    waitForVarToChange_withTimeout(connected2, value, timeout);
  }
}