NodeState getNodeState(int index) {
            NodeState ns = nodeStates.get(index);
            if (ns != null) return ns;
            return (index >= getLogicalNodeCount() || ! upNodes.get(index))
                    ? new NodeState(type, State.DOWN)
                    : new NodeState(type, State.UP);
        }
Maintains a bitset where all non-down nodes have a bit set. All nodes that differ from defaultUp
and defaultDown are stored explicitly in a hash map.
vdslib/src/main/java/com/yahoo/vdslib/state/ClusterState.java