code: public synchronized static IDGenerator generatorFor(int generatorId, int clusterId, Mode mode) {
          assertParameterWithinBounds("generatorId", 0, Blueprint.MAX_GENERATOR_ID, generatorId);
          assertParameterWithinBounds("clusterId", 0, Blueprint.MAX_CLUSTER_ID, clusterId);
          String generatorAndCluster = String.format("%d_%d", generatorId, clusterId);
          if (!instances.containsKey(generatorAndCluster)) {
              GeneratorIdentityHolder identityHolder = LocalGeneratorIdentity.with(clusterId, generatorId);
              instances.putIfAbsent(generatorAndCluster, new BaseUniqueIDGenerator(identityHolder, mode));
          }
          return instances.get(generatorAndCluster);
      }
docs: Return the UniqueIDGenerator instance for this specific generator-ID, cluster-ID combination. If one was
  already created, that is returned.
  
  @param generatorId Generator ID to use (0 ≤ n ≤ 255).
  @param clusterId   Cluster ID to use (0 ≤ n ≤ 15).
  @param mode        Generator mode.
  @return A thread-safe UniqueIDGenerator instance.