helix-core/src/main/java/org/apache/helix/controller/rebalancer/strategy/AutoRebalanceStrategy.java [422:485]:
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    Set<String> notAssigned = new LinkedHashSet<String>(preferenceList);
    for (int i = 0; i < replicas; i++) {
      String state = _stateMap.get(i);
      String node = getMinimumNodeForReplica(state, notAssigned, nodeReplicaCounts);
      newPreferenceList.add(node);
      notAssigned.remove(node);
      Map<String, Integer> counts = nodeReplicaCounts.get(node);
      counts.put(state, counts.get(state) + 1);
    }
    preferenceList.clear();
    preferenceList.addAll(newPreferenceList);
  }

  /**
   * Get the node which hosts the fewest of a given replica
   * @param state the state
   * @param nodes nodes to check
   * @param nodeReplicaCounts current assignment of replicas
   * @return the node most willing to accept the replica
   */
  private String getMinimumNodeForReplica(String state, Set<String> nodes,
      Map<String, Map<String, Integer>> nodeReplicaCounts) {
    String minimalNode = null;
    int minimalCount = Integer.MAX_VALUE;
    for (String node : nodes) {
      int count = getReplicaCountForNode(state, node, nodeReplicaCounts);
      if (count < minimalCount) {
        minimalCount = count;
        minimalNode = node;
      }
    }
    return minimalNode;
  }

  /**
   * Safe check for the number of replicas of a given id assiged to a node
   * @param state the state to assign
   * @param node the node to check
   * @param nodeReplicaCounts a map of node to replica id and counts
   * @return the number of currently assigned replicas of the given id
   */
  private int getReplicaCountForNode(String state, String node,
      Map<String, Map<String, Integer>> nodeReplicaCounts) {
    if (!nodeReplicaCounts.containsKey(node)) {
      Map<String, Integer> replicaCounts = new HashMap<String, Integer>();
      replicaCounts.put(state, 0);
      nodeReplicaCounts.put(node, replicaCounts);
      return 0;
    }
    Map<String, Integer> replicaCounts = nodeReplicaCounts.get(node);
    if (!replicaCounts.containsKey(state)) {
      replicaCounts.put(state, 0);
      return 0;
    }
    return replicaCounts.get(state);
  }

  /**
   * Compute the subset of the current mapping where replicas are not mapped according to their
   * existing preferred assignment.
   * @param currentMapping Current mapping of replicas to nodes
   * @return The current assignments that do not conform to the preferred assignment
   */
  private Map<Replica, Node> computeExistingNonPreferredPlacement(
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helix-core/src/main/java/org/apache/helix/controller/strategy/AutoRebalanceStrategy.java [358:421]:
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    Set<String> notAssigned = new LinkedHashSet<String>(preferenceList);
    for (int i = 0; i < replicas; i++) {
      String state = _stateMap.get(i);
      String node = getMinimumNodeForReplica(state, notAssigned, nodeReplicaCounts);
      newPreferenceList.add(node);
      notAssigned.remove(node);
      Map<String, Integer> counts = nodeReplicaCounts.get(node);
      counts.put(state, counts.get(state) + 1);
    }
    preferenceList.clear();
    preferenceList.addAll(newPreferenceList);
  }

  /**
   * Get the node which hosts the fewest of a given replica
   * @param state the state
   * @param nodes nodes to check
   * @param nodeReplicaCounts current assignment of replicas
   * @return the node most willing to accept the replica
   */
  private String getMinimumNodeForReplica(String state, Set<String> nodes,
      Map<String, Map<String, Integer>> nodeReplicaCounts) {
    String minimalNode = null;
    int minimalCount = Integer.MAX_VALUE;
    for (String node : nodes) {
      int count = getReplicaCountForNode(state, node, nodeReplicaCounts);
      if (count < minimalCount) {
        minimalCount = count;
        minimalNode = node;
      }
    }
    return minimalNode;
  }

  /**
   * Safe check for the number of replicas of a given id assiged to a node
   * @param state the state to assign
   * @param node the node to check
   * @param nodeReplicaCounts a map of node to replica id and counts
   * @return the number of currently assigned replicas of the given id
   */
  private int getReplicaCountForNode(String state, String node,
      Map<String, Map<String, Integer>> nodeReplicaCounts) {
    if (!nodeReplicaCounts.containsKey(node)) {
      Map<String, Integer> replicaCounts = new HashMap<String, Integer>();
      replicaCounts.put(state, 0);
      nodeReplicaCounts.put(node, replicaCounts);
      return 0;
    }
    Map<String, Integer> replicaCounts = nodeReplicaCounts.get(node);
    if (!replicaCounts.containsKey(state)) {
      replicaCounts.put(state, 0);
      return 0;
    }
    return replicaCounts.get(state);
  }

  /**
   * Compute the subset of the current mapping where replicas are not mapped according to their
   * preferred assignment.
   * @param currentMapping Current mapping of replicas to nodes
   * @return The current assignments that do not conform to the preferred assignment
   */
  private Map<Replica, Node> computeExistingNonPreferredPlacement(
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