/*
 * Licensed to the Apache Software Foundation (ASF) under one or more contributor license
 * agreements. See the NOTICE file distributed with this work for additional information regarding
 * copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the License. You may obtain a
 * copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */
package org.apache.geode.internal.concurrent;

/*
 * CompactConcurrentHashSet2 is derived from the JSR 166 ConcurrentHashMap class version 1.43
 * (current as of 27 March 2015). The modifications made for GemFire turn it into a Set instead of a
 * Map by removing the map's storage for values and removing the methods that are associated with
 * values.
 *
 * JSR 166 interest web site: http://gee.cs.oswego.edu/dl/concurrency-interest/
 *
 * File download location:
 * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jdk7/java/util/concurrent/
 * CompactConcurrentHashSet2.java?revision=1.43
 *
 * Original licensing notice: Written by Doug Lea with assistance from members of JCP JSR-166 Expert
 * Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */


import java.io.ObjectStreamField;
import java.io.Serializable;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;

import org.apache.geode.annotations.Immutable;
import org.apache.geode.annotations.internal.MakeNotStatic;
import org.apache.geode.unsafe.internal.sun.misc.Unsafe;

/**
 * <p>
 * This is the original javadoc describing ConcurrentHashMap. This class is actually a Set based on
 * Doug Lea's CHM implementation (see the source file for full attribution).
 * </p>
 * A hash map supporting full concurrency of retrievals and high expected concurrency for updates.
 * This class obeys the same functional specification as {@link java.util.HashMap}, and includes
 * versions of methods corresponding to each method of {@code HashMap}. However, even though all
 * operations are thread-safe, retrieval operations do <em>not</em> entail locking, and there is
 * <em>not</em> any support for locking the entire set in a way that prevents all access. This class
 * is fully interoperable with {@code HashMap} in programs that rely on its thread safety but not on
 * its synchronization details.
 *
 *
 * @since Geode 1.0
 * @author Originally Doug Lea
 * @param <V> the type of values held in the set
 */
public class CompactConcurrentHashSet2<V> extends AbstractSet<V> implements Set<V>, Serializable {
  private static final long serialVersionUID = 7249069246763182397L;

  /*
   * Overview:
   *
   * The primary design goal of this hash table is to maintain concurrent readability (typically
   * method get(), but also iterators and related methods) while minimizing update contention.
   * Secondary goals are to keep space consumption about the same or better than java.util.HashMap,
   * and to support high initial insertion rates on an empty table by many threads.
   *
   * This map usually acts as a binned (bucketed) hash table. Each key-value mapping is held in a
   * Node. Most nodes are instances of the basic Node class with hash, key, value, and next fields.
   * However, various subclasses exist: TreeNodes are arranged in balanced trees, not lists.
   * TreeBins hold the roots of sets of TreeNodes. ForwardingNodes are placed at the heads of bins
   * during resizing. ReservationNodes are used as placeholders while establishing values in
   * computeIfAbsent and related methods. The types TreeBin, ForwardingNode, and ReservationNode do
   * not hold normal user keys, values, or hashes, and are readily distinguishable during search etc
   * because they have negative hash fields and null key and value fields. (These special nodes are
   * either uncommon or transient, so the impact of carrying around some unused fields is
   * insignificant.)
   *
   * The table is lazily initialized to a power-of-two size upon the first insertion. Each bin in
   * the table normally contains a list of Nodes (most often, the list has only zero or one Node).
   * Table accesses require volatile/atomic reads, writes, and CASes. Because there is no other way
   * to arrange this without adding further indirections, we use intrinsics (sun.misc.Unsafe)
   * operations.
   *
   * We use the top (sign) bit of Node hash fields for control purposes -- it is available anyway
   * because of addressing constraints. Nodes with negative hash fields are specially handled or
   * ignored in map methods.
   *
   * Insertion (via put or its variants) of the first node in an empty bin is performed by just
   * CASing it to the bin. This is by far the most common case for put operations under most
   * key/hash distributions. Other update operations (insert, delete, and replace) require locks. We
   * do not want to waste the space required to associate a distinct lock object with each bin, so
   * instead use the first node of a bin list itself as a lock. Locking support for these locks
   * relies on builtin "synchronized" monitors.
   *
   * Using the first node of a list as a lock does not by itself suffice though: When a node is
   * locked, any update must first validate that it is still the first node after locking it, and
   * retry if not. Because new nodes are always appended to lists, once a node is first in a bin, it
   * remains first until deleted or the bin becomes invalidated (upon resizing).
   *
   * The main disadvantage of per-bin locks is that other update operations on other nodes in a bin
   * list protected by the same lock can stall, for example when user equals() or mapping functions
   * take a long time. However, statistically, under random hash codes, this is not a common
   * problem. Ideally, the frequency of nodes in bins follows a Poisson distribution
   * (http://en.wikipedia.org/wiki/Poisson_distribution) with a parameter of about 0.5 on average,
   * given the resizing threshold of 0.75, although with a large variance because of resizing
   * granularity. Ignoring variance, the expected occurrences of list size k are (exp(-0.5) *
   * pow(0.5, k) / factorial(k)). The first values are:
   *
   * 0: 0.60653066 1: 0.30326533 2: 0.07581633 3: 0.01263606 4: 0.00157952 5: 0.00015795 6:
   * 0.00001316 7: 0.00000094 8: 0.00000006 more: less than 1 in ten million
   *
   * Lock contention probability for two threads accessing distinct elements is roughly 1 / (8 *
   * #elements) under random hashes.
   *
   * Actual hash code distributions encountered in practice sometimes deviate significantly from
   * uniform randomness. This includes the case when N > (1<<30), so some keys MUST collide.
   * Similarly for dumb or hostile usages in which multiple keys are designed to have identical hash
   * codes or ones that differs only in masked-out high bits. So we use a secondary strategy that
   * applies when the number of nodes in a bin exceeds a threshold. These TreeBins use a balanced
   * tree to hold nodes (a specialized form of red-black trees), bounding search time to O(log N).
   * Each search step in a TreeBin is at least twice as slow as in a regular list, but given that N
   * cannot exceed (1<<64) (before running out of addresses) this bounds search steps, lock hold
   * times, etc, to reasonable constants (roughly 100 nodes inspected per operation worst case) so
   * long as keys are Comparable (which is very common -- String, Long, etc). TreeBin nodes
   * (TreeNodes) also maintain the same "next" traversal pointers as regular nodes, so can be
   * traversed in iterators in the same way.
   *
   * The table is resized when occupancy exceeds a percentage threshold (nominally, 0.75, but see
   * below). Any thread noticing an overfull bin may assist in resizing after the initiating thread
   * allocates and sets up the replacement array. However, rather than stalling, these other threads
   * may proceed with insertions etc. The use of TreeBins shields us from the worst case effects of
   * overfilling while resizes are in progress. Resizing proceeds by transferring bins, one by one,
   * from the table to the next table. However, threads claim small blocks of indices to transfer
   * (via field transferIndex) before doing so, reducing contention. A generation stamp in field
   * sizeCtl ensures that resizings do not overlap. Because we are using power-of-two expansion, the
   * elements from each bin must either stay at same index, or move with a power of two offset. We
   * eliminate unnecessary node creation by catching cases where old nodes can be reused because
   * their next fields won't change. On average, only about one-sixth of them need cloning when a
   * table doubles. The nodes they replace will be garbage collectable as soon as they are no longer
   * referenced by any reader thread that may be in the midst of concurrently traversing table. Upon
   * transfer, the old table bin contains only a special forwarding node (with hash field "MOVED")
   * that contains the next table as its key. On encountering a forwarding node, access and update
   * operations restart, using the new table.
   *
   * Each bin transfer requires its bin lock, which can stall waiting for locks while resizing.
   * However, because other threads can join in and help resize rather than contend for locks,
   * average aggregate waits become shorter as resizing progresses. The transfer operation must also
   * ensure that all accessible bins in both the old and new table are usable by any traversal. This
   * is arranged in part by proceeding from the last bin (table.length - 1) up towards the first.
   * Upon seeing a forwarding node, traversals (see class Traverser) arrange to move to the new
   * table without revisiting nodes. To ensure that no intervening nodes are skipped even when moved
   * out of order, a stack (see class TableStack) is created on first encounter of a forwarding node
   * during a traversal, to maintain its place if later processing the current table. The need for
   * these save/restore mechanics is relatively rare, but when one forwarding node is encountered,
   * typically many more will be. So Traversers use a simple caching scheme to avoid creating so
   * many new TableStack nodes. (Thanks to Peter Levart for suggesting use of a stack here.)
   *
   * The traversal scheme also applies to partial traversals of ranges of bins (via an alternate
   * Traverser constructor) to support partitioned aggregate operations. Also, read-only operations
   * give up if ever forwarded to a null table, which provides support for shutdown-style clearing,
   * which is also not currently implemented.
   *
   * Lazy table initialization minimizes footprint until first use, and also avoids resizings when
   * the first operation is from a putAll, constructor with map argument, or deserialization. These
   * cases attempt to override the initial capacity settings, but harmlessly fail to take effect in
   * cases of races.
   *
   * The element count is maintained using a specialization of LongAdder. We need to incorporate a
   * specialization rather than just use a LongAdder in order to access implicit contention-sensing
   * that leads to creation of multiple CounterCells. The counter mechanics avoid contention on
   * updates but can encounter cache thrashing if read too frequently during concurrent access. To
   * avoid reading so often, resizing under contention is attempted only upon adding to a bin
   * already holding two or more nodes. Under uniform hash distributions, the probability of this
   * occurring at threshold is around 13%, meaning that only about 1 in 8 puts check threshold (and
   * after resizing, many fewer do so).
   *
   * TreeBins use a special form of comparison for search and related operations (which is the main
   * reason we cannot use existing collections such as TreeMaps). TreeBins contain Comparable
   * elements, but may contain others, as well as elements that are Comparable but not necessarily
   * Comparable for the same T, so we cannot invoke compareTo among them. To handle this, the tree
   * is ordered primarily by hash value, then by Comparable.compareTo order if applicable. On lookup
   * at a node, if elements are not comparable or compare as 0 then both left and right children may
   * need to be searched in the case of tied hash values. (This corresponds to the full list search
   * that would be necessary if all elements were non-Comparable and had tied hashes.) On insertion,
   * to keep a total ordering (or as close as is required here) across rebalancings, we compare
   * classes and identityHashCodes as tie-breakers. The red-black balancing code is updated from
   * pre-jdk-collections (http://gee.cs.oswego.edu/dl/classes/collections/RBCell.java) based in turn
   * on Cormen, Leiserson, and Rivest "Introduction to Algorithms" (CLR).
   *
   * TreeBins also require an additional locking mechanism. While list traversal is always possible
   * by readers even during updates, tree traversal is not, mainly because of tree-rotations that
   * may change the root node and/or its linkages. TreeBins include a simple read-write lock
   * mechanism parasitic on the main bin-synchronization strategy: Structural adjustments associated
   * with an insertion or removal are already bin-locked (and so cannot conflict with other writers)
   * but must wait for ongoing readers to finish. Since there can be only one such waiter, we use a
   * simple scheme using a single "waiter" field to block writers. However, readers need never
   * block. If the root lock is held, they proceed along the slow traversal path (via next-pointers)
   * until the lock becomes available or the list is exhausted, whichever comes first. These cases
   * are not fast, but maximize aggregate expected throughput.
   *
   * Maintaining API and serialization compatibility with previous versions of this class introduces
   * several oddities. Mainly: We leave untouched but unused constructor arguments refering to
   * concurrencyLevel. We accept a loadFactor constructor argument, but apply it only to initial
   * table capacity (which is the only time that we can guarantee to honor it.) We also declare an
   * unused "Segment" class that is instantiated in minimal form only when serializing.
   *
   * Also, solely for compatibility with previous versions of this class, it extends AbstractMap,
   * even though all of its methods are overridden, so it is just useless baggage.
   *
   * This file is organized to make things a little easier to follow while reading than they might
   * otherwise: First the main static declarations and utilities, then fields, then main public
   * methods (with a few factorings of multiple public methods into internal ones), then sizing
   * methods, trees, traversers, and bulk operations.
   */


  /* ---------------- Constants -------------- */

  /**
   * The largest possible table capacity. This value must be exactly 1<<30 to stay within Java array
   * allocation and indexing bounds for power of two table sizes, and is further required because
   * the top two bits of 32bit hash fields are used for control purposes.
   */
  private static final int MAXIMUM_CAPACITY = 1 << 30;

  /**
   * The default initial table capacity. Must be a power of 2 (i.e., at least 1) and at most
   * MAXIMUM_CAPACITY.
   */
  private static final int DEFAULT_CAPACITY = 16;

  /**
   * The largest possible (non-power of two) array size. Needed by toArray and related methods.
   */
  static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

  /**
   * The default concurrency level for this table. Unused but defined for compatibility with
   * previous versions of this class.
   */
  private static final int DEFAULT_CONCURRENCY_LEVEL = 16;

  /**
   * The load factor for this table. Overrides of this value in constructors affect only the initial
   * table capacity. The actual floating point value isn't normally used -- it is simpler to use
   * expressions such as {@code n - (n >>> 2)} for the associated resizing threshold.
   */
  private static final float LOAD_FACTOR = 0.75f;

  /**
   * The bin count threshold for using a tree rather than list for a bin. Bins are converted to
   * trees when adding an element to a bin with at least this many nodes. The value must be greater
   * than 2, and should be at least 8 to mesh with assumptions in tree removal about conversion back
   * to plain bins upon shrinkage.
   */
  static final int TREEIFY_THRESHOLD = 8;

  /**
   * The bin count threshold for untreeifying a (split) bin during a resize operation. Should be
   * less than TREEIFY_THRESHOLD, and at most 6 to mesh with shrinkage detection under removal.
   */
  static final int UNTREEIFY_THRESHOLD = 6;

  /**
   * The smallest table capacity for which bins may be treeified. (Otherwise the table is resized if
   * too many nodes in a bin.) The value should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
   * between resizing and treeification thresholds.
   */
  static final int MIN_TREEIFY_CAPACITY = 64;

  /**
   * Minimum number of rebinnings per transfer step. Ranges are subdivided to allow multiple resizer
   * threads. This value serves as a lower bound to avoid resizers encountering excessive memory
   * contention. The value should be at least DEFAULT_CAPACITY.
   */
  private static final int MIN_TRANSFER_STRIDE = 16;

  /**
   * The number of bits used for generation stamp in sizeCtl. Must be at least 6 for 32bit arrays.
   */
  private static final int RESIZE_STAMP_BITS = 16;

  /**
   * The maximum number of threads that can help resize. Must fit in 32 - RESIZE_STAMP_BITS bits.
   */
  private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;

  /**
   * The bit shift for recording size stamp in sizeCtl.
   */
  private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;

  /*
   * Encodings for Node hash fields. See above for explanation.
   */
  static final int MOVED = 0x8fffffff; // (-1) hash for forwarding nodes
  static final int TREEBIN = 0x80000000; // hash for roots of trees
  static final int RESERVED = 0x80000001; // hash for transient reservations
  static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash

  /** Number of CPUS, to place bounds on some sizings */
  static final int NCPU = Runtime.getRuntime().availableProcessors();

  /** For serialization compatibility. */
  @Immutable
  private static final ObjectStreamField[] serialPersistentFields =
      {new ObjectStreamField("segments", Segment[].class),
          new ObjectStreamField("segmentMask", Integer.TYPE),
          new ObjectStreamField("segmentShift", Integer.TYPE)};

  /* ---------------- Nodes -------------- */

  /**
   * Key-value entry. This class is never exported out as a user-mutable Map.Entry (i.e., one
   * supporting setValue; see MapEntry below), but can be used for read-only traversals used in bulk
   * tasks. Subclasses of Node with a negative hash field are special, and contain null keys and
   * values (but are never exported). Otherwise, keys and vals are never null.
   */
  static class Node<K> {
    final int hash;
    final K key;
    Node<K> next;

    Node(int hash, K key, Node<K> next) {
      this.hash = hash;
      this.key = key;
      this.next = next;
    }

    public K getKey() {
      return key;
    }

    public int hashCode() {
      return key.hashCode();
    }

    public String toString() {
      return key.toString();
    }

    public boolean equals(Object o) {
      return ((o instanceof Node)
          && (o == key || o.equals(key)));
    }

    /**
     * Virtualized support for map.get(); overridden in subclasses.
     */
    Node<K> find(int h, Object k) {
      Node<K> e = this;
      if (k != null) {
        do {
          K ek;
          if (e.hash == h && ((ek = e.key) == k || (ek != null && k.equals(ek))))
            return e;
        } while ((e = e.next) != null);
      }
      return null;
    }
  }

  /* ---------------- Static utilities -------------- */

  /**
   * Spreads (XORs) higher bits of hash to lower and also forces top bit to 0. Because the table
   * uses power-of-two masking, sets of hashes that vary only in bits above the current mask will
   * always collide. (Among known examples are sets of Float keys holding consecutive whole numbers
   * in small tables.) So we apply a transform that spreads the impact of higher bits downward.
   * There is a tradeoff between speed, utility, and quality of bit-spreading. Because many common
   * sets of hashes are already reasonably distributed (so don't benefit from spreading), and
   * because we use trees to handle large sets of collisions in bins, we just XOR some shifted bits
   * in the cheapest possible way to reduce systematic lossage, as well as to incorporate impact of
   * the highest bits that would otherwise never be used in index calculations because of table
   * bounds.
   */
  static int spread(int h) {
    return (h ^ (h >>> 16)) & HASH_BITS;
  }

  /**
   * Returns a power of two table size for the given desired capacity. See Hackers Delight, sec 3.2
   */
  private static int tableSizeFor(int c) {
    int n = c - 1;
    n |= n >>> 1;
    n |= n >>> 2;
    n |= n >>> 4;
    n |= n >>> 8;
    n |= n >>> 16;
    return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
  }

  /**
   * Returns x's Class if it is of the form "class C implements Comparable<C>", else null.
   */
  static Class<?> comparableClassFor(Object x) {
    if (x instanceof Comparable) {
      Class<?> c;
      Type[] ts, as;
      Type t;
      ParameterizedType p;
      if ((c = x.getClass()) == String.class) // bypass checks
        return c;
      if ((ts = c.getGenericInterfaces()) != null) {
        for (int i = 0; i < ts.length; ++i) {
          if (((t = ts[i]) instanceof ParameterizedType)
              && ((p = (ParameterizedType) t).getRawType() == Comparable.class)
              && (as = p.getActualTypeArguments()) != null && as.length == 1 && as[0] == c) // type
                                                                                            // arg
                                                                                            // is c
            return c;
        }
      }
    }
    return null;
  }

  /**
   * Returns k.compareTo(x) if x matches kc (k's screened comparable class), else 0.
   */
  @SuppressWarnings({"rawtypes", "unchecked"}) // for cast to Comparable
  static int compareComparables(Class<?> kc, Object k, Object x) {
    return (x == null || x.getClass() != kc ? 0 : ((Comparable) k).compareTo(x));
  }

  /* ---------------- Table element access -------------- */

  /*
   * Volatile access methods are used for table elements as well as elements of in-progress next
   * table while resizing. All uses of the tab arguments must be null checked by callers. All
   * callers also paranoically precheck that tab's length is not zero (or an equivalent check), thus
   * ensuring that any index argument taking the form of a hash value anded with (length - 1) is a
   * valid index. Note that, to be correct wrt arbitrary concurrency errors by users, these checks
   * must operate on local variables, which accounts for some odd-looking inline assignments below.
   * Note that calls to setTabAt always occur within locked regions, and so do not need full
   * volatile semantics, but still require ordering to maintain concurrent readability.
   */

  @SuppressWarnings("unchecked")
  static <K> Node<K> tabAt(Node<K>[] tab, int i) {
    return (Node<K>) U.getObjectVolatile(tab, ((long) i << ASHIFT) + ABASE);
  }

  static <K> boolean casTabAt(Node<K>[] tab, int i, Node<K> c, Node<K> v) {
    return U.compareAndSwapObject(tab, ((long) i << ASHIFT) + ABASE, c, v);
  }

  static <K> void setTabAt(Node<K>[] tab, int i, Node<K> v) {
    U.putOrderedObject(tab, ((long) i << ASHIFT) + ABASE, v);
  }

  /* ---------------- Fields -------------- */

  /**
   * The array of bins. Lazily initialized upon first insertion. Size is always a power of two.
   * Accessed directly by iterators.
   */
  transient volatile Node<V>[] table;

  /**
   * The next table to use; non-null only while resizing.
   */
  private transient volatile Node<V>[] nextTable;

  /**
   * Base counter value, used mainly when there is no contention, but also as a fallback during
   * table initialization races. Updated via CAS.
   */
  private transient volatile long baseCount;

  /**
   * Table initialization and resizing control. When negative, the table is being initialized or
   * resized: -1 for initialization, else -(1 + the number of active resizing threads). Otherwise,
   * when table is null, holds the initial table size to use upon creation, or 0 for default. After
   * initialization, holds the next element count value upon which to resize the table.
   */
  private transient volatile int sizeCtl;

  /**
   * The next table index (plus one) to split while resizing.
   */
  private transient volatile int transferIndex;

  /**
   * Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
   */
  private transient volatile int cellsBusy;

  /**
   * Table of counter cells. When non-null, size is a power of 2.
   */
  private transient volatile CounterCell[] counterCells;


  /* ---------------- Public operations -------------- */

  /**
   * Creates a new, empty map with the default initial table size (16).
   */
  public CompactConcurrentHashSet2() {}

  /**
   * Creates a new, empty map with an initial table size accommodating the specified number of
   * elements without the need to dynamically resize.
   *
   * @param initialCapacity The implementation performs internal sizing to accommodate this many
   *        elements.
   * @throws IllegalArgumentException if the initial capacity of elements is negative
   */
  public CompactConcurrentHashSet2(int initialCapacity) {
    if (initialCapacity < 0)
      throw new IllegalArgumentException();
    int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY
        : tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
    this.sizeCtl = cap;
  }

  /**
   * Creates a new map with the same mappings as the given map.
   *
   * @param m the map
   */
  public CompactConcurrentHashSet2(Collection<? extends V> m) {
    this.sizeCtl = DEFAULT_CAPACITY;
    addAll(m);
  }

  /**
   * Creates a new, empty map with an initial table size based on the given number of elements
   * ({@code initialCapacity}) and initial table density ({@code loadFactor}).
   *
   * @param initialCapacity the initial capacity. The implementation performs internal sizing to
   *        accommodate this many elements, given the specified load factor.
   * @param loadFactor the load factor (table density) for establishing the initial table size
   * @throws IllegalArgumentException if the initial capacity of elements is negative or the load
   *         factor is nonpositive
   *
   * @since Geode 1.0
   */
  public CompactConcurrentHashSet2(int initialCapacity, float loadFactor) {
    this(initialCapacity, loadFactor, 1);
  }

  /**
   * Creates a new, empty map with an initial table size based on the given number of elements
   * ({@code initialCapacity}), table density ({@code loadFactor}), and number of concurrently
   * updating threads ({@code concurrencyLevel}).
   *
   * @param initialCapacity the initial capacity. The implementation performs internal sizing to
   *        accommodate this many elements, given the specified load factor.
   * @param loadFactor the load factor (table density) for establishing the initial table size
   * @param concurrencyLevel the estimated number of concurrently updating threads. The
   *        implementation may use this value as a sizing hint.
   * @throws IllegalArgumentException if the initial capacity is negative or the load factor or
   *         concurrencyLevel are nonpositive
   */
  public CompactConcurrentHashSet2(int initialCapacity, float loadFactor, int concurrencyLevel) {
    if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
      throw new IllegalArgumentException();
    if (initialCapacity < concurrencyLevel) // Use at least as many bins
      initialCapacity = concurrencyLevel; // as estimated threads
    long size = (long) (1.0 + (long) initialCapacity / loadFactor);
    int cap = (size >= (long) MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : tableSizeFor((int) size);
    this.sizeCtl = cap;
  }

  // Original (since JDK1.2) Map methods

  /**
   * {@inheritDoc}
   */
  @Override
  public int size() {
    long n = sumCount();
    return ((n < 0L) ? 0 : (n > (long) Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int) n);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public boolean isEmpty() {
    return sumCount() <= 0L; // ignore transient negative values
  }

  boolean _contains(Object key) {
    Node<V>[] tab;
    Node<V> e, p;
    int n, eh;
    V ek;
    if (key == null) {
      return false;
    }
    int h = spread(key.hashCode());
    if ((tab = table) != null && (n = tab.length) > 0 && (e = tabAt(tab, (n - 1) & h)) != null) {
      if ((eh = e.hash) == h) {
        if ((ek = e.key) == key || (ek != null && key.equals(ek)))
          return true;
      } else if (eh < 0)
        return ((p = e.find(h, key)) != null);
      while ((e = e.next) != null) {
        if (e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek))))
          return true;
      }
    }
    return false;
  }

  @Override
  public boolean add(V value) {
    return putKey(value, false);
  }

  /** Implementation for add */
  boolean putKey(V key, boolean onlyIfAbsent) {
    if (key == null)
      throw new NullPointerException();
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<V>[] tab = table;;) {
      Node<V> f;
      int n, i, fh;
      if (tab == null || (n = tab.length) == 0)
        tab = initTable();
      else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
        if (casTabAt(tab, i, null, new Node<V>(hash, key, null)))
          break; // no lock when adding to empty bin
      } else if ((fh = f.hash) == MOVED)
        tab = helpTransfer(tab, f);
      else {
        boolean wasPresent = false;
        synchronized (f) {
          if (tabAt(tab, i) == f) {
            if (fh >= 0) {
              binCount = 1;
              for (Node<V> e = f;; ++binCount) {
                V ek;
                if (e.hash == hash && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                  wasPresent = true;
                  break;
                }
                Node<V> pred = e;
                if ((e = e.next) == null) {
                  pred.next = new Node<V>(hash, key, null);
                  break;
                }
              }
            } else if (f instanceof TreeBin) {
              Node<V> p;
              binCount = 2;
              if ((p = ((TreeBin<V>) f).putTreeVal(hash, key)) != null) {
                wasPresent = true;
              }
            }
          }
        }
        if (binCount != 0) {
          if (binCount >= TREEIFY_THRESHOLD)
            treeifyBin(tab, i);
          if (wasPresent)
            return true;
          break;
        }
      }
    }
    addCount(1L, binCount);
    return false;
  }

  /**
   * Copies all of the mappings from the specified map to this one. These mappings replace any
   * mappings that this map had for any of the keys currently in the specified map.
   *
   * @param m mappings to be stored in this map
   */
  public void addAll(Set<? extends V> m) {
    tryPresize(m.size());
    Iterator<? extends V> it = m.iterator();
    while (it.hasNext()) {
      putKey(it.next(), false);
    }
  }

  @Override
  public boolean remove(Object key) {
    return removeNode(key);
  }

  boolean removeNode(Object key) {
    int hash = spread(key.hashCode());
    for (Node<V>[] tab = table;;) {
      Node<V> f;
      int n, i, fh;
      if (tab == null || (n = tab.length) == 0 || (f = tabAt(tab, i = (n - 1) & hash)) == null)
        break;
      else if ((fh = f.hash) == MOVED)
        tab = helpTransfer(tab, f);
      else {
        boolean wasPresent = false;
        boolean validated = false;
        synchronized (f) {
          if (tabAt(tab, i) == f) {
            if (fh >= 0) {
              validated = true;
              for (Node<V> e = f, pred = null;;) {
                V ek;
                if (e.hash == hash && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                  wasPresent = true;
                  if (pred != null)
                    pred.next = e.next;
                  else
                    setTabAt(tab, i, e.next);
                  break;
                }
                pred = e;
                if ((e = e.next) == null)
                  break;
              }
            } else if (f instanceof TreeBin) {
              validated = true;
              TreeBin<V> t = (TreeBin<V>) f;
              TreeNode<V> r, p;
              if ((r = t.root) != null && (p = r.findTreeNode(hash, key, null)) != null) {
                wasPresent = true;
                if (t.removeTreeNode(p))
                  setTabAt(tab, i, untreeify(t.first));
              }
            }
          }
        }
        if (validated) {
          if (wasPresent) {
            addCount(-1L, -1);
            return true;
          }
          break;
        }
      }
    }
    return false;
  }

  /**
   * Removes all of the mappings from this map.
   */
  @Override
  public void clear() {
    long delta = 0L; // negative number of deletions
    int i = 0;
    Node<V>[] tab = table;
    while (tab != null && i < tab.length) {
      int fh;
      Node<V> f = tabAt(tab, i);
      if (f == null)
        ++i;
      else if ((fh = f.hash) == MOVED) {
        tab = helpTransfer(tab, f);
        i = 0; // restart
      } else {
        synchronized (f) {
          if (tabAt(tab, i) == f) {
            Node<V> p = (fh >= 0 ? f : (f instanceof TreeBin) ? ((TreeBin<V>) f).first : null);
            while (p != null) {
              --delta;
              p = p.next;
            }
            setTabAt(tab, i++, null);
          }
        }
      }
    }
    if (delta != 0L)
      addCount(delta, -1);
  }

  // public int hashCode() {
  // int h = 0;
  // Node<V>[] t;
  // if ((t = table) != null) {
  // Traverser<V> it = new Traverser<V>(t, t.length, 0, t.length);
  // for (Node<V> p; (p = it.advance()) != null; )
  // h += p.key.hashCode();
  // }
  // return h;
  // }

  /**
   * Returns a string representation of this map. The string representation consists of a list of
   * key-value mappings (in no particular order) enclosed in braces (<code>{}</code>). Adjacent
   * mappings are separated by the characters <code>", "</code> (comma and space). Each key-value
   * mapping is rendered as the key followed by an equals sign (<code>=</code>) followed by the
   * associated value.
   *
   * @return a string representation of this map
   */
  public String toString() {
    Node<V>[] t;
    int f = (t = table) == null ? 0 : t.length;
    Traverser<V> it = new Traverser<V>(t, f, 0, f);
    StringBuilder sb = new StringBuilder();
    sb.append('{');
    Node<V> p;
    if ((p = it.advance()) != null) {
      for (;;) {
        V k = p.key;
        sb.append(k == this ? "(this Set)" : k);
        if ((p = it.advance()) == null)
          break;
        sb.append(',').append(' ');
      }
    }
    return sb.append('}').toString();
  }

  // public boolean equals(Object o) {
  // if (o != this) {
  // if (!(o instanceof Set))
  // return false;
  // Set<?> m = (Set<?>) o;
  // Node<V>[] t;
  // int f = (t = table) == null ? 0 : t.length;
  // Traverser<V> it = new Traverser<V>(t, f, 0, f);
  // for (Node<V> p; (p = it.advance()) != null; ) {
  // if (!m.contains(p.key)) {
  // return false;
  // }
  // }
  // Iterator<?> itr = m.iterator();
  // while (itr.hasNext()) {
  // if (!contains(itr.next())) {
  // return false;
  // }
  // }
  // }
  // return true;
  // }

  /**
   * Stripped-down version of helper class used in previous version, declared for the sake of
   * serialization compatibility
   */
  static class Segment<K> extends ReentrantLock implements Serializable {
    private static final long serialVersionUID = 2249069246763182397L;
    final float loadFactor;

    Segment(float lf) {
      this.loadFactor = lf;
    }
  }

  /**
   * Saves the state of the {@code CompactConcurrentHashSet2} instance to a stream (i.e., serializes
   * it).
   *
   * @param s the stream
   * @serialData the key (Object) and value (Object) for each key-value mapping, followed by a null
   *             pair. The key-value mappings are emitted in no particular order.
   */
  private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
    // For serialization compatibility
    // Emulate segment calculation from previous version of this class
    int sshift = 0;
    int ssize = 1;
    while (ssize < DEFAULT_CONCURRENCY_LEVEL) {
      ++sshift;
      ssize <<= 1;
    }
    int segmentShift = 32 - sshift;
    int segmentMask = ssize - 1;
    @SuppressWarnings("unchecked")
    Segment<V>[] segments = (Segment<V>[]) new Segment<?>[DEFAULT_CONCURRENCY_LEVEL];
    for (int i = 0; i < segments.length; ++i)
      segments[i] = new Segment<V>(LOAD_FACTOR);
    java.io.ObjectOutputStream.PutField streamFields = s.putFields();
    streamFields.put("segments", segments);
    streamFields.put("segmentShift", segmentShift);
    streamFields.put("segmentMask", segmentMask);
    s.writeFields();

    Node<V>[] t;
    if ((t = table) != null) {
      Traverser<V> it = new Traverser<V>(t, t.length, 0, t.length);
      for (Node<V> p; (p = it.advance()) != null;) {
        s.writeObject(p.key);
      }
    }
    s.writeObject(null);
    s.writeObject(null);
    segments = null; // throw away
  }

  /**
   * Reconstitutes the instance from a stream (that is, deserializes it).
   *
   * @param s the stream
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    /*
     * To improve performance in typical cases, we create nodes while reading, then place in table
     * once size is known. However, we must also validate uniqueness and deal with overpopulated
     * bins while doing so, which requires specialized versions of putVal mechanics.
     */
    sizeCtl = -1; // force exclusion for table construction
    s.defaultReadObject();
    long size = 0L;
    Node<V> p = null;
    for (;;) {
      @SuppressWarnings("unchecked")
      V k = (V) s.readObject();
      if (k != null) {
        p = new Node<V>(spread(k.hashCode()), k, p);
        ++size;
      } else
        break;
    }
    if (size == 0L)
      sizeCtl = 0;
    else {
      int n;
      if (size >= (long) (MAXIMUM_CAPACITY >>> 1))
        n = MAXIMUM_CAPACITY;
      else {
        int sz = (int) size;
        n = tableSizeFor(sz + (sz >>> 1) + 1);
      }
      @SuppressWarnings("unchecked")
      Node<V>[] tab = (Node<V>[]) new Node<?>[n];
      int mask = n - 1;
      long added = 0L;
      while (p != null) {
        boolean insertAtFront;
        Node<V> next = p.next, first;
        int h = p.hash, j = h & mask;
        if ((first = tabAt(tab, j)) == null)
          insertAtFront = true;
        else {
          V k = p.key;
          if (first.hash < 0) {
            TreeBin<V> t = (TreeBin<V>) first;
            if (t.putTreeVal(h, k) == null)
              ++added;
            insertAtFront = false;
          } else {
            int binCount = 0;
            insertAtFront = true;
            Node<V> q;
            V qk;
            for (q = first; q != null; q = q.next) {
              if (q.hash == h && ((qk = q.key) == k || (qk != null && k.equals(qk)))) {
                insertAtFront = false;
                break;
              }
              ++binCount;
            }
            if (insertAtFront && binCount >= TREEIFY_THRESHOLD) {
              insertAtFront = false;
              ++added;
              p.next = first;
              TreeNode<V> hd = null, tl = null;
              for (q = p; q != null; q = q.next) {
                TreeNode<V> t = new TreeNode<V>(q.hash, q.key, null, null);
                if ((t.prev = tl) == null)
                  hd = t;
                else
                  tl.next = t;
                tl = t;
              }
              setTabAt(tab, j, new TreeBin<V>(hd));
            }
          }
        }
        if (insertAtFront) {
          ++added;
          p.next = first;
          setTabAt(tab, j, p);
        }
        p = next;
      }
      table = tab;
      sizeCtl = n - (n >>> 2);
      baseCount = added;
    }
  }


  @Override
  public boolean contains(Object value) {
    return _contains(value);
  }

  @Override
  public Iterator<V> iterator() {
    Node<V>[] t;
    CompactConcurrentHashSet2<V> m = this;
    int f = (t = m.table) == null ? 0 : t.length;
    return new KeyIterator<V>(t, f, 0, f, m);
  }

  // CompactConcurrentHashSet2-only methods

  /**
   * Returns the number of mappings. This method should be used instead of {@link #size} because a
   * CompactConcurrentHashSet2 may contain more mappings than can be represented as an int. The
   * value returned is an estimate; the actual count may differ if there are concurrent insertions
   * or removals.
   *
   * @return the number of mappings
   * @since Geode 1.0
   */
  public long mappingCount() {
    long n = sumCount();
    return (n < 0L) ? 0L : n; // ignore transient negative values
  }


  /* ---------------- Special Nodes -------------- */

  /**
   * A node inserted at head of bins during transfer operations.
   */
  static class ForwardingNode<K> extends Node<K> {
    final Node<K>[] nextTable;

    ForwardingNode(Node<K>[] tab) {
      super(MOVED, null, null);
      this.nextTable = tab;
    }

    @Override
    Node<K> find(int h, Object k) {
      Node<K> e;
      int n;
      Node<K>[] tab = nextTable;
      if (k != null && tab != null && (n = tab.length) > 0
          && (e = tabAt(tab, (n - 1) & h)) != null) {
        do {
          int eh;
          K ek;
          if ((eh = e.hash) == h && ((ek = e.key) == k || (ek != null && k.equals(ek))))
            return e;
          if (eh < 0)
            return e.find(h, k);
        } while ((e = e.next) != null);
      }
      return null;
    }
  }

  /**
   * A place-holder node used in computeIfAbsent and compute
   */
  static class ReservationNode<K> extends Node<K> {
    ReservationNode() {
      super(RESERVED, null, null);
    }

    @Override
    Node<K> find(int h, Object k) {
      return null;
    }
  }

  /* ---------------- Table Initialization and Resizing -------------- */

  /**
   * Returns the stamp bits for resizing a table of size n. Must be negative when shifted left by
   * RESIZE_STAMP_SHIFT.
   */
  static int resizeStamp(int n) {
    return Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1));
  }

  /**
   * Initializes table, using the size recorded in sizeCtl.
   */
  private Node<V>[] initTable() {
    Node<V>[] tab;
    int sc;
    while ((tab = table) == null || tab.length == 0) {
      if ((sc = sizeCtl) < 0)
        Thread.yield(); // lost initialization race; just spin
      else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
        try {
          if ((tab = table) == null || tab.length == 0) {
            int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
            @SuppressWarnings("unchecked")
            Node<V>[] nt = (Node<V>[]) new Node<?>[n];
            table = tab = nt;
            sc = n - (n >>> 2);
          }
        } finally {
          sizeCtl = sc;
        }
        break;
      }
    }
    return tab;
  }

  /**
   * Adds to count, and if table is too small and not already resizing, initiates transfer. If
   * already resizing, helps perform transfer if work is available. Rechecks occupancy after a
   * transfer to see if another resize is already needed because resizings are lagging additions.
   *
   * @param x the count to add
   * @param check if <0, don't check resize, if <= 1 only check if uncontended
   */
  private void addCount(long x, int check) {
    CounterCell[] as;
    long b, s;
    if ((as = counterCells) != null
        || !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
      CounterHashCode hc;
      CounterCell a;
      long v;
      int m;
      boolean uncontended = true;
      if ((hc = threadCounterHashCode.get()) == null || as == null || (m = as.length - 1) < 0
          || (a = as[m & hc.code]) == null
          || !(uncontended = U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) {
        fullAddCount(x, hc, uncontended);
        return;
      }
      if (check <= 1)
        return;
      s = sumCount();
    }
    if (check >= 0) {
      Node<V>[] tab, nt;
      int n, sc;
      while (s >= (long) (sc = sizeCtl) && (tab = table) != null
          && (n = tab.length) < MAXIMUM_CAPACITY) {
        int rs = resizeStamp(n);
        if (sc < 0) {
          if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 || sc == rs + MAX_RESIZERS
              || (nt = nextTable) == null || transferIndex <= 0)
            break;
          if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
            transfer(tab, nt);
        } else if (U.compareAndSwapInt(this, SIZECTL, sc, (rs << RESIZE_STAMP_SHIFT) + 2))
          transfer(tab, null);
        s = sumCount();
      }
    }
  }

  /**
   * Helps transfer if a resize is in progress.
   */
  Node<V>[] helpTransfer(Node<V>[] tab, Node<V> f) {
    Node<V>[] nextTab;
    int sc;
    if (tab != null && (f instanceof ForwardingNode)
        && (nextTab = ((ForwardingNode<V>) f).nextTable) != null) {
      int rs = resizeStamp(tab.length);
      while (nextTab == nextTable && table == tab && (sc = sizeCtl) < 0) {
        if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 || sc == rs + MAX_RESIZERS
            || transferIndex <= 0)
          break;
        if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
          transfer(tab, nextTab);
          break;
        }
      }
      return nextTab;
    }
    return table;
  }

  /**
   * Tries to presize table to accommodate the given number of elements.
   *
   * @param size number of elements (doesn't need to be perfectly accurate)
   */
  private void tryPresize(int size) {
    int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY
        : tableSizeFor(size + (size >>> 1) + 1);
    int sc;
    while ((sc = sizeCtl) >= 0) {
      Node<V>[] tab = table;
      int n;
      if (tab == null || (n = tab.length) == 0) {
        n = (sc > c) ? sc : c;
        if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
          try {
            if (table == tab) {
              @SuppressWarnings("unchecked")
              Node<V>[] nt = (Node<V>[]) new Node<?>[n];
              table = nt;
              sc = n - (n >>> 2);
            }
          } finally {
            sizeCtl = sc;
          }
        }
      } else if (c <= sc || n >= MAXIMUM_CAPACITY)
        break;
      else if (tab == table) {
        int rs = resizeStamp(n);
        if (U.compareAndSwapInt(this, SIZECTL, sc, (rs << RESIZE_STAMP_SHIFT) + 2))
          transfer(tab, null);
      }
    }
  }

  /**
   * Moves and/or copies the nodes in each bin to new table. See above for explanation.
   */
  private void transfer(Node<V>[] tab, Node<V>[] nextTab) {
    int n = tab.length, stride;
    if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
      stride = MIN_TRANSFER_STRIDE; // subdivide range
    if (nextTab == null) { // initiating
      try {
        @SuppressWarnings("unchecked")
        Node<V>[] nt = (Node<V>[]) new Node<?>[n << 1];
        nextTab = nt;
      } catch (Throwable ex) { // try to cope with OOME
        sizeCtl = Integer.MAX_VALUE;
        return;
      }
      nextTable = nextTab;
      transferIndex = n;
    }
    int nextn = nextTab.length;
    ForwardingNode<V> fwd = new ForwardingNode<V>(nextTab);
    boolean advance = true;
    boolean finishing = false; // to ensure sweep before committing nextTab
    for (int i = 0, bound = 0;;) {
      Node<V> f;
      int fh;
      while (advance) {
        int nextIndex, nextBound;
        if (--i >= bound || finishing)
          advance = false;
        else if ((nextIndex = transferIndex) <= 0) {
          i = -1;
          advance = false;
        } else if (U.compareAndSwapInt(this, TRANSFERINDEX, nextIndex,
            nextBound = (nextIndex > stride ? nextIndex - stride : 0))) {
          bound = nextBound;
          i = nextIndex - 1;
          advance = false;
        }
      }
      if (i < 0 || i >= n || i + n >= nextn) {
        int sc;
        if (finishing) {
          nextTable = null;
          table = nextTab;
          sizeCtl = (n << 1) - (n >>> 1);
          return;
        }
        if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
          if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
            return;
          finishing = advance = true;
          i = n; // recheck before commit
        }
      } else if ((f = tabAt(tab, i)) == null)
        advance = casTabAt(tab, i, null, fwd);
      else if ((fh = f.hash) == MOVED)
        advance = true; // already processed
      else {
        synchronized (f) {
          if (tabAt(tab, i) == f) {
            Node<V> ln, hn;
            if (fh >= 0) {
              int runBit = fh & n;
              Node<V> lastRun = f;
              for (Node<V> p = f.next; p != null; p = p.next) {
                int b = p.hash & n;
                if (b != runBit) {
                  runBit = b;
                  lastRun = p;
                }
              }
              if (runBit == 0) {
                ln = lastRun;
                hn = null;
              } else {
                hn = lastRun;
                ln = null;
              }
              for (Node<V> p = f; p != lastRun; p = p.next) {
                int ph = p.hash;
                V pk = p.key;
                if ((ph & n) == 0)
                  ln = new Node<V>(ph, pk, ln);
                else
                  hn = new Node<V>(ph, pk, hn);
              }
              setTabAt(nextTab, i, ln);
              setTabAt(nextTab, i + n, hn);
              setTabAt(tab, i, fwd);
              advance = true;
            } else if (f instanceof TreeBin) {
              TreeBin<V> t = (TreeBin<V>) f;
              TreeNode<V> lo = null, loTail = null;
              TreeNode<V> hi = null, hiTail = null;
              int lc = 0, hc = 0;
              for (Node<V> e = t.first; e != null; e = e.next) {
                int h = e.hash;
                TreeNode<V> p = new TreeNode<V>(h, e.key, null, null);
                if ((h & n) == 0) {
                  if ((p.prev = loTail) == null)
                    lo = p;
                  else
                    loTail.next = p;
                  loTail = p;
                  ++lc;
                } else {
                  if ((p.prev = hiTail) == null)
                    hi = p;
                  else
                    hiTail.next = p;
                  hiTail = p;
                  ++hc;
                }
              }
              ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) : (hc != 0) ? new TreeBin<V>(lo) : t;
              hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) : (lc != 0) ? new TreeBin<V>(hi) : t;
              setTabAt(nextTab, i, ln);
              setTabAt(nextTab, i + n, hn);
              setTabAt(tab, i, fwd);
              advance = true;
            }
          }
        }
      }
    }
  }

  /* ---------------- Conversion from/to TreeBins -------------- */

  /**
   * Replaces all linked nodes in bin at given index unless table is too small, in which case
   * resizes instead.
   */
  private void treeifyBin(Node<V>[] tab, int index) {
    Node<V> b;
    int n, sc;
    if (tab != null) {
      if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
        tryPresize(n << 1);
      else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
        synchronized (b) {
          if (tabAt(tab, index) == b) {
            TreeNode<V> hd = null, tl = null;
            for (Node<V> e = b; e != null; e = e.next) {
              TreeNode<V> p = new TreeNode<V>(e.hash, e.key, null, null);
              if ((p.prev = tl) == null)
                hd = p;
              else
                tl.next = p;
              tl = p;
            }
            setTabAt(tab, index, new TreeBin<V>(hd));
          }
        }
      }
    }
  }

  /**
   * Returns a list on non-TreeNodes replacing those in given list.
   */
  static <K> Node<K> untreeify(Node<K> b) {
    Node<K> hd = null, tl = null;
    for (Node<K> q = b; q != null; q = q.next) {
      Node<K> p = new Node<K>(q.hash, q.key, null);
      if (tl == null)
        hd = p;
      else
        tl.next = p;
      tl = p;
    }
    return hd;
  }

  /* ---------------- TreeNodes -------------- */

  /**
   * Nodes for use in TreeBins
   */
  static class TreeNode<K> extends Node<K> {
    TreeNode<K> parent; // red-black tree links
    TreeNode<K> left;
    TreeNode<K> right;
    TreeNode<K> prev; // needed to unlink next upon deletion
    boolean red;

    TreeNode(int hash, K key, Node<K> next, TreeNode<K> parent) {
      super(hash, key, next);
      this.parent = parent;
    }

    @Override
    Node<K> find(int h, Object k) {
      return findTreeNode(h, k, null);
    }

    /**
     * Returns the TreeNode (or null if not found) for the given key starting at given root.
     */
    TreeNode<K> findTreeNode(int h, Object k, Class<?> kc) {
      if (k != null) {
        TreeNode<K> p = this;
        do {
          int ph, dir;
          K pk;
          TreeNode<K> q;
          TreeNode<K> pl = p.left, pr = p.right;
          if ((ph = p.hash) > h)
            p = pl;
          else if (ph < h)
            p = pr;
          else if ((pk = p.key) == k || (pk != null && k.equals(pk)))
            return p;
          else if (pl == null)
            p = pr;
          else if (pr == null)
            p = pl;
          else if ((kc != null || (kc = comparableClassFor(k)) != null)
              && (dir = compareComparables(kc, k, pk)) != 0)
            p = (dir < 0) ? pl : pr;
          else if ((q = pr.findTreeNode(h, k, kc)) != null)
            return q;
          else
            p = pl;
        } while (p != null);
      }
      return null;
    }
  }


  /* ---------------- TreeBins -------------- */

  /**
   * TreeNodes used at the heads of bins. TreeBins do not hold user keys or values, but instead
   * point to list of TreeNodes and their root. They also maintain a parasitic read-write lock
   * forcing writers (who hold bin lock) to wait for readers (who do not) to complete before tree
   * restructuring operations.
   */
  static class TreeBin<K> extends Node<K> {
    TreeNode<K> root;
    volatile TreeNode<K> first;
    volatile Thread waiter;
    volatile int lockState;
    // values for lockState
    static final int WRITER = 1; // set while holding write lock
    static final int WAITER = 2; // set when waiting for write lock
    static final int READER = 4; // increment value for setting read lock

    /**
     * Tie-breaking utility for ordering insertions when equal hashCodes and non-comparable. We
     * don't require a total order, just a consistent insertion rule to maintain equivalence across
     * rebalancings. Tie-breaking further than necessary simplifies testing a bit.
     */
    static int tieBreakOrder(Object a, Object b) {
      int d;
      if (a == null || b == null
          || (d = a.getClass().getName().compareTo(b.getClass().getName())) == 0)
        d = (System.identityHashCode(a) <= System.identityHashCode(b) ? -1 : 1);
      return d;
    }

    /**
     * Creates bin with initial set of nodes headed by b.
     */
    TreeBin(TreeNode<K> b) {
      super(TREEBIN, null, null);
      this.first = b;
      TreeNode<K> r = null;
      for (TreeNode<K> x = b, next; x != null; x = next) {
        next = (TreeNode<K>) x.next;
        x.left = x.right = null;
        if (r == null) {
          x.parent = null;
          x.red = false;
          r = x;
        } else {
          K k = x.key;
          int h = x.hash;
          Class<?> kc = null;
          for (TreeNode<K> p = r;;) {
            int dir, ph;
            K pk = p.key;
            if ((ph = p.hash) > h)
              dir = -1;
            else if (ph < h)
              dir = 1;
            else if ((kc == null && (kc = comparableClassFor(k)) == null)
                || (dir = compareComparables(kc, k, pk)) == 0)
              dir = tieBreakOrder(k, pk);
            TreeNode<K> xp = p;
            if ((p = (dir <= 0) ? p.left : p.right) == null) {
              x.parent = xp;
              if (dir <= 0)
                xp.left = x;
              else
                xp.right = x;
              r = balanceInsertion(r, x);
              break;
            }
          }
        }
      }
      this.root = r;
      assert checkInvariants(root);
    }

    /**
     * Acquires write lock for tree restructuring.
     */
    private void lockRoot() {
      if (!U.compareAndSwapInt(this, LOCKSTATE, 0, WRITER))
        contendedLock(); // offload to separate method
    }

    /**
     * Releases write lock for tree restructuring.
     */
    private void unlockRoot() {
      lockState = 0;
    }

    /**
     * Possibly blocks awaiting root lock.
     */
    private void contendedLock() {
      boolean waiting = false;
      for (int s;;) {
        if (((s = lockState) & ~WAITER) == 0) {
          if (U.compareAndSwapInt(this, LOCKSTATE, s, WRITER)) {
            if (waiting)
              waiter = null;
            return;
          }
        } else if ((s & WAITER) == 0) {
          if (U.compareAndSwapInt(this, LOCKSTATE, s, s | WAITER)) {
            waiting = true;
            waiter = Thread.currentThread();
          }
        } else if (waiting)
          LockSupport.park(this);
      }
    }

    /**
     * Returns matching node or null if none. Tries to search using tree comparisons from root, but
     * continues linear search when lock not available.
     */
    @Override
    Node<K> find(int h, Object k) {
      if (k != null) {
        for (Node<K> e = first; e != null;) {
          int s;
          K ek;
          if (((s = lockState) & (WAITER | WRITER)) != 0) {
            if (e.hash == h && ((ek = e.key) == k || (ek != null && k.equals(ek))))
              return e;
            e = e.next;
          } else if (U.compareAndSwapInt(this, LOCKSTATE, s, s + READER)) {
            TreeNode<K> r, p;
            try {
              p = ((r = root) == null ? null : r.findTreeNode(h, k, null));
            } finally {

              Thread w;
              int ls;
              do {
              } while (!U.compareAndSwapInt(this, LOCKSTATE, ls = lockState, ls - READER));
              if (ls == (READER | WAITER) && (w = waiter) != null)
                LockSupport.unpark(w);
            }
            return p;
          }
        }
      }
      return null;
    }

    /**
     * Finds or adds a node.
     *
     * @return null if added
     */
    /**
     * Finds or adds a node.
     *
     * @return null if added
     */
    TreeNode<K> putTreeVal(int h, K k) {
      Class<?> kc = null;
      boolean searched = false;
      for (TreeNode<K> p = root;;) {
        int dir, ph;
        K pk;
        if (p == null) {
          first = root = new TreeNode<K>(h, k, null, null);
          break;
        } else if ((ph = p.hash) > h)
          dir = -1;
        else if (ph < h)
          dir = 1;
        else if ((pk = p.key) == k || (pk != null && k.equals(pk)))
          return p;
        else if ((kc == null && (kc = comparableClassFor(k)) == null)
            || (dir = compareComparables(kc, k, pk)) == 0) {
          if (!searched) {
            TreeNode<K> q, ch;
            searched = true;
            if (((ch = p.left) != null && (q = ch.findTreeNode(h, k, kc)) != null)
                || ((ch = p.right) != null && (q = ch.findTreeNode(h, k, kc)) != null))
              return q;
          }
          dir = tieBreakOrder(k, pk);
        }

        TreeNode<K> xp = p;
        if ((p = (dir <= 0) ? p.left : p.right) == null) {
          TreeNode<K> x, f = first;
          first = x = new TreeNode<K>(h, k, f, xp);
          if (f != null)
            f.prev = x;
          if (dir <= 0)
            xp.left = x;
          else
            xp.right = x;
          if (!xp.red)
            x.red = true;
          else {
            lockRoot();
            try {
              root = balanceInsertion(root, x);
            } finally {
              unlockRoot();
            }
          }
          break;
        }
      }
      assert checkInvariants(root);
      return null;
    }

    /**
     * Removes the given node, that must be present before this call. This is messier than typical
     * red-black deletion code because we cannot swap the contents of an interior node with a leaf
     * successor that is pinned by "next" pointers that are accessible independently of lock. So
     * instead we swap the tree linkages.
     *
     * @return true if now too small, so should be untreeified
     */
    boolean removeTreeNode(TreeNode<K> p) {
      TreeNode<K> next = (TreeNode<K>) p.next;
      TreeNode<K> pred = p.prev; // unlink traversal pointers
      TreeNode<K> r, rl;
      if (pred == null)
        first = next;
      else
        pred.next = next;
      if (next != null)
        next.prev = pred;
      if (first == null) {
        root = null;
        return true;
      }
      if ((r = root) == null || r.right == null || // too small
          (rl = r.left) == null || rl.left == null)
        return true;
      lockRoot();
      try {
        TreeNode<K> replacement;
        TreeNode<K> pl = p.left;
        TreeNode<K> pr = p.right;
        if (pl != null && pr != null) {
          TreeNode<K> s = pr, sl;
          while ((sl = s.left) != null) // find successor
            s = sl;
          boolean c = s.red;
          s.red = p.red;
          p.red = c; // swap colors
          TreeNode<K> sr = s.right;
          TreeNode<K> pp = p.parent;
          if (s == pr) { // p was s's direct parent
            p.parent = s;
            s.right = p;
          } else {
            TreeNode<K> sp = s.parent;
            if ((p.parent = sp) != null) {
              if (s == sp.left)
                sp.left = p;
              else
                sp.right = p;
            }
            s.right = pr;
            pr.parent = s;
          }
          p.left = null;
          if ((p.right = sr) != null)
            sr.parent = p;
          s.left = pl;
          pl.parent = s;
          if ((s.parent = pp) == null)
            r = s;
          else if (p == pp.left)
            pp.left = s;
          else
            pp.right = s;
          if (sr != null)
            replacement = sr;
          else
            replacement = p;
        } else if (pl != null)
          replacement = pl;
        else if (pr != null)
          replacement = pr;
        else
          replacement = p;
        if (replacement != p) {
          TreeNode<K> pp = replacement.parent = p.parent;
          if (pp == null)
            r = replacement;
          else if (p == pp.left)
            pp.left = replacement;
          else
            pp.right = replacement;
          p.left = p.right = p.parent = null;
        }

        root = (p.red) ? r : balanceDeletion(r, replacement);

        if (p == replacement) { // detach pointers
          TreeNode<K> pp;
          if ((pp = p.parent) != null) {
            if (p == pp.left)
              pp.left = null;
            else if (p == pp.right)
              pp.right = null;
            p.parent = null;
          }
        }
      } finally {
        unlockRoot();
      }
      assert checkInvariants(root);
      return false;
    }

    /* ------------------------------------------------------------ */
    // Red-black tree methods, all adapted from CLR

    static <K> TreeNode<K> rotateLeft(TreeNode<K> root, TreeNode<K> p) {
      TreeNode<K> r, pp, rl;
      if (p != null && (r = p.right) != null) {
        if ((rl = p.right = r.left) != null)
          rl.parent = p;
        if ((pp = r.parent = p.parent) == null)
          (root = r).red = false;
        else if (pp.left == p)
          pp.left = r;
        else
          pp.right = r;
        r.left = p;
        p.parent = r;
      }
      return root;
    }

    static <K> TreeNode<K> rotateRight(TreeNode<K> root, TreeNode<K> p) {
      TreeNode<K> l, pp, lr;
      if (p != null && (l = p.left) != null) {
        if ((lr = p.left = l.right) != null)
          lr.parent = p;
        if ((pp = l.parent = p.parent) == null)
          (root = l).red = false;
        else if (pp.right == p)
          pp.right = l;
        else
          pp.left = l;
        l.right = p;
        p.parent = l;
      }
      return root;
    }

    static <K> TreeNode<K> balanceInsertion(TreeNode<K> root, TreeNode<K> x) {
      x.red = true;
      for (TreeNode<K> xp, xpp, xppl, xppr;;) {
        if ((xp = x.parent) == null) {
          x.red = false;
          return x;
        } else if (!xp.red || (xpp = xp.parent) == null)
          return root;
        if (xp == (xppl = xpp.left)) {
          if ((xppr = xpp.right) != null && xppr.red) {
            xppr.red = false;
            xp.red = false;
            xpp.red = true;
            x = xpp;
          } else {
            if (x == xp.right) {
              root = rotateLeft(root, x = xp);
              xpp = (xp = x.parent) == null ? null : xp.parent;
            }
            if (xp != null) {
              xp.red = false;
              if (xpp != null) {
                xpp.red = true;
                root = rotateRight(root, xpp);
              }
            }
          }
        } else {
          if (xppl != null && xppl.red) {
            xppl.red = false;
            xp.red = false;
            xpp.red = true;
            x = xpp;
          } else {
            if (x == xp.left) {
              root = rotateRight(root, x = xp);
              xpp = (xp = x.parent) == null ? null : xp.parent;
            }
            if (xp != null) {
              xp.red = false;
              if (xpp != null) {
                xpp.red = true;
                root = rotateLeft(root, xpp);
              }
            }
          }
        }
      }
    }

    static <K> TreeNode<K> balanceDeletion(TreeNode<K> root, TreeNode<K> x) {
      for (TreeNode<K> xp, xpl, xpr;;) {
        if (x == null || x == root)
          return root;
        else if ((xp = x.parent) == null) {
          x.red = false;
          return x;
        } else if (x.red) {
          x.red = false;
          return root;
        } else if ((xpl = xp.left) == x) {
          if ((xpr = xp.right) != null && xpr.red) {
            xpr.red = false;
            xp.red = true;
            root = rotateLeft(root, xp);
            xpr = (xp = x.parent) == null ? null : xp.right;
          }
          if (xpr == null)
            x = xp;
          else {
            TreeNode<K> sl = xpr.left, sr = xpr.right;
            if ((sr == null || !sr.red) && (sl == null || !sl.red)) {
              xpr.red = true;
              x = xp;
            } else {
              if (sr == null || !sr.red) {
                if (sl != null)
                  sl.red = false;
                xpr.red = true;
                root = rotateRight(root, xpr);
                xpr = (xp = x.parent) == null ? null : xp.right;
              }
              if (xpr != null) {
                xpr.red = (xp == null) ? false : xp.red;
                if ((sr = xpr.right) != null)
                  sr.red = false;
              }
              if (xp != null) {
                xp.red = false;
                root = rotateLeft(root, xp);
              }
              x = root;
            }
          }
        } else { // symmetric
          if (xpl != null && xpl.red) {
            xpl.red = false;
            xp.red = true;
            root = rotateRight(root, xp);
            xpl = (xp = x.parent) == null ? null : xp.left;
          }
          if (xpl == null)
            x = xp;
          else {
            TreeNode<K> sl = xpl.left, sr = xpl.right;
            if ((sl == null || !sl.red) && (sr == null || !sr.red)) {
              xpl.red = true;
              x = xp;
            } else {
              if (sl == null || !sl.red) {
                if (sr != null)
                  sr.red = false;
                xpl.red = true;
                root = rotateLeft(root, xpl);
                xpl = (xp = x.parent) == null ? null : xp.left;
              }
              if (xpl != null) {
                xpl.red = (xp == null) ? false : xp.red;
                if ((sl = xpl.left) != null)
                  sl.red = false;
              }
              if (xp != null) {
                xp.red = false;
                root = rotateRight(root, xp);
              }
              x = root;
            }
          }
        }
      }
    }

    /**
     * Recursive invariant check
     */
    static <K> boolean checkInvariants(TreeNode<K> t) {
      TreeNode<K> tp = t.parent, tl = t.left, tr = t.right, tb = t.prev, tn = (TreeNode<K>) t.next;
      if (tb != null && tb.next != t)
        return false;
      if (tn != null && tn.prev != t)
        return false;
      if (tp != null && t != tp.left && t != tp.right)
        return false;
      if (tl != null && (tl.parent != t || tl.hash > t.hash))
        return false;
      if (tr != null && (tr.parent != t || tr.hash < t.hash))
        return false;
      if (t.red && tl != null && tl.red && tr != null && tr.red)
        return false;
      if (tl != null && !checkInvariants(tl))
        return false;
      if (tr != null && !checkInvariants(tr))
        return false;
      return true;
    }

    @Immutable
    private static final long LOCKSTATE;
    static {
      try {
        Class<?> k = TreeBin.class;
        LOCKSTATE = U.objectFieldOffset(k.getDeclaredField("lockState"));
      } catch (Exception e) {
        throw new Error(e);
      }
    }
  }

  /* ----------------Table Traversal -------------- */

  /**
   * Records the table, its length, and current traversal index for a traverser that must process a
   * region of a forwarded table before proceeding with current table.
   */
  static class TableStack<K> {
    int length;
    int index;
    Node<K>[] tab;
    TableStack<K> next;
  }

  /**
   * Encapsulates traversal for methods such as containsValue; also serves as a base class for other
   * iterators and spliterators.
   *
   * Method advance visits once each still-valid node that was reachable upon iterator construction.
   * It might miss some that were added to a bin after the bin was visited, which is OK wrt
   * consistency guarantees. Maintaining this property in the face of possible ongoing resizes
   * requires a fair amount of bookkeeping state that is difficult to optimize away amidst volatile
   * accesses. Even so, traversal maintains reasonable throughput.
   *
   * Normally, iteration proceeds bin-by-bin traversing lists. However, if the table has been
   * resized, then all future steps must traverse both the bin at the current index as well as at
   * (index + baseSize); and so on for further resizings. To paranoically cope with potential
   * sharing by users of iterators across threads, iteration terminates if a bounds checks fails for
   * a table read.
   */
  static class Traverser<K> {
    Node<K>[] tab; // current table; updated if resized
    Node<K> next; // the next entry to use
    TableStack<K> stack, spare; // to save/restore on ForwardingNodes
    int index; // index of bin to use next
    int baseIndex; // current index of initial table
    int baseLimit; // index bound for initial table
    final int baseSize; // initial table size

    Traverser(Node<K>[] tab, int size, int index, int limit) {
      this.tab = tab;
      this.baseSize = size;
      this.baseIndex = this.index = index;
      this.baseLimit = limit;
      this.next = null;
    }

    /**
     * Advances if possible, returning next valid node, or null if none.
     */
    Node<K> advance() {
      Node<K> e;
      if ((e = next) != null)
        e = e.next;
      for (;;) {
        Node<K>[] t;
        int i, n; // must use locals in checks
        if (e != null)
          return next = e;
        if (baseIndex >= baseLimit || (t = tab) == null || (n = t.length) <= (i = index) || i < 0)
          return next = null;
        if ((e = tabAt(t, i)) != null && e.hash < 0) {
          if (e instanceof ForwardingNode) {
            tab = ((ForwardingNode<K>) e).nextTable;
            e = null;
            pushState(t, i, n);
            continue;
          } else if (e instanceof TreeBin)
            e = ((TreeBin<K>) e).first;
          else
            e = null;
        }
        if (stack != null)
          recoverState(n);
        else if ((index = i + baseSize) >= n)
          index = ++baseIndex; // visit upper slots if present
      }
    }

    /**
     * Saves traversal state upon encountering a forwarding node.
     */
    private void pushState(Node<K>[] t, int i, int n) {
      TableStack<K> s = spare; // reuse if possible
      if (s != null)
        spare = s.next;
      else
        s = new TableStack<K>();
      s.tab = t;
      s.length = n;
      s.index = i;
      s.next = stack;
      stack = s;
    }

    /**
     * Possibly pops traversal state.
     *
     * @param n length of current table
     */
    private void recoverState(int n) {
      TableStack<K> s;
      int len;
      while ((s = stack) != null && (index += (len = s.length)) >= n) {
        n = len;
        index = s.index;
        tab = s.tab;
        s.tab = null;
        TableStack<K> next = s.next;
        s.next = spare; // save for reuse
        stack = next;
        spare = s;
      }
      if (s == null && (index += baseSize) >= n)
        index = ++baseIndex;
    }
  }

  /**
   * Base of key, value, and entry Iterators. Adds fields to Traverser to support iterator.remove.
   */
  static class BaseIterator<K> extends Traverser<K> {
    final CompactConcurrentHashSet2<K> set;
    Node<K> lastReturned;

    BaseIterator(Node<K>[] tab, int size, int index, int limit, CompactConcurrentHashSet2<K> map) {
      super(tab, size, index, limit);
      this.set = map;
      advance();
    }

    public boolean hasNext() {
      return next != null;
    }

    public boolean hasMoreElements() {
      return next != null;
    }

    public void remove() {
      Node<K> p;
      if ((p = lastReturned) == null)
        throw new IllegalStateException();
      lastReturned = null;
      set.removeNode(p.key);
    }
  }

  static class KeyIterator<K> extends BaseIterator<K> implements Iterator<K>, Enumeration<K> {
    KeyIterator(Node<K>[] tab, int index, int size, int limit, CompactConcurrentHashSet2<K> map) {
      super(tab, index, size, limit, map);
    }

    @Override
    public K next() {
      Node<K> p;
      if ((p = next) == null)
        throw new NoSuchElementException();
      K k = p.key;
      lastReturned = p;
      advance();
      return k;
    }

    @Override
    public K nextElement() {
      return next();
    }
  }


  /* ---------------- Counters -------------- */

  // Adapted from LongAdder and Striped64.
  // See their internal docs for explanation.

  // A padded cell for distributing counts
  static class CounterCell {
    volatile long p0, p1, p2, p3, p4, p5, p6;
    volatile long value;
    volatile long q0, q1, q2, q3, q4, q5, q6;

    CounterCell(long x) {
      value = x;
    }
  }

  /**
   * Holder for the thread-local hash code determining which CounterCell to use. The code is
   * initialized via the counterHashCodeGenerator, but may be moved upon collisions.
   */
  static class CounterHashCode {
    int code;
  }

  /**
   * Generates initial value for per-thread CounterHashCodes.
   */
  @MakeNotStatic("Possible ok singleton?")
  static final AtomicInteger counterHashCodeGenerator = new AtomicInteger();

  /**
   * Increment for counterHashCodeGenerator. See class ThreadLocal for explanation.
   */
  static final int SEED_INCREMENT = 0x61c88647;

  /**
   * Per-thread counter hash codes. Shared across all instances.
   */
  static final ThreadLocal<CounterHashCode> threadCounterHashCode =
      new ThreadLocal<CounterHashCode>();

  long sumCount() {
    CounterCell[] as = counterCells;
    CounterCell a;
    long sum = baseCount;
    if (as != null) {
      for (int i = 0; i < as.length; ++i) {
        if ((a = as[i]) != null)
          sum += a.value;
      }
    }
    return sum;
  }

  // See LongAdder version for explanation
  private void fullAddCount(long x, CounterHashCode hc, boolean wasUncontended) {
    int h;
    if (hc == null) {
      hc = new CounterHashCode();
      int s = counterHashCodeGenerator.addAndGet(SEED_INCREMENT);
      h = hc.code = (s == 0) ? 1 : s; // Avoid zero
      threadCounterHashCode.set(hc);
    } else
      h = hc.code;
    boolean collide = false; // True if last slot nonempty
    for (;;) {
      CounterCell[] as;
      CounterCell a;
      int n;
      long v;
      if ((as = counterCells) != null && (n = as.length) > 0) {
        if ((a = as[(n - 1) & h]) == null) {
          if (cellsBusy == 0) { // Try to attach new Cell
            CounterCell r = new CounterCell(x); // Optimistic create
            if (cellsBusy == 0 && U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
              boolean created = false;
              try { // Recheck under lock
                CounterCell[] rs;
                int m, j;
                if ((rs = counterCells) != null && (m = rs.length) > 0
                    && rs[j = (m - 1) & h] == null) {
                  rs[j] = r;
                  created = true;
                }
              } finally {
                cellsBusy = 0;
              }
              if (created)
                break;
              continue; // Slot is now non-empty
            }
          }
          collide = false;
        } else if (!wasUncontended) // CAS already known to fail
          wasUncontended = true; // Continue after rehash
        else if (U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))
          break;
        else if (counterCells != as || n >= NCPU)
          collide = false; // At max size or stale
        else if (!collide)
          collide = true;
        else if (cellsBusy == 0 && U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
          try {
            if (counterCells == as) {// Expand table unless stale
              CounterCell[] rs = new CounterCell[n << 1];
              for (int i = 0; i < n; ++i)
                rs[i] = as[i];
              counterCells = rs;
            }
          } finally {
            cellsBusy = 0;
          }
          collide = false;
          continue; // Retry with expanded table
        }
        h ^= h << 13; // Rehash
        h ^= h >>> 17;
        h ^= h << 5;
      } else if (cellsBusy == 0 && counterCells == as
          && U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
        boolean init = false;
        try { // Initialize table
          if (counterCells == as) {
            CounterCell[] rs = new CounterCell[2];
            rs[h & 1] = new CounterCell(x);
            counterCells = rs;
            init = true;
          }
        } finally {
          cellsBusy = 0;
        }
        if (init)
          break;
      } else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x))
        break; // Fall back on using base
    }
    hc.code = h; // Record index for next time
  }

  // Unsafe mechanics
  @Immutable
  private static final Unsafe U = new Unsafe();
  private static final long SIZECTL;
  private static final long TRANSFERINDEX;
  private static final long BASECOUNT;
  private static final long CELLSBUSY;
  private static final long CELLVALUE;
  private static final long ABASE;
  private static final int ASHIFT;

  static {
    try {
      Class<?> k = CompactConcurrentHashSet2.class;
      SIZECTL = U.objectFieldOffset(k.getDeclaredField("sizeCtl"));
      TRANSFERINDEX = U.objectFieldOffset(k.getDeclaredField("transferIndex"));
      BASECOUNT = U.objectFieldOffset(k.getDeclaredField("baseCount"));
      CELLSBUSY = U.objectFieldOffset(k.getDeclaredField("cellsBusy"));
      Class<?> ck = CounterCell.class;
      CELLVALUE = U.objectFieldOffset(ck.getDeclaredField("value"));
      Class<?> ak = Node[].class;
      ABASE = U.arrayBaseOffset(ak);
      int scale = U.arrayIndexScale(ak);
      if ((scale & (scale - 1)) != 0)
        throw new Error("data type scale not a power of two");
      ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
    } catch (Exception e) {
      throw new Error(e);
    }

    // Reduce the risk of rare disastrous classloading in first call to
    // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
    Class<?> ensureLoaded = LockSupport.class;
  }

}