package org.jetbrains.kotlin.com.intellij.util.containers;

// Copyright 2000-2021 JetBrains s.r.o. and contributors. Use of this source code is governed by the
// Apache 2.0 license that can be found in the LICENSE file.

import androidx.annotation.NonNull;
import java.util.Arrays;
import java.util.Collection;
import java.util.Enumeration;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.locks.LockSupport;
import sun.misc.Unsafe;

/**
 * Adapted from Doug Lea ConcurrentHashMap (see
 * http://gee.cs.oswego.edu/dl/concurrency-interest/index.html) to int keys with following
 * additions/changes: - added hashing strategy argument - added cacheOrGet convenience method - Null
 * values are NOT allowed
 *
 * @author Doug Lea
 * @param <V> the type of mapped values
 * @deprecated Use {@link ConcurrentCollectionFactory#createConcurrentIntObjectMap()} instead
 */
@Deprecated
public final class ConcurrentIntObjectHashMap<V> implements ConcurrentIntObjectMap<V> {
  /**
   * 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 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 = -1; // hash for forwarding nodes
  static final int TREEBIN = -2; // hash for roots of trees
  static final int RESERVED = -3; // 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();

  /* ---------------- 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<V> implements Entry<V> {
    final int hash;
    final int key;
    volatile V val;
    volatile Node<V> next;

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

    @Override
    public final int getKey() {
      return key;
    }

    @NonNull
    @Override
    public final V getValue() {
      return val;
    }

    @Override
    public final int hashCode() {
      return (key ^ val.hashCode());
    }

    @Override
    public final String toString() {
      return key + "=" + val;
    }

    @Override
    public final boolean equals(Object o) {
      Object v;
      Object u;
      Entry<?> e;
      return ((o instanceof Entry)
          && (e = (Entry<?>) o).getKey() == key
          && (v = e.getValue()) != null
          && (v == (u = val) || v.equals(u)));
    }

    /** Virtualized support for map.get(); overridden in subclasses. */
    Node<V> find(int h, int k) {
      Node<V> e = this;
      do {
        if ((e.key == k)) {
          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;
  }

  /* ---------------- 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 in principle require only release ordering, not
   * full volatile semantics, but are currently coded as volatile
   * writes to be conservative.
   */

  @SuppressWarnings("unchecked")
  static <V> Node<V> tabAt(Node<V>[] tab, int i) {
    try {
      Object o = theUnsafe.getObjectVolatile(tab, ((long) i << ASHIFT) + ABASE);
      return (Node<V>) o;
    } catch (Throwable throwable) {
      throw new RuntimeException(throwable);
    }
  }

  static <V> boolean casTabAt(Node<V>[] tab, int i, Node<V> c, Node<V> v) {
    try {
      return theUnsafe.compareAndSwapObject(tab, ((long) i << ASHIFT) + ABASE, c, v);
    } catch (Throwable throwable) {
      throw new RuntimeException(throwable);
    }
  }

  static <V> void setTabAt(Node<V>[] tab, int i, Node<V> v) {
    try {
      theUnsafe.putObjectVolatile((Object) tab, ((long) i << ASHIFT) + ABASE, (Object) v);
    } catch (Throwable throwable) {
      throw new RuntimeException(throwable);
    }
  }

  /* ---------------- 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.
   */
  @SuppressWarnings("UnusedDeclaration")
  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;

  /**
   * A padded cell for distributing counts. Adapted from LongAdder and Striped64. See their internal
   * docs for explanation.
   */
  static final class CounterCell {
    volatile long p0;
    volatile long p1;
    volatile long p2;
    volatile long p3;
    volatile long p4;
    volatile long p5;
    volatile long p6;
    volatile long value;
    volatile long q0;
    volatile long q1;
    volatile long q2;
    volatile long q3;
    volatile long q4;
    volatile long q5;
    volatile long q6;

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

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

  // views
  private transient ValuesView<V> values;
  private transient EntrySetView<V> entrySet;

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

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

  /**
   * 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
   */
  ConcurrentIntObjectHashMap(int initialCapacity) {
    if (initialCapacity < 0) {
      throw new IllegalArgumentException();
    }
    int cap =
        ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1))
            ? MAXIMUM_CAPACITY
            : tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
    sizeCtl = cap;
  }

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

  /**
   * Returns the value to which the specified key is mapped, or {@code null} if this map contains no
   * mapping for the key.
   *
   * <p>
   *
   * <p>More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such
   * that {@code key.equals(k)}, then this method returns {@code v}; otherwise it returns {@code
   * null}. (There can be at most one such mapping.)
   */
  @Override
  public V get(int key) {
    Node<V>[] tab;
    Node<V> e;
    Node<V> p;
    int n, eh;
    int h = spread(key);
    if ((tab = table) != null && (n = tab.length) > 0 && (e = tabAt(tab, (n - 1) & h)) != null) {
      if ((eh = e.hash) == h) {
        if (e.key == key) {
          return e.val;
        }
      } else if (eh < 0) {
        return (p = e.find(h, key)) != null ? p.val : null;
      }
      while ((e = e.next) != null) {
        if (e.hash == h && (e.key == key)) {
          return e.val;
        }
      }
    }
    return null;
  }

  /**
   * Tests if the specified object is a key in this table.
   *
   * @param key possible key
   * @return {@code true} if and only if the specified object is a key in this table, as determined
   *     by the {@code equals} method; {@code false} otherwise
   */
  public boolean containsKey(int key) {
    return get(key) != null;
  }

  /**
   * Returns {@code true} if this map maps one or more keys to the specified value. Note: This
   * method may require a full traversal of the map, and is much slower than method {@code
   * containsKey}.
   *
   * @param value value whose presence in this map is to be tested
   * @return {@code true} if this map maps one or more keys to the specified value
   */
  public boolean containsValue(@NonNull Object value) {
    Node<V>[] t;
    if ((t = table) != null) {
      Traverser<V> it = new Traverser<>(t, t.length, 0, t.length);
      for (Node<V> p; (p = it.advance()) != null; ) {
        V v;
        if ((v = p.val) == value || (v != null && value.equals(v))) {
          return true;
        }
      }
    }
    return false;
  }

  /**
   * Maps the specified key to the specified value in this table. Neither the key nor the value can
   * be null.
   *
   * <p>
   *
   * <p>The value can be retrieved by calling the {@code get} method with a key that is equal to the
   * original key.
   *
   * @param key key with which the specified value is to be associated
   * @param value value to be associated with the specified key
   * @return the previous value associated with {@code key}, or {@code null} if there was no mapping
   *     for {@code key}
   */
  @Override
  public V put(int key, @NonNull V value) {
    return putVal(key, value, false);
  }

  /** Implementation for put and putIfAbsent */
  V putVal(int key, @NonNull V value, boolean onlyIfAbsent) {
    int hash = spread(key);
    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<>(hash, key, value, null))) {
          break; // no lock when adding to empty bin
        }
      } else if ((fh = f.hash) == MOVED) {
        tab = helpTransfer(tab, f);
      } else {
        V oldVal = null;
        synchronized (f) {
          if (tabAt(tab, i) == f) {
            if (fh >= 0) {
              binCount = 1;
              for (Node<V> e = f; ; ++binCount) {
                if ((e.key == key)) {
                  oldVal = e.val;
                  if (!onlyIfAbsent) {
                    e.val = value;
                  }
                  break;
                }
                Node<V> pred = e;
                if ((e = e.next) == null) {
                  pred.next = new Node<>(hash, key, value, null);
                  break;
                }
              }
            } else if (f instanceof TreeBin) {
              Node<V> p;
              binCount = 2;
              if ((p = ((TreeBin<V>) f).putTreeVal(hash, key, value)) != null) {
                oldVal = p.val;
                if (!onlyIfAbsent) {
                  p.val = value;
                }
              }
            }
          }
        }
        if (binCount != 0) {
          if (binCount >= TREEIFY_THRESHOLD) {
            treeifyBin(tab, i);
          }
          if (oldVal != null) {
            return oldVal;
          }
          break;
        }
      }
    }
    addCount(1L, binCount);
    return null;
  }

  /**
   * Removes the key (and its corresponding value) from this map. This method does nothing if the
   * key is not in the map.
   *
   * @param key the key that needs to be removed
   * @return the previous value associated with {@code key}, or {@code null} if there was no mapping
   *     for {@code key}
   */
  @Override
  public V remove(int key) {
    return replaceNode(key, null, null);
  }

  /**
   * Implementation for the four public remove/replace methods: Replaces node value with v,
   * conditional upon match of cv if non-null. If resulting value is null, delete.
   */
  V replaceNode(int key, V value, Object cv) {
    int hash = spread(key);
    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 {
        V oldVal = null;
        boolean validated = false;
        synchronized (f) {
          if (tabAt(tab, i) == f) {
            if (fh >= 0) {
              validated = true;
              for (Node<V> e = f, pred = null; ; ) {
                if ((e.key == key)) {
                  V ev = e.val;
                  if (cv == null || cv == ev || (ev != null && cv.equals(ev))) {
                    oldVal = ev;
                    if (value != null) {
                      e.val = value;
                    } else 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) {
                V pv = p.val;
                if (cv == null || cv == pv || (pv != null && cv.equals(pv))) {
                  oldVal = pv;
                  if (value != null) {
                    p.val = value;
                  } else if (t.removeTreeNode(p)) {
                    setTabAt(tab, i, untreeify(t.first));
                  }
                }
              }
            }
          }
        }
        if (validated) {
          if (oldVal != null) {
            if (value == null) {
              addCount(-1L, -1);
            }
            return oldVal;
          }
          break;
        }
      }
    }
    return null;
  }

  /** Removes all of the mappings from this map. */
  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);
    }
  }

  /**
   * Returns a {@link Collection} view of the values contained in this map. The collection is backed
   * by the map, so changes to the map are reflected in the collection, and vice-versa. The
   * collection supports element removal, which removes the corresponding mapping from this map, via
   * the {@code Iterator.remove}, {@code Collection.remove}, {@code removeAll}, {@code retainAll},
   * and {@code clear} operations. It does not support the {@code add} or {@code addAll} operations.
   *
   * <p>
   *
   * <p>The view's iterators and spliterators are <a href="package-summary.html#Weakly"><i>weakly
   * consistent</i></a>.
   *
   * <p>
   *
   * @return the collection view
   */
  @NonNull
  public Collection<V> values() {
    ValuesView<V> vs;
    return (vs = values) != null ? vs : (values = new ValuesView<>(this));
  }

  /**
   * Returns a {@link Set} view of the mappings contained in this map. The set is backed by the map,
   * so changes to the map are reflected in the set, and vice-versa. The set supports element
   * removal, which removes the corresponding mapping from the map, via the {@code Iterator.remove},
   * {@code Set.remove}, {@code removeAll}, {@code retainAll}, and {@code clear} operations.
   *
   * <p>
   *
   * <p>The view's iterators and spliterators are <a href="package-summary.html#Weakly"><i>weakly
   * consistent</i></a>.
   *
   * <p>
   *
   * @return the set view
   */
  @Override
  @NonNull
  public Set<Entry<V>> entrySet() {
    EntrySetView<V> es;
    return (es = entrySet) != null ? es : (entrySet = new EntrySetView<>(this));
  }

  /**
   * Returns the hash code value for this {@link Map}, i.e., the sum of, for each key-value pair in
   * the map, {@code key.hashCode() ^ value.hashCode()}.
   *
   * @return the hash code value for this map
   */
  @Override
  public int hashCode() {
    int h = 0;
    Node<V>[] t;
    if ((t = table) != null) {
      Traverser<V> it = new Traverser<>(t, t.length, 0, t.length);
      for (Node<V> p; (p = it.advance()) != null; ) {
        h += p.key ^ p.val.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 {}}"). Adjacent
   * mappings are separated by the characters {@code ", "} (comma and space). Each key-value mapping
   * is rendered as the key followed by an equals sign ("{@code =}") followed by the associated
   * value.
   *
   * @return a string representation of this map
   */
  @Override
  public String toString() {
    Node<V>[] t;
    int f = (t = table) == null ? 0 : t.length;
    Traverser<V> it = new Traverser<>(t, f, 0, f);
    StringBuilder sb = new StringBuilder();
    sb.append('{');
    Node<V> p;
    if ((p = it.advance()) != null) {
      for (; ; ) {
        int k = p.key;
        V v = p.val;
        sb.append(k);
        sb.append('=');
        sb.append(v == this ? "(this Map)" : v);
        if ((p = it.advance()) == null) {
          break;
        }
        sb.append(',').append(' ');
      }
    }
    return sb.append('}').toString();
  }

  /**
   * Compares the specified object with this map for equality. Returns {@code true} if the given
   * object is a map with the same mappings as this map. This operation may return misleading
   * results if either map is concurrently modified during execution of this method.
   *
   * @param o object to be compared for equality with this map
   * @return {@code true} if the specified object is equal to this map
   */
  @Override
  public boolean equals(Object o) {
    if (o != this) {
      if (!(o instanceof ConcurrentIntObjectMap)) {
        return false;
      }
      IntObjectMap<?> m = (IntObjectMap) o;
      Node<V>[] t;
      int f = (t = table) == null ? 0 : t.length;
      Traverser<V> it = new Traverser<>(t, f, 0, f);
      for (Node<V> p; (p = it.advance()) != null; ) {
        V val = p.val;
        Object v = m.get(p.key);
        if (v == null || (v != val && !v.equals(val))) {
          return false;
        }
      }
      for (Entry e : m.entrySet()) {
        int mk = e.getKey();
        Object mv;
        Object v;
        if ((mv = e.getValue()) == null || (v = get(mk)) == null || (mv != v && !mv.equals(v))) {
          return false;
        }
      }
    }
    return true;
  }

  // ConcurrentMap methods

  /**
   * {@inheritDoc}
   *
   * @return the previous value associated with the specified key, or {@code null} if there was no
   *     mapping for the key
   */
  @Override
  public V putIfAbsent(int key, @NonNull V value) {
    return putVal(key, value, true);
  }

  /** {@inheritDoc} */
  @Override
  public boolean remove(int key, @NonNull Object value) {
    return replaceNode(key, null, value) != null;
  }

  /** {@inheritDoc} */
  @Override
  public boolean replace(int key, @NonNull V oldValue, @NonNull V newValue) {
    return replaceNode(key, newValue, oldValue) != null;
  }

  /**
   * {@inheritDoc}
   *
   * @return the previous value associated with the specified key, or {@code null} if there was no
   *     mapping for the key
   */
  public V replace(int key, @NonNull V value) {
    return replaceNode(key, value, null);
  }

  // Overrides of JDK8+ Map extension method defaults

  /**
   * Returns the value to which the specified key is mapped, or the given default value if this map
   * contains no mapping for the key.
   *
   * @param key the key whose associated value is to be returned
   * @param defaultValue the value to return if this map contains no mapping for the given key
   * @return the mapping for the key, if present; else the default value
   */
  public V getOrDefault(int key, V defaultValue) {
    V v;
    return (v = get(key)) == null ? defaultValue : v;
  }

  // Hashtable legacy methods

  /**
   * Legacy method testing if some key maps into the specified value in this table. This method is
   * identical in functionality to {@link #containsValue(Object)}, and exists solely to ensure full
   * compatibility with class {@link Hashtable}, which supported this method prior to introduction
   * of the Java Collections framework.
   *
   * @param value a value to search for
   * @return {@code true} if and only if some key maps to the {@code value} argument in this table
   *     as determined by the {@code equals} method; {@code false} otherwise
   */
  public boolean contains(Object value) {
    return containsValue(value);
  }

  /**
   * Returns an enumeration of the keys in this table.
   *
   * @return an enumeration of the keys in this table
   */
  public int[] keys() {
    Object[] entries = new EntrySetView<>(this).toArray();
    int[] result = new int[entries.length];
    for (int i = 0; i < entries.length; i++) {
      Entry<V> entry = (Entry<V>) entries[i];
      result[i] = entry.getKey();
    }
    return result;
  }

  /**
   * Returns an enumeration of the values in this table.
   *
   * @return an enumeration of the values in this table
   * @see #values()
   */
  @NonNull
  @Override
  public Enumeration<V> elements() {
    Node<V>[] t;
    int f = (t = table) == null ? 0 : t.length;
    return new ValueIterator<>(t, f, 0, f, this);
  }

  // ConcurrentHashMap-only methods

  /**
   * Returns the number of mappings. This method should be used instead of {@link #size} because a
   * ConcurrentHashMap 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
   */
  public long mappingCount() {
    long n = sumCount();
    return Math.max(n, 0L); // ignore transient negative values
  }

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

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

    ForwardingNode(Node<V>[] tab) {
      super(MOVED, 0, null, null);
      nextTable = tab;
    }

    @Override
    Node<V> find(int h, int k) {
      // loop to avoid arbitrarily deep recursion on forwarding nodes
      outer:
      for (Node<V>[] tab = nextTable; ; ) {
        Node<V> e;
        int n;
        if (tab == null || (n = tab.length) == 0 || (e = tabAt(tab, (n - 1) & h)) == null) {
          return null;
        }
        for (; ; ) {
          if ((e.key == k)) {
            return e;
          }
          if (e.hash < 0) {
            if (e instanceof ForwardingNode) {
              tab = ((ForwardingNode<V>) e).nextTable;
              continue outer;
            } else {
              return e.find(h, k);
            }
          }
          if ((e = e.next) == null) {
            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 (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
        || !compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
      CounterCell a;
      long v;
      int m;
      boolean uncontended = true;
      if (as == null
          || (m = as.length - 1) < 0
          || (a = as[ThreadLocalRandom.getProbe() & m]) == null
          || !(uncontended = compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) {
        fullAddCount(x, 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 (compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
            transfer(tab, nt);
          }
        } else if (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 (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 = Math.max(sc, c);
        if (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 (sc < 0) {
          Node<V>[] nt;
          if ((sc >>> RESIZE_STAMP_SHIFT) != rs
              || sc == rs + 1
              || sc == rs + MAX_RESIZERS
              || (nt = nextTable) == null
              || transferIndex <= 0) {
            break;
          }
          if (compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
            transfer(tab, nt);
          }
        } else if (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<>(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 (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 (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;
                int pk = p.key;
                V pv = p.val;
                if ((ph & n) == 0) {
                  ln = new Node<>(ph, pk, pv, ln);
                } else {
                  hn = new Node<>(ph, pk, pv, 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<>(h, e.key, e.val, 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<>(lo) : t;
              hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) : (lc != 0) ? new TreeBin<>(hi) : t;
              setTabAt(nextTab, i, ln);
              setTabAt(nextTab, i + n, hn);
              setTabAt(tab, i, fwd);
              advance = true;
            }
          }
        }
      }
    }
  }

  /* ---------------- Counter support -------------- */

  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, boolean wasUncontended) {
    int h;
    if ((h = ThreadLocalRandom.getProbe()) == 0) {
      ThreadLocalRandom.localInit(); // force initialization
      h = ThreadLocalRandom.getProbe();
      wasUncontended = true;
    }
    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 && 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 (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 && 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 = ThreadLocalRandom.advanceProbe(h);
      } else if (cellsBusy == 0 && counterCells == as && 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 (compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x)) {
        break; // Fall back on using base
      }
    }
  }

  /* ---------------- 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<>(e.hash, e.key, e.val, null, null);
              if ((p.prev = tl) == null) {
                hd = p;
              } else {
                tl.next = p;
              }
              tl = p;
            }
            setTabAt(tab, index, new TreeBin<>(hd));
          }
        }
      }
    }
  }

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

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

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

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

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

    /** Returns the TreeNode (or null if not found) for the given key starting at given root. */
    TreeNode<V> findTreeNode(int h, int k) {
      TreeNode<V> p = this;
      do {
        int ph;
        TreeNode<V> q;
        TreeNode<V> pl = p.left;
        TreeNode<V> pr = p.right;
        if ((ph = p.hash) > h) {
          p = pl;
        } else if (ph < h) {
          p = pr;
        } else if (p.key == k) {
          return p;
        } else if (pl == null) {
          p = pr;
        } else if (pr == null) {
          p = pl;
        } else if ((q = pr.findTreeNode(h, k)) != 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 final class TreeBin<V> extends Node<V> {
    TreeNode<V> root;
    volatile TreeNode<V> 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

    /** Creates bin with initial set of nodes headed by b. */
    TreeBin(TreeNode<V> b) {
      super(TREEBIN, 0, null, null);
      first = b;
      TreeNode<V> r = null;
      for (TreeNode<V> x = b, next; x != null; x = next) {
        next = (TreeNode<V>) x.next;
        x.left = x.right = null;
        if (r == null) {
          x.parent = null;
          x.red = false;
          r = x;
        } else {
          int h = x.hash;
          for (TreeNode<V> p = r; ; ) {
            int dir, ph;
            if ((ph = p.hash) > h) {
              dir = -1;
            } else if (ph < h) {
              dir = 1;
            } else {
              dir = 0;
            }
            TreeNode<V> 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;
            }
          }
        }
      }
      root = r;
      assert checkInvariants(root);
    }

    /** Acquires write lock for tree restructuring. */
    private void lockRoot() {
      if (!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 (compareAndSwapInt(this, LOCKSTATE, s, WRITER)) {
            if (waiting) {
              waiter = null;
            }
            return;
          }
        } else if ((s & WAITER) == 0) {
          if (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<V> find(int h, int k) {
      for (Node<V> e = first; e != null; ) {
        int s;
        if (((s = lockState) & (WAITER | WRITER)) != 0) {
          if ((e.key == k)) {
            return e;
          }
          e = e.next;
        } else if (compareAndSwapInt(this, LOCKSTATE, s, s + READER)) {
          TreeNode<V> r;
          TreeNode<V> p;
          try {
            p = ((r = root) == null ? null : r.findTreeNode(h, k));
          } finally {
            Thread w;
            if (getAndAddInt(this, LOCKSTATE, -READER) == (READER | WAITER)
                && (w = waiter) != null) {
              LockSupport.unpark(w);
            }
          }
          return p;
        }
      }
      return null;
    }

    private static int getAndAddInt(Object object, long offset, int v) {
      try {
        return theUnsafe.getAndAddInt(object, offset, v);
      } catch (Throwable t) {
        throw new RuntimeException(t);
      }
    }

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

        TreeNode<V> xp = p;
        if ((p = (dir <= 0) ? p.left : p.right) == null) {
          TreeNode<V> x, f = first;
          first = x = new TreeNode<>(h, k, v, 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<V> p) {
      TreeNode<V> next = (TreeNode<V>) p.next;
      TreeNode<V> pred = p.prev; // unlink traversal pointers
      TreeNode<V> 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<V> replacement;
        TreeNode<V> pl = p.left;
        TreeNode<V> pr = p.right;
        if (pl != null && pr != null) {
          TreeNode<V> 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<V> sr = s.right;
          TreeNode<V> pp = p.parent;
          if (s == pr) { // p was s's direct parent
            p.parent = s;
            s.right = p;
          } else {
            TreeNode<V> sp = s.parent;
            if ((p.parent = sp) != null) {
              if (s == sp.left) {
                sp.left = p;
              } else {
                sp.right = p;
              }
            }
            if ((s.right = pr) != null) {
              pr.parent = s;
            }
          }
          p.left = null;
          if ((p.right = sr) != null) {
            sr.parent = p;
          }
          if ((s.left = pl) != null) {
            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<V> 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<V> 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 <V> TreeNode<V> rotateLeft(TreeNode<V> root, TreeNode<V> p) {
      TreeNode<V> 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 <V> TreeNode<V> rotateRight(TreeNode<V> root, TreeNode<V> p) {
      TreeNode<V> 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 <V> TreeNode<V> balanceInsertion(TreeNode<V> root, TreeNode<V> x) {
      x.red = true;
      for (TreeNode<V> 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 <V> TreeNode<V> balanceDeletion(TreeNode<V> root, TreeNode<V> x) {
      for (TreeNode<V> 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<V> 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<V> 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 <V> boolean checkInvariants(TreeNode<V> t) {
      TreeNode<V> tp = t.parent, tl = t.left, tr = t.right, tb = t.prev, tn = (TreeNode<V>) 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;
    }

    private static final long LOCKSTATE;

    static {
      try {
        Unsafe unsafe = UnsafeUtil.findUnsafe();
        Class<?> k = TreeBin.class;
        LOCKSTATE = unsafe.objectFieldOffset(k.getDeclaredField("lockState"));
      } catch (Throwable t) {
        throw new Error(t);
      }
    }
  }

  /* ----------------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 final class TableStack<V> {
    int length;
    int index;
    Node<V>[] tab;
    TableStack<V> next;
  }

  /**
   * Encapsulates traversal for methods such as containsValue; also serves as a base class for other
   * iterators and spliterators.
   *
   * <p>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.
   *
   * <p>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<V> {
    Node<V>[] tab; // current table; updated if resized
    Node<V> next; // the next entry to use
    TableStack<V> stack;
    TableStack<V> 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<V>[] tab, int size, int index, int limit) {
      this.tab = tab;
      baseSize = size;
      baseIndex = this.index = index;
      baseLimit = limit;
      next = null;
    }

    /** Advances if possible, returning next valid node, or null if none. */
    final Node<V> advance() {
      Node<V> e;
      if ((e = next) != null) {
        e = e.next;
      }
      for (; ; ) {
        Node<V>[] t;
        int i; // must use locals in checks
        int n;
        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<V>) e).nextTable;
            e = null;
            pushState(t, i, n);
            continue;
          } else if (e instanceof TreeBin) {
            e = ((TreeBin<V>) 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<V>[] t, int i, int n) {
      TableStack<V> s = spare; // reuse if possible
      if (s != null) {
        spare = s.next;
      } else {
        s = new TableStack<>();
      }
      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<V> s;
      int len;
      while ((s = stack) != null && (index += (len = s.length)) >= n) {
        n = len;
        index = s.index;
        tab = s.tab;
        s.tab = null;
        TableStack<V> 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<V> extends Traverser<V> {
    final ConcurrentIntObjectHashMap<V> map;
    Node<V> lastReturned;

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

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

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

    public final void remove() {
      Node<V> p;
      if ((p = lastReturned) == null) {
        throw new IllegalStateException();
      }
      lastReturned = null;
      // IJ patched:
      // see https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8078645
      map.replaceNode(p.key, null, p.val);
    }
  }

  static final class ValueIterator<V> extends BaseIterator<V>
      implements Iterator<V>, Enumeration<V> {
    ValueIterator(
        Node<V>[] tab, int index, int size, int limit, ConcurrentIntObjectHashMap<V> map) {
      super(tab, index, size, limit, map);
    }

    @Override
    public V next() {
      Node<V> p;
      if ((p = next) == null) {
        throw new NoSuchElementException();
      }
      V v = p.val;
      lastReturned = p;
      advance();
      return v;
    }

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

  static final class EntryIterator<V> extends BaseIterator<V> implements Iterator<Entry<V>> {
    EntryIterator(
        Node<V>[] tab, int index, int size, int limit, ConcurrentIntObjectHashMap<V> map) {
      super(tab, index, size, limit, map);
    }

    @Override
    public Entry<V> next() {
      Node<V> p;
      if ((p = next) == null) {
        throw new NoSuchElementException();
      }
      final int k = p.key;
      final V v = p.val;
      lastReturned = p;
      advance();
      return new SimpleEntry<>(k, v);
    }
  }

  // Parallel bulk operations

  /* ----------------Views -------------- */

  /** Base class for views. */
  abstract static class CollectionView<V, E> implements Collection<E> {
    final ConcurrentIntObjectHashMap<V> map;

    CollectionView(ConcurrentIntObjectHashMap<V> map) {
      this.map = map;
    }

    /**
     * Returns the map backing this view.
     *
     * @return the map backing this view
     */
    public ConcurrentIntObjectHashMap<V> getMap() {
      return map;
    }

    /**
     * Removes all of the elements from this view, by removing all the mappings from the map backing
     * this view.
     */
    @Override
    public final void clear() {
      map.clear();
    }

    @Override
    public final int size() {
      return map.size();
    }

    @Override
    public final boolean isEmpty() {
      return map.isEmpty();
    }

    // implementations below rely on concrete classes supplying these
    // abstract methods

    /**
     * Returns an iterator over the elements in this collection.
     *
     * <p>
     *
     * <p>The returned iterator is <a href="package-summary.html#Weakly"><i>weakly
     * consistent</i></a>.
     *
     * @return an iterator over the elements in this collection
     */
    @NonNull
    @Override
    public abstract Iterator<E> iterator();

    @Override
    public abstract boolean contains(Object o);

    @Override
    public abstract boolean remove(Object o);

    private static final String oomeMsg = "Required array size too large";

    @Override
    public final Object[] toArray() {
      long sz = map.mappingCount();
      if (sz > MAX_ARRAY_SIZE) {
        throw new OutOfMemoryError(oomeMsg);
      }
      int n = (int) sz;
      Object[] r = new Object[n];
      int i = 0;
      for (E e : this) {
        if (i == n) {
          if (n >= MAX_ARRAY_SIZE) {
            throw new OutOfMemoryError(oomeMsg);
          }
          if (n >= MAX_ARRAY_SIZE - (MAX_ARRAY_SIZE >>> 1) - 1) {
            n = MAX_ARRAY_SIZE;
          } else {
            n += (n >>> 1) + 1;
          }
          r = Arrays.copyOf(r, n);
        }
        r[i++] = e;
      }
      return (i == n) ? r : Arrays.copyOf(r, i);
    }

    @Override
    @SuppressWarnings("unchecked")
    public final <T> T[] toArray(@NonNull T[] a) {
      long sz = map.mappingCount();
      if (sz > MAX_ARRAY_SIZE) {
        throw new OutOfMemoryError(oomeMsg);
      }
      int m = (int) sz;
      T[] r =
          (a.length >= m)
              ? a
              : (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), m);
      int n = r.length;
      int i = 0;
      for (E e : this) {
        if (i == n) {
          if (n >= MAX_ARRAY_SIZE) {
            throw new OutOfMemoryError(oomeMsg);
          }
          if (n >= MAX_ARRAY_SIZE - (MAX_ARRAY_SIZE >>> 1) - 1) {
            n = MAX_ARRAY_SIZE;
          } else {
            n += (n >>> 1) + 1;
          }
          r = Arrays.copyOf(r, n);
        }
        r[i++] = (T) e;
      }
      if (a == r && i < n) {
        r[i] = null; // null-terminate
        return r;
      }
      return (i == n) ? r : Arrays.copyOf(r, i);
    }

    /**
     * Returns a string representation of this collection. The string representation consists of the
     * string representations of the collection's elements in the order they are returned by its
     * iterator, enclosed in square brackets ({@code "[]"}). Adjacent elements are separated by the
     * characters {@code ", "} (comma and space). Elements are converted to strings as by {@link
     * String#valueOf(Object)}.
     *
     * @return a string representation of this collection
     */
    @Override
    public final String toString() {
      StringBuilder sb = new StringBuilder();
      sb.append('[');
      Iterator<E> it = iterator();
      if (it.hasNext()) {
        for (; ; ) {
          Object e = it.next();
          sb.append(e == this ? "(this Collection)" : e);
          if (!it.hasNext()) {
            break;
          }
          sb.append(',').append(' ');
        }
      }
      return sb.append(']').toString();
    }

    @Override
    public final boolean containsAll(@NonNull Collection<?> c) {
      if (c != this) {
        for (Object e : c) {
          if (e == null || !contains(e)) {
            return false;
          }
        }
      }
      return true;
    }

    @Override
    public final boolean removeAll(@NonNull Collection<?> c) {
      boolean modified = false;
      for (Iterator<E> it = iterator(); it.hasNext(); ) {
        if (c.contains(it.next())) {
          it.remove();
          modified = true;
        }
      }
      return modified;
    }

    @Override
    public final boolean retainAll(@NonNull Collection<?> c) {
      boolean modified = false;
      for (Iterator<E> it = iterator(); it.hasNext(); ) {
        if (!c.contains(it.next())) {
          it.remove();
          modified = true;
        }
      }
      return modified;
    }
  }

  /**
   * A view of a ConcurrentHashMap as a {@link Collection} of values, in which additions are
   * disabled. This class cannot be directly instantiated. See {@link #values()}.
   */
  static final class ValuesView<V> extends CollectionView<V, V> implements Collection<V> {

    ValuesView(ConcurrentIntObjectHashMap<V> map) {
      super(map);
    }

    @Override
    public boolean contains(Object o) {
      return map.containsValue(o);
    }

    @Override
    public boolean remove(Object o) {
      if (o != null) {
        for (Iterator<V> it = iterator(); it.hasNext(); ) {
          if (o.equals(it.next())) {
            it.remove();
            return true;
          }
        }
      }
      return false;
    }

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

    @Override
    public boolean add(V e) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean addAll(@NonNull Collection<? extends V> c) {
      throw new UnsupportedOperationException();
    }
  }

  /**
   * A view of a ConcurrentHashMap as a {@link Set} of (key, value) entries. This class cannot be
   * directly instantiated. See {@link #entrySet()}.
   */
  static final class EntrySetView<V> extends CollectionView<V, Entry<V>> implements Set<Entry<V>> {

    EntrySetView(ConcurrentIntObjectHashMap<V> map) {
      super(map);
    }

    @Override
    public boolean contains(Object o) {
      Object v;
      Object r;
      Entry<?> e;
      return ((o instanceof IntObjectMap.Entry)
          && (r = map.get((e = (Entry) o).getKey())) != null
          && (v = e.getValue()) != null
          && (v == r || v.equals(r)));
    }

    @Override
    public boolean remove(Object o) {
      Object v;
      Entry<?> e;
      return ((o instanceof Map.Entry)
          && (e = (Entry<?>) o) != null
          && (v = e.getValue()) != null
          && map.remove(e.getKey(), v));
    }

    /**
     * @return an iterator over the entries of the backing map
     */
    @NonNull
    @Override
    public Iterator<Entry<V>> iterator() {
      ConcurrentIntObjectHashMap<V> m = map;
      Node<V>[] t;
      int f = (t = m.table) == null ? 0 : t.length;
      return new EntryIterator<>(t, f, 0, f, m);
    }

    @Override
    public boolean add(Entry<V> e) {
      return map.putVal(e.getKey(), e.getValue(), false) == null;
    }

    @Override
    public boolean addAll(Collection<? extends Entry<V>> c) {
      boolean added = false;
      for (Entry<V> e : c) {
        if (add(e)) {
          added = true;
        }
      }
      return added;
    }

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

    @Override
    public boolean equals(Object o) {
      Set<?> c;
      return ((o instanceof Set)
          && ((c = (Set<?>) o) == this || (containsAll(c) && c.containsAll(this))));
    }
  }

  // -------------------------------------------------------

  // Unsafe mechanics
  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;

  private static Unsafe theUnsafe;

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

  private static boolean compareAndSwapInt(Object object, long offset, int expected, int value) {
    try {
      return theUnsafe.compareAndSwapInt(object, offset, expected, value);
    } catch (Throwable throwable) {
      throw new RuntimeException(throwable);
    }
  }

  private static boolean compareAndSwapLong(Object object, long offset, long expected, long value) {
    try {
      return theUnsafe.compareAndSwapLong(object, offset, expected, value);
    } catch (Throwable throwable) {
      throw new RuntimeException(throwable);
    }
  }

  /**
   * @return value if there is no entry in the map, or corresponding value if entry already exists
   */
  @Override
  @NonNull
  public V cacheOrGet(final int key, @NonNull final V defaultValue) {
    V v = get(key);
    if (v != null) return v;
    V prev = putIfAbsent(key, defaultValue);
    return prev == null ? defaultValue : prev;
  }
}
