/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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/
 */

package java.util.concurrent;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NavigableSet;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.SortedMap;
import java.util.Spliterator;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.concurrent.atomic.LongAdder;

A scalable concurrent ConcurrentNavigableMap implementation. The map is sorted according to the natural
ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used. 
This class implements a concurrent variant of SkipLists
providing expected average log(n) time cost for the containsKey, get, put and remove operations and their variants. Insertion, removal, update, and access operations safely execute concurrently by multiple threads. 
Iterators and spliterators are
weakly consistent.
Ascending key ordered views and their iterators are faster than
descending ones.
All Map.Entry pairs returned by methods in this class and its views represent snapshots of mappings at the time they were produced. They do not support the Entry.setValue method. (Note however that it is possible to change mappings in the associated map using put, putIfAbsent, or replace, depending on exactly which effect you need.) 
Beware that bulk operations putAll, equals, toArray, containsValue, and clear are not guaranteed to be performed atomically. For example, an iterator operating concurrently with a putAll operation might view only some of the added elements. 
This class and its views and iterators implement all of the
optional methods of the Map and Iterator interfaces. Like most other concurrent collections, this class does not permit the use of null keys or values because some null return values cannot be reliably distinguished from the absence of elements. 
This class is a member of the

Java Collections Framework.
Author:
Doug Lea
Type parameters:
<K> – the type of keys maintained by this map
<V> – the type of mapped values
Since:
1.6/**
 * A scalable concurrent {@link ConcurrentNavigableMap} implementation.
 * The map is sorted according to the {@linkplain Comparable natural
 * ordering} of its keys, or by a {@link Comparator} provided at map
 * creation time, depending on which constructor is used.
 *
 * <p>This class implements a concurrent variant of <a
 * href="http://en.wikipedia.org/wiki/Skip_list" target="_top">SkipLists</a>
 * providing expected average <i>log(n)</i> time cost for the
 * {@code containsKey}, {@code get}, {@code put} and
 * {@code remove} operations and their variants.  Insertion, removal,
 * update, and access operations safely execute concurrently by
 * multiple threads.
 *
 * <p>Iterators and spliterators are
 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
 *
 * <p>Ascending key ordered views and their iterators are faster than
 * descending ones.
 *
 * <p>All {@code Map.Entry} pairs returned by methods in this class
 * and its views represent snapshots of mappings at the time they were
 * produced. They do <em>not</em> support the {@code Entry.setValue}
 * method. (Note however that it is possible to change mappings in the
 * associated map using {@code put}, {@code putIfAbsent}, or
 * {@code replace}, depending on exactly which effect you need.)
 *
 * <p>Beware that bulk operations {@code putAll}, {@code equals},
 * {@code toArray}, {@code containsValue}, and {@code clear} are
 * <em>not</em> guaranteed to be performed atomically. For example, an
 * iterator operating concurrently with a {@code putAll} operation
 * might view only some of the added elements.
 *
 * <p>This class and its views and iterators implement all of the
 * <em>optional</em> methods of the {@link Map} and {@link Iterator}
 * interfaces. Like most other concurrent collections, this class does
 * <em>not</em> permit the use of {@code null} keys or values because some
 * null return values cannot be reliably distinguished from the absence of
 * elements.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @author Doug Lea
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 * @since 1.6
 */
public class ConcurrentSkipListMap<K,V> extends AbstractMap<K,V>
    implements ConcurrentNavigableMap<K,V>, Cloneable, Serializable {
    /*
     * This class implements a tree-like two-dimensionally linked skip
     * list in which the index levels are represented in separate
     * nodes from the base nodes holding data.  There are two reasons
     * for taking this approach instead of the usual array-based
     * structure: 1) Array based implementations seem to encounter
     * more complexity and overhead 2) We can use cheaper algorithms
     * for the heavily-traversed index lists than can be used for the
     * base lists.  Here's a picture of some of the basics for a
     * possible list with 2 levels of index:
     *
     * Head nodes          Index nodes
     * +-+    right        +-+                      +-+
     * |2|---------------->| |--------------------->| |->null
     * +-+                 +-+                      +-+
     *  | down              |                        |
     *  v                   v                        v
     * +-+            +-+  +-+       +-+            +-+       +-+
     * |1|----------->| |->| |------>| |----------->| |------>| |->null
     * +-+            +-+  +-+       +-+            +-+       +-+
     *  v              |    |         |              |         |
     * Nodes  next     v    v         v              v         v
     * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
     * | |->|A|->|B|->|C|->|D|->|E|->|F|->|G|->|H|->|I|->|J|->|K|->null
     * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
     *
     * The base lists use a variant of the HM linked ordered set
     * algorithm. See Tim Harris, "A pragmatic implementation of
     * non-blocking linked lists"
     * http://www.cl.cam.ac.uk/~tlh20/publications.html and Maged
     * Michael "High Performance Dynamic Lock-Free Hash Tables and
     * List-Based Sets"
     * http://www.research.ibm.com/people/m/michael/pubs.htm.  The
     * basic idea in these lists is to mark the "next" pointers of
     * deleted nodes when deleting to avoid conflicts with concurrent
     * insertions, and when traversing to keep track of triples
     * (predecessor, node, successor) in order to detect when and how
     * to unlink these deleted nodes.
     *
     * Rather than using mark-bits to mark list deletions (which can
     * be slow and space-intensive using AtomicMarkedReference), nodes
     * use direct CAS'able next pointers.  On deletion, instead of
     * marking a pointer, they splice in another node that can be
     * thought of as standing for a marked pointer (see method
     * unlinkNode).  Using plain nodes acts roughly like "boxed"
     * implementations of marked pointers, but uses new nodes only
     * when nodes are deleted, not for every link.  This requires less
     * space and supports faster traversal. Even if marked references
     * were better supported by JVMs, traversal using this technique
     * might still be faster because any search need only read ahead
     * one more node than otherwise required (to check for trailing
     * marker) rather than unmasking mark bits or whatever on each
     * read.
     *
     * This approach maintains the essential property needed in the HM
     * algorithm of changing the next-pointer of a deleted node so
     * that any other CAS of it will fail, but implements the idea by
     * changing the pointer to point to a different node (with
     * otherwise illegal null fields), not by marking it.  While it
     * would be possible to further squeeze space by defining marker
     * nodes not to have key/value fields, it isn't worth the extra
     * type-testing overhead.  The deletion markers are rarely
     * encountered during traversal, are easily detected via null
     * checks that are needed anyway, and are normally quickly garbage
     * collected. (Note that this technique would not work well in
     * systems without garbage collection.)
     *
     * In addition to using deletion markers, the lists also use
     * nullness of value fields to indicate deletion, in a style
     * similar to typical lazy-deletion schemes.  If a node's value is
     * null, then it is considered logically deleted and ignored even
     * though it is still reachable.
     *
     * Here's the sequence of events for a deletion of node n with
     * predecessor b and successor f, initially:
     *
     *        +------+       +------+      +------+
     *   ...  |   b  |------>|   n  |----->|   f  | ...
     *        +------+       +------+      +------+
     *
     * 1. CAS n's value field from non-null to null.
     *    Traversals encountering a node with null value ignore it.
     *    However, ongoing insertions and deletions might still modify
     *    n's next pointer.
     *
     * 2. CAS n's next pointer to point to a new marker node.
     *    From this point on, no other nodes can be appended to n.
     *    which avoids deletion errors in CAS-based linked lists.
     *
     *        +------+       +------+      +------+       +------+
     *   ...  |   b  |------>|   n  |----->|marker|------>|   f  | ...
     *        +------+       +------+      +------+       +------+
     *
     * 3. CAS b's next pointer over both n and its marker.
     *    From this point on, no new traversals will encounter n,
     *    and it can eventually be GCed.
     *        +------+                                    +------+
     *   ...  |   b  |----------------------------------->|   f  | ...
     *        +------+                                    +------+
     *
     * A failure at step 1 leads to simple retry due to a lost race
     * with another operation. Steps 2-3 can fail because some other
     * thread noticed during a traversal a node with null value and
     * helped out by marking and/or unlinking.  This helping-out
     * ensures that no thread can become stuck waiting for progress of
     * the deleting thread.
     *
     * Skip lists add indexing to this scheme, so that the base-level
     * traversals start close to the locations being found, inserted
     * or deleted -- usually base level traversals only traverse a few
     * nodes. This doesn't change the basic algorithm except for the
     * need to make sure base traversals start at predecessors (here,
     * b) that are not (structurally) deleted, otherwise retrying
     * after processing the deletion.
     *
     * Index levels are maintained using CAS to link and unlink
     * successors ("right" fields).  Races are allowed in index-list
     * operations that can (rarely) fail to link in a new index node.
     * (We can't do this of course for data nodes.)  However, even
     * when this happens, the index lists correctly guide search.
     * This can impact performance, but since skip lists are
     * probabilistic anyway, the net result is that under contention,
     * the effective "p" value may be lower than its nominal value.
     *
     * Index insertion and deletion sometimes require a separate
     * traversal pass occurring after the base-level action, to add or
     * remove index nodes.  This adds to single-threaded overhead, but
     * improves contended multithreaded performance by narrowing
     * interference windows, and allows deletion to ensure that all
     * index nodes will be made unreachable upon return from a public
     * remove operation, thus avoiding unwanted garbage retention.
     *
     * Indexing uses skip list parameters that maintain good search
     * performance while using sparser-than-usual indices: The
     * hardwired parameters k=1, p=0.5 (see method doPut) mean that
     * about one-quarter of the nodes have indices. Of those that do,
     * half have one level, a quarter have two, and so on (see Pugh's
     * Skip List Cookbook, sec 3.4), up to a maximum of 62 levels
     * (appropriate for up to 2^63 elements).  The expected total
     * space requirement for a map is slightly less than for the
     * current implementation of java.util.TreeMap.
     *
     * Changing the level of the index (i.e, the height of the
     * tree-like structure) also uses CAS.  Creation of an index with
     * height greater than the current level adds a level to the head
     * index by CAS'ing on a new top-most head. To maintain good
     * performance after a lot of removals, deletion methods
     * heuristically try to reduce the height if the topmost levels
     * appear to be empty.  This may encounter races in which it is
     * possible (but rare) to reduce and "lose" a level just as it is
     * about to contain an index (that will then never be
     * encountered). This does no structural harm, and in practice
     * appears to be a better option than allowing unrestrained growth
     * of levels.
     *
     * This class provides concurrent-reader-style memory consistency,
     * ensuring that read-only methods report status and/or values no
     * staler than those holding at method entry. This is done by
     * performing all publication and structural updates using
     * (volatile) CAS, placing an acquireFence in a few access
     * methods, and ensuring that linked objects are transitively
     * acquired via dependent reads (normally once) unless performing
     * a volatile-mode CAS operation (that also acts as an acquire and
     * release).  This form of fence-hoisting is similar to RCU and
     * related techniques (see McKenney's online book
     * https://www.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.html)
     * It minimizes overhead that may otherwise occur when using so
     * many volatile-mode reads. Using explicit acquireFences is
     * logistically easier than targeting particular fields to be read
     * in acquire mode: fences are just hoisted up as far as possible,
     * to the entry points or loop headers of a few methods. A
     * potential disadvantage is that these few remaining fences are
     * not easily optimized away by compilers under exclusively
     * single-thread use.  It requires some care to avoid volatile
     * mode reads of other fields. (Note that the memory semantics of
     * a reference dependently read in plain mode exactly once are
     * equivalent to those for atomic opaque mode.)  Iterators and
     * other traversals encounter each node and value exactly once.
     * Other operations locate an element (or position to insert an
     * element) via a sequence of dereferences. This search is broken
     * into two parts. Method findPredecessor (and its specialized
     * embeddings) searches index nodes only, returning a base-level
     * predecessor of the key. Callers carry out the base-level
     * search, restarting if encountering a marker preventing link
     * modification.  In some cases, it is possible to encounter a
     * node multiple times while descending levels. For mutative
     * operations, the reported value is validated using CAS (else
     * retrying), preserving linearizability with respect to each
     * other. Others may return any (non-null) value holding in the
     * course of the method call.  (Search-based methods also include
     * some useless-looking explicit null checks designed to allow
     * more fields to be nulled out upon removal, to reduce floating
     * garbage, but which is not currently done, pending discovery of
     * a way to do this with less impact on other operations.)
     *
     * To produce random values without interference across threads,
     * we use within-JDK thread local random support (via the
     * "secondary seed", to avoid interference with user-level
     * ThreadLocalRandom.)
     *
     * For explanation of algorithms sharing at least a couple of
     * features with this one, see Mikhail Fomitchev's thesis
     * (http://www.cs.yorku.ca/~mikhail/), Keir Fraser's thesis
     * (http://www.cl.cam.ac.uk/users/kaf24/), and Hakan Sundell's
     * thesis (http://www.cs.chalmers.se/~phs/).
     *
     * Notation guide for local variables
     * Node:         b, n, f, p for  predecessor, node, successor, aux
     * Index:        q, r, d    for index node, right, down.
     * Head:         h
     * Keys:         k, key
     * Values:       v, value
     * Comparisons:  c
     */

    private static final long serialVersionUID = -8627078645895051609L;

    
The comparator used to maintain order in this map, or null if
using natural ordering.  (Non-private to simplify access in
nested classes.)
@serial
/**
     * The comparator used to maintain order in this map, or null if
     * using natural ordering.  (Non-private to simplify access in
     * nested classes.)
     * @serial
     */
    final Comparator<? super K> comparator;

    
Lazily initialized topmost index of the skiplist. /** Lazily initialized topmost index of the skiplist. */
    private transient Index<K,V> head;
    
Lazily initialized element count /** Lazily initialized element count */
    private transient LongAdder adder;
    
Lazily initialized key set /** Lazily initialized key set */
    private transient KeySet<K,V> keySet;
    
Lazily initialized values collection /** Lazily initialized values collection */
    private transient Values<K,V> values;
    
Lazily initialized entry set /** Lazily initialized entry set */
    private transient EntrySet<K,V> entrySet;
    
Lazily initialized descending map /** Lazily initialized descending map */
    private transient SubMap<K,V> descendingMap;

    
Nodes hold keys and values, and are singly linked in sorted
order, possibly with some intervening marker nodes. The list is
headed by a header node accessible as head.node. Headers and
marker nodes have null keys. The val field (but currently not
the key field) is nulled out upon deletion.
/**
     * Nodes hold keys and values, and are singly linked in sorted
     * order, possibly with some intervening marker nodes. The list is
     * headed by a header node accessible as head.node. Headers and
     * marker nodes have null keys. The val field (but currently not
     * the key field) is nulled out upon deletion.
     */
    static final class Node<K,V> {
        final K key; // currently, never detached
        V val;
        Node<K,V> next;
        Node(K key, V value, Node<K,V> next) {
            this.key = key;
            this.val = value;
            this.next = next;
        }
    }

    
Index nodes represent the levels of the skip list.
/**
     * Index nodes represent the levels of the skip list.
     */
    static final class Index<K,V> {
        final Node<K,V> node;  // currently, never detached
        final Index<K,V> down;
        Index<K,V> right;
        Index(Node<K,V> node, Index<K,V> down, Index<K,V> right) {
            this.node = node;
            this.down = down;
            this.right = right;
        }
    }

    /* ----------------  Utilities -------------- */

    
Compares using comparator or natural ordering if null.
Called only by methods that have performed required type checks.
/**
     * Compares using comparator or natural ordering if null.
     * Called only by methods that have performed required type checks.
     */
    @SuppressWarnings({"unchecked", "rawtypes"})
    static int cpr(Comparator c, Object x, Object y) {
        return (c != null) ? c.compare(x, y) : ((Comparable)x).compareTo(y);
    }

    
Returns the header for base node list, or null if uninitialized
/**
     * Returns the header for base node list, or null if uninitialized
     */
    final Node<K,V> baseHead() {
        Index<K,V> h;
        VarHandle.acquireFence();
        return ((h = head) == null) ? null : h.node;
    }

    
Tries to unlink deleted node n from predecessor b (if both
exist), by first splicing in a marker if not already present.
Upon return, node n is sure to be unlinked from b, possibly
via the actions of some other thread.
Params:
b – if nonnull, predecessor
n – if nonnull, node known to be deleted/**
     * Tries to unlink deleted node n from predecessor b (if both
     * exist), by first splicing in a marker if not already present.
     * Upon return, node n is sure to be unlinked from b, possibly
     * via the actions of some other thread.
     *
     * @param b if nonnull, predecessor
     * @param n if nonnull, node known to be deleted
     */
    static <K,V> void unlinkNode(Node<K,V> b, Node<K,V> n) {
        if (b != null && n != null) {
            Node<K,V> f, p;
            for (;;) {
                if ((f = n.next) != null && f.key == null) {
                    p = f.next;               // already marked
                    break;
                }
                else if (NEXT.compareAndSet(n, f,
                                            new Node<K,V>(null, null, f))) {
                    p = f;                    // add marker
                    break;
                }
            }
            NEXT.compareAndSet(b, n, p);
        }
    }

    
Adds to element count, initializing adder if necessary
Params:
c – count to add/**
     * Adds to element count, initializing adder if necessary
     *
     * @param c count to add
     */
    private void addCount(long c) {
        LongAdder a;
        do {} while ((a = adder) == null &&
                     !ADDER.compareAndSet(this, null, a = new LongAdder()));
        a.add(c);
    }

    
Returns element count, initializing adder if necessary.
/**
     * Returns element count, initializing adder if necessary.
     */
    final long getAdderCount() {
        LongAdder a; long c;
        do {} while ((a = adder) == null &&
                     !ADDER.compareAndSet(this, null, a = new LongAdder()));
        return ((c = a.sum()) <= 0L) ? 0L : c; // ignore transient negatives
    }

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

    
Returns an index node with key strictly less than given key.
Also unlinks indexes to deleted nodes found along the way.
Callers rely on this side-effect of clearing indices to deleted
nodes.
Params:
key – if nonnull the key
Returns:
a predecessor node of key, or null if uninitialized or null key/**
     * Returns an index node with key strictly less than given key.
     * Also unlinks indexes to deleted nodes found along the way.
     * Callers rely on this side-effect of clearing indices to deleted
     * nodes.
     *
     * @param key if nonnull the key
     * @return a predecessor node of key, or null if uninitialized or null key
     */
    private Node<K,V> findPredecessor(Object key, Comparator<? super K> cmp) {
        Index<K,V> q;
        VarHandle.acquireFence();
        if ((q = head) == null || key == null)
            return null;
        else {
            for (Index<K,V> r, d;;) {
                while ((r = q.right) != null) {
                    Node<K,V> p; K k;
                    if ((p = r.node) == null || (k = p.key) == null ||
                        p.val == null)  // unlink index to deleted node
                        RIGHT.compareAndSet(q, r, r.right);
                    else if (cpr(cmp, key, k) > 0)
                        q = r;
                    else
                        break;
                }
                if ((d = q.down) != null)
                    q = d;
                else
                    return q.node;
            }
        }
    }

    
Returns node holding key or null if no such, clearing out any
deleted nodes seen along the way.  Repeatedly traverses at
base-level looking for key starting at predecessor returned
from findPredecessor, processing base-level deletions as
encountered. Restarts occur, at traversal step encountering
node n, if n's key field is null, indicating it is a marker, so
its predecessor is deleted before continuing, which we help do
by re-finding a valid predecessor.  The traversal loops in
doPut, doRemove, and findNear all include the same checks.
Params:
key – the key
Returns:
node holding key, or null if no such/**
     * Returns node holding key or null if no such, clearing out any
     * deleted nodes seen along the way.  Repeatedly traverses at
     * base-level looking for key starting at predecessor returned
     * from findPredecessor, processing base-level deletions as
     * encountered. Restarts occur, at traversal step encountering
     * node n, if n's key field is null, indicating it is a marker, so
     * its predecessor is deleted before continuing, which we help do
     * by re-finding a valid predecessor.  The traversal loops in
     * doPut, doRemove, and findNear all include the same checks.
     *
     * @param key the key
     * @return node holding key, or null if no such
     */
    private Node<K,V> findNode(Object key) {
        if (key == null)
            throw new NullPointerException(); // don't postpone errors
        Comparator<? super K> cmp = comparator;
        Node<K,V> b;
        outer: while ((b = findPredecessor(key, cmp)) != null) {
            for (;;) {
                Node<K,V> n; K k; V v; int c;
                if ((n = b.next) == null)
                    break outer;               // empty
                else if ((k = n.key) == null)
                    break;                     // b is deleted
                else if ((v = n.val) == null)
                    unlinkNode(b, n);          // n is deleted
                else if ((c = cpr(cmp, key, k)) > 0)
                    b = n;
                else if (c == 0)
                    return n;
                else
                    break outer;
            }
        }
        return null;
    }

    
Gets value for key. Same idea as findNode, except skips over
deletions and markers, and returns first encountered value to
avoid possibly inconsistent rereads.
Params:
key – the key
Returns:
the value, or null if absent/**
     * Gets value for key. Same idea as findNode, except skips over
     * deletions and markers, and returns first encountered value to
     * avoid possibly inconsistent rereads.
     *
     * @param key the key
     * @return the value, or null if absent
     */
    private V doGet(Object key) {
        Index<K,V> q;
        VarHandle.acquireFence();
        if (key == null)
            throw new NullPointerException();
        Comparator<? super K> cmp = comparator;
        V result = null;
        if ((q = head) != null) {
            outer: for (Index<K,V> r, d;;) {
                while ((r = q.right) != null) {
                    Node<K,V> p; K k; V v; int c;
                    if ((p = r.node) == null || (k = p.key) == null ||
                        (v = p.val) == null)
                        RIGHT.compareAndSet(q, r, r.right);
                    else if ((c = cpr(cmp, key, k)) > 0)
                        q = r;
                    else if (c == 0) {
                        result = v;
                        break outer;
                    }
                    else
                        break;
                }
                if ((d = q.down) != null)
                    q = d;
                else {
                    Node<K,V> b, n;
                    if ((b = q.node) != null) {
                        while ((n = b.next) != null) {
                            V v; int c;
                            K k = n.key;
                            if ((v = n.val) == null || k == null ||
                                (c = cpr(cmp, key, k)) > 0)
                                b = n;
                            else {
                                if (c == 0)
                                    result = v;
                                break;
                            }
                        }
                    }
                    break;
                }
            }
        }
        return result;
    }

    /* ---------------- Insertion -------------- */

    
Main insertion method.  Adds element if not present, or
replaces value if present and onlyIfAbsent is false.
Params:
key – the key
value – the value that must be associated with key
onlyIfAbsent – if should not insert if already present
Returns:
the old value, or null if newly inserted/**
     * Main insertion method.  Adds element if not present, or
     * replaces value if present and onlyIfAbsent is false.
     *
     * @param key the key
     * @param value the value that must be associated with key
     * @param onlyIfAbsent if should not insert if already present
     * @return the old value, or null if newly inserted
     */
    private V doPut(K key, V value, boolean onlyIfAbsent) {
        if (key == null)
            throw new NullPointerException();
        Comparator<? super K> cmp = comparator;
        for (;;) {
            Index<K,V> h; Node<K,V> b;
            VarHandle.acquireFence();
            int levels = 0;                    // number of levels descended
            if ((h = head) == null) {          // try to initialize
                Node<K,V> base = new Node<K,V>(null, null, null);
                h = new Index<K,V>(base, null, null);
                b = (HEAD.compareAndSet(this, null, h)) ? base : null;
            }
            else {
                for (Index<K,V> q = h, r, d;;) { // count while descending
                    while ((r = q.right) != null) {
                        Node<K,V> p; K k;
                        if ((p = r.node) == null || (k = p.key) == null ||
                            p.val == null)
                            RIGHT.compareAndSet(q, r, r.right);
                        else if (cpr(cmp, key, k) > 0)
                            q = r;
                        else
                            break;
                    }
                    if ((d = q.down) != null) {
                        ++levels;
                        q = d;
                    }
                    else {
                        b = q.node;
                        break;
                    }
                }
            }
            if (b != null) {
                Node<K,V> z = null;              // new node, if inserted
                for (;;) {                       // find insertion point
                    Node<K,V> n, p; K k; V v; int c;
                    if ((n = b.next) == null) {
                        if (b.key == null)       // if empty, type check key now
                            cpr(cmp, key, key);
                        c = -1;
                    }
                    else if ((k = n.key) == null)
                        break;                   // can't append; restart
                    else if ((v = n.val) == null) {
                        unlinkNode(b, n);
                        c = 1;
                    }
                    else if ((c = cpr(cmp, key, k)) > 0)
                        b = n;
                    else if (c == 0 &&
                             (onlyIfAbsent || VAL.compareAndSet(n, v, value)))
                        return v;

                    if (c < 0 &&
                        NEXT.compareAndSet(b, n,
                                           p = new Node<K,V>(key, value, n))) {
                        z = p;
                        break;
                    }
                }

                if (z != null) {
                    int lr = ThreadLocalRandom.nextSecondarySeed();
                    if ((lr & 0x3) == 0) {       // add indices with 1/4 prob
                        int hr = ThreadLocalRandom.nextSecondarySeed();
                        long rnd = ((long)hr << 32) | ((long)lr & 0xffffffffL);
                        int skips = levels;      // levels to descend before add
                        Index<K,V> x = null;
                        for (;;) {               // create at most 62 indices
                            x = new Index<K,V>(z, x, null);
                            if (rnd >= 0L || --skips < 0)
                                break;
                            else
                                rnd <<= 1;
                        }
                        if (addIndices(h, skips, x, cmp) && skips < 0 &&
                            head == h) {         // try to add new level
                            Index<K,V> hx = new Index<K,V>(z, x, null);
                            Index<K,V> nh = new Index<K,V>(h.node, h, hx);
                            HEAD.compareAndSet(this, h, nh);
                        }
                        if (z.val == null)       // deleted while adding indices
                            findPredecessor(key, cmp); // clean
                    }
                    addCount(1L);
                    return null;
                }
            }
        }
    }

    
Add indices after an insertion. Descends iteratively to the
highest level of insertion, then recursively, to chain index
nodes to lower ones. Returns null on (staleness) failure,
disabling higher-level insertions. Recursion depths are
exponentially less probable.
Params:
q – starting index for current level
skips – levels to skip before inserting
x – index for this insertion
cmp – comparator/**
     * Add indices after an insertion. Descends iteratively to the
     * highest level of insertion, then recursively, to chain index
     * nodes to lower ones. Returns null on (staleness) failure,
     * disabling higher-level insertions. Recursion depths are
     * exponentially less probable.
     *
     * @param q starting index for current level
     * @param skips levels to skip before inserting
     * @param x index for this insertion
     * @param cmp comparator
     */
    static <K,V> boolean addIndices(Index<K,V> q, int skips, Index<K,V> x,
                                    Comparator<? super K> cmp) {
        Node<K,V> z; K key;
        if (x != null && (z = x.node) != null && (key = z.key) != null &&
            q != null) {                            // hoist checks
            boolean retrying = false;
            for (;;) {                              // find splice point
                Index<K,V> r, d; int c;
                if ((r = q.right) != null) {
                    Node<K,V> p; K k;
                    if ((p = r.node) == null || (k = p.key) == null ||
                        p.val == null) {
                        RIGHT.compareAndSet(q, r, r.right);
                        c = 0;
                    }
                    else if ((c = cpr(cmp, key, k)) > 0)
                        q = r;
                    else if (c == 0)
                        break;                      // stale
                }
                else
                    c = -1;

                if (c < 0) {
                    if ((d = q.down) != null && skips > 0) {
                        --skips;
                        q = d;
                    }
                    else if (d != null && !retrying &&
                             !addIndices(d, 0, x.down, cmp))
                        break;
                    else {
                        x.right = r;
                        if (RIGHT.compareAndSet(q, r, x))
                            return true;
                        else
                            retrying = true;         // re-find splice point
                    }
                }
            }
        }
        return false;
    }

    /* ---------------- Deletion -------------- */

    
Main deletion method. Locates node, nulls value, appends a
deletion marker, unlinks predecessor, removes associated index
nodes, and possibly reduces head index level.
Params:
key – the key
value – if non-null, the value that must be
associated with key
Returns:
the node, or null if not found/**
     * Main deletion method. Locates node, nulls value, appends a
     * deletion marker, unlinks predecessor, removes associated index
     * nodes, and possibly reduces head index level.
     *
     * @param key the key
     * @param value if non-null, the value that must be
     * associated with key
     * @return the node, or null if not found
     */
    final V doRemove(Object key, Object value) {
        if (key == null)
            throw new NullPointerException();
        Comparator<? super K> cmp = comparator;
        V result = null;
        Node<K,V> b;
        outer: while ((b = findPredecessor(key, cmp)) != null &&
                      result == null) {
            for (;;) {
                Node<K,V> n; K k; V v; int c;
                if ((n = b.next) == null)
                    break outer;
                else if ((k = n.key) == null)
                    break;
                else if ((v = n.val) == null)
                    unlinkNode(b, n);
                else if ((c = cpr(cmp, key, k)) > 0)
                    b = n;
                else if (c < 0)
                    break outer;
                else if (value != null && !value.equals(v))
                    break outer;
                else if (VAL.compareAndSet(n, v, null)) {
                    result = v;
                    unlinkNode(b, n);
                    break; // loop to clean up
                }
            }
        }
        if (result != null) {
            tryReduceLevel();
            addCount(-1L);
        }
        return result;
    }

    
Possibly reduce head level if it has no nodes.  This method can
(rarely) make mistakes, in which case levels can disappear even
though they are about to contain index nodes. This impacts
performance, not correctness.  To minimize mistakes as well as
to reduce hysteresis, the level is reduced by one only if the
topmost three levels look empty. Also, if the removed level
looks non-empty after CAS, we try to change it back quick
before anyone notices our mistake! (This trick works pretty
well because this method will practically never make mistakes
unless current thread stalls immediately before first CAS, in
which case it is very unlikely to stall again immediately
afterwards, so will recover.)
We put up with all this rather than just let levels grow
because otherwise, even a small map that has undergone a large
number of insertions and removals will have a lot of levels,
slowing down access more than would an occasional unwanted
reduction.
/**
     * Possibly reduce head level if it has no nodes.  This method can
     * (rarely) make mistakes, in which case levels can disappear even
     * though they are about to contain index nodes. This impacts
     * performance, not correctness.  To minimize mistakes as well as
     * to reduce hysteresis, the level is reduced by one only if the
     * topmost three levels look empty. Also, if the removed level
     * looks non-empty after CAS, we try to change it back quick
     * before anyone notices our mistake! (This trick works pretty
     * well because this method will practically never make mistakes
     * unless current thread stalls immediately before first CAS, in
     * which case it is very unlikely to stall again immediately
     * afterwards, so will recover.)
     *
     * We put up with all this rather than just let levels grow
     * because otherwise, even a small map that has undergone a large
     * number of insertions and removals will have a lot of levels,
     * slowing down access more than would an occasional unwanted
     * reduction.
     */
    private void tryReduceLevel() {
        Index<K,V> h, d, e;
        if ((h = head) != null && h.right == null &&
            (d = h.down) != null && d.right == null &&
            (e = d.down) != null && e.right == null &&
            HEAD.compareAndSet(this, h, d) &&
            h.right != null)   // recheck
            HEAD.compareAndSet(this, d, h);  // try to backout
    }

    /* ---------------- Finding and removing first element -------------- */

    
Gets first valid node, unlinking deleted nodes if encountered.
Returns:
first node or null if empty/**
     * Gets first valid node, unlinking deleted nodes if encountered.
     * @return first node or null if empty
     */
    final Node<K,V> findFirst() {
        Node<K,V> b, n;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                if (n.val == null)
                    unlinkNode(b, n);
                else
                    return n;
            }
        }
        return null;
    }

    
Entry snapshot version of findFirst
/**
     * Entry snapshot version of findFirst
     */
    final AbstractMap.SimpleImmutableEntry<K,V> findFirstEntry() {
        Node<K,V> b, n; V v;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                if ((v = n.val) == null)
                    unlinkNode(b, n);
                else
                    return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
            }
        }
        return null;
    }

    
Removes first entry; returns its snapshot.
Returns:
null if empty, else snapshot of first entry/**
     * Removes first entry; returns its snapshot.
     * @return null if empty, else snapshot of first entry
     */
    private AbstractMap.SimpleImmutableEntry<K,V> doRemoveFirstEntry() {
        Node<K,V> b, n; V v;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                if ((v = n.val) == null || VAL.compareAndSet(n, v, null)) {
                    K k = n.key;
                    unlinkNode(b, n);
                    if (v != null) {
                        tryReduceLevel();
                        findPredecessor(k, comparator); // clean index
                        addCount(-1L);
                        return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
                    }
                }
            }
        }
        return null;
    }

    /* ---------------- Finding and removing last element -------------- */

    
Specialized version of find to get last valid node.
Returns:
last node or null if empty/**
     * Specialized version of find to get last valid node.
     * @return last node or null if empty
     */
    final Node<K,V> findLast() {
        outer: for (;;) {
            Index<K,V> q; Node<K,V> b;
            VarHandle.acquireFence();
            if ((q = head) == null)
                break;
            for (Index<K,V> r, d;;) {
                while ((r = q.right) != null) {
                    Node<K,V> p;
                    if ((p = r.node) == null || p.val == null)
                        RIGHT.compareAndSet(q, r, r.right);
                    else
                        q = r;
                }
                if ((d = q.down) != null)
                    q = d;
                else {
                    b = q.node;
                    break;
                }
            }
            if (b != null) {
                for (;;) {
                    Node<K,V> n;
                    if ((n = b.next) == null) {
                        if (b.key == null) // empty
                            break outer;
                        else
                            return b;
                    }
                    else if (n.key == null)
                        break;
                    else if (n.val == null)
                        unlinkNode(b, n);
                    else
                        b = n;
                }
            }
        }
        return null;
    }

    
Entry version of findLast
Returns:
Entry for last node or null if empty/**
     * Entry version of findLast
     * @return Entry for last node or null if empty
     */
    final AbstractMap.SimpleImmutableEntry<K,V> findLastEntry() {
        for (;;) {
            Node<K,V> n; V v;
            if ((n = findLast()) == null)
                return null;
            if ((v = n.val) != null)
                return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
        }
    }

    
Removes last entry; returns its snapshot.
Specialized variant of doRemove.
Returns:
null if empty, else snapshot of last entry/**
     * Removes last entry; returns its snapshot.
     * Specialized variant of doRemove.
     * @return null if empty, else snapshot of last entry
     */
    private Map.Entry<K,V> doRemoveLastEntry() {
        outer: for (;;) {
            Index<K,V> q; Node<K,V> b;
            VarHandle.acquireFence();
            if ((q = head) == null)
                break;
            for (;;) {
                Index<K,V> d, r; Node<K,V> p;
                while ((r = q.right) != null) {
                    if ((p = r.node) == null || p.val == null)
                        RIGHT.compareAndSet(q, r, r.right);
                    else if (p.next != null)
                        q = r;  // continue only if a successor
                    else
                        break;
                }
                if ((d = q.down) != null)
                    q = d;
                else {
                    b = q.node;
                    break;
                }
            }
            if (b != null) {
                for (;;) {
                    Node<K,V> n; K k; V v;
                    if ((n = b.next) == null) {
                        if (b.key == null) // empty
                            break outer;
                        else
                            break; // retry
                    }
                    else if ((k = n.key) == null)
                        break;
                    else if ((v = n.val) == null)
                        unlinkNode(b, n);
                    else if (n.next != null)
                        b = n;
                    else if (VAL.compareAndSet(n, v, null)) {
                        unlinkNode(b, n);
                        tryReduceLevel();
                        findPredecessor(k, comparator); // clean index
                        addCount(-1L);
                        return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
                    }
                }
            }
        }
        return null;
    }

    /* ---------------- Relational operations -------------- */

    // Control values OR'ed as arguments to findNear

    private static final int EQ = 1;
    private static final int LT = 2;
    private static final int GT = 0; // Actually checked as !LT

    
Utility for ceiling, floor, lower, higher methods.
Params:
key – the key
rel – the relation -- OR'ed combination of EQ, LT, GT
Returns:
nearest node fitting relation, or null if no such/**
     * Utility for ceiling, floor, lower, higher methods.
     * @param key the key
     * @param rel the relation -- OR'ed combination of EQ, LT, GT
     * @return nearest node fitting relation, or null if no such
     */
    final Node<K,V> findNear(K key, int rel, Comparator<? super K> cmp) {
        if (key == null)
            throw new NullPointerException();
        Node<K,V> result;
        outer: for (Node<K,V> b;;) {
            if ((b = findPredecessor(key, cmp)) == null) {
                result = null;
                break;                   // empty
            }
            for (;;) {
                Node<K,V> n; K k; int c;
                if ((n = b.next) == null) {
                    result = ((rel & LT) != 0 && b.key != null) ? b : null;
                    break outer;
                }
                else if ((k = n.key) == null)
                    break;
                else if (n.val == null)
                    unlinkNode(b, n);
                else if (((c = cpr(cmp, key, k)) == 0 && (rel & EQ) != 0) ||
                         (c < 0 && (rel & LT) == 0)) {
                    result = n;
                    break outer;
                }
                else if (c <= 0 && (rel & LT) != 0) {
                    result = (b.key != null) ? b : null;
                    break outer;
                }
                else
                    b = n;
            }
        }
        return result;
    }

    
Variant of findNear returning SimpleImmutableEntry
Params:
key – the key
rel – the relation -- OR'ed combination of EQ, LT, GT
Returns:
Entry fitting relation, or null if no such/**
     * Variant of findNear returning SimpleImmutableEntry
     * @param key the key
     * @param rel the relation -- OR'ed combination of EQ, LT, GT
     * @return Entry fitting relation, or null if no such
     */
    final AbstractMap.SimpleImmutableEntry<K,V> findNearEntry(K key, int rel,
                                                              Comparator<? super K> cmp) {
        for (;;) {
            Node<K,V> n; V v;
            if ((n = findNear(key, rel, cmp)) == null)
                return null;
            if ((v = n.val) != null)
                return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
        }
    }

    /* ---------------- Constructors -------------- */

    
Constructs a new, empty map, sorted according to the natural ordering of the keys. /**
     * Constructs a new, empty map, sorted according to the
     * {@linkplain Comparable natural ordering} of the keys.
     */
    public ConcurrentSkipListMap() {
        this.comparator = null;
    }

    
Constructs a new, empty map, sorted according to the specified
comparator.
Params:
comparator – the comparator that will be used to order this map. If null, the natural
       ordering of the keys will be used./**
     * Constructs a new, empty map, sorted according to the specified
     * comparator.
     *
     * @param comparator the comparator that will be used to order this map.
     *        If {@code null}, the {@linkplain Comparable natural
     *        ordering} of the keys will be used.
     */
    public ConcurrentSkipListMap(Comparator<? super K> comparator) {
        this.comparator = comparator;
    }

    
Constructs a new map containing the same mappings as the given map, sorted according to the natural ordering of the keys. 
Params:
m – the map whose mappings are to be placed in this map
Throws:
ClassCastException – if the keys in m are not Comparable, or are not mutually comparable
NullPointerException – if the specified map or any of its keys
        or values are null/**
     * Constructs a new map containing the same mappings as the given map,
     * sorted according to the {@linkplain Comparable natural ordering} of
     * the keys.
     *
     * @param  m the map whose mappings are to be placed in this map
     * @throws ClassCastException if the keys in {@code m} are not
     *         {@link Comparable}, or are not mutually comparable
     * @throws NullPointerException if the specified map or any of its keys
     *         or values are null
     */
    public ConcurrentSkipListMap(Map<? extends K, ? extends V> m) {
        this.comparator = null;
        putAll(m);
    }

    
Constructs a new map containing the same mappings and using the
same ordering as the specified sorted map.
Params:
m – the sorted map whose mappings are to be placed in this
       map, and whose comparator is to be used to sort this map
Throws:
NullPointerException – if the specified sorted map or any of
        its keys or values are null/**
     * Constructs a new map containing the same mappings and using the
     * same ordering as the specified sorted map.
     *
     * @param m the sorted map whose mappings are to be placed in this
     *        map, and whose comparator is to be used to sort this map
     * @throws NullPointerException if the specified sorted map or any of
     *         its keys or values are null
     */
    public ConcurrentSkipListMap(SortedMap<K, ? extends V> m) {
        this.comparator = m.comparator();
        buildFromSorted(m); // initializes transients
    }

    
Returns a shallow copy of this ConcurrentSkipListMap instance. (The keys and values themselves are not cloned.) 
Returns:
a shallow copy of this map/**
     * Returns a shallow copy of this {@code ConcurrentSkipListMap}
     * instance. (The keys and values themselves are not cloned.)
     *
     * @return a shallow copy of this map
     */
    public ConcurrentSkipListMap<K,V> clone() {
        try {
            @SuppressWarnings("unchecked")
            ConcurrentSkipListMap<K,V> clone =
                (ConcurrentSkipListMap<K,V>) super.clone();
            clone.keySet = null;
            clone.entrySet = null;
            clone.values = null;
            clone.descendingMap = null;
            clone.adder = null;
            clone.buildFromSorted(this);
            return clone;
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }
    }

    
Streamlined bulk insertion to initialize from elements of
given sorted map.  Call only from constructor or clone
method.
/**
     * Streamlined bulk insertion to initialize from elements of
     * given sorted map.  Call only from constructor or clone
     * method.
     */
    private void buildFromSorted(SortedMap<K, ? extends V> map) {
        if (map == null)
            throw new NullPointerException();
        Iterator<? extends Map.Entry<? extends K, ? extends V>> it =
            map.entrySet().iterator();

        /*
         * Add equally spaced indices at log intervals, using the bits
         * of count during insertion. The maximum possible resulting
         * level is less than the number of bits in a long (64). The
         * preds array tracks the current rightmost node at each
         * level.
         */
        @SuppressWarnings("unchecked")
        Index<K,V>[] preds = (Index<K,V>[])new Index<?,?>[64];
        Node<K,V> bp = new Node<K,V>(null, null, null);
        Index<K,V> h = preds[0] = new Index<K,V>(bp, null, null);
        long count = 0;

        while (it.hasNext()) {
            Map.Entry<? extends K, ? extends V> e = it.next();
            K k = e.getKey();
            V v = e.getValue();
            if (k == null || v == null)
                throw new NullPointerException();
            Node<K,V> z = new Node<K,V>(k, v, null);
            bp = bp.next = z;
            if ((++count & 3L) == 0L) {
                long m = count >>> 2;
                int i = 0;
                Index<K,V> idx = null, q;
                do {
                    idx = new Index<K,V>(z, idx, null);
                    if ((q = preds[i]) == null)
                        preds[i] = h = new Index<K,V>(h.node, h, idx);
                    else
                        preds[i] = q.right = idx;
                } while (++i < preds.length && ((m >>>= 1) & 1L) != 0L);
            }
        }
        if (count != 0L) {
            VarHandle.releaseFence(); // emulate volatile stores
            addCount(count);
            head = h;
            VarHandle.fullFence();
        }
    }

    /* ---------------- Serialization -------------- */

    
Saves this map to a stream (that is, serializes it).
Params:
s – the stream
Throws:
IOException – if an I/O error occurs
@serialData
The key (Object) and value (Object) for each key-value mapping represented by the map, followed by null. The key-value mappings are emitted in key-order (as determined by the Comparator, or by the keys' natural ordering if no Comparator)./**
     * Saves this map to a stream (that is, serializes it).
     *
     * @param s the stream
     * @throws java.io.IOException if an I/O error occurs
     * @serialData The key (Object) and value (Object) for each
     * key-value mapping represented by the map, followed by
     * {@code null}. The key-value mappings are emitted in key-order
     * (as determined by the Comparator, or by the keys' natural
     * ordering if no Comparator).
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out the Comparator and any hidden stuff
        s.defaultWriteObject();

        // Write out keys and values (alternating)
        Node<K,V> b, n; V v;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                if ((v = n.val) != null) {
                    s.writeObject(n.key);
                    s.writeObject(v);
                }
                b = n;
            }
        }
        s.writeObject(null);
    }

    
Reconstitutes this map from a stream (that is, deserializes it).
Params:
s – the stream
Throws:
ClassNotFoundException – if the class of a serialized object
        could not be found
IOException – if an I/O error occurs/**
     * Reconstitutes this map from a stream (that is, deserializes it).
     * @param s the stream
     * @throws ClassNotFoundException if the class of a serialized object
     *         could not be found
     * @throws java.io.IOException if an I/O error occurs
     */
    @SuppressWarnings("unchecked")
    private void readObject(final java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in the Comparator and any hidden stuff
        s.defaultReadObject();

        // Same idea as buildFromSorted
        @SuppressWarnings("unchecked")
        Index<K,V>[] preds = (Index<K,V>[])new Index<?,?>[64];
        Node<K,V> bp = new Node<K,V>(null, null, null);
        Index<K,V> h = preds[0] = new Index<K,V>(bp, null, null);
        Comparator<? super K> cmp = comparator;
        K prevKey = null;
        long count = 0;

        for (;;) {
            K k = (K)s.readObject();
            if (k == null)
                break;
            V v = (V)s.readObject();
            if (v == null)
                throw new NullPointerException();
            if (prevKey != null && cpr(cmp, prevKey, k) > 0)
                throw new IllegalStateException("out of order");
            prevKey = k;
            Node<K,V> z = new Node<K,V>(k, v, null);
            bp = bp.next = z;
            if ((++count & 3L) == 0L) {
                long m = count >>> 2;
                int i = 0;
                Index<K,V> idx = null, q;
                do {
                    idx = new Index<K,V>(z, idx, null);
                    if ((q = preds[i]) == null)
                        preds[i] = h = new Index<K,V>(h.node, h, idx);
                    else
                        preds[i] = q.right = idx;
                } while (++i < preds.length && ((m >>>= 1) & 1L) != 0L);
            }
        }
        if (count != 0L) {
            VarHandle.releaseFence();
            addCount(count);
            head = h;
            VarHandle.fullFence();
        }
    }

    /* ------ Map API methods ------ */

    
Returns true if this map contains a mapping for the specified key. 
Params:
key – key whose presence in this map is to be tested
Throws:
ClassCastException – if the specified key cannot be compared
        with the keys currently in the map
NullPointerException – if the specified key is null
Returns:
true if this map contains a mapping for the specified key/**
     * Returns {@code true} if this map contains a mapping for the specified
     * key.
     *
     * @param key key whose presence in this map is to be tested
     * @return {@code true} if this map contains a mapping for the specified key
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if the specified key is null
     */
    public boolean containsKey(Object key) {
        return doGet(key) != null;
    }

    
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. 
More formally, if this map contains a mapping from a key k to a value v such that key compares equal to k according to the map's ordering, then this method returns v; otherwise it returns null. (There can be at most one such mapping.) 
Throws:
ClassCastException – if the specified key cannot be compared
        with the keys currently in the map
NullPointerException – if the specified key is null/**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code key} compares
     * equal to {@code k} according to the map's ordering, then this
     * method returns {@code v}; otherwise it returns {@code null}.
     * (There can be at most one such mapping.)
     *
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if the specified key is null
     */
    public V get(Object key) {
        return doGet(key);
    }

    
Returns the value to which the specified key is mapped,
or the given defaultValue if this map contains no mapping for the key.
Params:
key – the key
defaultValue – the value to return if this map contains
no mapping for the given key
Throws:
NullPointerException – if the specified key is null
Returns:
the mapping for the key, if present; else the defaultValue
Since:
1.8/**
     * Returns the value to which the specified key is mapped,
     * or the given defaultValue if this map contains no mapping for the key.
     *
     * @param key the key
     * @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 defaultValue
     * @throws NullPointerException if the specified key is null
     * @since 1.8
     */
    public V getOrDefault(Object key, V defaultValue) {
        V v;
        return (v = doGet(key)) == null ? defaultValue : v;
    }

    
Associates the specified value with the specified key in this map.
If the map previously contained a mapping for the key, the old
value is replaced.
Params:
key – key with which the specified value is to be associated
value – value to be associated with the specified key
Throws:
ClassCastException – if the specified key cannot be compared
        with the keys currently in the map
NullPointerException – if the specified key or value is null
Returns:
the previous value associated with the specified key, or null if there was no mapping for the key/**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @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 the specified key, or
     *         {@code null} if there was no mapping for the key
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if the specified key or value is null
     */
    public V put(K key, V value) {
        if (value == null)
            throw new NullPointerException();
        return doPut(key, value, false);
    }

    
Removes the mapping for the specified key from this map if present.
Params:
key – key for which mapping should be removed
Throws:
ClassCastException – if the specified key cannot be compared
        with the keys currently in the map
NullPointerException – if the specified key is null
Returns:
the previous value associated with the specified key, or null if there was no mapping for the key/**
     * Removes the mapping for the specified key from this map if present.
     *
     * @param  key key for which mapping should be removed
     * @return the previous value associated with the specified key, or
     *         {@code null} if there was no mapping for the key
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if the specified key is null
     */
    public V remove(Object key) {
        return doRemove(key, null);
    }

    
Returns true if this map maps one or more keys to the specified value. This operation requires time linear in the map size. Additionally, it is possible for the map to change during execution of this method, in which case the returned result may be inaccurate. 
Params:
value – value whose presence in this map is to be tested
Throws:
NullPointerException – if the specified value is null
Returns:
true if a mapping to value exists; false otherwise/**
     * Returns {@code true} if this map maps one or more keys to the
     * specified value.  This operation requires time linear in the
     * map size. Additionally, it is possible for the map to change
     * during execution of this method, in which case the returned
     * result may be inaccurate.
     *
     * @param value value whose presence in this map is to be tested
     * @return {@code true} if a mapping to {@code value} exists;
     *         {@code false} otherwise
     * @throws NullPointerException if the specified value is null
     */
    public boolean containsValue(Object value) {
        if (value == null)
            throw new NullPointerException();
        Node<K,V> b, n; V v;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                if ((v = n.val) != null && value.equals(v))
                    return true;
                else
                    b = n;
            }
        }
        return false;
    }

    
{@inheritDoc}
/**
     * {@inheritDoc}
     */
    public int size() {
        long c;
        return ((baseHead() == null) ? 0 :
                ((c = getAdderCount()) >= Integer.MAX_VALUE) ?
                Integer.MAX_VALUE : (int) c);
    }

    
{@inheritDoc}
/**
     * {@inheritDoc}
     */
    public boolean isEmpty() {
        return findFirst() == null;
    }

    
Removes all of the mappings from this map.
/**
     * Removes all of the mappings from this map.
     */
    public void clear() {
        Index<K,V> h, r, d; Node<K,V> b;
        VarHandle.acquireFence();
        while ((h = head) != null) {
            if ((r = h.right) != null)        // remove indices
                RIGHT.compareAndSet(h, r, null);
            else if ((d = h.down) != null)    // remove levels
                HEAD.compareAndSet(this, h, d);
            else {
                long count = 0L;
                if ((b = h.node) != null) {    // remove nodes
                    Node<K,V> n; V v;
                    while ((n = b.next) != null) {
                        if ((v = n.val) != null &&
                            VAL.compareAndSet(n, v, null)) {
                            --count;
                            v = null;
                        }
                        if (v == null)
                            unlinkNode(b, n);
                    }
                }
                if (count != 0L)
                    addCount(count);
                else
                    break;
            }
        }
    }

    
If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map unless null. The function is NOT guaranteed to be applied once atomically only
if the value is not present.
Params:
key – key with which the specified value is to be associated
mappingFunction – the function to compute a value
Throws:
NullPointerException – if the specified key is null
        or the mappingFunction is null
Returns:
the current (existing or computed) value associated with
        the specified key, or null if the computed value is null
Since:
1.8/**
     * If the specified key is not already associated with a value,
     * attempts to compute its value using the given mapping function
     * and enters it into this map unless {@code null}.  The function
     * is <em>NOT</em> guaranteed to be applied once atomically only
     * if the value is not present.
     *
     * @param key key with which the specified value is to be associated
     * @param mappingFunction the function to compute a value
     * @return the current (existing or computed) value associated with
     *         the specified key, or null if the computed value is null
     * @throws NullPointerException if the specified key is null
     *         or the mappingFunction is null
     * @since 1.8
     */
    public V computeIfAbsent(K key,
                             Function<? super K, ? extends V> mappingFunction) {
        if (key == null || mappingFunction == null)
            throw new NullPointerException();
        V v, p, r;
        if ((v = doGet(key)) == null &&
            (r = mappingFunction.apply(key)) != null)
            v = (p = doPut(key, r, true)) == null ? r : p;
        return v;
    }

    
If the value for the specified key is present, attempts to
compute a new mapping given the key and its current mapped
value. The function is NOT guaranteed to be applied
once atomically.
Params:
key – key with which a value may be associated
remappingFunction – the function to compute a value
Throws:
NullPointerException – if the specified key is null
        or the remappingFunction is null
Returns:
the new value associated with the specified key, or null if none
Since:
1.8/**
     * If the value for the specified key is present, attempts to
     * compute a new mapping given the key and its current mapped
     * value. The function is <em>NOT</em> guaranteed to be applied
     * once atomically.
     *
     * @param key key with which a value may be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null
     *         or the remappingFunction is null
     * @since 1.8
     */
    public V computeIfPresent(K key,
                              BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        if (key == null || remappingFunction == null)
            throw new NullPointerException();
        Node<K,V> n; V v;
        while ((n = findNode(key)) != null) {
            if ((v = n.val) != null) {
                V r = remappingFunction.apply(key, v);
                if (r != null) {
                    if (VAL.compareAndSet(n, v, r))
                        return r;
                }
                else if (doRemove(key, v) != null)
                    break;
            }
        }
        return null;
    }

    
Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). The function is NOT guaranteed to be applied
once atomically.
Params:
key – key with which the specified value is to be associated
remappingFunction – the function to compute a value
Throws:
NullPointerException – if the specified key is null
        or the remappingFunction is null
Returns:
the new value associated with the specified key, or null if none
Since:
1.8/**
     * Attempts to compute a mapping for the specified key and its
     * current mapped value (or {@code null} if there is no current
     * mapping). The function is <em>NOT</em> guaranteed to be applied
     * once atomically.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null
     *         or the remappingFunction is null
     * @since 1.8
     */
    public V compute(K key,
                     BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        if (key == null || remappingFunction == null)
            throw new NullPointerException();
        for (;;) {
            Node<K,V> n; V v; V r;
            if ((n = findNode(key)) == null) {
                if ((r = remappingFunction.apply(key, null)) == null)
                    break;
                if (doPut(key, r, true) == null)
                    return r;
            }
            else if ((v = n.val) != null) {
                if ((r = remappingFunction.apply(key, v)) != null) {
                    if (VAL.compareAndSet(n, v, r))
                        return r;
                }
                else if (doRemove(key, v) != null)
                    break;
            }
        }
        return null;
    }

    
If the specified key is not already associated with a value, associates it with the given value. Otherwise, replaces the value with the results of the given remapping function, or removes if null. The function is NOT
guaranteed to be applied once atomically.
Params:
key – key with which the specified value is to be associated
value – the value to use if absent
remappingFunction – the function to recompute a value if present
Throws:
NullPointerException – if the specified key or value is null
        or the remappingFunction is null
Returns:
the new value associated with the specified key, or null if none
Since:
1.8/**
     * If the specified key is not already associated with a value,
     * associates it with the given value.  Otherwise, replaces the
     * value with the results of the given remapping function, or
     * removes if {@code null}. The function is <em>NOT</em>
     * guaranteed to be applied once atomically.
     *
     * @param key key with which the specified value is to be associated
     * @param value the value to use if absent
     * @param remappingFunction the function to recompute a value if present
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key or value is null
     *         or the remappingFunction is null
     * @since 1.8
     */
    public V merge(K key, V value,
                   BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        if (key == null || value == null || remappingFunction == null)
            throw new NullPointerException();
        for (;;) {
            Node<K,V> n; V v; V r;
            if ((n = findNode(key)) == null) {
                if (doPut(key, value, true) == null)
                    return value;
            }
            else if ((v = n.val) != null) {
                if ((r = remappingFunction.apply(v, value)) != null) {
                    if (VAL.compareAndSet(n, v, r))
                        return r;
                }
                else if (doRemove(key, v) != null)
                    return null;
            }
        }
    }

    /* ---------------- View methods -------------- */

    /*
     * Note: Lazy initialization works for views because view classes
     * are stateless/immutable so it doesn't matter wrt correctness if
     * more than one is created (which will only rarely happen).  Even
     * so, the following idiom conservatively ensures that the method
     * returns the one it created if it does so, not one created by
     * another racing thread.
     */

    
Returns a NavigableSet view of the keys contained in this map. 
The set's iterator returns the keys in ascending order. The set's spliterator additionally reports Spliterator.CONCURRENT, Spliterator.NONNULL, Spliterator.SORTED and Spliterator.ORDERED, with an encounter order that is ascending key order. 
The spliterator's comparator is null if the map's comparator is null. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the map's comparator. 
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 Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations. 
The view's iterators and spliterators are
weakly consistent.
This method is equivalent to method navigableKeySet. 
Returns:
a navigable set view of the keys in this map/**
     * Returns a {@link NavigableSet} view of the keys contained in this map.
     *
     * <p>The set's iterator returns the keys in ascending order.
     * The set's spliterator additionally reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#NONNULL}, {@link Spliterator#SORTED} and
     * {@link Spliterator#ORDERED}, with an encounter order that is ascending
     * key order.
     *
     * <p>The {@linkplain Spliterator#getComparator() spliterator's comparator}
     * is {@code null} if the {@linkplain #comparator() map's comparator}
     * is {@code null}.
     * Otherwise, the spliterator's comparator is the same as or imposes the
     * same total ordering as the map's comparator.
     *
     * <p>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.  It does not support the {@code add} or {@code addAll}
     * operations.
     *
     * <p>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * <p>This method is equivalent to method {@code navigableKeySet}.
     *
     * @return a navigable set view of the keys in this map
     */
    public NavigableSet<K> keySet() {
        KeySet<K,V> ks;
        if ((ks = keySet) != null) return ks;
        return keySet = new KeySet<>(this);
    }

    public NavigableSet<K> navigableKeySet() {
        KeySet<K,V> ks;
        if ((ks = keySet) != null) return ks;
        return keySet = new KeySet<>(this);
    }

    
Returns a Collection view of the values contained in this map. 
The collection's iterator returns the values in ascending order of the corresponding keys. The collections's spliterator additionally reports Spliterator.CONCURRENT, Spliterator.NONNULL and Spliterator.ORDERED, with an encounter order that is ascending order of the corresponding keys. 
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 the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. 
The view's iterators and spliterators are
weakly consistent.
/**
     * Returns a {@link Collection} view of the values contained in this map.
     * <p>The collection's iterator returns the values in ascending order
     * of the corresponding keys. The collections's spliterator additionally
     * reports {@link Spliterator#CONCURRENT}, {@link Spliterator#NONNULL} and
     * {@link Spliterator#ORDERED}, with an encounter order that is ascending
     * order of the corresponding keys.
     *
     * <p>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 the 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>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     */
    public Collection<V> values() {
        Values<K,V> vs;
        if ((vs = values) != null) return vs;
        return values = new Values<>(this);
    }

    
Returns a Set view of the mappings contained in this map. 
The set's iterator returns the entries in ascending key order. The set's spliterator additionally reports Spliterator.CONCURRENT, Spliterator.NONNULL, Spliterator.SORTED and Spliterator.ORDERED, with an encounter order that is ascending key order. 
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 Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. 
The view's iterators and spliterators are
weakly consistent.
The Map.Entry elements traversed by the iterator or spliterator do not support the setValue operation. 
Returns:
a set view of the mappings contained in this map,
        sorted in ascending key order/**
     * Returns a {@link Set} view of the mappings contained in this map.
     *
     * <p>The set's iterator returns the entries in ascending key order.  The
     * set's spliterator additionally reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#NONNULL}, {@link Spliterator#SORTED} and
     * {@link Spliterator#ORDERED}, with an encounter order that is ascending
     * key order.
     *
     * <p>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.  It does not support the {@code add} or
     * {@code addAll} operations.
     *
     * <p>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * <p>The {@code Map.Entry} elements traversed by the {@code iterator}
     * or {@code spliterator} do <em>not</em> support the {@code setValue}
     * operation.
     *
     * @return a set view of the mappings contained in this map,
     *         sorted in ascending key order
     */
    public Set<Map.Entry<K,V>> entrySet() {
        EntrySet<K,V> es;
        if ((es = entrySet) != null) return es;
        return entrySet = new EntrySet<K,V>(this);
    }

    public ConcurrentNavigableMap<K,V> descendingMap() {
        ConcurrentNavigableMap<K,V> dm;
        if ((dm = descendingMap) != null) return dm;
        return descendingMap =
            new SubMap<K,V>(this, null, false, null, false, true);
    }

    public NavigableSet<K> descendingKeySet() {
        return descendingMap().navigableKeySet();
    }

    /* ---------------- AbstractMap Overrides -------------- */

    
Compares the specified object with this map for equality. Returns true if the given object is also a map and the two maps represent the same mappings. More formally, two maps m1 and m2 represent the same mappings if m1.entrySet().equals(m2.entrySet()). This operation may return misleading results if either map is concurrently modified during execution of this method. 
Params:
o – object to be compared for equality with this map
Returns:
true if the specified object is equal to this map/**
     * Compares the specified object with this map for equality.
     * Returns {@code true} if the given object is also a map and the
     * two maps represent the same mappings.  More formally, two maps
     * {@code m1} and {@code m2} represent the same mappings if
     * {@code m1.entrySet().equals(m2.entrySet())}.  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
     */
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof Map))
            return false;
        Map<?,?> m = (Map<?,?>) o;
        try {
            Comparator<? super K> cmp = comparator;
            // See JDK-8223553 for Iterator type wildcard rationale
            Iterator<? extends Map.Entry<?,?>> it = m.entrySet().iterator();
            if (m instanceof SortedMap &&
                ((SortedMap<?,?>)m).comparator() == cmp) {
                Node<K,V> b, n;
                if ((b = baseHead()) != null) {
                    while ((n = b.next) != null) {
                        K k; V v;
                        if ((v = n.val) != null && (k = n.key) != null) {
                            if (!it.hasNext())
                                return false;
                            Map.Entry<?,?> e = it.next();
                            Object mk = e.getKey();
                            Object mv = e.getValue();
                            if (mk == null || mv == null)
                                return false;
                            try {
                                if (cpr(cmp, k, mk) != 0)
                                    return false;
                            } catch (ClassCastException cce) {
                                return false;
                            }
                            if (!mv.equals(v))
                                return false;
                        }
                        b = n;
                    }
                }
                return !it.hasNext();
            }
            else {
                while (it.hasNext()) {
                    V v;
                    Map.Entry<?,?> e = it.next();
                    Object mk = e.getKey();
                    Object mv = e.getValue();
                    if (mk == null || mv == null ||
                        (v = get(mk)) == null || !v.equals(mv))
                        return false;
                }
                Node<K,V> b, n;
                if ((b = baseHead()) != null) {
                    K k; V v; Object mv;
                    while ((n = b.next) != null) {
                        if ((v = n.val) != null && (k = n.key) != null &&
                            ((mv = m.get(k)) == null || !mv.equals(v)))
                            return false;
                        b = n;
                    }
                }
                return true;
            }
        } catch (ClassCastException | NullPointerException unused) {
            return false;
        }
    }

    /* ------ ConcurrentMap API methods ------ */

    
{@inheritDoc}
Throws:
ClassCastException – if the specified key cannot be compared
        with the keys currently in the map
NullPointerException – if the specified key or value is null
Returns:
the previous value associated with the specified key, or null if there was no mapping for the key/**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     *         or {@code null} if there was no mapping for the key
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if the specified key or value is null
     */
    public V putIfAbsent(K key, V value) {
        if (value == null)
            throw new NullPointerException();
        return doPut(key, value, true);
    }

    
{@inheritDoc}
Throws:
ClassCastException – if the specified key cannot be compared
        with the keys currently in the map
NullPointerException – if the specified key is null/**
     * {@inheritDoc}
     *
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if the specified key is null
     */
    public boolean remove(Object key, Object value) {
        if (key == null)
            throw new NullPointerException();
        return value != null && doRemove(key, value) != null;
    }

    
{@inheritDoc}
Throws:
ClassCastException – if the specified key cannot be compared
        with the keys currently in the map
NullPointerException – if any of the arguments are null/**
     * {@inheritDoc}
     *
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if any of the arguments are null
     */
    public boolean replace(K key, V oldValue, V newValue) {
        if (key == null || oldValue == null || newValue == null)
            throw new NullPointerException();
        for (;;) {
            Node<K,V> n; V v;
            if ((n = findNode(key)) == null)
                return false;
            if ((v = n.val) != null) {
                if (!oldValue.equals(v))
                    return false;
                if (VAL.compareAndSet(n, v, newValue))
                    return true;
            }
        }
    }

    
{@inheritDoc}
Throws:
ClassCastException – if the specified key cannot be compared
        with the keys currently in the map
NullPointerException – if the specified key or value is null
Returns:
the previous value associated with the specified key, or null if there was no mapping for the key/**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     *         or {@code null} if there was no mapping for the key
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if the specified key or value is null
     */
    public V replace(K key, V value) {
        if (key == null || value == null)
            throw new NullPointerException();
        for (;;) {
            Node<K,V> n; V v;
            if ((n = findNode(key)) == null)
                return null;
            if ((v = n.val) != null && VAL.compareAndSet(n, v, value))
                return v;
        }
    }

    /* ------ SortedMap API methods ------ */

    public Comparator<? super K> comparator() {
        return comparator;
    }

    
Throws:
NoSuchElementException – {@inheritDoc}/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public K firstKey() {
        Node<K,V> n = findFirst();
        if (n == null)
            throw new NoSuchElementException();
        return n.key;
    }

    
Throws:
NoSuchElementException – {@inheritDoc}/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public K lastKey() {
        Node<K,V> n = findLast();
        if (n == null)
            throw new NoSuchElementException();
        return n.key;
    }

    
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if fromKey or toKey is null
IllegalArgumentException – {@inheritDoc}/**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code fromKey} or {@code toKey} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public ConcurrentNavigableMap<K,V> subMap(K fromKey,
                                              boolean fromInclusive,
                                              K toKey,
                                              boolean toInclusive) {
        if (fromKey == null || toKey == null)
            throw new NullPointerException();
        return new SubMap<K,V>
            (this, fromKey, fromInclusive, toKey, toInclusive, false);
    }

    
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if toKey is null
IllegalArgumentException – {@inheritDoc}/**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code toKey} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public ConcurrentNavigableMap<K,V> headMap(K toKey,
                                               boolean inclusive) {
        if (toKey == null)
            throw new NullPointerException();
        return new SubMap<K,V>
            (this, null, false, toKey, inclusive, false);
    }

    
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if fromKey is null
IllegalArgumentException – {@inheritDoc}/**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code fromKey} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public ConcurrentNavigableMap<K,V> tailMap(K fromKey,
                                               boolean inclusive) {
        if (fromKey == null)
            throw new NullPointerException();
        return new SubMap<K,V>
            (this, fromKey, inclusive, null, false, false);
    }

    
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if fromKey or toKey is null
IllegalArgumentException – {@inheritDoc}/**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code fromKey} or {@code toKey} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public ConcurrentNavigableMap<K,V> subMap(K fromKey, K toKey) {
        return subMap(fromKey, true, toKey, false);
    }

    
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if toKey is null
IllegalArgumentException – {@inheritDoc}/**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code toKey} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public ConcurrentNavigableMap<K,V> headMap(K toKey) {
        return headMap(toKey, false);
    }

    
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if fromKey is null
IllegalArgumentException – {@inheritDoc}/**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code fromKey} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public ConcurrentNavigableMap<K,V> tailMap(K fromKey) {
        return tailMap(fromKey, true);
    }

    /* ---------------- Relational operations -------------- */

    
Returns a key-value mapping associated with the greatest key strictly less than the given key, or null if there is no such key. The returned entry does not support the Entry.setValue method. 
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if the specified key is null/**
     * Returns a key-value mapping associated with the greatest key
     * strictly less than the given key, or {@code null} if there is
     * no such key. The returned entry does <em>not</em> support the
     * {@code Entry.setValue} method.
     *
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    public Map.Entry<K,V> lowerEntry(K key) {
        return findNearEntry(key, LT, comparator);
    }

    
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if the specified key is null/**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    public K lowerKey(K key) {
        Node<K,V> n = findNear(key, LT, comparator);
        return (n == null) ? null : n.key;
    }

    
Returns a key-value mapping associated with the greatest key less than or equal to the given key, or null if there is no such key. The returned entry does not support the Entry.setValue method. 
Params:
key – the key
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if the specified key is null/**
     * Returns a key-value mapping associated with the greatest key
     * less than or equal to the given key, or {@code null} if there
     * is no such key. The returned entry does <em>not</em> support
     * the {@code Entry.setValue} method.
     *
     * @param key the key
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    public Map.Entry<K,V> floorEntry(K key) {
        return findNearEntry(key, LT|EQ, comparator);
    }

    
Params:
key – the key
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if the specified key is null/**
     * @param key the key
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    public K floorKey(K key) {
        Node<K,V> n = findNear(key, LT|EQ, comparator);
        return (n == null) ? null : n.key;
    }

    
Returns a key-value mapping associated with the least key greater than or equal to the given key, or null if there is no such entry. The returned entry does not support the Entry.setValue method. 
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if the specified key is null/**
     * Returns a key-value mapping associated with the least key
     * greater than or equal to the given key, or {@code null} if
     * there is no such entry. The returned entry does <em>not</em>
     * support the {@code Entry.setValue} method.
     *
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    public Map.Entry<K,V> ceilingEntry(K key) {
        return findNearEntry(key, GT|EQ, comparator);
    }

    
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if the specified key is null/**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    public K ceilingKey(K key) {
        Node<K,V> n = findNear(key, GT|EQ, comparator);
        return (n == null) ? null : n.key;
    }

    
Returns a key-value mapping associated with the least key strictly greater than the given key, or null if there is no such key. The returned entry does not support the Entry.setValue method. 
Params:
key – the key
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if the specified key is null/**
     * Returns a key-value mapping associated with the least key
     * strictly greater than the given key, or {@code null} if there
     * is no such key. The returned entry does <em>not</em> support
     * the {@code Entry.setValue} method.
     *
     * @param key the key
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    public Map.Entry<K,V> higherEntry(K key) {
        return findNearEntry(key, GT, comparator);
    }

    
Params:
key – the key
Throws:
ClassCastException – {@inheritDoc}
NullPointerException – if the specified key is null/**
     * @param key the key
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified key is null
     */
    public K higherKey(K key) {
        Node<K,V> n = findNear(key, GT, comparator);
        return (n == null) ? null : n.key;
    }

    
Returns a key-value mapping associated with the least key in this map, or null if the map is empty. The returned entry does not support the Entry.setValue method. /**
     * Returns a key-value mapping associated with the least
     * key in this map, or {@code null} if the map is empty.
     * The returned entry does <em>not</em> support
     * the {@code Entry.setValue} method.
     */
    public Map.Entry<K,V> firstEntry() {
        return findFirstEntry();
    }

    
Returns a key-value mapping associated with the greatest key in this map, or null if the map is empty. The returned entry does not support the Entry.setValue method. /**
     * Returns a key-value mapping associated with the greatest
     * key in this map, or {@code null} if the map is empty.
     * The returned entry does <em>not</em> support
     * the {@code Entry.setValue} method.
     */
    public Map.Entry<K,V> lastEntry() {
        return findLastEntry();
    }

    
Removes and returns a key-value mapping associated with the least key in this map, or null if the map is empty. The returned entry does not support the Entry.setValue method. /**
     * Removes and returns a key-value mapping associated with
     * the least key in this map, or {@code null} if the map is empty.
     * The returned entry does <em>not</em> support
     * the {@code Entry.setValue} method.
     */
    public Map.Entry<K,V> pollFirstEntry() {
        return doRemoveFirstEntry();
    }

    
Removes and returns a key-value mapping associated with the greatest key in this map, or null if the map is empty. The returned entry does not support the Entry.setValue method. /**
     * Removes and returns a key-value mapping associated with
     * the greatest key in this map, or {@code null} if the map is empty.
     * The returned entry does <em>not</em> support
     * the {@code Entry.setValue} method.
     */
    public Map.Entry<K,V> pollLastEntry() {
        return doRemoveLastEntry();
    }

    /* ---------------- Iterators -------------- */

    
Base of iterator classes
/**
     * Base of iterator classes
     */
    abstract class Iter<T> implements Iterator<T> {
        
the last node returned by next() /** the last node returned by next() */
        Node<K,V> lastReturned;
        
the next node to return from next(); /** the next node to return from next(); */
        Node<K,V> next;
        
Cache of next value field to maintain weak consistency /** Cache of next value field to maintain weak consistency */
        V nextValue;

        
Initializes ascending iterator for entire range. /** Initializes ascending iterator for entire range. */
        Iter() {
            advance(baseHead());
        }

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

        
Advances next to higher entry. /** Advances next to higher entry. */
        final void advance(Node<K,V> b) {
            Node<K,V> n = null;
            V v = null;
            if ((lastReturned = b) != null) {
                while ((n = b.next) != null && (v = n.val) == null)
                    b = n;
            }
            nextValue = v;
            next = n;
        }

        public final void remove() {
            Node<K,V> n; K k;
            if ((n = lastReturned) == null || (k = n.key) == null)
                throw new IllegalStateException();
            // It would not be worth all of the overhead to directly
            // unlink from here. Using remove is fast enough.
            ConcurrentSkipListMap.this.remove(k);
            lastReturned = null;
        }
    }

    final class ValueIterator extends Iter<V> {
        public V next() {
            V v;
            if ((v = nextValue) == null)
                throw new NoSuchElementException();
            advance(next);
            return v;
        }
    }

    final class KeyIterator extends Iter<K> {
        public K next() {
            Node<K,V> n;
            if ((n = next) == null)
                throw new NoSuchElementException();
            K k = n.key;
            advance(n);
            return k;
        }
    }

    final class EntryIterator extends Iter<Map.Entry<K,V>> {
        public Map.Entry<K,V> next() {
            Node<K,V> n;
            if ((n = next) == null)
                throw new NoSuchElementException();
            K k = n.key;
            V v = nextValue;
            advance(n);
            return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
        }
    }

    /* ---------------- View Classes -------------- */

    /*
     * View classes are static, delegating to a ConcurrentNavigableMap
     * to allow use by SubMaps, which outweighs the ugliness of
     * needing type-tests for Iterator methods.
     */

    static final <E> List<E> toList(Collection<E> c) {
        // Using size() here would be a pessimization.
        ArrayList<E> list = new ArrayList<E>();
        for (E e : c)
            list.add(e);
        return list;
    }

    static final class KeySet<K,V>
            extends AbstractSet<K> implements NavigableSet<K> {
        final ConcurrentNavigableMap<K,V> m;
        KeySet(ConcurrentNavigableMap<K,V> map) { m = map; }
        public int size() { return m.size(); }
        public boolean isEmpty() { return m.isEmpty(); }
        public boolean contains(Object o) { return m.containsKey(o); }
        public boolean remove(Object o) { return m.remove(o) != null; }
        public void clear() { m.clear(); }
        public K lower(K e) { return m.lowerKey(e); }
        public K floor(K e) { return m.floorKey(e); }
        public K ceiling(K e) { return m.ceilingKey(e); }
        public K higher(K e) { return m.higherKey(e); }
        public Comparator<? super K> comparator() { return m.comparator(); }
        public K first() { return m.firstKey(); }
        public K last() { return m.lastKey(); }
        public K pollFirst() {
            Map.Entry<K,V> e = m.pollFirstEntry();
            return (e == null) ? null : e.getKey();
        }
        public K pollLast() {
            Map.Entry<K,V> e = m.pollLastEntry();
            return (e == null) ? null : e.getKey();
        }
        public Iterator<K> iterator() {
            return (m instanceof ConcurrentSkipListMap)
                ? ((ConcurrentSkipListMap<K,V>)m).new KeyIterator()
                : ((SubMap<K,V>)m).new SubMapKeyIterator();
        }
        public boolean equals(Object o) {
            if (o == this)
                return true;
            if (!(o instanceof Set))
                return false;
            Collection<?> c = (Collection<?>) o;
            try {
                return containsAll(c) && c.containsAll(this);
            } catch (ClassCastException | NullPointerException unused) {
                return false;
            }
        }
        public Object[] toArray()     { return toList(this).toArray();  }
        public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }
        public Iterator<K> descendingIterator() {
            return descendingSet().iterator();
        }
        public NavigableSet<K> subSet(K fromElement,
                                      boolean fromInclusive,
                                      K toElement,
                                      boolean toInclusive) {
            return new KeySet<>(m.subMap(fromElement, fromInclusive,
                                         toElement,   toInclusive));
        }
        public NavigableSet<K> headSet(K toElement, boolean inclusive) {
            return new KeySet<>(m.headMap(toElement, inclusive));
        }
        public NavigableSet<K> tailSet(K fromElement, boolean inclusive) {
            return new KeySet<>(m.tailMap(fromElement, inclusive));
        }
        public NavigableSet<K> subSet(K fromElement, K toElement) {
            return subSet(fromElement, true, toElement, false);
        }
        public NavigableSet<K> headSet(K toElement) {
            return headSet(toElement, false);
        }
        public NavigableSet<K> tailSet(K fromElement) {
            return tailSet(fromElement, true);
        }
        public NavigableSet<K> descendingSet() {
            return new KeySet<>(m.descendingMap());
        }

        public Spliterator<K> spliterator() {
            return (m instanceof ConcurrentSkipListMap)
                ? ((ConcurrentSkipListMap<K,V>)m).keySpliterator()
                : ((SubMap<K,V>)m).new SubMapKeyIterator();
        }
    }

    static final class Values<K,V> extends AbstractCollection<V> {
        final ConcurrentNavigableMap<K,V> m;
        Values(ConcurrentNavigableMap<K,V> map) {
            m = map;
        }
        public Iterator<V> iterator() {
            return (m instanceof ConcurrentSkipListMap)
                ? ((ConcurrentSkipListMap<K,V>)m).new ValueIterator()
                : ((SubMap<K,V>)m).new SubMapValueIterator();
        }
        public int size() { return m.size(); }
        public boolean isEmpty() { return m.isEmpty(); }
        public boolean contains(Object o) { return m.containsValue(o); }
        public void clear() { m.clear(); }
        public Object[] toArray()     { return toList(this).toArray();  }
        public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }

        public Spliterator<V> spliterator() {
            return (m instanceof ConcurrentSkipListMap)
                ? ((ConcurrentSkipListMap<K,V>)m).valueSpliterator()
                : ((SubMap<K,V>)m).new SubMapValueIterator();
        }

        public boolean removeIf(Predicate<? super V> filter) {
            if (filter == null) throw new NullPointerException();
            if (m instanceof ConcurrentSkipListMap)
                return ((ConcurrentSkipListMap<K,V>)m).removeValueIf(filter);
            // else use iterator
            Iterator<Map.Entry<K,V>> it =
                ((SubMap<K,V>)m).new SubMapEntryIterator();
            boolean removed = false;
            while (it.hasNext()) {
                Map.Entry<K,V> e = it.next();
                V v = e.getValue();
                if (filter.test(v) && m.remove(e.getKey(), v))
                    removed = true;
            }
            return removed;
        }
    }

    static final class EntrySet<K,V> extends AbstractSet<Map.Entry<K,V>> {
        final ConcurrentNavigableMap<K,V> m;
        EntrySet(ConcurrentNavigableMap<K,V> map) {
            m = map;
        }
        public Iterator<Map.Entry<K,V>> iterator() {
            return (m instanceof ConcurrentSkipListMap)
                ? ((ConcurrentSkipListMap<K,V>)m).new EntryIterator()
                : ((SubMap<K,V>)m).new SubMapEntryIterator();
        }

        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
            V v = m.get(e.getKey());
            return v != null && v.equals(e.getValue());
        }
        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
            return m.remove(e.getKey(),
                            e.getValue());
        }
        public boolean isEmpty() {
            return m.isEmpty();
        }
        public int size() {
            return m.size();
        }
        public void clear() {
            m.clear();
        }
        public boolean equals(Object o) {
            if (o == this)
                return true;
            if (!(o instanceof Set))
                return false;
            Collection<?> c = (Collection<?>) o;
            try {
                return containsAll(c) && c.containsAll(this);
            } catch (ClassCastException | NullPointerException unused) {
                return false;
            }
        }
        public Object[] toArray()     { return toList(this).toArray();  }
        public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }

        public Spliterator<Map.Entry<K,V>> spliterator() {
            return (m instanceof ConcurrentSkipListMap)
                ? ((ConcurrentSkipListMap<K,V>)m).entrySpliterator()
                : ((SubMap<K,V>)m).new SubMapEntryIterator();
        }
        public boolean removeIf(Predicate<? super Entry<K,V>> filter) {
            if (filter == null) throw new NullPointerException();
            if (m instanceof ConcurrentSkipListMap)
                return ((ConcurrentSkipListMap<K,V>)m).removeEntryIf(filter);
            // else use iterator
            Iterator<Map.Entry<K,V>> it =
                ((SubMap<K,V>)m).new SubMapEntryIterator();
            boolean removed = false;
            while (it.hasNext()) {
                Map.Entry<K,V> e = it.next();
                if (filter.test(e) && m.remove(e.getKey(), e.getValue()))
                    removed = true;
            }
            return removed;
        }
    }

    
Submaps returned by ConcurrentSkipListMap submap operations represent a subrange of mappings of their underlying maps. Instances of this class support all methods of their underlying maps, differing in that mappings outside their range are ignored, and attempts to add mappings outside their ranges result in IllegalArgumentException. Instances of this class are constructed only using the subMap, headMap, and tailMap methods of their underlying maps. 
@serial
include/**
     * Submaps returned by {@link ConcurrentSkipListMap} submap operations
     * represent a subrange of mappings of their underlying maps.
     * Instances of this class support all methods of their underlying
     * maps, differing in that mappings outside their range are ignored,
     * and attempts to add mappings outside their ranges result in {@link
     * IllegalArgumentException}.  Instances of this class are constructed
     * only using the {@code subMap}, {@code headMap}, and {@code tailMap}
     * methods of their underlying maps.
     *
     * @serial include
     */
    static final class SubMap<K,V> extends AbstractMap<K,V>
        implements ConcurrentNavigableMap<K,V>, Serializable {
        private static final long serialVersionUID = -7647078645895051609L;

        
Underlying map /** Underlying map */
        final ConcurrentSkipListMap<K,V> m;
        
lower bound key, or null if from start /** lower bound key, or null if from start */
        private final K lo;
        
upper bound key, or null if to end /** upper bound key, or null if to end */
        private final K hi;
        
inclusion flag for lo /** inclusion flag for lo */
        private final boolean loInclusive;
        
inclusion flag for hi /** inclusion flag for hi */
        private final boolean hiInclusive;
        
direction /** direction */
        final boolean isDescending;

        // Lazily initialized view holders
        private transient KeySet<K,V> keySetView;
        private transient Values<K,V> valuesView;
        private transient EntrySet<K,V> entrySetView;

        
Creates a new submap, initializing all fields.
/**
         * Creates a new submap, initializing all fields.
         */
        SubMap(ConcurrentSkipListMap<K,V> map,
               K fromKey, boolean fromInclusive,
               K toKey, boolean toInclusive,
               boolean isDescending) {
            Comparator<? super K> cmp = map.comparator;
            if (fromKey != null && toKey != null &&
                cpr(cmp, fromKey, toKey) > 0)
                throw new IllegalArgumentException("inconsistent range");
            this.m = map;
            this.lo = fromKey;
            this.hi = toKey;
            this.loInclusive = fromInclusive;
            this.hiInclusive = toInclusive;
            this.isDescending = isDescending;
        }

        /* ----------------  Utilities -------------- */

        boolean tooLow(Object key, Comparator<? super K> cmp) {
            int c;
            return (lo != null && ((c = cpr(cmp, key, lo)) < 0 ||
                                   (c == 0 && !loInclusive)));
        }

        boolean tooHigh(Object key, Comparator<? super K> cmp) {
            int c;
            return (hi != null && ((c = cpr(cmp, key, hi)) > 0 ||
                                   (c == 0 && !hiInclusive)));
        }

        boolean inBounds(Object key, Comparator<? super K> cmp) {
            return !tooLow(key, cmp) && !tooHigh(key, cmp);
        }

        void checkKeyBounds(K key, Comparator<? super K> cmp) {
            if (key == null)
                throw new NullPointerException();
            if (!inBounds(key, cmp))
                throw new IllegalArgumentException("key out of range");
        }

        
Returns true if node key is less than upper bound of range.
/**
         * Returns true if node key is less than upper bound of range.
         */
        boolean isBeforeEnd(ConcurrentSkipListMap.Node<K,V> n,
                            Comparator<? super K> cmp) {
            if (n == null)
                return false;
            if (hi == null)
                return true;
            K k = n.key;
            if (k == null) // pass by markers and headers
                return true;
            int c = cpr(cmp, k, hi);
            return c < 0 || (c == 0 && hiInclusive);
        }

        
Returns lowest node. This node might not be in range, so
most usages need to check bounds.
/**
         * Returns lowest node. This node might not be in range, so
         * most usages need to check bounds.
         */
        ConcurrentSkipListMap.Node<K,V> loNode(Comparator<? super K> cmp) {
            if (lo == null)
                return m.findFirst();
            else if (loInclusive)
                return m.findNear(lo, GT|EQ, cmp);
            else
                return m.findNear(lo, GT, cmp);
        }

        
Returns highest node. This node might not be in range, so
most usages need to check bounds.
/**
         * Returns highest node. This node might not be in range, so
         * most usages need to check bounds.
         */
        ConcurrentSkipListMap.Node<K,V> hiNode(Comparator<? super K> cmp) {
            if (hi == null)
                return m.findLast();
            else if (hiInclusive)
                return m.findNear(hi, LT|EQ, cmp);
            else
                return m.findNear(hi, LT, cmp);
        }

        
Returns lowest absolute key (ignoring directionality).
/**
         * Returns lowest absolute key (ignoring directionality).
         */
        K lowestKey() {
            Comparator<? super K> cmp = m.comparator;
            ConcurrentSkipListMap.Node<K,V> n = loNode(cmp);
            if (isBeforeEnd(n, cmp))
                return n.key;
            else
                throw new NoSuchElementException();
        }

        
Returns highest absolute key (ignoring directionality).
/**
         * Returns highest absolute key (ignoring directionality).
         */
        K highestKey() {
            Comparator<? super K> cmp = m.comparator;
            ConcurrentSkipListMap.Node<K,V> n = hiNode(cmp);
            if (n != null) {
                K last = n.key;
                if (inBounds(last, cmp))
                    return last;
            }
            throw new NoSuchElementException();
        }

        Map.Entry<K,V> lowestEntry() {
            Comparator<? super K> cmp = m.comparator;
            for (;;) {
                ConcurrentSkipListMap.Node<K,V> n; V v;
                if ((n = loNode(cmp)) == null || !isBeforeEnd(n, cmp))
                    return null;
                else if ((v = n.val) != null)
                    return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
            }
        }

        Map.Entry<K,V> highestEntry() {
            Comparator<? super K> cmp = m.comparator;
            for (;;) {
                ConcurrentSkipListMap.Node<K,V> n; V v;
                if ((n = hiNode(cmp)) == null || !inBounds(n.key, cmp))
                    return null;
                else if ((v = n.val) != null)
                    return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
            }
        }

        Map.Entry<K,V> removeLowest() {
            Comparator<? super K> cmp = m.comparator;
            for (;;) {
                ConcurrentSkipListMap.Node<K,V> n; K k; V v;
                if ((n = loNode(cmp)) == null)
                    return null;
                else if (!inBounds((k = n.key), cmp))
                    return null;
                else if ((v = m.doRemove(k, null)) != null)
                    return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
            }
        }

        Map.Entry<K,V> removeHighest() {
            Comparator<? super K> cmp = m.comparator;
            for (;;) {
                ConcurrentSkipListMap.Node<K,V> n; K k; V v;
                if ((n = hiNode(cmp)) == null)
                    return null;
                else if (!inBounds((k = n.key), cmp))
                    return null;
                else if ((v = m.doRemove(k, null)) != null)
                    return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
            }
        }

        
Submap version of ConcurrentSkipListMap.findNearEntry.
/**
         * Submap version of ConcurrentSkipListMap.findNearEntry.
         */
        Map.Entry<K,V> getNearEntry(K key, int rel) {
            Comparator<? super K> cmp = m.comparator;
            if (isDescending) { // adjust relation for direction
                if ((rel & LT) == 0)
                    rel |= LT;
                else
                    rel &= ~LT;
            }
            if (tooLow(key, cmp))
                return ((rel & LT) != 0) ? null : lowestEntry();
            if (tooHigh(key, cmp))
                return ((rel & LT) != 0) ? highestEntry() : null;
            AbstractMap.SimpleImmutableEntry<K,V> e =
                m.findNearEntry(key, rel, cmp);
            if (e == null || !inBounds(e.getKey(), cmp))
                return null;
            else
                return e;
        }

        // Almost the same as getNearEntry, except for keys
        K getNearKey(K key, int rel) {
            Comparator<? super K> cmp = m.comparator;
            if (isDescending) { // adjust relation for direction
                if ((rel & LT) == 0)
                    rel |= LT;
                else
                    rel &= ~LT;
            }
            if (tooLow(key, cmp)) {
                if ((rel & LT) == 0) {
                    ConcurrentSkipListMap.Node<K,V> n = loNode(cmp);
                    if (isBeforeEnd(n, cmp))
                        return n.key;
                }
                return null;
            }
            if (tooHigh(key, cmp)) {
                if ((rel & LT) != 0) {
                    ConcurrentSkipListMap.Node<K,V> n = hiNode(cmp);
                    if (n != null) {
                        K last = n.key;
                        if (inBounds(last, cmp))
                            return last;
                    }
                }
                return null;
            }
            for (;;) {
                Node<K,V> n = m.findNear(key, rel, cmp);
                if (n == null || !inBounds(n.key, cmp))
                    return null;
                if (n.val != null)
                    return n.key;
            }
        }

        /* ----------------  Map API methods -------------- */

        public boolean containsKey(Object key) {
            if (key == null) throw new NullPointerException();
            return inBounds(key, m.comparator) && m.containsKey(key);
        }

        public V get(Object key) {
            if (key == null) throw new NullPointerException();
            return (!inBounds(key, m.comparator)) ? null : m.get(key);
        }

        public V put(K key, V value) {
            checkKeyBounds(key, m.comparator);
            return m.put(key, value);
        }

        public V remove(Object key) {
            return (!inBounds(key, m.comparator)) ? null : m.remove(key);
        }

        public int size() {
            Comparator<? super K> cmp = m.comparator;
            long count = 0;
            for (ConcurrentSkipListMap.Node<K,V> n = loNode(cmp);
                 isBeforeEnd(n, cmp);
                 n = n.next) {
                if (n.val != null)
                    ++count;
            }
            return count >= Integer.MAX_VALUE ? Integer.MAX_VALUE : (int)count;
        }

        public boolean isEmpty() {
            Comparator<? super K> cmp = m.comparator;
            return !isBeforeEnd(loNode(cmp), cmp);
        }

        public boolean containsValue(Object value) {
            if (value == null)
                throw new NullPointerException();
            Comparator<? super K> cmp = m.comparator;
            for (ConcurrentSkipListMap.Node<K,V> n = loNode(cmp);
                 isBeforeEnd(n, cmp);
                 n = n.next) {
                V v = n.val;
                if (v != null && value.equals(v))
                    return true;
            }
            return false;
        }

        public void clear() {
            Comparator<? super K> cmp = m.comparator;
            for (ConcurrentSkipListMap.Node<K,V> n = loNode(cmp);
                 isBeforeEnd(n, cmp);
                 n = n.next) {
                if (n.val != null)
                    m.remove(n.key);
            }
        }

        /* ----------------  ConcurrentMap API methods -------------- */

        public V putIfAbsent(K key, V value) {
            checkKeyBounds(key, m.comparator);
            return m.putIfAbsent(key, value);
        }

        public boolean remove(Object key, Object value) {
            return inBounds(key, m.comparator) && m.remove(key, value);
        }

        public boolean replace(K key, V oldValue, V newValue) {
            checkKeyBounds(key, m.comparator);
            return m.replace(key, oldValue, newValue);
        }

        public V replace(K key, V value) {
            checkKeyBounds(key, m.comparator);
            return m.replace(key, value);
        }

        /* ----------------  SortedMap API methods -------------- */

        public Comparator<? super K> comparator() {
            Comparator<? super K> cmp = m.comparator();
            if (isDescending)
                return Collections.reverseOrder(cmp);
            else
                return cmp;
        }

        
Utility to create submaps, where given bounds override
unbounded(null) ones and/or are checked against bounded ones.
/**
         * Utility to create submaps, where given bounds override
         * unbounded(null) ones and/or are checked against bounded ones.
         */
        SubMap<K,V> newSubMap(K fromKey, boolean fromInclusive,
                              K toKey, boolean toInclusive) {
            Comparator<? super K> cmp = m.comparator;
            if (isDescending) { // flip senses
                K tk = fromKey;
                fromKey = toKey;
                toKey = tk;
                boolean ti = fromInclusive;
                fromInclusive = toInclusive;
                toInclusive = ti;
            }
            if (lo != null) {
                if (fromKey == null) {
                    fromKey = lo;
                    fromInclusive = loInclusive;
                }
                else {
                    int c = cpr(cmp, fromKey, lo);
                    if (c < 0 || (c == 0 && !loInclusive && fromInclusive))
                        throw new IllegalArgumentException("key out of range");
                }
            }
            if (hi != null) {
                if (toKey == null) {
                    toKey = hi;
                    toInclusive = hiInclusive;
                }
                else {
                    int c = cpr(cmp, toKey, hi);
                    if (c > 0 || (c == 0 && !hiInclusive && toInclusive))
                        throw new IllegalArgumentException("key out of range");
                }
            }
            return new SubMap<K,V>(m, fromKey, fromInclusive,
                                   toKey, toInclusive, isDescending);
        }

        public SubMap<K,V> subMap(K fromKey, boolean fromInclusive,
                                  K toKey, boolean toInclusive) {
            if (fromKey == null || toKey == null)
                throw new NullPointerException();
            return newSubMap(fromKey, fromInclusive, toKey, toInclusive);
        }

        public SubMap<K,V> headMap(K toKey, boolean inclusive) {
            if (toKey == null)
                throw new NullPointerException();
            return newSubMap(null, false, toKey, inclusive);
        }

        public SubMap<K,V> tailMap(K fromKey, boolean inclusive) {
            if (fromKey == null)
                throw new NullPointerException();
            return newSubMap(fromKey, inclusive, null, false);
        }

        public SubMap<K,V> subMap(K fromKey, K toKey) {
            return subMap(fromKey, true, toKey, false);
        }

        public SubMap<K,V> headMap(K toKey) {
            return headMap(toKey, false);
        }

        public SubMap<K,V> tailMap(K fromKey) {
            return tailMap(fromKey, true);
        }

        public SubMap<K,V> descendingMap() {
            return new SubMap<K,V>(m, lo, loInclusive,
                                   hi, hiInclusive, !isDescending);
        }

        /* ----------------  Relational methods -------------- */

        public Map.Entry<K,V> ceilingEntry(K key) {
            return getNearEntry(key, GT|EQ);
        }

        public K ceilingKey(K key) {
            return getNearKey(key, GT|EQ);
        }

        public Map.Entry<K,V> lowerEntry(K key) {
            return getNearEntry(key, LT);
        }

        public K lowerKey(K key) {
            return getNearKey(key, LT);
        }

        public Map.Entry<K,V> floorEntry(K key) {
            return getNearEntry(key, LT|EQ);
        }

        public K floorKey(K key) {
            return getNearKey(key, LT|EQ);
        }

        public Map.Entry<K,V> higherEntry(K key) {
            return getNearEntry(key, GT);
        }

        public K higherKey(K key) {
            return getNearKey(key, GT);
        }

        public K firstKey() {
            return isDescending ? highestKey() : lowestKey();
        }

        public K lastKey() {
            return isDescending ? lowestKey() : highestKey();
        }

        public Map.Entry<K,V> firstEntry() {
            return isDescending ? highestEntry() : lowestEntry();
        }

        public Map.Entry<K,V> lastEntry() {
            return isDescending ? lowestEntry() : highestEntry();
        }

        public Map.Entry<K,V> pollFirstEntry() {
            return isDescending ? removeHighest() : removeLowest();
        }

        public Map.Entry<K,V> pollLastEntry() {
            return isDescending ? removeLowest() : removeHighest();
        }

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

        public NavigableSet<K> keySet() {
            KeySet<K,V> ks;
            if ((ks = keySetView) != null) return ks;
            return keySetView = new KeySet<>(this);
        }

        public NavigableSet<K> navigableKeySet() {
            KeySet<K,V> ks;
            if ((ks = keySetView) != null) return ks;
            return keySetView = new KeySet<>(this);
        }

        public Collection<V> values() {
            Values<K,V> vs;
            if ((vs = valuesView) != null) return vs;
            return valuesView = new Values<>(this);
        }

        public Set<Map.Entry<K,V>> entrySet() {
            EntrySet<K,V> es;
            if ((es = entrySetView) != null) return es;
            return entrySetView = new EntrySet<K,V>(this);
        }

        public NavigableSet<K> descendingKeySet() {
            return descendingMap().navigableKeySet();
        }

        
Variant of main Iter class to traverse through submaps.
Also serves as back-up Spliterator for views.
/**
         * Variant of main Iter class to traverse through submaps.
         * Also serves as back-up Spliterator for views.
         */
        abstract class SubMapIter<T> implements Iterator<T>, Spliterator<T> {
            
the last node returned by next() /** the last node returned by next() */
            Node<K,V> lastReturned;
            
the next node to return from next(); /** the next node to return from next(); */
            Node<K,V> next;
            
Cache of next value field to maintain weak consistency /** Cache of next value field to maintain weak consistency */
            V nextValue;

            SubMapIter() {
                VarHandle.acquireFence();
                Comparator<? super K> cmp = m.comparator;
                for (;;) {
                    next = isDescending ? hiNode(cmp) : loNode(cmp);
                    if (next == null)
                        break;
                    V x = next.val;
                    if (x != null) {
                        if (! inBounds(next.key, cmp))
                            next = null;
                        else
                            nextValue = x;
                        break;
                    }
                }
            }

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

            final void advance() {
                if (next == null)
                    throw new NoSuchElementException();
                lastReturned = next;
                if (isDescending)
                    descend();
                else
                    ascend();
            }

            private void ascend() {
                Comparator<? super K> cmp = m.comparator;
                for (;;) {
                    next = next.next;
                    if (next == null)
                        break;
                    V x = next.val;
                    if (x != null) {
                        if (tooHigh(next.key, cmp))
                            next = null;
                        else
                            nextValue = x;
                        break;
                    }
                }
            }

            private void descend() {
                Comparator<? super K> cmp = m.comparator;
                for (;;) {
                    next = m.findNear(lastReturned.key, LT, cmp);
                    if (next == null)
                        break;
                    V x = next.val;
                    if (x != null) {
                        if (tooLow(next.key, cmp))
                            next = null;
                        else
                            nextValue = x;
                        break;
                    }
                }
            }

            public void remove() {
                Node<K,V> l = lastReturned;
                if (l == null)
                    throw new IllegalStateException();
                m.remove(l.key);
                lastReturned = null;
            }

            public Spliterator<T> trySplit() {
                return null;
            }

            public boolean tryAdvance(Consumer<? super T> action) {
                if (hasNext()) {
                    action.accept(next());
                    return true;
                }
                return false;
            }

            public void forEachRemaining(Consumer<? super T> action) {
                while (hasNext())
                    action.accept(next());
            }

            public long estimateSize() {
                return Long.MAX_VALUE;
            }

        }

        final class SubMapValueIterator extends SubMapIter<V> {
            public V next() {
                V v = nextValue;
                advance();
                return v;
            }
            public int characteristics() {
                return 0;
            }
        }

        final class SubMapKeyIterator extends SubMapIter<K> {
            public K next() {
                Node<K,V> n = next;
                advance();
                return n.key;
            }
            public int characteristics() {
                return Spliterator.DISTINCT | Spliterator.ORDERED |
                    Spliterator.SORTED;
            }
            public final Comparator<? super K> getComparator() {
                return SubMap.this.comparator();
            }
        }

        final class SubMapEntryIterator extends SubMapIter<Map.Entry<K,V>> {
            public Map.Entry<K,V> next() {
                Node<K,V> n = next;
                V v = nextValue;
                advance();
                return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
            }
            public int characteristics() {
                return Spliterator.DISTINCT;
            }
        }
    }

    // default Map method overrides

    public void forEach(BiConsumer<? super K, ? super V> action) {
        if (action == null) throw new NullPointerException();
        Node<K,V> b, n; V v;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                if ((v = n.val) != null)
                    action.accept(n.key, v);
                b = n;
            }
        }
    }

    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        if (function == null) throw new NullPointerException();
        Node<K,V> b, n; V v;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                while ((v = n.val) != null) {
                    V r = function.apply(n.key, v);
                    if (r == null) throw new NullPointerException();
                    if (VAL.compareAndSet(n, v, r))
                        break;
                }
                b = n;
            }
        }
    }

    
Helper method for EntrySet.removeIf.
/**
     * Helper method for EntrySet.removeIf.
     */
    boolean removeEntryIf(Predicate<? super Entry<K,V>> function) {
        if (function == null) throw new NullPointerException();
        boolean removed = false;
        Node<K,V> b, n; V v;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                if ((v = n.val) != null) {
                    K k = n.key;
                    Map.Entry<K,V> e = new AbstractMap.SimpleImmutableEntry<>(k, v);
                    if (function.test(e) && remove(k, v))
                        removed = true;
                }
                b = n;
            }
        }
        return removed;
    }

    
Helper method for Values.removeIf.
/**
     * Helper method for Values.removeIf.
     */
    boolean removeValueIf(Predicate<? super V> function) {
        if (function == null) throw new NullPointerException();
        boolean removed = false;
        Node<K,V> b, n; V v;
        if ((b = baseHead()) != null) {
            while ((n = b.next) != null) {
                if ((v = n.val) != null && function.test(v) && remove(n.key, v))
                    removed = true;
                b = n;
            }
        }
        return removed;
    }

    
Base class providing common structure for Spliterators.
(Although not all that much common functionality; as usual for
view classes, details annoyingly vary in key, value, and entry
subclasses in ways that are not worth abstracting out for
internal classes.)
The basic split strategy is to recursively descend from top
level, row by row, descending to next row when either split
off, or the end of row is encountered. Control of the number of
splits relies on some statistical estimation: The expected
remaining number of elements of a skip list when advancing
either across or down decreases by about 25%.
/**
     * Base class providing common structure for Spliterators.
     * (Although not all that much common functionality; as usual for
     * view classes, details annoyingly vary in key, value, and entry
     * subclasses in ways that are not worth abstracting out for
     * internal classes.)
     *
     * The basic split strategy is to recursively descend from top
     * level, row by row, descending to next row when either split
     * off, or the end of row is encountered. Control of the number of
     * splits relies on some statistical estimation: The expected
     * remaining number of elements of a skip list when advancing
     * either across or down decreases by about 25%.
     */
    abstract static class CSLMSpliterator<K,V> {
        final Comparator<? super K> comparator;
        final K fence;     // exclusive upper bound for keys, or null if to end
        Index<K,V> row;    // the level to split out
        Node<K,V> current; // current traversal node; initialize at origin
        long est;          // size estimate
        CSLMSpliterator(Comparator<? super K> comparator, Index<K,V> row,
                        Node<K,V> origin, K fence, long est) {
            this.comparator = comparator; this.row = row;
            this.current = origin; this.fence = fence; this.est = est;
        }

        public final long estimateSize() { return est; }
    }

    static final class KeySpliterator<K,V> extends CSLMSpliterator<K,V>
        implements Spliterator<K> {
        KeySpliterator(Comparator<? super K> comparator, Index<K,V> row,
                       Node<K,V> origin, K fence, long est) {
            super(comparator, row, origin, fence, est);
        }

        public KeySpliterator<K,V> trySplit() {
            Node<K,V> e; K ek;
            Comparator<? super K> cmp = comparator;
            K f = fence;
            if ((e = current) != null && (ek = e.key) != null) {
                for (Index<K,V> q = row; q != null; q = row = q.down) {
                    Index<K,V> s; Node<K,V> b, n; K sk;
                    if ((s = q.right) != null && (b = s.node) != null &&
                        (n = b.next) != null && n.val != null &&
                        (sk = n.key) != null && cpr(cmp, sk, ek) > 0 &&
                        (f == null || cpr(cmp, sk, f) < 0)) {
                        current = n;
                        Index<K,V> r = q.down;
                        row = (s.right != null) ? s : s.down;
                        est -= est >>> 2;
                        return new KeySpliterator<K,V>(cmp, r, e, sk, est);
                    }
                }
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super K> action) {
            if (action == null) throw new NullPointerException();
            Comparator<? super K> cmp = comparator;
            K f = fence;
            Node<K,V> e = current;
            current = null;
            for (; e != null; e = e.next) {
                K k;
                if ((k = e.key) != null && f != null && cpr(cmp, f, k) <= 0)
                    break;
                if (e.val != null)
                    action.accept(k);
            }
        }

        public boolean tryAdvance(Consumer<? super K> action) {
            if (action == null) throw new NullPointerException();
            Comparator<? super K> cmp = comparator;
            K f = fence;
            Node<K,V> e = current;
            for (; e != null; e = e.next) {
                K k;
                if ((k = e.key) != null && f != null && cpr(cmp, f, k) <= 0) {
                    e = null;
                    break;
                }
                if (e.val != null) {
                    current = e.next;
                    action.accept(k);
                    return true;
                }
            }
            current = e;
            return false;
        }

        public int characteristics() {
            return Spliterator.DISTINCT | Spliterator.SORTED |
                Spliterator.ORDERED | Spliterator.CONCURRENT |
                Spliterator.NONNULL;
        }

        public final Comparator<? super K> getComparator() {
            return comparator;
        }
    }
    // factory method for KeySpliterator
    final KeySpliterator<K,V> keySpliterator() {
        Index<K,V> h; Node<K,V> n; long est;
        VarHandle.acquireFence();
        if ((h = head) == null) {
            n = null;
            est = 0L;
        }
        else {
            n = h.node;
            est = getAdderCount();
        }
        return new KeySpliterator<K,V>(comparator, h, n, null, est);
    }

    static final class ValueSpliterator<K,V> extends CSLMSpliterator<K,V>
        implements Spliterator<V> {
        ValueSpliterator(Comparator<? super K> comparator, Index<K,V> row,
                       Node<K,V> origin, K fence, long est) {
            super(comparator, row, origin, fence, est);
        }

        public ValueSpliterator<K,V> trySplit() {
            Node<K,V> e; K ek;
            Comparator<? super K> cmp = comparator;
            K f = fence;
            if ((e = current) != null && (ek = e.key) != null) {
                for (Index<K,V> q = row; q != null; q = row = q.down) {
                    Index<K,V> s; Node<K,V> b, n; K sk;
                    if ((s = q.right) != null && (b = s.node) != null &&
                        (n = b.next) != null && n.val != null &&
                        (sk = n.key) != null && cpr(cmp, sk, ek) > 0 &&
                        (f == null || cpr(cmp, sk, f) < 0)) {
                        current = n;
                        Index<K,V> r = q.down;
                        row = (s.right != null) ? s : s.down;
                        est -= est >>> 2;
                        return new ValueSpliterator<K,V>(cmp, r, e, sk, est);
                    }
                }
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super V> action) {
            if (action == null) throw new NullPointerException();
            Comparator<? super K> cmp = comparator;
            K f = fence;
            Node<K,V> e = current;
            current = null;
            for (; e != null; e = e.next) {
                K k; V v;
                if ((k = e.key) != null && f != null && cpr(cmp, f, k) <= 0)
                    break;
                if ((v = e.val) != null)
                    action.accept(v);
            }
        }

        public boolean tryAdvance(Consumer<? super V> action) {
            if (action == null) throw new NullPointerException();
            Comparator<? super K> cmp = comparator;
            K f = fence;
            Node<K,V> e = current;
            for (; e != null; e = e.next) {
                K k; V v;
                if ((k = e.key) != null && f != null && cpr(cmp, f, k) <= 0) {
                    e = null;
                    break;
                }
                if ((v = e.val) != null) {
                    current = e.next;
                    action.accept(v);
                    return true;
                }
            }
            current = e;
            return false;
        }

        public int characteristics() {
            return Spliterator.CONCURRENT | Spliterator.ORDERED |
                Spliterator.NONNULL;
        }
    }

    // Almost the same as keySpliterator()
    final ValueSpliterator<K,V> valueSpliterator() {
        Index<K,V> h; Node<K,V> n; long est;
        VarHandle.acquireFence();
        if ((h = head) == null) {
            n = null;
            est = 0L;
        }
        else {
            n = h.node;
            est = getAdderCount();
        }
        return new ValueSpliterator<K,V>(comparator, h, n, null, est);
    }

    static final class EntrySpliterator<K,V> extends CSLMSpliterator<K,V>
        implements Spliterator<Map.Entry<K,V>> {
        EntrySpliterator(Comparator<? super K> comparator, Index<K,V> row,
                         Node<K,V> origin, K fence, long est) {
            super(comparator, row, origin, fence, est);
        }

        public EntrySpliterator<K,V> trySplit() {
            Node<K,V> e; K ek;
            Comparator<? super K> cmp = comparator;
            K f = fence;
            if ((e = current) != null && (ek = e.key) != null) {
                for (Index<K,V> q = row; q != null; q = row = q.down) {
                    Index<K,V> s; Node<K,V> b, n; K sk;
                    if ((s = q.right) != null && (b = s.node) != null &&
                        (n = b.next) != null && n.val != null &&
                        (sk = n.key) != null && cpr(cmp, sk, ek) > 0 &&
                        (f == null || cpr(cmp, sk, f) < 0)) {
                        current = n;
                        Index<K,V> r = q.down;
                        row = (s.right != null) ? s : s.down;
                        est -= est >>> 2;
                        return new EntrySpliterator<K,V>(cmp, r, e, sk, est);
                    }
                }
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super Map.Entry<K,V>> action) {
            if (action == null) throw new NullPointerException();
            Comparator<? super K> cmp = comparator;
            K f = fence;
            Node<K,V> e = current;
            current = null;
            for (; e != null; e = e.next) {
                K k; V v;
                if ((k = e.key) != null && f != null && cpr(cmp, f, k) <= 0)
                    break;
                if ((v = e.val) != null) {
                    action.accept
                        (new AbstractMap.SimpleImmutableEntry<K,V>(k, v));
                }
            }
        }

        public boolean tryAdvance(Consumer<? super Map.Entry<K,V>> action) {
            if (action == null) throw new NullPointerException();
            Comparator<? super K> cmp = comparator;
            K f = fence;
            Node<K,V> e = current;
            for (; e != null; e = e.next) {
                K k; V v;
                if ((k = e.key) != null && f != null && cpr(cmp, f, k) <= 0) {
                    e = null;
                    break;
                }
                if ((v = e.val) != null) {
                    current = e.next;
                    action.accept
                        (new AbstractMap.SimpleImmutableEntry<K,V>(k, v));
                    return true;
                }
            }
            current = e;
            return false;
        }

        public int characteristics() {
            return Spliterator.DISTINCT | Spliterator.SORTED |
                Spliterator.ORDERED | Spliterator.CONCURRENT |
                Spliterator.NONNULL;
        }

        public final Comparator<Map.Entry<K,V>> getComparator() {
            // Adapt or create a key-based comparator
            if (comparator != null) {
                return Map.Entry.comparingByKey(comparator);
            }
            else {
                return (Comparator<Map.Entry<K,V>> & Serializable) (e1, e2) -> {
                    @SuppressWarnings("unchecked")
                    Comparable<? super K> k1 = (Comparable<? super K>) e1.getKey();
                    return k1.compareTo(e2.getKey());
                };
            }
        }
    }

    // Almost the same as keySpliterator()
    final EntrySpliterator<K,V> entrySpliterator() {
        Index<K,V> h; Node<K,V> n; long est;
        VarHandle.acquireFence();
        if ((h = head) == null) {
            n = null;
            est = 0L;
        }
        else {
            n = h.node;
            est = getAdderCount();
        }
        return new EntrySpliterator<K,V>(comparator, h, n, null, est);
    }

    // VarHandle mechanics
    private static final VarHandle HEAD;
    private static final VarHandle ADDER;
    private static final VarHandle NEXT;
    private static final VarHandle VAL;
    private static final VarHandle RIGHT;
    static {
        try {
            MethodHandles.Lookup l = MethodHandles.lookup();
            HEAD = l.findVarHandle(ConcurrentSkipListMap.class, "head",
                                   Index.class);
            ADDER = l.findVarHandle(ConcurrentSkipListMap.class, "adder",
                                    LongAdder.class);
            NEXT = l.findVarHandle(Node.class, "next", Node.class);
            VAL = l.findVarHandle(Node.class, "val", Object.class);
            RIGHT = l.findVarHandle(Index.class, "right", Index.class);
        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(e);
        }
    }
}