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From hinde...@apache.org
Subject svn commit: r800934 [2/6] - in /harmony/enhanced/classlib/branches/java6/modules/concurrent/src/main/java/java/util/concurrent: ./ locks/
Date Tue, 04 Aug 2009 19:39:25 GMT
Added: harmony/enhanced/classlib/branches/java6/modules/concurrent/src/main/java/java/util/concurrent/ConcurrentSkipListMap.java
URL: http://svn.apache.org/viewvc/harmony/enhanced/classlib/branches/java6/modules/concurrent/src/main/java/java/util/concurrent/ConcurrentSkipListMap.java?rev=800934&view=auto
==============================================================================
--- harmony/enhanced/classlib/branches/java6/modules/concurrent/src/main/java/java/util/concurrent/ConcurrentSkipListMap.java (added)
+++ harmony/enhanced/classlib/branches/java6/modules/concurrent/src/main/java/java/util/concurrent/ConcurrentSkipListMap.java Tue Aug  4 19:39:24 2009
@@ -0,0 +1,3115 @@
+/*
+ * 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/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+import java.util.concurrent.atomic.*;
+
+/**
+ * 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://www.cs.umd.edu/~pugh/">SkipLists</a> providing
+ * expected average <i>log(n)</i> time cost for the
+ * <tt>containsKey</tt>, <tt>get</tt>, <tt>put</tt> and
+ * <tt>remove</tt> operations and their variants.  Insertion, removal,
+ * update, and access operations safely execute concurrently by
+ * multiple threads.  Iterators are <i>weakly consistent</i>, returning
+ * elements reflecting the state of the map at some point at or since
+ * the creation of the iterator.  They do <em>not</em> throw {@link
+ * ConcurrentModificationException}, and may proceed concurrently with
+ * other operations. Ascending key ordered views and their iterators
+ * are faster than descending ones.
+ *
+ * <p>All <tt>Map.Entry</tt> 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 <tt>Entry.setValue</tt>
+ * method. (Note however that it is possible to change mappings in the
+ * associated map using <tt>put</tt>, <tt>putIfAbsent</tt>, or
+ * <tt>replace</tt>, depending on exactly which effect you need.)
+ *
+ * <p>Beware that, unlike in most collections, the <tt>size</tt>
+ * method is <em>not</em> a constant-time operation. Because of the
+ * asynchronous nature of these maps, determining the current number
+ * of elements requires a traversal of the elements.  Additionally,
+ * the bulk operations <tt>putAll</tt>, <tt>equals</tt>, and
+ * <tt>clear</tt> are <em>not</em> guaranteed to be performed
+ * atomically. For example, an iterator operating concurrently with a
+ * <tt>putAll</tt> 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 <tt>null</tt> 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}/../technotes/guides/collections/index.html">
+ * 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,
+               java.io.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 (indicating this by
+     * using otherwise impossible field values).  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, 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 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. This maintains proper control of
+     * concurrent replace vs delete operations -- an attempted replace
+     * must fail if a delete beat it by nulling field, and a delete
+     * must return the last non-null value held in the field. (Note:
+     * Null, rather than some special marker, is used for value fields
+     * here because it just so happens to mesh with the Map API
+     * requirement that method get returns null if there is no
+     * mapping, which allows nodes to remain concurrently readable
+     * even when deleted. Using any other marker value here would be
+     * messy at best.)
+     *
+     * 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.
+     *    From this point on, no public operations encountering
+     *    the node consider this mapping to exist. However, other
+     *    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.  The use of marker nodes slightly
+     * complicates helping-out code because traversals must track
+     * consistent reads of up to four nodes (b, n, marker, f), not
+     * just (b, n, f), although the next field of a marker is
+     * immutable, and once a next field is CAS'ed to point to a
+     * marker, it never again changes, so this requires less care.
+     *
+     * 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 as lists with volatile next fields,
+     * using CAS to link and unlink.  Races are allowed in index-list
+     * operations that can (rarely) fail to link in a new index node
+     * or delete one. (We can't do this of course for data nodes.)
+     * However, even when this happens, the index lists remain sorted,
+     * so correctly serve as indices.  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. And race windows are kept small enough
+     * that in practice these failures are rare, even under a lot of
+     * contention.
+     *
+     * The fact that retries (for both base and index lists) are
+     * relatively cheap due to indexing allows some minor
+     * simplifications of retry logic. Traversal restarts are
+     * performed after most "helping-out" CASes. This isn't always
+     * strictly necessary, but the implicit backoffs tend to help
+     * reduce other downstream failed CAS's enough to outweigh restart
+     * cost.  This worsens the worst case, but seems to improve even
+     * highly contended cases.
+     *
+     * Unlike most skip-list implementations, index insertion and
+     * deletion here require a separate traversal pass occuring 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. This is more important
+     * here than in some other data structures because we cannot null
+     * out node fields referencing user keys since they might still be
+     * read by other ongoing traversals.
+     *
+     * 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 randomLevel) 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).  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. The head index has initial
+     * level/height of one. 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 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.
+     *
+     * The code for all this is more verbose than you'd like. Most
+     * operations entail locating an element (or position to insert an
+     * element). The code to do this can't be nicely factored out
+     * because subsequent uses require a snapshot of predecessor
+     * and/or successor and/or value fields which can't be returned
+     * all at once, at least not without creating yet another object
+     * to hold them -- creating such little objects is an especially
+     * bad idea for basic internal search operations because it adds
+     * to GC overhead.  (This is one of the few times I've wished Java
+     * had macros.) Instead, some traversal code is interleaved within
+     * insertion and removal operations.  The control logic to handle
+     * all the retry conditions is sometimes twisty. Most search is
+     * broken into 2 parts. findPredecessor() searches index nodes
+     * only, returning a base-level predecessor of the key. findNode()
+     * finishes out the base-level search. Even with this factoring,
+     * there is a fair amount of near-duplication of code to handle
+     * variants.
+     *
+     * 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/).
+     *
+     * Given the use of tree-like index nodes, you might wonder why
+     * this doesn't use some kind of search tree instead, which would
+     * support somewhat faster search operations. The reason is that
+     * there are no known efficient lock-free insertion and deletion
+     * algorithms for search trees. The immutability of the "down"
+     * links of index nodes (as opposed to mutable "left" fields in
+     * true trees) makes this tractable using only CAS operations.
+     *
+     * Notation guide for local variables
+     * Node:         b, n, f    for  predecessor, node, successor
+     * Index:        q, r, d    for index node, right, down.
+     *               t          for another index node
+     * Head:         h
+     * Levels:       j
+     * Keys:         k, key
+     * Values:       v, value
+     * Comparisons:  c
+     */
+
+    private static final long serialVersionUID = -8627078645895051609L;
+
+    /**
+     * Generates the initial random seed for the cheaper per-instance
+     * random number generators used in randomLevel.
+     */
+    private static final Random seedGenerator = new Random();
+
+    /**
+     * Special value used to identify base-level header
+     */
+    private static final Object BASE_HEADER = new Object();
+
+    /**
+     * The topmost head index of the skiplist.
+     */
+    private transient volatile HeadIndex<K,V> head;
+
+    /**
+     * The comparator used to maintain order in this map, or null
+     * if using natural ordering.
+     * @serial
+     */
+    private final Comparator<? super K> comparator;
+
+    /**
+     * Seed for simple random number generator.  Not volatile since it
+     * doesn't matter too much if different threads don't see updates.
+     */
+    private transient int randomSeed;
+
+    /** Lazily initialized key set */
+    private transient KeySet keySet;
+    /** Lazily initialized entry set */
+    private transient EntrySet entrySet;
+    /** Lazily initialized values collection */
+    private transient Values values;
+    /** Lazily initialized descending key set */
+    private transient ConcurrentNavigableMap<K,V> descendingMap;
+
+    /**
+     * Initializes or resets state. Needed by constructors, clone,
+     * clear, readObject. and ConcurrentSkipListSet.clone.
+     * (Note that comparator must be separately initialized.)
+     */
+    final void initialize() {
+        keySet = null;
+        entrySet = null;
+        values = null;
+        descendingMap = null;
+        randomSeed = seedGenerator.nextInt() | 0x0100; // ensure nonzero
+        head = new HeadIndex<K,V>(new Node<K,V>(null, BASE_HEADER, null),
+                                  null, null, 1);
+    }
+
+    /** Updater for casHead */
+    private static final
+        AtomicReferenceFieldUpdater<ConcurrentSkipListMap, HeadIndex>
+        headUpdater = AtomicReferenceFieldUpdater.newUpdater
+        (ConcurrentSkipListMap.class, HeadIndex.class, "head");
+
+    /**
+     * compareAndSet head node
+     */
+    private boolean casHead(HeadIndex<K,V> cmp, HeadIndex<K,V> val) {
+        return headUpdater.compareAndSet(this, cmp, val);
+    }
+
+    /* ---------------- Nodes -------------- */
+
+    /**
+     * Nodes hold keys and values, and are singly linked in sorted
+     * order, possibly with some intervening marker nodes. The list is
+     * headed by a dummy node accessible as head.node. The value field
+     * is declared only as Object because it takes special non-V
+     * values for marker and header nodes.
+     */
+    static final class Node<K,V> {
+        final K key;
+        volatile Object value;
+        volatile Node<K,V> next;
+
+        /**
+         * Creates a new regular node.
+         */
+        Node(K key, Object value, Node<K,V> next) {
+            this.key = key;
+            this.value = value;
+            this.next = next;
+        }
+
+        /**
+         * Creates a new marker node. A marker is distinguished by
+         * having its value field point to itself.  Marker nodes also
+         * have null keys, a fact that is exploited in a few places,
+         * but this doesn't distinguish markers from the base-level
+         * header node (head.node), which also has a null key.
+         */
+        Node(Node<K,V> next) {
+            this.key = null;
+            this.value = this;
+            this.next = next;
+        }
+
+        /** Updater for casNext */
+        static final AtomicReferenceFieldUpdater<Node, Node>
+            nextUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (Node.class, Node.class, "next");
+
+        /** Updater for casValue */
+        static final AtomicReferenceFieldUpdater<Node, Object>
+            valueUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (Node.class, Object.class, "value");
+
+        /**
+         * compareAndSet value field
+         */
+        boolean casValue(Object cmp, Object val) {
+            return valueUpdater.compareAndSet(this, cmp, val);
+        }
+
+        /**
+         * compareAndSet next field
+         */
+        boolean casNext(Node<K,V> cmp, Node<K,V> val) {
+            return nextUpdater.compareAndSet(this, cmp, val);
+        }
+
+        /**
+         * Returns true if this node is a marker. This method isn't
+         * actually called in any current code checking for markers
+         * because callers will have already read value field and need
+         * to use that read (not another done here) and so directly
+         * test if value points to node.
+         * @param n a possibly null reference to a node
+         * @return true if this node is a marker node
+         */
+        boolean isMarker() {
+            return value == this;
+        }
+
+        /**
+         * Returns true if this node is the header of base-level list.
+         * @return true if this node is header node
+         */
+        boolean isBaseHeader() {
+            return value == BASE_HEADER;
+        }
+
+        /**
+         * Tries to append a deletion marker to this node.
+         * @param f the assumed current successor of this node
+         * @return true if successful
+         */
+        boolean appendMarker(Node<K,V> f) {
+            return casNext(f, new Node<K,V>(f));
+        }
+
+        /**
+         * Helps out a deletion by appending marker or unlinking from
+         * predecessor. This is called during traversals when value
+         * field seen to be null.
+         * @param b predecessor
+         * @param f successor
+         */
+        void helpDelete(Node<K,V> b, Node<K,V> f) {
+            /*
+             * Rechecking links and then doing only one of the
+             * help-out stages per call tends to minimize CAS
+             * interference among helping threads.
+             */
+            if (f == next && this == b.next) {
+                if (f == null || f.value != f) // not already marked
+                    appendMarker(f);
+                else
+                    b.casNext(this, f.next);
+            }
+        }
+
+        /**
+         * Returns value if this node contains a valid key-value pair,
+         * else null.
+         * @return this node's value if it isn't a marker or header or
+         * is deleted, else null.
+         */
+        V getValidValue() {
+            Object v = value;
+            if (v == this || v == BASE_HEADER)
+                return null;
+            return (V)v;
+        }
+
+        /**
+         * Creates and returns a new SimpleImmutableEntry holding current
+         * mapping if this node holds a valid value, else null.
+         * @return new entry or null
+         */
+        AbstractMap.SimpleImmutableEntry<K,V> createSnapshot() {
+            V v = getValidValue();
+            if (v == null)
+                return null;
+            return new AbstractMap.SimpleImmutableEntry<K,V>(key, v);
+        }
+    }
+
+    /* ---------------- Indexing -------------- */
+
+    /**
+     * Index nodes represent the levels of the skip list.  Note that
+     * even though both Nodes and Indexes have forward-pointing
+     * fields, they have different types and are handled in different
+     * ways, that can't nicely be captured by placing field in a
+     * shared abstract class.
+     */
+    static class Index<K,V> {
+        final Node<K,V> node;
+        final Index<K,V> down;
+        volatile Index<K,V> right;
+
+        /**
+         * Creates index node with given values.
+         */
+        Index(Node<K,V> node, Index<K,V> down, Index<K,V> right) {
+            this.node = node;
+            this.down = down;
+            this.right = right;
+        }
+
+        /** Updater for casRight */
+        static final AtomicReferenceFieldUpdater<Index, Index>
+            rightUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (Index.class, Index.class, "right");
+
+        /**
+         * compareAndSet right field
+         */
+        final boolean casRight(Index<K,V> cmp, Index<K,V> val) {
+            return rightUpdater.compareAndSet(this, cmp, val);
+        }
+
+        /**
+         * Returns true if the node this indexes has been deleted.
+         * @return true if indexed node is known to be deleted
+         */
+        final boolean indexesDeletedNode() {
+            return node.value == null;
+        }
+
+        /**
+         * Tries to CAS newSucc as successor.  To minimize races with
+         * unlink that may lose this index node, if the node being
+         * indexed is known to be deleted, it doesn't try to link in.
+         * @param succ the expected current successor
+         * @param newSucc the new successor
+         * @return true if successful
+         */
+        final boolean link(Index<K,V> succ, Index<K,V> newSucc) {
+            Node<K,V> n = node;
+            newSucc.right = succ;
+            return n.value != null && casRight(succ, newSucc);
+        }
+
+        /**
+         * Tries to CAS right field to skip over apparent successor
+         * succ.  Fails (forcing a retraversal by caller) if this node
+         * is known to be deleted.
+         * @param succ the expected current successor
+         * @return true if successful
+         */
+        final boolean unlink(Index<K,V> succ) {
+            return !indexesDeletedNode() && casRight(succ, succ.right);
+        }
+    }
+
+    /* ---------------- Head nodes -------------- */
+
+    /**
+     * Nodes heading each level keep track of their level.
+     */
+    static final class HeadIndex<K,V> extends Index<K,V> {
+        final int level;
+        HeadIndex(Node<K,V> node, Index<K,V> down, Index<K,V> right, int level) {
+            super(node, down, right);
+            this.level = level;
+        }
+    }
+
+    /* ---------------- Comparison utilities -------------- */
+
+    /**
+     * Represents a key with a comparator as a Comparable.
+     *
+     * Because most sorted collections seem to use natural ordering on
+     * Comparables (Strings, Integers, etc), most internal methods are
+     * geared to use them. This is generally faster than checking
+     * per-comparison whether to use comparator or comparable because
+     * it doesn't require a (Comparable) cast for each comparison.
+     * (Optimizers can only sometimes remove such redundant checks
+     * themselves.) When Comparators are used,
+     * ComparableUsingComparators are created so that they act in the
+     * same way as natural orderings. This penalizes use of
+     * Comparators vs Comparables, which seems like the right
+     * tradeoff.
+     */
+    static final class ComparableUsingComparator<K> implements Comparable<K> {
+        final K actualKey;
+        final Comparator<? super K> cmp;
+        ComparableUsingComparator(K key, Comparator<? super K> cmp) {
+            this.actualKey = key;
+            this.cmp = cmp;
+        }
+        public int compareTo(K k2) {
+            return cmp.compare(actualKey, k2);
+        }
+    }
+
+    /**
+     * If using comparator, return a ComparableUsingComparator, else
+     * cast key as Comparable, which may cause ClassCastException,
+     * which is propagated back to caller.
+     */
+    private Comparable<? super K> comparable(Object key) throws ClassCastException {
+        if (key == null)
+            throw new NullPointerException();
+        if (comparator != null)
+            return new ComparableUsingComparator<K>((K)key, comparator);
+        else
+            return (Comparable<? super K>)key;
+    }
+
+    /**
+     * Compares using comparator or natural ordering. Used when the
+     * ComparableUsingComparator approach doesn't apply.
+     */
+    int compare(K k1, K k2) throws ClassCastException {
+        Comparator<? super K> cmp = comparator;
+        if (cmp != null)
+            return cmp.compare(k1, k2);
+        else
+            return ((Comparable<? super K>)k1).compareTo(k2);
+    }
+
+    /**
+     * Returns true if given key greater than or equal to least and
+     * strictly less than fence, bypassing either test if least or
+     * fence are null. Needed mainly in submap operations.
+     */
+    boolean inHalfOpenRange(K key, K least, K fence) {
+        if (key == null)
+            throw new NullPointerException();
+        return ((least == null || compare(key, least) >= 0) &&
+                (fence == null || compare(key, fence) <  0));
+    }
+
+    /**
+     * Returns true if given key greater than or equal to least and less
+     * or equal to fence. Needed mainly in submap operations.
+     */
+    boolean inOpenRange(K key, K least, K fence) {
+        if (key == null)
+            throw new NullPointerException();
+        return ((least == null || compare(key, least) >= 0) &&
+                (fence == null || compare(key, fence) <= 0));
+    }
+
+    /* ---------------- Traversal -------------- */
+
+    /**
+     * Returns a base-level node with key strictly less than given key,
+     * or the base-level header if there is no such node.  Also
+     * unlinks indexes to deleted nodes found along the way.  Callers
+     * rely on this side-effect of clearing indices to deleted nodes.
+     * @param key the key
+     * @return a predecessor of key
+     */
+    private Node<K,V> findPredecessor(Comparable<? super K> key) {
+        if (key == null)
+            throw new NullPointerException(); // don't postpone errors
+        for (;;) {
+            Index<K,V> q = head;
+            Index<K,V> r = q.right;
+            for (;;) {
+                if (r != null) {
+                    Node<K,V> n = r.node;
+                    K k = n.key;
+                    if (n.value == null) {
+                        if (!q.unlink(r))
+                            break;           // restart
+                        r = q.right;         // reread r
+                        continue;
+                    }
+                    if (key.compareTo(k) > 0) {
+                        q = r;
+                        r = r.right;
+                        continue;
+                    }
+                }
+                Index<K,V> d = q.down;
+                if (d != null) {
+                    q = d;
+                    r = d.right;
+                } 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. Some callers rely on this side-effect of clearing
+     * deleted nodes.
+     *
+     * Restarts occur, at traversal step centered on node n, if:
+     *
+     *   (1) After reading n's next field, n is no longer assumed
+     *       predecessor b's current successor, which means that
+     *       we don't have a consistent 3-node snapshot and so cannot
+     *       unlink any subsequent deleted nodes encountered.
+     *
+     *   (2) n's value field is null, indicating n is deleted, in
+     *       which case we help out an ongoing structural deletion
+     *       before retrying.  Even though there are cases where such
+     *       unlinking doesn't require restart, they aren't sorted out
+     *       here because doing so would not usually outweigh cost of
+     *       restarting.
+     *
+     *   (3) n is a marker or n's predecessor's value field is null,
+     *       indicating (among other possibilities) that
+     *       findPredecessor returned a deleted node. We can't unlink
+     *       the node because we don't know its predecessor, so rely
+     *       on another call to findPredecessor to notice and return
+     *       some earlier predecessor, which it will do. This check is
+     *       only strictly needed at beginning of loop, (and the
+     *       b.value check isn't strictly needed at all) but is done
+     *       each iteration to help avoid contention with other
+     *       threads by callers that will fail to be able to change
+     *       links, and so will retry anyway.
+     *
+     * The traversal loops in doPut, doRemove, and findNear all
+     * include the same three kinds of checks. And specialized
+     * versions appear in findFirst, and findLast and their
+     * variants. They can't easily share code because each uses the
+     * reads of fields held in locals occurring in the orders they
+     * were performed.
+     *
+     * @param key the key
+     * @return node holding key, or null if no such
+     */
+    private Node<K,V> findNode(Comparable<? super K> key) {
+        for (;;) {
+            Node<K,V> b = findPredecessor(key);
+            Node<K,V> n = b.next;
+            for (;;) {
+                if (n == null)
+                    return null;
+                Node<K,V> f = n.next;
+                if (n != b.next)                // inconsistent read
+                    break;
+                Object v = n.value;
+                if (v == null) {                // n is deleted
+                    n.helpDelete(b, f);
+                    break;
+                }
+                if (v == n || b.value == null)  // b is deleted
+                    break;
+                int c = key.compareTo(n.key);
+                if (c == 0)
+                    return n;
+                if (c < 0)
+                    return null;
+                b = n;
+                n = f;
+            }
+        }
+    }
+
+    /**
+     * Specialized variant of findNode to perform Map.get. Does a weak
+     * traversal, not bothering to fix any deleted index nodes,
+     * returning early if it happens to see key in index, and passing
+     * over any deleted base nodes, falling back to getUsingFindNode
+     * only if it would otherwise return value from an ongoing
+     * deletion. Also uses "bound" to eliminate need for some
+     * comparisons (see Pugh Cookbook). Also folds uses of null checks
+     * and node-skipping because markers have null keys.
+     * @param okey the key
+     * @return the value, or null if absent
+     */
+    private V doGet(Object okey) {
+        Comparable<? super K> key = comparable(okey);
+        Node<K,V> bound = null;
+        Index<K,V> q = head;
+        Index<K,V> r = q.right;
+        Node<K,V> n;
+        K k;
+        int c;
+        for (;;) {
+            Index<K,V> d;
+            // Traverse rights
+            if (r != null && (n = r.node) != bound && (k = n.key) != null) {
+                if ((c = key.compareTo(k)) > 0) {
+                    q = r;
+                    r = r.right;
+                    continue;
+                } else if (c == 0) {
+                    Object v = n.value;
+                    return (v != null)? (V)v : getUsingFindNode(key);
+                } else
+                    bound = n;
+            }
+
+            // Traverse down
+            if ((d = q.down) != null) {
+                q = d;
+                r = d.right;
+            } else
+                break;
+        }
+
+        // Traverse nexts
+        for (n = q.node.next;  n != null; n = n.next) {
+            if ((k = n.key) != null) {
+                if ((c = key.compareTo(k)) == 0) {
+                    Object v = n.value;
+                    return (v != null)? (V)v : getUsingFindNode(key);
+                } else if (c < 0)
+                    break;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Performs map.get via findNode.  Used as a backup if doGet
+     * encounters an in-progress deletion.
+     * @param key the key
+     * @return the value, or null if absent
+     */
+    private V getUsingFindNode(Comparable<? super K> key) {
+        /*
+         * Loop needed here and elsewhere in case value field goes
+         * null just as it is about to be returned, in which case we
+         * lost a race with a deletion, so must retry.
+         */
+        for (;;) {
+            Node<K,V> n = findNode(key);
+            if (n == null)
+                return null;
+            Object v = n.value;
+            if (v != null)
+                return (V)v;
+        }
+    }
+
+    /* ---------------- Insertion -------------- */
+
+    /**
+     * Main insertion method.  Adds element if not present, or
+     * replaces value if present and onlyIfAbsent is false.
+     * @param kkey 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 kkey, V value, boolean onlyIfAbsent) {
+        Comparable<? super K> key = comparable(kkey);
+        for (;;) {
+            Node<K,V> b = findPredecessor(key);
+            Node<K,V> n = b.next;
+            for (;;) {
+                if (n != null) {
+                    Node<K,V> f = n.next;
+                    if (n != b.next)               // inconsistent read
+                        break;
+                    Object v = n.value;
+                    if (v == null) {               // n is deleted
+                        n.helpDelete(b, f);
+                        break;
+                    }
+                    if (v == n || b.value == null) // b is deleted
+                        break;
+                    int c = key.compareTo(n.key);
+                    if (c > 0) {
+                        b = n;
+                        n = f;
+                        continue;
+                    }
+                    if (c == 0) {
+                        if (onlyIfAbsent || n.casValue(v, value))
+                            return (V)v;
+                        else
+                            break; // restart if lost race to replace value
+                    }
+                    // else c < 0; fall through
+                }
+
+                Node<K,V> z = new Node<K,V>(kkey, value, n);
+                if (!b.casNext(n, z))
+                    break;         // restart if lost race to append to b
+                int level = randomLevel();
+                if (level > 0)
+                    insertIndex(z, level);
+                return null;
+            }
+        }
+    }
+
+    /**
+     * Returns a random level for inserting a new node.
+     * Hardwired to k=1, p=0.5, max 31 (see above and
+     * Pugh's "Skip List Cookbook", sec 3.4).
+     *
+     * This uses the simplest of the generators described in George
+     * Marsaglia's "Xorshift RNGs" paper.  This is not a high-quality
+     * generator but is acceptable here.
+     */
+    private int randomLevel() {
+        int x = randomSeed;
+        x ^= x << 13;
+        x ^= x >>> 17;
+        randomSeed = x ^= x << 5;
+        if ((x & 0x8001) != 0) // test highest and lowest bits
+            return 0;
+        int level = 1;
+        while (((x >>>= 1) & 1) != 0) ++level;
+        return level;
+    }
+
+    /**
+     * Creates and adds index nodes for the given node.
+     * @param z the node
+     * @param level the level of the index
+     */
+    private void insertIndex(Node<K,V> z, int level) {
+        HeadIndex<K,V> h = head;
+        int max = h.level;
+
+        if (level <= max) {
+            Index<K,V> idx = null;
+            for (int i = 1; i <= level; ++i)
+                idx = new Index<K,V>(z, idx, null);
+            addIndex(idx, h, level);
+
+        } else { // Add a new level
+            /*
+             * To reduce interference by other threads checking for
+             * empty levels in tryReduceLevel, new levels are added
+             * with initialized right pointers. Which in turn requires
+             * keeping levels in an array to access them while
+             * creating new head index nodes from the opposite
+             * direction.
+             */
+            level = max + 1;
+            Index<K,V>[] idxs = (Index<K,V>[])new Index[level+1];
+            Index<K,V> idx = null;
+            for (int i = 1; i <= level; ++i)
+                idxs[i] = idx = new Index<K,V>(z, idx, null);
+
+            HeadIndex<K,V> oldh;
+            int k;
+            for (;;) {
+                oldh = head;
+                int oldLevel = oldh.level;
+                if (level <= oldLevel) { // lost race to add level
+                    k = level;
+                    break;
+                }
+                HeadIndex<K,V> newh = oldh;
+                Node<K,V> oldbase = oldh.node;
+                for (int j = oldLevel+1; j <= level; ++j)
+                    newh = new HeadIndex<K,V>(oldbase, newh, idxs[j], j);
+                if (casHead(oldh, newh)) {
+                    k = oldLevel;
+                    break;
+                }
+            }
+            addIndex(idxs[k], oldh, k);
+        }
+    }
+
+    /**
+     * Adds given index nodes from given level down to 1.
+     * @param idx the topmost index node being inserted
+     * @param h the value of head to use to insert. This must be
+     * snapshotted by callers to provide correct insertion level
+     * @param indexLevel the level of the index
+     */
+    private void addIndex(Index<K,V> idx, HeadIndex<K,V> h, int indexLevel) {
+        // Track next level to insert in case of retries
+        int insertionLevel = indexLevel;
+        Comparable<? super K> key = comparable(idx.node.key);
+        if (key == null) throw new NullPointerException();
+
+        // Similar to findPredecessor, but adding index nodes along
+        // path to key.
+        for (;;) {
+            int j = h.level;
+            Index<K,V> q = h;
+            Index<K,V> r = q.right;
+            Index<K,V> t = idx;
+            for (;;) {
+                if (r != null) {
+                    Node<K,V> n = r.node;
+                    // compare before deletion check avoids needing recheck
+                    int c = key.compareTo(n.key);
+                    if (n.value == null) {
+                        if (!q.unlink(r))
+                            break;
+                        r = q.right;
+                        continue;
+                    }
+                    if (c > 0) {
+                        q = r;
+                        r = r.right;
+                        continue;
+                    }
+                }
+
+                if (j == insertionLevel) {
+                    // Don't insert index if node already deleted
+                    if (t.indexesDeletedNode()) {
+                        findNode(key); // cleans up
+                        return;
+                    }
+                    if (!q.link(r, t))
+                        break; // restart
+                    if (--insertionLevel == 0) {
+                        // need final deletion check before return
+                        if (t.indexesDeletedNode())
+                            findNode(key);
+                        return;
+                    }
+                }
+
+                if (--j >= insertionLevel && j < indexLevel)
+                    t = t.down;
+                q = q.down;
+                r = q.right;
+            }
+        }
+    }
+
+    /* ---------------- Deletion -------------- */
+
+    /**
+     * Main deletion method. Locates node, nulls value, appends a
+     * deletion marker, unlinks predecessor, removes associated index
+     * nodes, and possibly reduces head index level.
+     *
+     * Index nodes are cleared out simply by calling findPredecessor.
+     * which unlinks indexes to deleted nodes found along path to key,
+     * which will include the indexes to this node.  This is done
+     * unconditionally. We can't check beforehand whether there are
+     * index nodes because it might be the case that some or all
+     * indexes hadn't been inserted yet for this node during initial
+     * search for it, and we'd like to ensure lack of garbage
+     * retention, so must call to be sure.
+     *
+     * @param okey 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 okey, Object value) {
+        Comparable<? super K> key = comparable(okey);
+        for (;;) {
+            Node<K,V> b = findPredecessor(key);
+            Node<K,V> n = b.next;
+            for (;;) {
+                if (n == null)
+                    return null;
+                Node<K,V> f = n.next;
+                if (n != b.next)                    // inconsistent read
+                    break;
+                Object v = n.value;
+                if (v == null) {                    // n is deleted
+                    n.helpDelete(b, f);
+                    break;
+                }
+                if (v == n || b.value == null)      // b is deleted
+                    break;
+                int c = key.compareTo(n.key);
+                if (c < 0)
+                    return null;
+                if (c > 0) {
+                    b = n;
+                    n = f;
+                    continue;
+                }
+                if (value != null && !value.equals(v))
+                    return null;
+                if (!n.casValue(v, null))
+                    break;
+                if (!n.appendMarker(f) || !b.casNext(n, f))
+                    findNode(key);                  // Retry via findNode
+                else {
+                    findPredecessor(key);           // Clean index
+                    if (head.right == null)
+                        tryReduceLevel();
+                }
+                return (V)v;
+            }
+        }
+    }
+
+    /**
+     * 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() {
+        HeadIndex<K,V> h = head;
+        HeadIndex<K,V> d;
+        HeadIndex<K,V> e;
+        if (h.level > 3 &&
+            (d = (HeadIndex<K,V>)h.down) != null &&
+            (e = (HeadIndex<K,V>)d.down) != null &&
+            e.right == null &&
+            d.right == null &&
+            h.right == null &&
+            casHead(h, d) && // try to set
+            h.right != null) // recheck
+            casHead(d, h);   // try to backout
+    }
+
+    /* ---------------- Finding and removing first element -------------- */
+
+    /**
+     * Specialized variant of findNode to get first valid node.
+     * @return first node or null if empty
+     */
+    Node<K,V> findFirst() {
+        for (;;) {
+            Node<K,V> b = head.node;
+            Node<K,V> n = b.next;
+            if (n == null)
+                return null;
+            if (n.value != null)
+                return n;
+            n.helpDelete(b, n.next);
+        }
+    }
+
+    /**
+     * Removes first entry; returns its snapshot.
+     * @return null if empty, else snapshot of first entry
+     */
+    Map.Entry<K,V> doRemoveFirstEntry() {
+        for (;;) {
+            Node<K,V> b = head.node;
+            Node<K,V> n = b.next;
+            if (n == null)
+                return null;
+            Node<K,V> f = n.next;
+            if (n != b.next)
+                continue;
+            Object v = n.value;
+            if (v == null) {
+                n.helpDelete(b, f);
+                continue;
+            }
+            if (!n.casValue(v, null))
+                continue;
+            if (!n.appendMarker(f) || !b.casNext(n, f))
+                findFirst(); // retry
+            clearIndexToFirst();
+            return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, (V)v);
+        }
+    }
+
+    /**
+     * Clears out index nodes associated with deleted first entry.
+     */
+    private void clearIndexToFirst() {
+        for (;;) {
+            Index<K,V> q = head;
+            for (;;) {
+                Index<K,V> r = q.right;
+                if (r != null && r.indexesDeletedNode() && !q.unlink(r))
+                    break;
+                if ((q = q.down) == null) {
+                    if (head.right == null)
+                        tryReduceLevel();
+                    return;
+                }
+            }
+        }
+    }
+
+
+    /* ---------------- Finding and removing last element -------------- */
+
+    /**
+     * Specialized version of find to get last valid node.
+     * @return last node or null if empty
+     */
+    Node<K,V> findLast() {
+        /*
+         * findPredecessor can't be used to traverse index level
+         * because this doesn't use comparisons.  So traversals of
+         * both levels are folded together.
+         */
+        Index<K,V> q = head;
+        for (;;) {
+            Index<K,V> d, r;
+            if ((r = q.right) != null) {
+                if (r.indexesDeletedNode()) {
+                    q.unlink(r);
+                    q = head; // restart
+                }
+                else
+                    q = r;
+            } else if ((d = q.down) != null) {
+                q = d;
+            } else {
+                Node<K,V> b = q.node;
+                Node<K,V> n = b.next;
+                for (;;) {
+                    if (n == null)
+                        return (b.isBaseHeader())? null : b;
+                    Node<K,V> f = n.next;            // inconsistent read
+                    if (n != b.next)
+                        break;
+                    Object v = n.value;
+                    if (v == null) {                 // n is deleted
+                        n.helpDelete(b, f);
+                        break;
+                    }
+                    if (v == n || b.value == null)   // b is deleted
+                        break;
+                    b = n;
+                    n = f;
+                }
+                q = head; // restart
+            }
+        }
+    }
+
+    /**
+     * Specialized variant of findPredecessor to get predecessor of last
+     * valid node.  Needed when removing the last entry.  It is possible
+     * that all successors of returned node will have been deleted upon
+     * return, in which case this method can be retried.
+     * @return likely predecessor of last node
+     */
+    private Node<K,V> findPredecessorOfLast() {
+        for (;;) {
+            Index<K,V> q = head;
+            for (;;) {
+                Index<K,V> d, r;
+                if ((r = q.right) != null) {
+                    if (r.indexesDeletedNode()) {
+                        q.unlink(r);
+                        break;    // must restart
+                    }
+                    // proceed as far across as possible without overshooting
+                    if (r.node.next != null) {
+                        q = r;
+                        continue;
+                    }
+                }
+                if ((d = q.down) != null)
+                    q = d;
+                else
+                    return q.node;
+            }
+        }
+    }
+
+    /**
+     * Removes last entry; returns its snapshot.
+     * Specialized variant of doRemove.
+     * @return null if empty, else snapshot of last entry
+     */
+    Map.Entry<K,V> doRemoveLastEntry() {
+        for (;;) {
+            Node<K,V> b = findPredecessorOfLast();
+            Node<K,V> n = b.next;
+            if (n == null) {
+                if (b.isBaseHeader())               // empty
+                    return null;
+                else
+                    continue; // all b's successors are deleted; retry
+            }
+            for (;;) {
+                Node<K,V> f = n.next;
+                if (n != b.next)                    // inconsistent read
+                    break;
+                Object v = n.value;
+                if (v == null) {                    // n is deleted
+                    n.helpDelete(b, f);
+                    break;
+                }
+                if (v == n || b.value == null)      // b is deleted
+                    break;
+                if (f != null) {
+                    b = n;
+                    n = f;
+                    continue;
+                }
+                if (!n.casValue(v, null))
+                    break;
+                K key = n.key;
+                Comparable<? super K> ck = comparable(key);
+                if (!n.appendMarker(f) || !b.casNext(n, f))
+                    findNode(ck);                  // Retry via findNode
+                else {
+                    findPredecessor(ck);           // Clean index
+                    if (head.right == null)
+                        tryReduceLevel();
+                }
+                return new AbstractMap.SimpleImmutableEntry<K,V>(key, (V)v);
+            }
+        }
+    }
+
+    /* ---------------- 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.
+     * @param kkey the key
+     * @param rel the relation -- OR'ed combination of EQ, LT, GT
+     * @return nearest node fitting relation, or null if no such
+     */
+    Node<K,V> findNear(K kkey, int rel) {
+        Comparable<? super K> key = comparable(kkey);
+        for (;;) {
+            Node<K,V> b = findPredecessor(key);
+            Node<K,V> n = b.next;
+            for (;;) {
+                if (n == null)
+                    return ((rel & LT) == 0 || b.isBaseHeader())? null : b;
+                Node<K,V> f = n.next;
+                if (n != b.next)                  // inconsistent read
+                    break;
+                Object v = n.value;
+                if (v == null) {                  // n is deleted
+                    n.helpDelete(b, f);
+                    break;
+                }
+                if (v == n || b.value == null)    // b is deleted
+                    break;
+                int c = key.compareTo(n.key);
+                if ((c == 0 && (rel & EQ) != 0) ||
+                    (c <  0 && (rel & LT) == 0))
+                    return n;
+                if ( c <= 0 && (rel & LT) != 0)
+                    return (b.isBaseHeader())? null : b;
+                b = n;
+                n = f;
+            }
+        }
+    }
+
+    /**
+     * Returns SimpleImmutableEntry for results of findNear.
+     * @param key the key
+     * @param rel the relation -- OR'ed combination of EQ, LT, GT
+     * @return Entry fitting relation, or null if no such
+     */
+    AbstractMap.SimpleImmutableEntry<K,V> getNear(K key, int rel) {
+        for (;;) {
+            Node<K,V> n = findNear(key, rel);
+            if (n == null)
+                return null;
+            AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
+            if (e != null)
+                return e;
+        }
+    }
+
+
+    /* ---------------- Constructors -------------- */
+
+    /**
+     * Constructs a new, empty map, sorted according to the
+     * {@linkplain Comparable natural ordering} of the keys.
+     */
+    public ConcurrentSkipListMap() {
+        this.comparator = null;
+        initialize();
+    }
+
+    /**
+     * Constructs a new, empty map, sorted according to the specified
+     * comparator.
+     *
+     * @param comparator the comparator that will be used to order this map.
+     *        If <tt>null</tt>, the {@linkplain Comparable natural
+     *        ordering} of the keys will be used.
+     */
+    public ConcurrentSkipListMap(Comparator<? super K> comparator) {
+        this.comparator = comparator;
+        initialize();
+    }
+
+    /**
+     * 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 <tt>m</tt> 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;
+        initialize();
+        putAll(m);
+    }
+
+    /**
+     * 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();
+        initialize();
+        buildFromSorted(m);
+    }
+
+    /**
+     * Returns a shallow copy of this <tt>ConcurrentSkipListMap</tt>
+     * instance. (The keys and values themselves are not cloned.)
+     *
+     * @return a shallow copy of this map
+     */
+    public ConcurrentSkipListMap<K,V> clone() {
+        ConcurrentSkipListMap<K,V> clone = null;
+        try {
+            clone = (ConcurrentSkipListMap<K,V>) super.clone();
+        } catch (CloneNotSupportedException e) {
+            throw new InternalError();
+        }
+
+        clone.initialize();
+        clone.buildFromSorted(this);
+        return clone;
+    }
+
+    /**
+     * 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();
+
+        HeadIndex<K,V> h = head;
+        Node<K,V> basepred = h.node;
+
+        // Track the current rightmost node at each level. Uses an
+        // ArrayList to avoid committing to initial or maximum level.
+        ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>();
+
+        // initialize
+        for (int i = 0; i <= h.level; ++i)
+            preds.add(null);
+        Index<K,V> q = h;
+        for (int i = h.level; i > 0; --i) {
+            preds.set(i, q);
+            q = q.down;
+        }
+
+        Iterator<? extends Map.Entry<? extends K, ? extends V>> it =
+            map.entrySet().iterator();
+        while (it.hasNext()) {
+            Map.Entry<? extends K, ? extends V> e = it.next();
+            int j = randomLevel();
+            if (j > h.level) j = h.level + 1;
+            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);
+            basepred.next = z;
+            basepred = z;
+            if (j > 0) {
+                Index<K,V> idx = null;
+                for (int i = 1; i <= j; ++i) {
+                    idx = new Index<K,V>(z, idx, null);
+                    if (i > h.level)
+                        h = new HeadIndex<K,V>(h.node, h, idx, i);
+
+                    if (i < preds.size()) {
+                        preds.get(i).right = idx;
+                        preds.set(i, idx);
+                    } else
+                        preds.add(idx);
+                }
+            }
+        }
+        head = h;
+    }
+
+    /* ---------------- Serialization -------------- */
+
+    /**
+     * Save the state of this map to a stream.
+     *
+     * @serialData The key (Object) and value (Object) for each
+     * key-value mapping represented by the map, followed by
+     * <tt>null</tt>. 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)
+        for (Node<K,V> n = findFirst(); n != null; n = n.next) {
+            V v = n.getValidValue();
+            if (v != null) {
+                s.writeObject(n.key);
+                s.writeObject(v);
+            }
+        }
+        s.writeObject(null);
+    }
+
+    /**
+     * Reconstitute the map from a stream.
+     */
+    private void readObject(final java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        // Read in the Comparator and any hidden stuff
+        s.defaultReadObject();
+        // Reset transients
+        initialize();
+
+        /*
+         * This is nearly identical to buildFromSorted, but is
+         * distinct because readObject calls can't be nicely adapted
+         * as the kind of iterator needed by buildFromSorted. (They
+         * can be, but doing so requires type cheats and/or creation
+         * of adaptor classes.) It is simpler to just adapt the code.
+         */
+
+        HeadIndex<K,V> h = head;
+        Node<K,V> basepred = h.node;
+        ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>();
+        for (int i = 0; i <= h.level; ++i)
+            preds.add(null);
+        Index<K,V> q = h;
+        for (int i = h.level; i > 0; --i) {
+            preds.set(i, q);
+            q = q.down;
+        }
+
+        for (;;) {
+            Object k = s.readObject();
+            if (k == null)
+                break;
+            Object v = s.readObject();
+            if (v == null)
+                throw new NullPointerException();
+            K key = (K) k;
+            V val = (V) v;
+            int j = randomLevel();
+            if (j > h.level) j = h.level + 1;
+            Node<K,V> z = new Node<K,V>(key, val, null);
+            basepred.next = z;
+            basepred = z;
+            if (j > 0) {
+                Index<K,V> idx = null;
+                for (int i = 1; i <= j; ++i) {
+                    idx = new Index<K,V>(z, idx, null);
+                    if (i > h.level)
+                        h = new HeadIndex<K,V>(h.node, h, idx, i);
+
+                    if (i < preds.size()) {
+                        preds.get(i).right = idx;
+                        preds.set(i, idx);
+                    } else
+                        preds.add(idx);
+                }
+            }
+        }
+        head = h;
+    }
+
+    /* ------ Map API methods ------ */
+
+    /**
+     * Returns <tt>true</tt> if this map contains a mapping for the specified
+     * key.
+     *
+     * @param key key whose presence in this map is to be tested
+     * @return <tt>true</tt> 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 {@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);
+    }
+
+    /**
+     * 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
+     *         <tt>null</tt> 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.
+     *
+     * @param  key key for which mapping should be removed
+     * @return the previous value associated with the specified key, or
+     *         <tt>null</tt> 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 <tt>true</tt> if this map maps one or more keys to the
+     * specified value.  This operation requires time linear in the
+     * map size.
+     *
+     * @param value value whose presence in this map is to be tested
+     * @return <tt>true</tt> if a mapping to <tt>value</tt> exists;
+     *         <tt>false</tt> otherwise
+     * @throws NullPointerException if the specified value is null
+     */
+    public boolean containsValue(Object value) {
+        if (value == null)
+            throw new NullPointerException();
+        for (Node<K,V> n = findFirst(); n != null; n = n.next) {
+            V v = n.getValidValue();
+            if (v != null && value.equals(v))
+                return true;
+        }
+        return false;
+    }
+
+    /**
+     * Returns the number of key-value mappings in this map.  If this map
+     * contains more than <tt>Integer.MAX_VALUE</tt> elements, it
+     * returns <tt>Integer.MAX_VALUE</tt>.
+     *
+     * <p>Beware that, unlike in most collections, this method is
+     * <em>NOT</em> a constant-time operation. Because of the
+     * asynchronous nature of these maps, determining the current
+     * number of elements requires traversing them all to count them.
+     * Additionally, it is possible for the size to change during
+     * execution of this method, in which case the returned result
+     * will be inaccurate. Thus, this method is typically not very
+     * useful in concurrent applications.
+     *
+     * @return the number of elements in this map
+     */
+    public int size() {
+        long count = 0;
+        for (Node<K,V> n = findFirst(); n != null; n = n.next) {
+            if (n.getValidValue() != null)
+                ++count;
+        }
+        return (count >= Integer.MAX_VALUE)? Integer.MAX_VALUE : (int)count;
+    }
+
+    /**
+     * Returns <tt>true</tt> if this map contains no key-value mappings.
+     * @return <tt>true</tt> if this map contains no key-value mappings
+     */
+    public boolean isEmpty() {
+        return findFirst() == null;
+    }
+
+    /**
+     * Removes all of the mappings from this map.
+     */
+    public void clear() {
+        initialize();
+    }
+
+    /* ---------------- 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 {@link NavigableSet} view of the keys contained in this map.
+     * The set's iterator returns the keys in ascending 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 {@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 {@code iterator} is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * <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 ks = keySet;
+        return (ks != null) ? ks : (keySet = new KeySet(this));
+    }
+
+    public NavigableSet<K> navigableKeySet() {
+        KeySet ks = keySet;
+        return (ks != null) ? ks : (keySet = new KeySet(this));
+    }
+
+    /**
+     * Returns a {@link Collection} view of the values contained in this map.
+     * The collection's iterator returns the values in 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 <tt>Iterator.remove</tt>,
+     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
+     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
+     * support the <tt>add</tt> or <tt>addAll</tt> operations.
+     *
+     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     */
+    public Collection<V> values() {
+        Values vs = values;
+        return (vs != null) ? vs : (values = new Values(this));
+    }
+
+    /**
+     * Returns a {@link Set} view of the mappings contained in this map.
+     * The set's iterator returns the entries in 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 <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
+     * <tt>removeAll</tt>, <tt>retainAll</tt> and <tt>clear</tt>
+     * operations.  It does not support the <tt>add</tt> or
+     * <tt>addAll</tt> operations.
+     *
+     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * <p>The <tt>Map.Entry</tt> elements returned by
+     * <tt>iterator.next()</tt> do <em>not</em> support the
+     * <tt>setValue</tt> 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 es = entrySet;
+        return (es != null) ? es : (entrySet = new EntrySet(this));
+    }
+
+    public ConcurrentNavigableMap<K,V> descendingMap() {
+        ConcurrentNavigableMap<K,V> dm = descendingMap;
+        return (dm != null) ? dm : (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 <tt>true</tt> if the given object is also a map and the
+     * two maps represent the same mappings.  More formally, two maps
+     * <tt>m1</tt> and <tt>m2</tt> represent the same mappings if
+     * <tt>m1.entrySet().equals(m2.entrySet())</tt>.  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 <tt>true</tt> 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 {
+            for (Map.Entry<K,V> e : this.entrySet())
+                if (! e.getValue().equals(m.get(e.getKey())))
+                    return false;
+            for (Map.Entry<?,?> e : m.entrySet()) {
+                Object k = e.getKey();
+                Object v = e.getValue();
+                if (k == null || v == null || !v.equals(get(k)))
+                    return false;
+            }
+            return true;
+        } catch (ClassCastException unused) {
+            return false;
+        } catch (NullPointerException unused) {
+            return false;
+        }
+    }
+
+    /* ------ ConcurrentMap API methods ------ */
+
+    /**
+     * {@inheritDoc}
+     *
+     * @return the previous value associated with the specified key,
+     *         or <tt>null</tt> 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
+     * @throws NullPointerException if the specified key is null
+     */
+    public boolean remove(Object key, Object value) {
+        if (key == null)
+            throw new NullPointerException();
+        if (value == null)
+            return false;
+        return doRemove(key, value) != 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 (oldValue == null || newValue == null)
+            throw new NullPointerException();
+        Comparable<? super K> k = comparable(key);
+        for (;;) {
+            Node<K,V> n = findNode(k);
+            if (n == null)
+                return false;
+            Object v = n.value;
+            if (v != null) {
+                if (!oldValue.equals(v))
+                    return false;
+                if (n.casValue(v, newValue))
+                    return true;
+            }
+        }
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @return the previous value associated with the specified key,
+     *         or <tt>null</tt> 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 (value == null)
+            throw new NullPointerException();
+        Comparable<? super K> k = comparable(key);
+        for (;;) {
+            Node<K,V> n = findNode(k);
+            if (n == null)
+                return null;
+            Object v = n.value;
+            if (v != null && n.casValue(v, value))
+                return (V)v;
+        }
+    }
+
+    /* ------ SortedMap API methods ------ */
+
+    public Comparator<? super K> comparator() {
+        return comparator;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public K firstKey() {
+        Node<K,V> n = findFirst();
+        if (n == null)
+            throw new NoSuchElementException();
+        return n.key;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public K lastKey() {
+        Node<K,V> n = findLast();
+        if (n == null)
+            throw new NoSuchElementException();
+        return n.key;
+    }
+
+    /**
+     * @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}
+     * @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}
+     * @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}
+     * @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}
+     * @throws NullPointerException if {@code toKey} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public ConcurrentNavigableMap<K,V> headMap(K toKey) {
+        return headMap(toKey, false);
+    }
+
+    /**
+     * @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 <tt>null</tt> if there is
+     * no such key. The returned entry does <em>not</em> support the
+     * <tt>Entry.setValue</tt> method.
+     *
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public Map.Entry<K,V> lowerEntry(K key) {
+        return getNear(key, LT);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public K lowerKey(K key) {
+        Node<K,V> n = findNear(key, LT);
+        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 <tt>null</tt> if there
+     * is no such key. The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> 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 getNear(key, LT|EQ);
+    }
+
+    /**
+     * @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);
+        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 <tt>null</tt> if
+     * there is no such entry. The returned entry does <em>not</em>
+     * support the <tt>Entry.setValue</tt> method.
+     *
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public Map.Entry<K,V> ceilingEntry(K key) {
+        return getNear(key, GT|EQ);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public K ceilingKey(K key) {
+        Node<K,V> n = findNear(key, GT|EQ);
+        return (n == null)? null : n.key;
+    }
+
+    /**
+     * Returns a key-value mapping associated with the least key
+     * strictly greater than the given key, or <tt>null</tt> if there
+     * is no such key. The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> 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 getNear(key, GT);
+    }
+
+    /**
+     * @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);
+        return (n == null)? null : n.key;
+    }
+
+    /**
+     * Returns a key-value mapping associated with the least
+     * key in this map, or <tt>null</tt> if the map is empty.
+     * The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     */
+    public Map.Entry<K,V> firstEntry() {
+        for (;;) {
+            Node<K,V> n = findFirst();
+            if (n == null)
+                return null;
+            AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
+            if (e != null)
+                return e;
+        }
+    }
+
+    /**
+     * Returns a key-value mapping associated with the greatest
+     * key in this map, or <tt>null</tt> if the map is empty.
+     * The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     */
+    public Map.Entry<K,V> lastEntry() {
+        for (;;) {
+            Node<K,V> n = findLast();
+            if (n == null)
+                return null;
+            AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
+            if (e != null)
+                return e;
+        }
+    }
+
+    /**
+     * Removes and returns a key-value mapping associated with
+     * the least key in this map, or <tt>null</tt> if the map is empty.
+     * The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> 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 <tt>null</tt> if the map is empty.
+     * The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     */
+    public Map.Entry<K,V> pollLastEntry() {
+        return doRemoveLastEntry();
+    }
+
+
+    /* ---------------- Iterators -------------- */
+
+    /**
+     * Base of iterator classes:
+     */
+    abstract class Iter<T> implements Iterator<T> {
+        /** the last node returned by next() */
+        Node<K,V> lastReturned;
+        /** the next node to return from next(); */
+        Node<K,V> next;
+        /** Cache of next value field to maintain weak consistency */
+        V nextValue;
+
+        /** Initializes ascending iterator for entire range. */
+        Iter() {
+            for (;;) {
+                next = findFirst();
+                if (next == null)
+                    break;
+                Object x = next.value;
+                if (x != null && x != next) {
+                    nextValue = (V) x;
+                    break;
+                }
+            }
+        }
+
+        public final boolean hasNext() {
+            return next != null;
+        }
+
+        /** Advances next to higher entry. */
+        final void advance() {
+            if (next == null)
+                throw new NoSuchElementException();
+            lastReturned = next;
+            for (;;) {
+                next = next.next;
+                if (next == null)
+                    break;
+                Object x = next.value;
+                if (x != null && x != next) {
+                    nextValue = (V) x;
+                    break;
+                }
+            }
+        }
+
+        public void remove() {
+            Node<K,V> l = lastReturned;
+            if (l == 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(l.key);
+            lastReturned = null;
+        }
+
+    }
+
+    final class ValueIterator extends Iter<V> {
+        public V next() {
+            V v = nextValue;
+            advance();
+            return v;
+        }
+    }
+
+    final class KeyIterator extends Iter<K> {
+        public K next() {
+            Node<K,V> n = next;
+            advance();
+            return n.key;
+        }
+    }
+
+    final class EntryIterator extends Iter<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);
+        }
+    }
+
+    // Factory methods for iterators needed by ConcurrentSkipListSet etc
+
+    Iterator<K> keyIterator() {
+        return new KeyIterator();
+    }
+
+    Iterator<V> valueIterator() {
+        return new ValueIterator();
+    }
+
+    Iterator<Map.Entry<K,V>> entryIterator() {
+        return new EntryIterator();
+    }
+
+    /* ---------------- 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.
+        List<E> list = new ArrayList<E>();
+        for (E e : c)

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