Source for java.util.concurrent.ConcurrentSkipListMap

   1: /*
   2:  * Written by Doug Lea with assistance from members of JCP JSR-166
   3:  * Expert Group and released to the public domain, as explained at
   4:  * http://creativecommons.org/licenses/publicdomain
   5:  */
   6: 
   7: package java.util.concurrent;
   8: import java.util.*;
   9: import java.util.concurrent.atomic.*;
  10: 
  11: /**
  12:  * A scalable concurrent {@link ConcurrentNavigableMap} implementation.
  13:  * The map is sorted according to the {@linkplain Comparable natural
  14:  * ordering} of its keys, or by a {@link Comparator} provided at map
  15:  * creation time, depending on which constructor is used.
  16:  *
  17:  * <p>This class implements a concurrent variant of <a
  18:  * href="http://www.cs.umd.edu/~pugh/">SkipLists</a> providing
  19:  * expected average <i>log(n)</i> time cost for the
  20:  * <tt>containsKey</tt>, <tt>get</tt>, <tt>put</tt> and
  21:  * <tt>remove</tt> operations and their variants.  Insertion, removal,
  22:  * update, and access operations safely execute concurrently by
  23:  * multiple threads.  Iterators are <i>weakly consistent</i>, returning
  24:  * elements reflecting the state of the map at some point at or since
  25:  * the creation of the iterator.  They do <em>not</em> throw {@link
  26:  * ConcurrentModificationException}, and may proceed concurrently with
  27:  * other operations. Ascending key ordered views and their iterators
  28:  * are faster than descending ones.
  29:  *
  30:  * <p>All <tt>Map.Entry</tt> pairs returned by methods in this class
  31:  * and its views represent snapshots of mappings at the time they were
  32:  * produced. They do <em>not</em> support the <tt>Entry.setValue</tt>
  33:  * method. (Note however that it is possible to change mappings in the
  34:  * associated map using <tt>put</tt>, <tt>putIfAbsent</tt>, or
  35:  * <tt>replace</tt>, depending on exactly which effect you need.)
  36:  *
  37:  * <p>Beware that, unlike in most collections, the <tt>size</tt>
  38:  * method is <em>not</em> a constant-time operation. Because of the
  39:  * asynchronous nature of these maps, determining the current number
  40:  * of elements requires a traversal of the elements.  Additionally,
  41:  * the bulk operations <tt>putAll</tt>, <tt>equals</tt>, and
  42:  * <tt>clear</tt> are <em>not</em> guaranteed to be performed
  43:  * atomically. For example, an iterator operating concurrently with a
  44:  * <tt>putAll</tt> operation might view only some of the added
  45:  * elements.
  46:  *
  47:  * <p>This class and its views and iterators implement all of the
  48:  * <em>optional</em> methods of the {@link Map} and {@link Iterator}
  49:  * interfaces. Like most other concurrent collections, this class does
  50:  * <em>not</em> permit the use of <tt>null</tt> keys or values because some
  51:  * null return values cannot be reliably distinguished from the absence of
  52:  * elements.
  53:  *
  54:  * <p>This class is a member of the
  55:  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  56:  * Java Collections Framework</a>.
  57:  *
  58:  * @author Doug Lea
  59:  * @param <K> the type of keys maintained by this map
  60:  * @param <V> the type of mapped values
  61:  * @since 1.6
  62:  */
  63: public class ConcurrentSkipListMap<K,V> extends AbstractMap<K,V>
  64:     implements ConcurrentNavigableMap<K,V>,
  65:                Cloneable,
  66:                java.io.Serializable {
  67:     /*
  68:      * This class implements a tree-like two-dimensionally linked skip
  69:      * list in which the index levels are represented in separate
  70:      * nodes from the base nodes holding data.  There are two reasons
  71:      * for taking this approach instead of the usual array-based
  72:      * structure: 1) Array based implementations seem to encounter
  73:      * more complexity and overhead 2) We can use cheaper algorithms
  74:      * for the heavily-traversed index lists than can be used for the
  75:      * base lists.  Here's a picture of some of the basics for a
  76:      * possible list with 2 levels of index:
  77:      *
  78:      * Head nodes          Index nodes
  79:      * +-+    right        +-+                      +-+
  80:      * |2|---------------->| |--------------------->| |->null
  81:      * +-+                 +-+                      +-+
  82:      *  | down              |                        |
  83:      *  v                   v                        v
  84:      * +-+            +-+  +-+       +-+            +-+       +-+
  85:      * |1|----------->| |->| |------>| |----------->| |------>| |->null
  86:      * +-+            +-+  +-+       +-+            +-+       +-+
  87:      *  v              |    |         |              |         |
  88:      * Nodes  next     v    v         v              v         v
  89:      * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
  90:      * | |->|A|->|B|->|C|->|D|->|E|->|F|->|G|->|H|->|I|->|J|->|K|->null
  91:      * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
  92:      *
  93:      * The base lists use a variant of the HM linked ordered set
  94:      * algorithm. See Tim Harris, "A pragmatic implementation of
  95:      * non-blocking linked lists"
  96:      * http://www.cl.cam.ac.uk/~tlh20/publications.html and Maged
  97:      * Michael "High Performance Dynamic Lock-Free Hash Tables and
  98:      * List-Based Sets"
  99:      * http://www.research.ibm.com/people/m/michael/pubs.htm.  The
 100:      * basic idea in these lists is to mark the "next" pointers of
 101:      * deleted nodes when deleting to avoid conflicts with concurrent
 102:      * insertions, and when traversing to keep track of triples
 103:      * (predecessor, node, successor) in order to detect when and how
 104:      * to unlink these deleted nodes.
 105:      *
 106:      * Rather than using mark-bits to mark list deletions (which can
 107:      * be slow and space-intensive using AtomicMarkedReference), nodes
 108:      * use direct CAS'able next pointers.  On deletion, instead of
 109:      * marking a pointer, they splice in another node that can be
 110:      * thought of as standing for a marked pointer (indicating this by
 111:      * using otherwise impossible field values).  Using plain nodes
 112:      * acts roughly like "boxed" implementations of marked pointers,
 113:      * but uses new nodes only when nodes are deleted, not for every
 114:      * link.  This requires less space and supports faster
 115:      * traversal. Even if marked references were better supported by
 116:      * JVMs, traversal using this technique might still be faster
 117:      * because any search need only read ahead one more node than
 118:      * otherwise required (to check for trailing marker) rather than
 119:      * unmasking mark bits or whatever on each read.
 120:      *
 121:      * This approach maintains the essential property needed in the HM
 122:      * algorithm of changing the next-pointer of a deleted node so
 123:      * that any other CAS of it will fail, but implements the idea by
 124:      * changing the pointer to point to a different node, not by
 125:      * marking it.  While it would be possible to further squeeze
 126:      * space by defining marker nodes not to have key/value fields, it
 127:      * isn't worth the extra type-testing overhead.  The deletion
 128:      * markers are rarely encountered during traversal and are
 129:      * normally quickly garbage collected. (Note that this technique
 130:      * would not work well in systems without garbage collection.)
 131:      *
 132:      * In addition to using deletion markers, the lists also use
 133:      * nullness of value fields to indicate deletion, in a style
 134:      * similar to typical lazy-deletion schemes.  If a node's value is
 135:      * null, then it is considered logically deleted and ignored even
 136:      * though it is still reachable. This maintains proper control of
 137:      * concurrent replace vs delete operations -- an attempted replace
 138:      * must fail if a delete beat it by nulling field, and a delete
 139:      * must return the last non-null value held in the field. (Note:
 140:      * Null, rather than some special marker, is used for value fields
 141:      * here because it just so happens to mesh with the Map API
 142:      * requirement that method get returns null if there is no
 143:      * mapping, which allows nodes to remain concurrently readable
 144:      * even when deleted. Using any other marker value here would be
 145:      * messy at best.)
 146:      *
 147:      * Here's the sequence of events for a deletion of node n with
 148:      * predecessor b and successor f, initially:
 149:      *
 150:      *        +------+       +------+      +------+
 151:      *   ...  |   b  |------>|   n  |----->|   f  | ...
 152:      *        +------+       +------+      +------+
 153:      *
 154:      * 1. CAS n's value field from non-null to null.
 155:      *    From this point on, no public operations encountering
 156:      *    the node consider this mapping to exist. However, other
 157:      *    ongoing insertions and deletions might still modify
 158:      *    n's next pointer.
 159:      *
 160:      * 2. CAS n's next pointer to point to a new marker node.
 161:      *    From this point on, no other nodes can be appended to n.
 162:      *    which avoids deletion errors in CAS-based linked lists.
 163:      *
 164:      *        +------+       +------+      +------+       +------+
 165:      *   ...  |   b  |------>|   n  |----->|marker|------>|   f  | ...
 166:      *        +------+       +------+      +------+       +------+
 167:      *
 168:      * 3. CAS b's next pointer over both n and its marker.
 169:      *    From this point on, no new traversals will encounter n,
 170:      *    and it can eventually be GCed.
 171:      *        +------+                                    +------+
 172:      *   ...  |   b  |----------------------------------->|   f  | ...
 173:      *        +------+                                    +------+
 174:      *
 175:      * A failure at step 1 leads to simple retry due to a lost race
 176:      * with another operation. Steps 2-3 can fail because some other
 177:      * thread noticed during a traversal a node with null value and
 178:      * helped out by marking and/or unlinking.  This helping-out
 179:      * ensures that no thread can become stuck waiting for progress of
 180:      * the deleting thread.  The use of marker nodes slightly
 181:      * complicates helping-out code because traversals must track
 182:      * consistent reads of up to four nodes (b, n, marker, f), not
 183:      * just (b, n, f), although the next field of a marker is
 184:      * immutable, and once a next field is CAS'ed to point to a
 185:      * marker, it never again changes, so this requires less care.
 186:      *
 187:      * Skip lists add indexing to this scheme, so that the base-level
 188:      * traversals start close to the locations being found, inserted
 189:      * or deleted -- usually base level traversals only traverse a few
 190:      * nodes. This doesn't change the basic algorithm except for the
 191:      * need to make sure base traversals start at predecessors (here,
 192:      * b) that are not (structurally) deleted, otherwise retrying
 193:      * after processing the deletion.
 194:      *
 195:      * Index levels are maintained as lists with volatile next fields,
 196:      * using CAS to link and unlink.  Races are allowed in index-list
 197:      * operations that can (rarely) fail to link in a new index node
 198:      * or delete one. (We can't do this of course for data nodes.)
 199:      * However, even when this happens, the index lists remain sorted,
 200:      * so correctly serve as indices.  This can impact performance,
 201:      * but since skip lists are probabilistic anyway, the net result
 202:      * is that under contention, the effective "p" value may be lower
 203:      * than its nominal value. And race windows are kept small enough
 204:      * that in practice these failures are rare, even under a lot of
 205:      * contention.
 206:      *
 207:      * The fact that retries (for both base and index lists) are
 208:      * relatively cheap due to indexing allows some minor
 209:      * simplifications of retry logic. Traversal restarts are
 210:      * performed after most "helping-out" CASes. This isn't always
 211:      * strictly necessary, but the implicit backoffs tend to help
 212:      * reduce other downstream failed CAS's enough to outweigh restart
 213:      * cost.  This worsens the worst case, but seems to improve even
 214:      * highly contended cases.
 215:      *
 216:      * Unlike most skip-list implementations, index insertion and
 217:      * deletion here require a separate traversal pass occuring after
 218:      * the base-level action, to add or remove index nodes.  This adds
 219:      * to single-threaded overhead, but improves contended
 220:      * multithreaded performance by narrowing interference windows,
 221:      * and allows deletion to ensure that all index nodes will be made
 222:      * unreachable upon return from a public remove operation, thus
 223:      * avoiding unwanted garbage retention. This is more important
 224:      * here than in some other data structures because we cannot null
 225:      * out node fields referencing user keys since they might still be
 226:      * read by other ongoing traversals.
 227:      *
 228:      * Indexing uses skip list parameters that maintain good search
 229:      * performance while using sparser-than-usual indices: The
 230:      * hardwired parameters k=1, p=0.5 (see method randomLevel) mean
 231:      * that about one-quarter of the nodes have indices. Of those that
 232:      * do, half have one level, a quarter have two, and so on (see
 233:      * Pugh's Skip List Cookbook, sec 3.4).  The expected total space
 234:      * requirement for a map is slightly less than for the current
 235:      * implementation of java.util.TreeMap.
 236:      *
 237:      * Changing the level of the index (i.e, the height of the
 238:      * tree-like structure) also uses CAS. The head index has initial
 239:      * level/height of one. Creation of an index with height greater
 240:      * than the current level adds a level to the head index by
 241:      * CAS'ing on a new top-most head. To maintain good performance
 242:      * after a lot of removals, deletion methods heuristically try to
 243:      * reduce the height if the topmost levels appear to be empty.
 244:      * This may encounter races in which it possible (but rare) to
 245:      * reduce and "lose" a level just as it is about to contain an
 246:      * index (that will then never be encountered). This does no
 247:      * structural harm, and in practice appears to be a better option
 248:      * than allowing unrestrained growth of levels.
 249:      *
 250:      * The code for all this is more verbose than you'd like. Most
 251:      * operations entail locating an element (or position to insert an
 252:      * element). The code to do this can't be nicely factored out
 253:      * because subsequent uses require a snapshot of predecessor
 254:      * and/or successor and/or value fields which can't be returned
 255:      * all at once, at least not without creating yet another object
 256:      * to hold them -- creating such little objects is an especially
 257:      * bad idea for basic internal search operations because it adds
 258:      * to GC overhead.  (This is one of the few times I've wished Java
 259:      * had macros.) Instead, some traversal code is interleaved within
 260:      * insertion and removal operations.  The control logic to handle
 261:      * all the retry conditions is sometimes twisty. Most search is
 262:      * broken into 2 parts. findPredecessor() searches index nodes
 263:      * only, returning a base-level predecessor of the key. findNode()
 264:      * finishes out the base-level search. Even with this factoring,
 265:      * there is a fair amount of near-duplication of code to handle
 266:      * variants.
 267:      *
 268:      * For explanation of algorithms sharing at least a couple of
 269:      * features with this one, see Mikhail Fomitchev's thesis
 270:      * (http://www.cs.yorku.ca/~mikhail/), Keir Fraser's thesis
 271:      * (http://www.cl.cam.ac.uk/users/kaf24/), and Hakan Sundell's
 272:      * thesis (http://www.cs.chalmers.se/~phs/).
 273:      *
 274:      * Given the use of tree-like index nodes, you might wonder why
 275:      * this doesn't use some kind of search tree instead, which would
 276:      * support somewhat faster search operations. The reason is that
 277:      * there are no known efficient lock-free insertion and deletion
 278:      * algorithms for search trees. The immutability of the "down"
 279:      * links of index nodes (as opposed to mutable "left" fields in
 280:      * true trees) makes this tractable using only CAS operations.
 281:      *
 282:      * Notation guide for local variables
 283:      * Node:         b, n, f    for  predecessor, node, successor
 284:      * Index:        q, r, d    for index node, right, down.
 285:      *               t          for another index node
 286:      * Head:         h
 287:      * Levels:       j
 288:      * Keys:         k, key
 289:      * Values:       v, value
 290:      * Comparisons:  c
 291:      */
 292: 
 293:     private static final long serialVersionUID = -8627078645895051609L;
 294: 
 295:     /**
 296:      * Generates the initial random seed for the cheaper per-instance
 297:      * random number generators used in randomLevel.
 298:      */
 299:     private static final Random seedGenerator = new Random();
 300: 
 301:     /**
 302:      * Special value used to identify base-level header
 303:      */
 304:     private static final Object BASE_HEADER = new Object();
 305: 
 306:     /**
 307:      * The topmost head index of the skiplist.
 308:      */
 309:     private transient volatile HeadIndex<K,V> head;
 310: 
 311:     /**
 312:      * The comparator used to maintain order in this map, or null
 313:      * if using natural ordering.
 314:      * @serial
 315:      */
 316:     private final Comparator<? super K> comparator;
 317: 
 318:     /**
 319:      * Seed for simple random number generator.  Not volatile since it
 320:      * doesn't matter too much if different threads don't see updates.
 321:      */
 322:     private transient int randomSeed;
 323: 
 324:     /** Lazily initialized key set */
 325:     private transient KeySet keySet;
 326:     /** Lazily initialized entry set */
 327:     private transient EntrySet entrySet;
 328:     /** Lazily initialized values collection */
 329:     private transient Values values;
 330:     /** Lazily initialized descending key set */
 331:     private transient ConcurrentNavigableMap<K,V> descendingMap;
 332: 
 333:     /**
 334:      * Initializes or resets state. Needed by constructors, clone,
 335:      * clear, readObject. and ConcurrentSkipListSet.clone.
 336:      * (Note that comparator must be separately initialized.)
 337:      */
 338:     final void initialize() {
 339:         keySet = null;
 340:         entrySet = null;
 341:         values = null;
 342:         descendingMap = null;
 343:         randomSeed = seedGenerator.nextInt() | 0x0100; // ensure nonzero
 344:         head = new HeadIndex<K,V>(new Node<K,V>(null, BASE_HEADER, null),
 345:                                   null, null, 1);
 346:     }
 347: 
 348:     /** Updater for casHead */
 349:     private static final
 350:         AtomicReferenceFieldUpdater<ConcurrentSkipListMap, HeadIndex>
 351:         headUpdater = AtomicReferenceFieldUpdater.newUpdater
 352:         (ConcurrentSkipListMap.class, HeadIndex.class, "head");
 353: 
 354:     /**
 355:      * compareAndSet head node
 356:      */
 357:     private boolean casHead(HeadIndex<K,V> cmp, HeadIndex<K,V> val) {
 358:         return headUpdater.compareAndSet(this, cmp, val);
 359:     }
 360: 
 361:     /* ---------------- Nodes -------------- */
 362: 
 363:     /**
 364:      * Nodes hold keys and values, and are singly linked in sorted
 365:      * order, possibly with some intervening marker nodes. The list is
 366:      * headed by a dummy node accessible as head.node. The value field
 367:      * is declared only as Object because it takes special non-V
 368:      * values for marker and header nodes.
 369:      */
 370:     static final class Node<K,V> {
 371:         final K key;
 372:         volatile Object value;
 373:         volatile Node<K,V> next;
 374: 
 375:         /**
 376:          * Creates a new regular node.
 377:          */
 378:         Node(K key, Object value, Node<K,V> next) {
 379:             this.key = key;
 380:             this.value = value;
 381:             this.next = next;
 382:         }
 383: 
 384:         /**
 385:          * Creates a new marker node. A marker is distinguished by
 386:          * having its value field point to itself.  Marker nodes also
 387:          * have null keys, a fact that is exploited in a few places,
 388:          * but this doesn't distinguish markers from the base-level
 389:          * header node (head.node), which also has a null key.
 390:          */
 391:         Node(Node<K,V> next) {
 392:             this.key = null;
 393:             this.value = this;
 394:             this.next = next;
 395:         }
 396: 
 397:         /** Updater for casNext */
 398:         static final AtomicReferenceFieldUpdater<Node, Node>
 399:             nextUpdater = AtomicReferenceFieldUpdater.newUpdater
 400:             (Node.class, Node.class, "next");
 401: 
 402:         /** Updater for casValue */
 403:         static final AtomicReferenceFieldUpdater<Node, Object>
 404:             valueUpdater = AtomicReferenceFieldUpdater.newUpdater
 405:             (Node.class, Object.class, "value");
 406: 
 407:         /**
 408:          * compareAndSet value field
 409:          */
 410:         boolean casValue(Object cmp, Object val) {
 411:             return valueUpdater.compareAndSet(this, cmp, val);
 412:         }
 413: 
 414:         /**
 415:          * compareAndSet next field
 416:          */
 417:         boolean casNext(Node<K,V> cmp, Node<K,V> val) {
 418:             return nextUpdater.compareAndSet(this, cmp, val);
 419:         }
 420: 
 421:         /**
 422:          * Returns true if this node is a marker. This method isn't
 423:          * actually called in any current code checking for markers
 424:          * because callers will have already read value field and need
 425:          * to use that read (not another done here) and so directly
 426:          * test if value points to node.
 427:          * @param n a possibly null reference to a node
 428:          * @return true if this node is a marker node
 429:          */
 430:         boolean isMarker() {
 431:             return value == this;
 432:         }
 433: 
 434:         /**
 435:          * Returns true if this node is the header of base-level list.
 436:          * @return true if this node is header node
 437:          */
 438:         boolean isBaseHeader() {
 439:             return value == BASE_HEADER;
 440:         }
 441: 
 442:         /**
 443:          * Tries to append a deletion marker to this node.
 444:          * @param f the assumed current successor of this node
 445:          * @return true if successful
 446:          */
 447:         boolean appendMarker(Node<K,V> f) {
 448:             return casNext(f, new Node<K,V>(f));
 449:         }
 450: 
 451:         /**
 452:          * Helps out a deletion by appending marker or unlinking from
 453:          * predecessor. This is called during traversals when value
 454:          * field seen to be null.
 455:          * @param b predecessor
 456:          * @param f successor
 457:          */
 458:         void helpDelete(Node<K,V> b, Node<K,V> f) {
 459:             /*
 460:              * Rechecking links and then doing only one of the
 461:              * help-out stages per call tends to minimize CAS
 462:              * interference among helping threads.
 463:              */
 464:             if (f == next && this == b.next) {
 465:                 if (f == null || f.value != f) // not already marked
 466:                     appendMarker(f);
 467:                 else
 468:                     b.casNext(this, f.next);
 469:             }
 470:         }
 471: 
 472:         /**
 473:          * Returns value if this node contains a valid key-value pair,
 474:          * else null.
 475:          * @return this node's value if it isn't a marker or header or
 476:          * is deleted, else null.
 477:          */
 478:         V getValidValue() {
 479:             Object v = value;
 480:             if (v == this || v == BASE_HEADER)
 481:                 return null;
 482:             return (V)v;
 483:         }
 484: 
 485:         /**
 486:          * Creates and returns a new SimpleImmutableEntry holding current
 487:          * mapping if this node holds a valid value, else null.
 488:          * @return new entry or null
 489:          */
 490:         AbstractMap.SimpleImmutableEntry<K,V> createSnapshot() {
 491:             V v = getValidValue();
 492:             if (v == null)
 493:                 return null;
 494:             return new AbstractMap.SimpleImmutableEntry<K,V>(key, v);
 495:         }
 496:     }
 497: 
 498:     /* ---------------- Indexing -------------- */
 499: 
 500:     /**
 501:      * Index nodes represent the levels of the skip list.  Note that
 502:      * even though both Nodes and Indexes have forward-pointing
 503:      * fields, they have different types and are handled in different
 504:      * ways, that can't nicely be captured by placing field in a
 505:      * shared abstract class.
 506:      */
 507:     static class Index<K,V> {
 508:         final Node<K,V> node;
 509:         final Index<K,V> down;
 510:         volatile Index<K,V> right;
 511: 
 512:         /**
 513:          * Creates index node with given values.
 514:          */
 515:         Index(Node<K,V> node, Index<K,V> down, Index<K,V> right) {
 516:             this.node = node;
 517:             this.down = down;
 518:             this.right = right;
 519:         }
 520: 
 521:         /** Updater for casRight */
 522:         static final AtomicReferenceFieldUpdater<Index, Index>
 523:             rightUpdater = AtomicReferenceFieldUpdater.newUpdater
 524:             (Index.class, Index.class, "right");
 525: 
 526:         /**
 527:          * compareAndSet right field
 528:          */
 529:         final boolean casRight(Index<K,V> cmp, Index<K,V> val) {
 530:             return rightUpdater.compareAndSet(this, cmp, val);
 531:         }
 532: 
 533:         /**
 534:          * Returns true if the node this indexes has been deleted.
 535:          * @return true if indexed node is known to be deleted
 536:          */
 537:         final boolean indexesDeletedNode() {
 538:             return node.value == null;
 539:         }
 540: 
 541:         /**
 542:          * Tries to CAS newSucc as successor.  To minimize races with
 543:          * unlink that may lose this index node, if the node being
 544:          * indexed is known to be deleted, it doesn't try to link in.
 545:          * @param succ the expected current successor
 546:          * @param newSucc the new successor
 547:          * @return true if successful
 548:          */
 549:         final boolean link(Index<K,V> succ, Index<K,V> newSucc) {
 550:             Node<K,V> n = node;
 551:             newSucc.right = succ;
 552:             return n.value != null && casRight(succ, newSucc);
 553:         }
 554: 
 555:         /**
 556:          * Tries to CAS right field to skip over apparent successor
 557:          * succ.  Fails (forcing a retraversal by caller) if this node
 558:          * is known to be deleted.
 559:          * @param succ the expected current successor
 560:          * @return true if successful
 561:          */
 562:         final boolean unlink(Index<K,V> succ) {
 563:             return !indexesDeletedNode() && casRight(succ, succ.right);
 564:         }
 565:     }
 566: 
 567:     /* ---------------- Head nodes -------------- */
 568: 
 569:     /**
 570:      * Nodes heading each level keep track of their level.
 571:      */
 572:     static final class HeadIndex<K,V> extends Index<K,V> {
 573:         final int level;
 574:         HeadIndex(Node<K,V> node, Index<K,V> down, Index<K,V> right, int level) {
 575:             super(node, down, right);
 576:             this.level = level;
 577:         }
 578:     }
 579: 
 580:     /* ---------------- Comparison utilities -------------- */
 581: 
 582:     /**
 583:      * Represents a key with a comparator as a Comparable.
 584:      *
 585:      * Because most sorted collections seem to use natural ordering on
 586:      * Comparables (Strings, Integers, etc), most internal methods are
 587:      * geared to use them. This is generally faster than checking
 588:      * per-comparison whether to use comparator or comparable because
 589:      * it doesn't require a (Comparable) cast for each comparison.
 590:      * (Optimizers can only sometimes remove such redundant checks
 591:      * themselves.) When Comparators are used,
 592:      * ComparableUsingComparators are created so that they act in the
 593:      * same way as natural orderings. This penalizes use of
 594:      * Comparators vs Comparables, which seems like the right
 595:      * tradeoff.
 596:      */
 597:     static final class ComparableUsingComparator<K> implements Comparable<K> {
 598:         final K actualKey;
 599:         final Comparator<? super K> cmp;
 600:         ComparableUsingComparator(K key, Comparator<? super K> cmp) {
 601:             this.actualKey = key;
 602:             this.cmp = cmp;
 603:         }
 604:         public int compareTo(K k2) {
 605:             return cmp.compare(actualKey, k2);
 606:         }
 607:     }
 608: 
 609:     /**
 610:      * If using comparator, return a ComparableUsingComparator, else
 611:      * cast key as Comparable, which may cause ClassCastException,
 612:      * which is propagated back to caller.
 613:      */
 614:     private Comparable<? super K> comparable(Object key) throws ClassCastException {
 615:         if (key == null)
 616:             throw new NullPointerException();
 617:         if (comparator != null)
 618:             return new ComparableUsingComparator<K>((K)key, comparator);
 619:         else
 620:             return (Comparable<? super K>)key;
 621:     }
 622: 
 623:     /**
 624:      * Compares using comparator or natural ordering. Used when the
 625:      * ComparableUsingComparator approach doesn't apply.
 626:      */
 627:     int compare(K k1, K k2) throws ClassCastException {
 628:         Comparator<? super K> cmp = comparator;
 629:         if (cmp != null)
 630:             return cmp.compare(k1, k2);
 631:         else
 632:             return ((Comparable<? super K>)k1).compareTo(k2);
 633:     }
 634: 
 635:     /**
 636:      * Returns true if given key greater than or equal to least and
 637:      * strictly less than fence, bypassing either test if least or
 638:      * fence are null. Needed mainly in submap operations.
 639:      */
 640:     boolean inHalfOpenRange(K key, K least, K fence) {
 641:         if (key == null)
 642:             throw new NullPointerException();
 643:         return ((least == null || compare(key, least) >= 0) &&
 644:                 (fence == null || compare(key, fence) <  0));
 645:     }
 646: 
 647:     /**
 648:      * Returns true if given key greater than or equal to least and less
 649:      * or equal to fence. Needed mainly in submap operations.
 650:      */
 651:     boolean inOpenRange(K key, K least, K fence) {
 652:         if (key == null)
 653:             throw new NullPointerException();
 654:         return ((least == null || compare(key, least) >= 0) &&
 655:                 (fence == null || compare(key, fence) <= 0));
 656:     }
 657: 
 658:     /* ---------------- Traversal -------------- */
 659: 
 660:     /**
 661:      * Returns a base-level node with key strictly less than given key,
 662:      * or the base-level header if there is no such node.  Also
 663:      * unlinks indexes to deleted nodes found along the way.  Callers
 664:      * rely on this side-effect of clearing indices to deleted nodes.
 665:      * @param key the key
 666:      * @return a predecessor of key
 667:      */
 668:     private Node<K,V> findPredecessor(Comparable<? super K> key) {
 669:         if (key == null)
 670:             throw new NullPointerException(); // don't postpone errors
 671:         for (;;) {
 672:             Index<K,V> q = head;
 673:             Index<K,V> r = q.right;
 674:             for (;;) {
 675:                 if (r != null) {
 676:                     Node<K,V> n = r.node;
 677:                     K k = n.key;
 678:                     if (n.value == null) {
 679:                         if (!q.unlink(r))
 680:                             break;           // restart
 681:                         r = q.right;         // reread r
 682:                         continue;
 683:                     }
 684:                     if (key.compareTo(k) > 0) {
 685:                         q = r;
 686:                         r = r.right;
 687:                         continue;
 688:                     }
 689:                 }
 690:                 Index<K,V> d = q.down;
 691:                 if (d != null) {
 692:                     q = d;
 693:                     r = d.right;
 694:                 } else
 695:                     return q.node;
 696:             }
 697:         }
 698:     }
 699: 
 700:     /**
 701:      * Returns node holding key or null if no such, clearing out any
 702:      * deleted nodes seen along the way.  Repeatedly traverses at
 703:      * base-level looking for key starting at predecessor returned
 704:      * from findPredecessor, processing base-level deletions as
 705:      * encountered. Some callers rely on this side-effect of clearing
 706:      * deleted nodes.
 707:      *
 708:      * Restarts occur, at traversal step centered on node n, if:
 709:      *
 710:      *   (1) After reading n's next field, n is no longer assumed
 711:      *       predecessor b's current successor, which means that
 712:      *       we don't have a consistent 3-node snapshot and so cannot
 713:      *       unlink any subsequent deleted nodes encountered.
 714:      *
 715:      *   (2) n's value field is null, indicating n is deleted, in
 716:      *       which case we help out an ongoing structural deletion
 717:      *       before retrying.  Even though there are cases where such
 718:      *       unlinking doesn't require restart, they aren't sorted out
 719:      *       here because doing so would not usually outweigh cost of
 720:      *       restarting.
 721:      *
 722:      *   (3) n is a marker or n's predecessor's value field is null,
 723:      *       indicating (among other possibilities) that
 724:      *       findPredecessor returned a deleted node. We can't unlink
 725:      *       the node because we don't know its predecessor, so rely
 726:      *       on another call to findPredecessor to notice and return
 727:      *       some earlier predecessor, which it will do. This check is
 728:      *       only strictly needed at beginning of loop, (and the
 729:      *       b.value check isn't strictly needed at all) but is done
 730:      *       each iteration to help avoid contention with other
 731:      *       threads by callers that will fail to be able to change
 732:      *       links, and so will retry anyway.
 733:      *
 734:      * The traversal loops in doPut, doRemove, and findNear all
 735:      * include the same three kinds of checks. And specialized
 736:      * versions appear in findFirst, and findLast and their
 737:      * variants. They can't easily share code because each uses the
 738:      * reads of fields held in locals occurring in the orders they
 739:      * were performed.
 740:      *
 741:      * @param key the key
 742:      * @return node holding key, or null if no such
 743:      */
 744:     private Node<K,V> findNode(Comparable<? super K> key) {
 745:         for (;;) {
 746:             Node<K,V> b = findPredecessor(key);
 747:             Node<K,V> n = b.next;
 748:             for (;;) {
 749:                 if (n == null)
 750:                     return null;
 751:                 Node<K,V> f = n.next;
 752:                 if (n != b.next)                // inconsistent read
 753:                     break;
 754:                 Object v = n.value;
 755:                 if (v == null) {                // n is deleted
 756:                     n.helpDelete(b, f);
 757:                     break;
 758:                 }
 759:                 if (v == n || b.value == null)  // b is deleted
 760:                     break;
 761:                 int c = key.compareTo(n.key);
 762:                 if (c == 0)
 763:                     return n;
 764:                 if (c < 0)
 765:                     return null;
 766:                 b = n;
 767:                 n = f;
 768:             }
 769:         }
 770:     }
 771: 
 772:     /**
 773:      * Specialized variant of findNode to perform Map.get. Does a weak
 774:      * traversal, not bothering to fix any deleted index nodes,
 775:      * returning early if it happens to see key in index, and passing
 776:      * over any deleted base nodes, falling back to getUsingFindNode
 777:      * only if it would otherwise return value from an ongoing
 778:      * deletion. Also uses "bound" to eliminate need for some
 779:      * comparisons (see Pugh Cookbook). Also folds uses of null checks
 780:      * and node-skipping because markers have null keys.
 781:      * @param okey the key
 782:      * @return the value, or null if absent
 783:      */
 784:     private V doGet(Object okey) {
 785:         Comparable<? super K> key = comparable(okey);
 786:         Node<K,V> bound = null;
 787:         Index<K,V> q = head;
 788:         Index<K,V> r = q.right;
 789:         Node<K,V> n;
 790:         K k;
 791:         int c;
 792:         for (;;) {
 793:             Index<K,V> d;
 794:             // Traverse rights
 795:             if (r != null && (n = r.node) != bound && (k = n.key) != null) {
 796:                 if ((c = key.compareTo(k)) > 0) {
 797:                     q = r;
 798:                     r = r.right;
 799:                     continue;
 800:                 } else if (c == 0) {
 801:                     Object v = n.value;
 802:                     return (v != null)? (V)v : getUsingFindNode(key);
 803:                 } else
 804:                     bound = n;
 805:             }
 806: 
 807:             // Traverse down
 808:             if ((d = q.down) != null) {
 809:                 q = d;
 810:                 r = d.right;
 811:             } else
 812:                 break;
 813:         }
 814: 
 815:         // Traverse nexts
 816:         for (n = q.node.next;  n != null; n = n.next) {
 817:             if ((k = n.key) != null) {
 818:                 if ((c = key.compareTo(k)) == 0) {
 819:                     Object v = n.value;
 820:                     return (v != null)? (V)v : getUsingFindNode(key);
 821:                 } else if (c < 0)
 822:                     break;
 823:             }
 824:         }
 825:         return null;
 826:     }
 827: 
 828:     /**
 829:      * Performs map.get via findNode.  Used as a backup if doGet
 830:      * encounters an in-progress deletion.
 831:      * @param key the key
 832:      * @return the value, or null if absent
 833:      */
 834:     private V getUsingFindNode(Comparable<? super K> key) {
 835:         /*
 836:          * Loop needed here and elsewhere in case value field goes
 837:          * null just as it is about to be returned, in which case we
 838:          * lost a race with a deletion, so must retry.
 839:          */
 840:         for (;;) {
 841:             Node<K,V> n = findNode(key);
 842:             if (n == null)
 843:                 return null;
 844:             Object v = n.value;
 845:             if (v != null)
 846:                 return (V)v;
 847:         }
 848:     }
 849: 
 850:     /* ---------------- Insertion -------------- */
 851: 
 852:     /**
 853:      * Main insertion method.  Adds element if not present, or
 854:      * replaces value if present and onlyIfAbsent is false.
 855:      * @param kkey the key
 856:      * @param value  the value that must be associated with key
 857:      * @param onlyIfAbsent if should not insert if already present
 858:      * @return the old value, or null if newly inserted
 859:      */
 860:     private V doPut(K kkey, V value, boolean onlyIfAbsent) {
 861:         Comparable<? super K> key = comparable(kkey);
 862:         for (;;) {
 863:             Node<K,V> b = findPredecessor(key);
 864:             Node<K,V> n = b.next;
 865:             for (;;) {
 866:                 if (n != null) {
 867:                     Node<K,V> f = n.next;
 868:                     if (n != b.next)               // inconsistent read
 869:                         break;
 870:                     Object v = n.value;
 871:                     if (v == null) {               // n is deleted
 872:                         n.helpDelete(b, f);
 873:                         break;
 874:                     }
 875:                     if (v == n || b.value == null) // b is deleted
 876:                         break;
 877:                     int c = key.compareTo(n.key);
 878:                     if (c > 0) {
 879:                         b = n;
 880:                         n = f;
 881:                         continue;
 882:                     }
 883:                     if (c == 0) {
 884:                         if (onlyIfAbsent || n.casValue(v, value))
 885:                             return (V)v;
 886:                         else
 887:                             break; // restart if lost race to replace value
 888:                     }
 889:                     // else c < 0; fall through
 890:                 }
 891: 
 892:                 Node<K,V> z = new Node<K,V>(kkey, value, n);
 893:                 if (!b.casNext(n, z))
 894:                     break;         // restart if lost race to append to b
 895:                 int level = randomLevel();
 896:                 if (level > 0)
 897:                     insertIndex(z, level);
 898:                 return null;
 899:             }
 900:         }
 901:     }
 902: 
 903:     /**
 904:      * Returns a random level for inserting a new node.
 905:      * Hardwired to k=1, p=0.5, max 31 (see above and
 906:      * Pugh's "Skip List Cookbook", sec 3.4).
 907:      *
 908:      * This uses the simplest of the generators described in George
 909:      * Marsaglia's "Xorshift RNGs" paper.  This is not a high-quality
 910:      * generator but is acceptable here.
 911:      */
 912:     private int randomLevel() {
 913:         int x = randomSeed;
 914:         x ^= x << 13;
 915:         x ^= x >>> 17;
 916:         randomSeed = x ^= x << 5;
 917:         if ((x & 0x8001) != 0) // test highest and lowest bits
 918:             return 0;
 919:         int level = 1;
 920:         while (((x >>>= 1) & 1) != 0) ++level;
 921:         return level;
 922:     }
 923: 
 924:     /**
 925:      * Creates and adds index nodes for the given node.
 926:      * @param z the node
 927:      * @param level the level of the index
 928:      */
 929:     private void insertIndex(Node<K,V> z, int level) {
 930:         HeadIndex<K,V> h = head;
 931:         int max = h.level;
 932: 
 933:         if (level <= max) {
 934:             Index<K,V> idx = null;
 935:             for (int i = 1; i <= level; ++i)
 936:                 idx = new Index<K,V>(z, idx, null);
 937:             addIndex(idx, h, level);
 938: 
 939:         } else { // Add a new level
 940:             /*
 941:              * To reduce interference by other threads checking for
 942:              * empty levels in tryReduceLevel, new levels are added
 943:              * with initialized right pointers. Which in turn requires
 944:              * keeping levels in an array to access them while
 945:              * creating new head index nodes from the opposite
 946:              * direction.
 947:              */
 948:             level = max + 1;
 949:             Index<K,V>[] idxs = (Index<K,V>[])new Index[level+1];
 950:             Index<K,V> idx = null;
 951:             for (int i = 1; i <= level; ++i)
 952:                 idxs[i] = idx = new Index<K,V>(z, idx, null);
 953: 
 954:             HeadIndex<K,V> oldh;
 955:             int k;
 956:             for (;;) {
 957:                 oldh = head;
 958:                 int oldLevel = oldh.level;
 959:                 if (level <= oldLevel) { // lost race to add level
 960:                     k = level;
 961:                     break;
 962:                 }
 963:                 HeadIndex<K,V> newh = oldh;
 964:                 Node<K,V> oldbase = oldh.node;
 965:                 for (int j = oldLevel+1; j <= level; ++j)
 966:                     newh = new HeadIndex<K,V>(oldbase, newh, idxs[j], j);
 967:                 if (casHead(oldh, newh)) {
 968:                     k = oldLevel;
 969:                     break;
 970:                 }
 971:             }
 972:             addIndex(idxs[k], oldh, k);
 973:         }
 974:     }
 975: 
 976:     /**
 977:      * Adds given index nodes from given level down to 1.
 978:      * @param idx the topmost index node being inserted
 979:      * @param h the value of head to use to insert. This must be
 980:      * snapshotted by callers to provide correct insertion level
 981:      * @param indexLevel the level of the index
 982:      */
 983:     private void addIndex(Index<K,V> idx, HeadIndex<K,V> h, int indexLevel) {
 984:         // Track next level to insert in case of retries
 985:         int insertionLevel = indexLevel;
 986:         Comparable<? super K> key = comparable(idx.node.key);
 987:         if (key == null) throw new NullPointerException();
 988: 
 989:         // Similar to findPredecessor, but adding index nodes along
 990:         // path to key.
 991:         for (;;) {
 992:             int j = h.level;
 993:             Index<K,V> q = h;
 994:             Index<K,V> r = q.right;
 995:             Index<K,V> t = idx;
 996:             for (;;) {
 997:                 if (r != null) {
 998:                     Node<K,V> n = r.node;
 999:                     // compare before deletion check avoids needing recheck
1000:                     int c = key.compareTo(n.key);
1001:                     if (n.value == null) {
1002:                         if (!q.unlink(r))
1003:                             break;
1004:                         r = q.right;
1005:                         continue;
1006:                     }
1007:                     if (c > 0) {
1008:                         q = r;
1009:                         r = r.right;
1010:                         continue;
1011:                     }
1012:                 }
1013: 
1014:                 if (j == insertionLevel) {
1015:                     // Don't insert index if node already deleted
1016:                     if (t.indexesDeletedNode()) {
1017:                         findNode(key); // cleans up
1018:                         return;
1019:                     }
1020:                     if (!q.link(r, t))
1021:                         break; // restart
1022:                     if (--insertionLevel == 0) {
1023:                         // need final deletion check before return
1024:                         if (t.indexesDeletedNode())
1025:                             findNode(key);
1026:                         return;
1027:                     }
1028:                 }
1029: 
1030:                 if (--j >= insertionLevel && j < indexLevel)
1031:                     t = t.down;
1032:                 q = q.down;
1033:                 r = q.right;
1034:             }
1035:         }
1036:     }
1037: 
1038:     /* ---------------- Deletion -------------- */
1039: 
1040:     /**
1041:      * Main deletion method. Locates node, nulls value, appends a
1042:      * deletion marker, unlinks predecessor, removes associated index
1043:      * nodes, and possibly reduces head index level.
1044:      *
1045:      * Index nodes are cleared out simply by calling findPredecessor.
1046:      * which unlinks indexes to deleted nodes found along path to key,
1047:      * which will include the indexes to this node.  This is done
1048:      * unconditionally. We can't check beforehand whether there are
1049:      * index nodes because it might be the case that some or all
1050:      * indexes hadn't been inserted yet for this node during initial
1051:      * search for it, and we'd like to ensure lack of garbage
1052:      * retention, so must call to be sure.
1053:      *
1054:      * @param okey the key
1055:      * @param value if non-null, the value that must be
1056:      * associated with key
1057:      * @return the node, or null if not found
1058:      */
1059:     final V doRemove(Object okey, Object value) {
1060:         Comparable<? super K> key = comparable(okey);
1061:         for (;;) {
1062:             Node<K,V> b = findPredecessor(key);
1063:             Node<K,V> n = b.next;
1064:             for (;;) {
1065:                 if (n == null)
1066:                     return null;
1067:                 Node<K,V> f = n.next;
1068:                 if (n != b.next)                    // inconsistent read
1069:                     break;
1070:                 Object v = n.value;
1071:                 if (v == null) {                    // n is deleted
1072:                     n.helpDelete(b, f);
1073:                     break;
1074:                 }
1075:                 if (v == n || b.value == null)      // b is deleted
1076:                     break;
1077:                 int c = key.compareTo(n.key);
1078:                 if (c < 0)
1079:                     return null;
1080:                 if (c > 0) {
1081:                     b = n;
1082:                     n = f;
1083:                     continue;
1084:                 }
1085:                 if (value != null && !value.equals(v))
1086:                     return null;
1087:                 if (!n.casValue(v, null))
1088:                     break;
1089:                 if (!n.appendMarker(f) || !b.casNext(n, f))
1090:                     findNode(key);                  // Retry via findNode
1091:                 else {
1092:                     findPredecessor(key);           // Clean index
1093:                     if (head.right == null)
1094:                         tryReduceLevel();
1095:                 }
1096:                 return (V)v;
1097:             }
1098:         }
1099:     }
1100: 
1101:     /**
1102:      * Possibly reduce head level if it has no nodes.  This method can
1103:      * (rarely) make mistakes, in which case levels can disappear even
1104:      * though they are about to contain index nodes. This impacts
1105:      * performance, not correctness.  To minimize mistakes as well as
1106:      * to reduce hysteresis, the level is reduced by one only if the
1107:      * topmost three levels look empty. Also, if the removed level
1108:      * looks non-empty after CAS, we try to change it back quick
1109:      * before anyone notices our mistake! (This trick works pretty
1110:      * well because this method will practically never make mistakes
1111:      * unless current thread stalls immediately before first CAS, in
1112:      * which case it is very unlikely to stall again immediately
1113:      * afterwards, so will recover.)
1114:      *
1115:      * We put up with all this rather than just let levels grow
1116:      * because otherwise, even a small map that has undergone a large
1117:      * number of insertions and removals will have a lot of levels,
1118:      * slowing down access more than would an occasional unwanted
1119:      * reduction.
1120:      */
1121:     private void tryReduceLevel() {
1122:         HeadIndex<K,V> h = head;
1123:         HeadIndex<K,V> d;
1124:         HeadIndex<K,V> e;
1125:         if (h.level > 3 &&
1126:             (d = (HeadIndex<K,V>)h.down) != null &&
1127:             (e = (HeadIndex<K,V>)d.down) != null &&
1128:             e.right == null &&
1129:             d.right == null &&
1130:             h.right == null &&
1131:             casHead(h, d) && // try to set
1132:             h.right != null) // recheck
1133:             casHead(d, h);   // try to backout
1134:     }
1135: 
1136:     /* ---------------- Finding and removing first element -------------- */
1137: 
1138:     /**
1139:      * Specialized variant of findNode to get first valid node.
1140:      * @return first node or null if empty
1141:      */
1142:     Node<K,V> findFirst() {
1143:         for (;;) {
1144:             Node<K,V> b = head.node;
1145:             Node<K,V> n = b.next;
1146:             if (n == null)
1147:                 return null;
1148:             if (n.value != null)
1149:                 return n;
1150:             n.helpDelete(b, n.next);
1151:         }
1152:     }
1153: 
1154:     /**
1155:      * Removes first entry; returns its snapshot.
1156:      * @return null if empty, else snapshot of first entry
1157:      */
1158:     Map.Entry<K,V> doRemoveFirstEntry() {
1159:         for (;;) {
1160:             Node<K,V> b = head.node;
1161:             Node<K,V> n = b.next;
1162:             if (n == null)
1163:                 return null;
1164:             Node<K,V> f = n.next;
1165:             if (n != b.next)
1166:                 continue;
1167:             Object v = n.value;
1168:             if (v == null) {
1169:                 n.helpDelete(b, f);
1170:                 continue;
1171:             }
1172:             if (!n.casValue(v, null))
1173:                 continue;
1174:             if (!n.appendMarker(f) || !b.casNext(n, f))
1175:                 findFirst(); // retry
1176:             clearIndexToFirst();
1177:             return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, (V)v);
1178:     }
1179:     }
1180: 
1181:     /**
1182:      * Clears out index nodes associated with deleted first entry.
1183:      */
1184:     private void clearIndexToFirst() {
1185:         for (;;) {
1186:             Index<K,V> q = head;
1187:             for (;;) {
1188:                 Index<K,V> r = q.right;
1189:                 if (r != null && r.indexesDeletedNode() && !q.unlink(r))
1190:                     break;
1191:                 if ((q = q.down) == null) {
1192:                     if (head.right == null)
1193:                         tryReduceLevel();
1194:                     return;
1195:                 }
1196:             }
1197:         }
1198:     }
1199: 
1200: 
1201:     /* ---------------- Finding and removing last element -------------- */
1202: 
1203:     /**
1204:      * Specialized version of find to get last valid node.
1205:      * @return last node or null if empty
1206:      */
1207:     Node<K,V> findLast() {
1208:         /*
1209:          * findPredecessor can't be used to traverse index level
1210:          * because this doesn't use comparisons.  So traversals of
1211:          * both levels are folded together.
1212:          */
1213:         Index<K,V> q = head;
1214:         for (;;) {
1215:             Index<K,V> d, r;
1216:             if ((r = q.right) != null) {
1217:                 if (r.indexesDeletedNode()) {
1218:                     q.unlink(r);
1219:                     q = head; // restart
1220:                 }
1221:                 else
1222:                     q = r;
1223:             } else if ((d = q.down) != null) {
1224:                 q = d;
1225:             } else {
1226:                 Node<K,V> b = q.node;
1227:                 Node<K,V> n = b.next;
1228:                 for (;;) {
1229:                     if (n == null)
1230:                         return (b.isBaseHeader())? null : b;
1231:                     Node<K,V> f = n.next;            // inconsistent read
1232:                     if (n != b.next)
1233:                         break;
1234:                     Object v = n.value;
1235:                     if (v == null) {                 // n is deleted
1236:                         n.helpDelete(b, f);
1237:                         break;
1238:                     }
1239:                     if (v == n || b.value == null)   // b is deleted
1240:                         break;
1241:                     b = n;
1242:                     n = f;
1243:                 }
1244:                 q = head; // restart
1245:             }
1246:         }
1247:     }
1248: 
1249:     /**
1250:      * Specialized variant of findPredecessor to get predecessor of last
1251:      * valid node.  Needed when removing the last entry.  It is possible
1252:      * that all successors of returned node will have been deleted upon
1253:      * return, in which case this method can be retried.
1254:      * @return likely predecessor of last node
1255:      */
1256:     private Node<K,V> findPredecessorOfLast() {
1257:         for (;;) {
1258:             Index<K,V> q = head;
1259:             for (;;) {
1260:                 Index<K,V> d, r;
1261:                 if ((r = q.right) != null) {
1262:                     if (r.indexesDeletedNode()) {
1263:                         q.unlink(r);
1264:                         break;    // must restart
1265:                     }
1266:                     // proceed as far across as possible without overshooting
1267:                     if (r.node.next != null) {
1268:                         q = r;
1269:                         continue;
1270:                     }
1271:                 }
1272:                 if ((d = q.down) != null)
1273:                     q = d;
1274:                 else
1275:                     return q.node;
1276:             }
1277:         }
1278:     }
1279: 
1280:     /**
1281:      * Removes last entry; returns its snapshot.
1282:      * Specialized variant of doRemove.
1283:      * @return null if empty, else snapshot of last entry
1284:      */
1285:     Map.Entry<K,V> doRemoveLastEntry() {
1286:         for (;;) {
1287:             Node<K,V> b = findPredecessorOfLast();
1288:             Node<K,V> n = b.next;
1289:             if (n == null) {
1290:                 if (b.isBaseHeader())               // empty
1291:                     return null;
1292:                 else
1293:                     continue; // all b's successors are deleted; retry
1294:             }
1295:             for (;;) {
1296:                 Node<K,V> f = n.next;
1297:                 if (n != b.next)                    // inconsistent read
1298:                     break;
1299:                 Object v = n.value;
1300:                 if (v == null) {                    // n is deleted
1301:                     n.helpDelete(b, f);
1302:                     break;
1303:                 }
1304:                 if (v == n || b.value == null)      // b is deleted
1305:                     break;
1306:                 if (f != null) {
1307:                     b = n;
1308:                     n = f;
1309:                     continue;
1310:                 }
1311:                 if (!n.casValue(v, null))
1312:                     break;
1313:                 K key = n.key;
1314:                 Comparable<? super K> ck = comparable(key);
1315:                 if (!n.appendMarker(f) || !b.casNext(n, f))
1316:                     findNode(ck);                  // Retry via findNode
1317:                 else {
1318:                     findPredecessor(ck);           // Clean index
1319:                     if (head.right == null)
1320:                         tryReduceLevel();
1321:                 }
1322:                 return new AbstractMap.SimpleImmutableEntry<K,V>(key, (V)v);
1323:             }
1324:         }
1325:     }
1326: 
1327:     /* ---------------- Relational operations -------------- */
1328: 
1329:     // Control values OR'ed as arguments to findNear
1330: 
1331:     private static final int EQ = 1;
1332:     private static final int LT = 2;
1333:     private static final int GT = 0; // Actually checked as !LT
1334: 
1335:     /**
1336:      * Utility for ceiling, floor, lower, higher methods.
1337:      * @param kkey the key
1338:      * @param rel the relation -- OR'ed combination of EQ, LT, GT
1339:      * @return nearest node fitting relation, or null if no such
1340:      */
1341:     Node<K,V> findNear(K kkey, int rel) {
1342:         Comparable<? super K> key = comparable(kkey);
1343:         for (;;) {
1344:             Node<K,V> b = findPredecessor(key);
1345:             Node<K,V> n = b.next;
1346:             for (;;) {
1347:                 if (n == null)
1348:                     return ((rel & LT) == 0 || b.isBaseHeader())? null : b;
1349:                 Node<K,V> f = n.next;
1350:                 if (n != b.next)                  // inconsistent read
1351:                     break;
1352:                 Object v = n.value;
1353:                 if (v == null) {                  // n is deleted
1354:                     n.helpDelete(b, f);
1355:                     break;
1356:                 }
1357:                 if (v == n || b.value == null)    // b is deleted
1358:                     break;
1359:                 int c = key.compareTo(n.key);
1360:                 if ((c == 0 && (rel & EQ) != 0) ||
1361:                     (c <  0 && (rel & LT) == 0))
1362:                     return n;
1363:                 if ( c <= 0 && (rel & LT) != 0)
1364:                     return (b.isBaseHeader())? null : b;
1365:                 b = n;
1366:                 n = f;
1367:             }
1368:         }
1369:     }
1370: 
1371:     /**
1372:      * Returns SimpleImmutableEntry for results of findNear.
1373:      * @param key the key
1374:      * @param rel the relation -- OR'ed combination of EQ, LT, GT
1375:      * @return Entry fitting relation, or null if no such
1376:      */
1377:     AbstractMap.SimpleImmutableEntry<K,V> getNear(K key, int rel) {
1378:         for (;;) {
1379:             Node<K,V> n = findNear(key, rel);
1380:             if (n == null)
1381:                 return null;
1382:             AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
1383:             if (e != null)
1384:                 return e;
1385:         }
1386:     }
1387: 
1388: 
1389:     /* ---------------- Constructors -------------- */
1390: 
1391:     /**
1392:      * Constructs a new, empty map, sorted according to the
1393:      * {@linkplain Comparable natural ordering} of the keys.
1394:      */
1395:     public ConcurrentSkipListMap() {
1396:         this.comparator = null;
1397:         initialize();
1398:     }
1399: 
1400:     /**
1401:      * Constructs a new, empty map, sorted according to the specified
1402:      * comparator.
1403:      *
1404:      * @param comparator the comparator that will be used to order this map.
1405:      *        If <tt>null</tt>, the {@linkplain Comparable natural
1406:      *        ordering} of the keys will be used.
1407:      */
1408:     public ConcurrentSkipListMap(Comparator<? super K> comparator) {
1409:         this.comparator = comparator;
1410:         initialize();
1411:     }
1412: 
1413:     /**
1414:      * Constructs a new map containing the same mappings as the given map,
1415:      * sorted according to the {@linkplain Comparable natural ordering} of
1416:      * the keys.
1417:      *
1418:      * @param  m the map whose mappings are to be placed in this map
1419:      * @throws ClassCastException if the keys in <tt>m</tt> are not
1420:      *         {@link Comparable}, or are not mutually comparable
1421:      * @throws NullPointerException if the specified map or any of its keys
1422:      *         or values are null
1423:      */
1424:     public ConcurrentSkipListMap(Map<? extends K, ? extends V> m) {
1425:         this.comparator = null;
1426:         initialize();
1427:         putAll(m);
1428:     }
1429: 
1430:     /**
1431:      * Constructs a new map containing the same mappings and using the
1432:      * same ordering as the specified sorted map.
1433:      *
1434:      * @param m the sorted map whose mappings are to be placed in this
1435:      *        map, and whose comparator is to be used to sort this map
1436:      * @throws NullPointerException if the specified sorted map or any of
1437:      *         its keys or values are null
1438:      */
1439:     public ConcurrentSkipListMap(SortedMap<K, ? extends V> m) {
1440:         this.comparator = m.comparator();
1441:         initialize();
1442:         buildFromSorted(m);
1443:     }
1444: 
1445:     /**
1446:      * Returns a shallow copy of this <tt>ConcurrentSkipListMap</tt>
1447:      * instance. (The keys and values themselves are not cloned.)
1448:      *
1449:      * @return a shallow copy of this map
1450:      */
1451:     public ConcurrentSkipListMap<K,V> clone() {
1452:         ConcurrentSkipListMap<K,V> clone = null;
1453:         try {
1454:             clone = (ConcurrentSkipListMap<K,V>) super.clone();
1455:         } catch (CloneNotSupportedException e) {
1456:             throw new InternalError();
1457:         }
1458: 
1459:         clone.initialize();
1460:         clone.buildFromSorted(this);
1461:         return clone;
1462:     }
1463: 
1464:     /**
1465:      * Streamlined bulk insertion to initialize from elements of
1466:      * given sorted map.  Call only from constructor or clone
1467:      * method.
1468:      */
1469:     private void buildFromSorted(SortedMap<K, ? extends V> map) {
1470:         if (map == null)
1471:             throw new NullPointerException();
1472: 
1473:         HeadIndex<K,V> h = head;
1474:         Node<K,V> basepred = h.node;
1475: 
1476:         // Track the current rightmost node at each level. Uses an
1477:         // ArrayList to avoid committing to initial or maximum level.
1478:         ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>();
1479: 
1480:         // initialize
1481:         for (int i = 0; i <= h.level; ++i)
1482:             preds.add(null);
1483:         Index<K,V> q = h;
1484:         for (int i = h.level; i > 0; --i) {
1485:             preds.set(i, q);
1486:             q = q.down;
1487:         }
1488: 
1489:         Iterator<? extends Map.Entry<? extends K, ? extends V>> it =
1490:             map.entrySet().iterator();
1491:         while (it.hasNext()) {
1492:             Map.Entry<? extends K, ? extends V> e = it.next();
1493:             int j = randomLevel();
1494:             if (j > h.level) j = h.level + 1;
1495:             K k = e.getKey();
1496:             V v = e.getValue();
1497:             if (k == null || v == null)
1498:                 throw new NullPointerException();
1499:             Node<K,V> z = new Node<K,V>(k, v, null);
1500:             basepred.next = z;
1501:             basepred = z;
1502:             if (j > 0) {
1503:                 Index<K,V> idx = null;
1504:                 for (int i = 1; i <= j; ++i) {
1505:                     idx = new Index<K,V>(z, idx, null);
1506:                     if (i > h.level)
1507:                         h = new HeadIndex<K,V>(h.node, h, idx, i);
1508: 
1509:                     if (i < preds.size()) {
1510:                         preds.get(i).right = idx;
1511:                         preds.set(i, idx);
1512:                     } else
1513:                         preds.add(idx);
1514:                 }
1515:             }
1516:         }
1517:         head = h;
1518:     }
1519: 
1520:     /* ---------------- Serialization -------------- */
1521: 
1522:     /**
1523:      * Save the state of this map to a stream.
1524:      *
1525:      * @serialData The key (Object) and value (Object) for each
1526:      * key-value mapping represented by the map, followed by
1527:      * <tt>null</tt>. The key-value mappings are emitted in key-order
1528:      * (as determined by the Comparator, or by the keys' natural
1529:      * ordering if no Comparator).
1530:      */
1531:     private void writeObject(java.io.ObjectOutputStream s)
1532:         throws java.io.IOException {
1533:         // Write out the Comparator and any hidden stuff
1534:         s.defaultWriteObject();
1535: 
1536:         // Write out keys and values (alternating)
1537:         for (Node<K,V> n = findFirst(); n != null; n = n.next) {
1538:             V v = n.getValidValue();
1539:             if (v != null) {
1540:                 s.writeObject(n.key);
1541:                 s.writeObject(v);
1542:             }
1543:         }
1544:         s.writeObject(null);
1545:     }
1546: 
1547:     /**
1548:      * Reconstitute the map from a stream.
1549:      */
1550:     private void readObject(final java.io.ObjectInputStream s)
1551:         throws java.io.IOException, ClassNotFoundException {
1552:         // Read in the Comparator and any hidden stuff
1553:         s.defaultReadObject();
1554:         // Reset transients
1555:         initialize();
1556: 
1557:         /*
1558:          * This is nearly identical to buildFromSorted, but is
1559:          * distinct because readObject calls can't be nicely adapted
1560:          * as the kind of iterator needed by buildFromSorted. (They
1561:          * can be, but doing so requires type cheats and/or creation
1562:          * of adaptor classes.) It is simpler to just adapt the code.
1563:          */
1564: 
1565:         HeadIndex<K,V> h = head;
1566:         Node<K,V> basepred = h.node;
1567:         ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>();
1568:         for (int i = 0; i <= h.level; ++i)
1569:             preds.add(null);
1570:         Index<K,V> q = h;
1571:         for (int i = h.level; i > 0; --i) {
1572:             preds.set(i, q);
1573:             q = q.down;
1574:         }
1575: 
1576:         for (;;) {
1577:             Object k = s.readObject();
1578:             if (k == null)
1579:                 break;
1580:             Object v = s.readObject();
1581:             if (v == null)
1582:                 throw new NullPointerException();
1583:             K key = (K) k;
1584:             V val = (V) v;
1585:             int j = randomLevel();
1586:             if (j > h.level) j = h.level + 1;
1587:             Node<K,V> z = new Node<K,V>(key, val, null);
1588:             basepred.next = z;
1589:             basepred = z;
1590:             if (j > 0) {
1591:                 Index<K,V> idx = null;
1592:                 for (int i = 1; i <= j; ++i) {
1593:                     idx = new Index<K,V>(z, idx, null);
1594:                     if (i > h.level)
1595:                         h = new HeadIndex<K,V>(h.node, h, idx, i);
1596: 
1597:                     if (i < preds.size()) {
1598:                         preds.get(i).right = idx;
1599:                         preds.set(i, idx);
1600:                     } else
1601:                         preds.add(idx);
1602:                 }
1603:             }
1604:         }
1605:         head = h;
1606:     }
1607: 
1608:     /* ------ Map API methods ------ */
1609: 
1610:     /**
1611:      * Returns <tt>true</tt> if this map contains a mapping for the specified
1612:      * key.
1613:      *
1614:      * @param key key whose presence in this map is to be tested
1615:      * @return <tt>true</tt> if this map contains a mapping for the specified key
1616:      * @throws ClassCastException if the specified key cannot be compared
1617:      *         with the keys currently in the map
1618:      * @throws NullPointerException if the specified key is null
1619:      */
1620:     public boolean containsKey(Object key) {
1621:         return doGet(key) != null;
1622:     }
1623: 
1624:     /**
1625:      * Returns the value to which the specified key is mapped,
1626:      * or {@code null} if this map contains no mapping for the key.
1627:      *
1628:      * <p>More formally, if this map contains a mapping from a key
1629:      * {@code k} to a value {@code v} such that {@code key} compares
1630:      * equal to {@code k} according to the map's ordering, then this
1631:      * method returns {@code v}; otherwise it returns {@code null}.
1632:      * (There can be at most one such mapping.)
1633:      *
1634:      * @throws ClassCastException if the specified key cannot be compared
1635:      *         with the keys currently in the map
1636:      * @throws NullPointerException if the specified key is null
1637:      */
1638:     public V get(Object key) {
1639:         return doGet(key);
1640:     }
1641: 
1642:     /**
1643:      * Associates the specified value with the specified key in this map.
1644:      * If the map previously contained a mapping for the key, the old
1645:      * value is replaced.
1646:      *
1647:      * @param key key with which the specified value is to be associated
1648:      * @param value value to be associated with the specified key
1649:      * @return the previous value associated with the specified key, or
1650:      *         <tt>null</tt> if there was no mapping for the key
1651:      * @throws ClassCastException if the specified key cannot be compared
1652:      *         with the keys currently in the map
1653:      * @throws NullPointerException if the specified key or value is null
1654:      */
1655:     public V put(K key, V value) {
1656:         if (value == null)
1657:             throw new NullPointerException();
1658:         return doPut(key, value, false);
1659:     }
1660: 
1661:     /**
1662:      * Removes the mapping for the specified key from this map if present.
1663:      *
1664:      * @param  key key for which mapping should be removed
1665:      * @return the previous value associated with the specified key, or
1666:      *         <tt>null</tt> if there was no mapping for the key
1667:      * @throws ClassCastException if the specified key cannot be compared
1668:      *         with the keys currently in the map
1669:      * @throws NullPointerException if the specified key is null
1670:      */
1671:     public V remove(Object key) {
1672:         return doRemove(key, null);
1673:     }
1674: 
1675:     /**
1676:      * Returns <tt>true</tt> if this map maps one or more keys to the
1677:      * specified value.  This operation requires time linear in the
1678:      * map size.
1679:      *
1680:      * @param value value whose presence in this map is to be tested
1681:      * @return <tt>true</tt> if a mapping to <tt>value</tt> exists;
1682:      *         <tt>false</tt> otherwise
1683:      * @throws NullPointerException if the specified value is null
1684:      */
1685:     public boolean containsValue(Object value) {
1686:         if (value == null)
1687:             throw new NullPointerException();
1688:         for (Node<K,V> n = findFirst(); n != null; n = n.next) {
1689:             V v = n.getValidValue();
1690:             if (v != null && value.equals(v))
1691:                 return true;
1692:         }
1693:         return false;
1694:     }
1695: 
1696:     /**
1697:      * Returns the number of key-value mappings in this map.  If this map
1698:      * contains more than <tt>Integer.MAX_VALUE</tt> elements, it
1699:      * returns <tt>Integer.MAX_VALUE</tt>.
1700:      *
1701:      * <p>Beware that, unlike in most collections, this method is
1702:      * <em>NOT</em> a constant-time operation. Because of the
1703:      * asynchronous nature of these maps, determining the current
1704:      * number of elements requires traversing them all to count them.
1705:      * Additionally, it is possible for the size to change during
1706:      * execution of this method, in which case the returned result
1707:      * will be inaccurate. Thus, this method is typically not very
1708:      * useful in concurrent applications.
1709:      *
1710:      * @return the number of elements in this map
1711:      */
1712:     public int size() {
1713:         long count = 0;
1714:         for (Node<K,V> n = findFirst(); n != null; n = n.next) {
1715:             if (n.getValidValue() != null)
1716:                 ++count;
1717:         }
1718:         return (count >= Integer.MAX_VALUE)? Integer.MAX_VALUE : (int)count;
1719:     }
1720: 
1721:     /**
1722:      * Returns <tt>true</tt> if this map contains no key-value mappings.
1723:      * @return <tt>true</tt> if this map contains no key-value mappings
1724:      */
1725:     public boolean isEmpty() {
1726:         return findFirst() == null;
1727:     }
1728: 
1729:     /**
1730:      * Removes all of the mappings from this map.
1731:      */
1732:     public void clear() {
1733:         initialize();
1734:     }
1735: 
1736:     /* ---------------- View methods -------------- */
1737: 
1738:     /*
1739:      * Note: Lazy initialization works for views because view classes
1740:      * are stateless/immutable so it doesn't matter wrt correctness if
1741:      * more than one is created (which will only rarely happen).  Even
1742:      * so, the following idiom conservatively ensures that the method
1743:      * returns the one it created if it does so, not one created by
1744:      * another racing thread.
1745:      */
1746: 
1747:     /**
1748:      * Returns a {@link NavigableSet} view of the keys contained in this map.
1749:      * The set's iterator returns the keys in ascending order.
1750:      * The set is backed by the map, so changes to the map are
1751:      * reflected in the set, and vice-versa.  The set supports element
1752:      * removal, which removes the corresponding mapping from the map,
1753:      * via the {@code Iterator.remove}, {@code Set.remove},
1754:      * {@code removeAll}, {@code retainAll}, and {@code clear}
1755:      * operations.  It does not support the {@code add} or {@code addAll}
1756:      * operations.
1757:      *
1758:      * <p>The view's {@code iterator} is a "weakly consistent" iterator
1759:      * that will never throw {@link ConcurrentModificationException},
1760:      * and guarantees to traverse elements as they existed upon
1761:      * construction of the iterator, and may (but is not guaranteed to)
1762:      * reflect any modifications subsequent to construction.
1763:      *
1764:      * <p>This method is equivalent to method {@code navigableKeySet}.
1765:      *
1766:      * @return a navigable set view of the keys in this map
1767:      */
1768:      public NavigableSet<K> keySet() {
1769:         KeySet ks = keySet;
1770:         return (ks != null) ? ks : (keySet = new KeySet(this));
1771:     }
1772: 
1773:     public NavigableSet<K> navigableKeySet() {
1774:         KeySet ks = keySet;
1775:         return (ks != null) ? ks : (keySet = new KeySet(this));
1776:     }
1777: 
1778:     /**
1779:      * Returns a {@link Collection} view of the values contained in this map.
1780:      * The collection's iterator returns the values in ascending order
1781:      * of the corresponding keys.
1782:      * The collection is backed by the map, so changes to the map are
1783:      * reflected in the collection, and vice-versa.  The collection
1784:      * supports element removal, which removes the corresponding
1785:      * mapping from the map, via the <tt>Iterator.remove</tt>,
1786:      * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
1787:      * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
1788:      * support the <tt>add</tt> or <tt>addAll</tt> operations.
1789:      *
1790:      * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
1791:      * that will never throw {@link ConcurrentModificationException},
1792:      * and guarantees to traverse elements as they existed upon
1793:      * construction of the iterator, and may (but is not guaranteed to)
1794:      * reflect any modifications subsequent to construction.
1795:      */
1796:     public Collection<V> values() {
1797:         Values vs = values;
1798:         return (vs != null) ? vs : (values = new Values(this));
1799:     }
1800: 
1801:     /**
1802:      * Returns a {@link Set} view of the mappings contained in this map.
1803:      * The set's iterator returns the entries in ascending key order.
1804:      * The set is backed by the map, so changes to the map are
1805:      * reflected in the set, and vice-versa.  The set supports element
1806:      * removal, which removes the corresponding mapping from the map,
1807:      * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
1808:      * <tt>removeAll</tt>, <tt>retainAll</tt> and <tt>clear</tt>
1809:      * operations.  It does not support the <tt>add</tt> or
1810:      * <tt>addAll</tt> operations.
1811:      *
1812:      * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
1813:      * that will never throw {@link ConcurrentModificationException},
1814:      * and guarantees to traverse elements as they existed upon
1815:      * construction of the iterator, and may (but is not guaranteed to)
1816:      * reflect any modifications subsequent to construction.
1817:      *
1818:      * <p>The <tt>Map.Entry</tt> elements returned by
1819:      * <tt>iterator.next()</tt> do <em>not</em> support the
1820:      * <tt>setValue</tt> operation.
1821:      *
1822:      * @return a set view of the mappings contained in this map,
1823:      *         sorted in ascending key order
1824:      */
1825:     public Set<Map.Entry<K,V>> entrySet() {
1826:         EntrySet es = entrySet;
1827:         return (es != null) ? es : (entrySet = new EntrySet(this));
1828:     }
1829: 
1830:     public ConcurrentNavigableMap<K,V> descendingMap() {
1831:         ConcurrentNavigableMap<K,V> dm = descendingMap;
1832:         return (dm != null) ? dm : (descendingMap = new SubMap<K,V>
1833:                                     (this, null, false, null, false, true));
1834:     }
1835: 
1836:     public NavigableSet<K> descendingKeySet() {
1837:         return descendingMap().navigableKeySet();
1838:     }
1839: 
1840:     /* ---------------- AbstractMap Overrides -------------- */
1841: 
1842:     /**
1843:      * Compares the specified object with this map for equality.
1844:      * Returns <tt>true</tt> if the given object is also a map and the
1845:      * two maps represent the same mappings.  More formally, two maps
1846:      * <tt>m1</tt> and <tt>m2</tt> represent the same mappings if
1847:      * <tt>m1.entrySet().equals(m2.entrySet())</tt>.  This
1848:      * operation may return misleading results if either map is
1849:      * concurrently modified during execution of this method.
1850:      *
1851:      * @param o object to be compared for equality with this map
1852:      * @return <tt>true</tt> if the specified object is equal to this map
1853:      */
1854:     public boolean equals(Object o) {
1855:     if (o == this)
1856:         return true;
1857:     if (!(o instanceof Map))
1858:         return false;
1859:     Map<?,?> m = (Map<?,?>) o;
1860:         try {
1861:         for (Map.Entry<K,V> e : this.entrySet())
1862:         if (! e.getValue().equals(m.get(e.getKey())))
1863:                     return false;
1864:         for (Map.Entry<?,?> e : m.entrySet()) {
1865:                 Object k = e.getKey();
1866:                 Object v = e.getValue();
1867:         if (k == null || v == null || !v.equals(get(k)))
1868:                     return false;
1869:             }
1870:             return true;
1871:         } catch (ClassCastException unused) {
1872:             return false;
1873:         } catch (NullPointerException unused) {
1874:             return false;
1875:         }
1876:     }
1877: 
1878:     /* ------ ConcurrentMap API methods ------ */
1879: 
1880:     /**
1881:      * {@inheritDoc}
1882:      *
1883:      * @return the previous value associated with the specified key,
1884:      *         or <tt>null</tt> if there was no mapping for the key
1885:      * @throws ClassCastException if the specified key cannot be compared
1886:      *         with the keys currently in the map
1887:      * @throws NullPointerException if the specified key or value is null
1888:      */
1889:     public V putIfAbsent(K key, V value) {
1890:         if (value == null)
1891:             throw new NullPointerException();
1892:         return doPut(key, value, true);
1893:     }
1894: 
1895:     /**
1896:      * {@inheritDoc}
1897:      *
1898:      * @throws ClassCastException if the specified key cannot be compared
1899:      *         with the keys currently in the map
1900:      * @throws NullPointerException if the specified key is null
1901:      */
1902:     public boolean remove(Object key, Object value) {
1903:         if (key == null)
1904:             throw new NullPointerException();
1905:         if (value == null)
1906:             return false;
1907:         return doRemove(key, value) != null;
1908:     }
1909: 
1910:     /**
1911:      * {@inheritDoc}
1912:      *
1913:      * @throws ClassCastException if the specified key cannot be compared
1914:      *         with the keys currently in the map
1915:      * @throws NullPointerException if any of the arguments are null
1916:      */
1917:     public boolean replace(K key, V oldValue, V newValue) {
1918:         if (oldValue == null || newValue == null)
1919:             throw new NullPointerException();
1920:         Comparable<? super K> k = comparable(key);
1921:         for (;;) {
1922:             Node<K,V> n = findNode(k);
1923:             if (n == null)
1924:                 return false;
1925:             Object v = n.value;
1926:             if (v != null) {
1927:                 if (!oldValue.equals(v))
1928:                     return false;
1929:                 if (n.casValue(v, newValue))
1930:                     return true;
1931:             }
1932:         }
1933:     }
1934: 
1935:     /**
1936:      * {@inheritDoc}
1937:      *
1938:      * @return the previous value associated with the specified key,
1939:      *         or <tt>null</tt> if there was no mapping for the key
1940:      * @throws ClassCastException if the specified key cannot be compared
1941:      *         with the keys currently in the map
1942:      * @throws NullPointerException if the specified key or value is null
1943:      */
1944:     public V replace(K key, V value) {
1945:         if (value == null)
1946:             throw new NullPointerException();
1947:         Comparable<? super K> k = comparable(key);
1948:         for (;;) {
1949:             Node<K,V> n = findNode(k);
1950:             if (n == null)
1951:                 return null;
1952:             Object v = n.value;
1953:             if (v != null && n.casValue(v, value))
1954:                 return (V)v;
1955:         }
1956:     }
1957: 
1958:     /* ------ SortedMap API methods ------ */
1959: 
1960:     public Comparator<? super K> comparator() {
1961:         return comparator;
1962:     }
1963: 
1964:     /**
1965:      * @throws NoSuchElementException {@inheritDoc}
1966:      */
1967:     public K firstKey() {
1968:         Node<K,V> n = findFirst();
1969:         if (n == null)
1970:             throw new NoSuchElementException();
1971:         return n.key;
1972:     }
1973: 
1974:     /**
1975:      * @throws NoSuchElementException {@inheritDoc}
1976:      */
1977:     public K lastKey() {
1978:         Node<K,V> n = findLast();
1979:         if (n == null)
1980:             throw new NoSuchElementException();
1981:         return n.key;
1982:     }
1983: 
1984:     /**
1985:      * @throws ClassCastException {@inheritDoc}
1986:      * @throws NullPointerException if {@code fromKey} or {@code toKey} is null
1987:      * @throws IllegalArgumentException {@inheritDoc}
1988:      */
1989:     public ConcurrentNavigableMap<K,V> subMap(K fromKey,
1990:                                               boolean fromInclusive,
1991:                                               K toKey,
1992:                                               boolean toInclusive) {
1993:         if (fromKey == null || toKey == null)
1994:             throw new NullPointerException();
1995:         return new SubMap<K,V>
1996:             (this, fromKey, fromInclusive, toKey, toInclusive, false);
1997:     }
1998: 
1999:     /**
2000:      * @throws ClassCastException {@inheritDoc}
2001:      * @throws NullPointerException if {@code toKey} is null
2002:      * @throws IllegalArgumentException {@inheritDoc}
2003:      */
2004:     public ConcurrentNavigableMap<K,V> headMap(K toKey,
2005:                                                boolean inclusive) {
2006:         if (toKey == null)
2007:             throw new NullPointerException();
2008:         return new SubMap<K,V>
2009:             (this, null, false, toKey, inclusive, false);
2010:     }
2011: 
2012:     /**
2013:      * @throws ClassCastException {@inheritDoc}
2014:      * @throws NullPointerException if {@code fromKey} is null
2015:      * @throws IllegalArgumentException {@inheritDoc}
2016:      */
2017:     public ConcurrentNavigableMap<K,V> tailMap(K fromKey,
2018:                                                boolean inclusive) {
2019:         if (fromKey == null)
2020:             throw new NullPointerException();
2021:         return new SubMap<K,V>
2022:             (this, fromKey, inclusive, null, false, false);
2023:     }
2024: 
2025:     /**
2026:      * @throws ClassCastException {@inheritDoc}
2027:      * @throws NullPointerException if {@code fromKey} or {@code toKey} is null
2028:      * @throws IllegalArgumentException {@inheritDoc}
2029:      */
2030:     public ConcurrentNavigableMap<K,V> subMap(K fromKey, K toKey) {
2031:         return subMap(fromKey, true, toKey, false);
2032:     }
2033: 
2034:     /**
2035:      * @throws ClassCastException {@inheritDoc}
2036:      * @throws NullPointerException if {@code toKey} is null
2037:      * @throws IllegalArgumentException {@inheritDoc}
2038:      */
2039:     public ConcurrentNavigableMap<K,V> headMap(K toKey) {
2040:         return headMap(toKey, false);
2041:     }
2042: 
2043:     /**
2044:      * @throws ClassCastException {@inheritDoc}
2045:      * @throws NullPointerException if {@code fromKey} is null
2046:      * @throws IllegalArgumentException {@inheritDoc}
2047:      */
2048:     public ConcurrentNavigableMap<K,V> tailMap(K fromKey) {
2049:         return tailMap(fromKey, true);
2050:     }
2051: 
2052:     /* ---------------- Relational operations -------------- */
2053: 
2054:     /**
2055:      * Returns a key-value mapping associated with the greatest key
2056:      * strictly less than the given key, or <tt>null</tt> if there is
2057:      * no such key. The returned entry does <em>not</em> support the
2058:      * <tt>Entry.setValue</tt> method.
2059:      *
2060:      * @throws ClassCastException {@inheritDoc}
2061:      * @throws NullPointerException if the specified key is null
2062:      */
2063:     public Map.Entry<K,V> lowerEntry(K key) {
2064:         return getNear(key, LT);
2065:     }
2066: 
2067:     /**
2068:      * @throws ClassCastException {@inheritDoc}
2069:      * @throws NullPointerException if the specified key is null
2070:      */
2071:     public K lowerKey(K key) {
2072:         Node<K,V> n = findNear(key, LT);
2073:         return (n == null)? null : n.key;
2074:     }
2075: 
2076:     /**
2077:      * Returns a key-value mapping associated with the greatest key
2078:      * less than or equal to the given key, or <tt>null</tt> if there
2079:      * is no such key. The returned entry does <em>not</em> support
2080:      * the <tt>Entry.setValue</tt> method.
2081:      *
2082:      * @param key the key
2083:      * @throws ClassCastException {@inheritDoc}
2084:      * @throws NullPointerException if the specified key is null
2085:      */
2086:     public Map.Entry<K,V> floorEntry(K key) {
2087:         return getNear(key, LT|EQ);
2088:     }
2089: 
2090:     /**
2091:      * @param key the key
2092:      * @throws ClassCastException {@inheritDoc}
2093:      * @throws NullPointerException if the specified key is null
2094:      */
2095:     public K floorKey(K key) {
2096:         Node<K,V> n = findNear(key, LT|EQ);
2097:         return (n == null)? null : n.key;
2098:     }
2099: 
2100:     /**
2101:      * Returns a key-value mapping associated with the least key
2102:      * greater than or equal to the given key, or <tt>null</tt> if
2103:      * there is no such entry. The returned entry does <em>not</em>
2104:      * support the <tt>Entry.setValue</tt> method.
2105:      *
2106:      * @throws ClassCastException {@inheritDoc}
2107:      * @throws NullPointerException if the specified key is null
2108:      */
2109:     public Map.Entry<K,V> ceilingEntry(K key) {
2110:         return getNear(key, GT|EQ);
2111:     }
2112: 
2113:     /**
2114:      * @throws ClassCastException {@inheritDoc}
2115:      * @throws NullPointerException if the specified key is null
2116:      */
2117:     public K ceilingKey(K key) {
2118:         Node<K,V> n = findNear(key, GT|EQ);
2119:         return (n == null)? null : n.key;
2120:     }
2121: 
2122:     /**
2123:      * Returns a key-value mapping associated with the least key
2124:      * strictly greater than the given key, or <tt>null</tt> if there
2125:      * is no such key. The returned entry does <em>not</em> support
2126:      * the <tt>Entry.setValue</tt> method.
2127:      *
2128:      * @param key the key
2129:      * @throws ClassCastException {@inheritDoc}
2130:      * @throws NullPointerException if the specified key is null
2131:      */
2132:     public Map.Entry<K,V> higherEntry(K key) {
2133:         return getNear(key, GT);
2134:     }
2135: 
2136:     /**
2137:      * @param key the key
2138:      * @throws ClassCastException {@inheritDoc}
2139:      * @throws NullPointerException if the specified key is null
2140:      */
2141:     public K higherKey(K key) {
2142:         Node<K,V> n = findNear(key, GT);
2143:         return (n == null)? null : n.key;
2144:     }
2145: 
2146:     /**
2147:      * Returns a key-value mapping associated with the least
2148:      * key in this map, or <tt>null</tt> if the map is empty.
2149:      * The returned entry does <em>not</em> support
2150:      * the <tt>Entry.setValue</tt> method.
2151:      */
2152:     public Map.Entry<K,V> firstEntry() {
2153:         for (;;) {
2154:             Node<K,V> n = findFirst();
2155:             if (n == null)
2156:                 return null;
2157:             AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
2158:             if (e != null)
2159:                 return e;
2160:         }
2161:     }
2162: 
2163:     /**
2164:      * Returns a key-value mapping associated with the greatest
2165:      * key in this map, or <tt>null</tt> if the map is empty.
2166:      * The returned entry does <em>not</em> support
2167:      * the <tt>Entry.setValue</tt> method.
2168:      */
2169:     public Map.Entry<K,V> lastEntry() {
2170:         for (;;) {
2171:             Node<K,V> n = findLast();
2172:             if (n == null)
2173:                 return null;
2174:             AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
2175:             if (e != null)
2176:                 return e;
2177:         }
2178:     }
2179: 
2180:     /**
2181:      * Removes and returns a key-value mapping associated with
2182:      * the least key in this map, or <tt>null</tt> if the map is empty.
2183:      * The returned entry does <em>not</em> support
2184:      * the <tt>Entry.setValue</tt> method.
2185:      */
2186:     public Map.Entry<K,V> pollFirstEntry() {
2187:         return doRemoveFirstEntry();
2188:     }
2189: 
2190:     /**
2191:      * Removes and returns a key-value mapping associated with
2192:      * the greatest key in this map, or <tt>null</tt> if the map is empty.
2193:      * The returned entry does <em>not</em> support
2194:      * the <tt>Entry.setValue</tt> method.
2195:      */
2196:     public Map.Entry<K,V> pollLastEntry() {
2197:         return doRemoveLastEntry();
2198:     }
2199: 
2200: 
2201:     /* ---------------- Iterators -------------- */
2202: 
2203:     /**
2204:      * Base of iterator classes:
2205:      */
2206:     abstract class Iter<T> implements Iterator<T> {
2207:         /** the last node returned by next() */
2208:         Node<K,V> lastReturned;
2209:         /** the next node to return from next(); */
2210:         Node<K,V> next;
2211:     /** Cache of next value field to maintain weak consistency */
2212:     V nextValue;
2213: 
2214:         /** Initializes ascending iterator for entire range. */
2215:         Iter() {
2216:             for (;;) {
2217:         next = findFirst();
2218:                 if (next == null)
2219:                     break;
2220:                 Object x = next.value;
2221:                 if (x != null && x != next) {
2222:             nextValue = (V) x;
2223:                     break;
2224:         }
2225:             }
2226:         }
2227: 
2228:         public final boolean hasNext() {
2229:             return next != null;
2230:         }
2231: 
2232:         /** Advances next to higher entry. */
2233:         final void advance() {
2234:             if ((lastReturned = next) == null)
2235:                 throw new NoSuchElementException();
2236:             for (;;) {
2237:         next = next.next;
2238:                 if (next == null)
2239:                     break;
2240:                 Object x = next.value;
2241:                 if (x != null && x != next) {
2242:             nextValue = (V) x;
2243:                     break;
2244:         }
2245:             }
2246:         }
2247: 
2248:         public void remove() {
2249:             Node<K,V> l = lastReturned;
2250:             if (l == null)
2251:                 throw new IllegalStateException();
2252:             // It would not be worth all of the overhead to directly
2253:             // unlink from here. Using remove is fast enough.
2254:             ConcurrentSkipListMap.this.remove(l.key);
2255:         lastReturned = null;
2256:         }
2257: 
2258:     }
2259: 
2260:     final class ValueIterator extends Iter<V> {
2261:         public V next() {
2262:             V v = nextValue;
2263:             advance();
2264:             return v;
2265:         }
2266:     }
2267: 
2268:     final class KeyIterator extends Iter<K> {
2269:         public K next() {
2270:             Node<K,V> n = next;
2271:             advance();
2272:             return n.key;
2273:         }
2274:     }
2275: 
2276:     final class EntryIterator extends Iter<Map.Entry<K,V>> {
2277:         public Map.Entry<K,V> next() {
2278:             Node<K,V> n = next;
2279:             V v = nextValue;
2280:             advance();
2281:             return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
2282:         }
2283:     }
2284: 
2285:     // Factory methods for iterators needed by ConcurrentSkipListSet etc
2286: 
2287:     Iterator<K> keyIterator() {
2288:         return new KeyIterator();
2289:     }
2290: 
2291:     Iterator<V> valueIterator() {
2292:         return new ValueIterator();
2293:     }
2294: 
2295:     Iterator<Map.Entry<K,V>> entryIterator() {
2296:         return new EntryIterator();
2297:     }
2298: 
2299:     /* ---------------- View Classes -------------- */
2300: 
2301:     /*
2302:      * View classes are static, delegating to a ConcurrentNavigableMap
2303:      * to allow use by SubMaps, which outweighs the ugliness of
2304:      * needing type-tests for Iterator methods.
2305:      */
2306: 
2307:     static final <E> List<E> toList(Collection<E> c) {
2308:     // Using size() here would be a pessimization.
2309:     List<E> list = new ArrayList<E>();
2310:     for (E e : c)
2311:         list.add(e);
2312:     return list;
2313:     }
2314: 
2315:     static final class KeySet<E> extends AbstractSet<E> implements NavigableSet<E> {
2316:         private final ConcurrentNavigableMap<E,Object> m;
2317:         KeySet(ConcurrentNavigableMap<E,Object> map) { m = map; }
2318:         public int size() { return m.size(); }
2319:         public boolean isEmpty() { return m.isEmpty(); }
2320:         public boolean contains(Object o) { return m.containsKey(o); }
2321:         public boolean remove(Object o) { return m.remove(o) != null; }
2322:         public void clear() { m.clear(); }
2323:         public E lower(E e) { return m.lowerKey(e); }
2324:         public E floor(E e) { return m.floorKey(e); }
2325:         public E ceiling(E e) { return m.ceilingKey(e); }
2326:         public E higher(E e) { return m.higherKey(e); }
2327:         public Comparator<? super E> comparator() { return m.comparator(); }
2328:         public E first() { return m.firstKey(); }
2329:         public E last() { return m.lastKey(); }
2330:         public E pollFirst() {
2331:             Map.Entry<E,Object> e = m.pollFirstEntry();
2332:             return e == null? null : e.getKey();
2333:         }
2334:         public E pollLast() {
2335:             Map.Entry<E,Object> e = m.pollLastEntry();
2336:             return e == null? null : e.getKey();
2337:         }
2338:         public Iterator<E> iterator() {
2339:             if (m instanceof ConcurrentSkipListMap)
2340:                 return ((ConcurrentSkipListMap<E,Object>)m).keyIterator();
2341:             else
2342:                 return ((ConcurrentSkipListMap.SubMap<E,Object>)m).keyIterator();
2343:         }
2344:         public boolean equals(Object o) {
2345:             if (o == this)
2346:                 return true;
2347:             if (!(o instanceof Set))
2348:                 return false;
2349:             Collection<?> c = (Collection<?>) o;
2350:             try {
2351:                 return containsAll(c) && c.containsAll(this);
2352:             } catch (ClassCastException unused)   {
2353:                 return false;
2354:             } catch (NullPointerException unused) {
2355:                 return false;
2356:             }
2357:         }
2358:     public Object[] toArray()     { return toList(this).toArray();  }
2359:     public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }
2360:         public Iterator<E> descendingIterator() {
2361:             return descendingSet().iterator();
2362:         }
2363:         public NavigableSet<E> subSet(E fromElement,
2364:                                       boolean fromInclusive,
2365:                                       E toElement,
2366:                                       boolean toInclusive) {
2367:             return new ConcurrentSkipListSet<E>
2368:                 (m.subMap(fromElement, fromInclusive,
2369:                           toElement,   toInclusive));
2370:         }
2371:         public NavigableSet<E> headSet(E toElement, boolean inclusive) {
2372:             return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
2373:         }
2374:         public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
2375:             return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
2376:         }
2377:         public NavigableSet<E> subSet(E fromElement, E toElement) {
2378:             return subSet(fromElement, true, toElement, false);
2379:         }
2380:         public NavigableSet<E> headSet(E toElement) {
2381:             return headSet(toElement, false);
2382:         }
2383:         public NavigableSet<E> tailSet(E fromElement) {
2384:             return tailSet(fromElement, true);
2385:         }
2386:         public NavigableSet<E> descendingSet() {
2387:             return new ConcurrentSkipListSet(m.descendingMap());
2388:         }
2389:     }
2390: 
2391:     static final class Values<E> extends AbstractCollection<E> {
2392:         private final ConcurrentNavigableMap<Object, E> m;
2393:         Values(ConcurrentNavigableMap<Object, E> map) {
2394:             m = map;
2395:         }
2396:         public Iterator<E> iterator() {
2397:             if (m instanceof ConcurrentSkipListMap)
2398:                 return ((ConcurrentSkipListMap<Object,E>)m).valueIterator();
2399:             else
2400:                 return ((SubMap<Object,E>)m).valueIterator();
2401:         }
2402:         public boolean isEmpty() {
2403:             return m.isEmpty();
2404:         }
2405:         public int size() {
2406:             return m.size();
2407:         }
2408:         public boolean contains(Object o) {
2409:             return m.containsValue(o);
2410:         }
2411:         public void clear() {
2412:             m.clear();
2413:         }
2414:     public Object[] toArray()     { return toList(this).toArray();  }
2415:     public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }
2416:     }
2417: 
2418:     static final class EntrySet<K1,V1> extends AbstractSet<Map.Entry<K1,V1>> {
2419:         private final ConcurrentNavigableMap<K1, V1> m;
2420:         EntrySet(ConcurrentNavigableMap<K1, V1> map) {
2421:             m = map;
2422:         }
2423: 
2424:         public Iterator<Map.Entry<K1,V1>> iterator() {
2425:             if (m instanceof ConcurrentSkipListMap)
2426:                 return ((ConcurrentSkipListMap<K1,V1>)m).entryIterator();
2427:             else
2428:                 return ((SubMap<K1,V1>)m).entryIterator();
2429:         }
2430: 
2431:         public boolean contains(Object o) {
2432:             if (!(o instanceof Map.Entry))
2433:                 return false;
2434:             Map.Entry<K1,V1> e = (Map.Entry<K1,V1>)o;
2435:             V1 v = m.get(e.getKey());
2436:             return v != null && v.equals(e.getValue());
2437:         }
2438:         public boolean remove(Object o) {
2439:             if (!(o instanceof Map.Entry))
2440:                 return false;
2441:             Map.Entry<K1,V1> e = (Map.Entry<K1,V1>)o;
2442:             return m.remove(e.getKey(),
2443:                             e.getValue());
2444:         }
2445:         public boolean isEmpty() {
2446:             return m.isEmpty();
2447:         }
2448:         public int size() {
2449:             return m.size();
2450:         }
2451:         public void clear() {
2452:             m.clear();
2453:         }
2454:         public boolean equals(Object o) {
2455:             if (o == this)
2456:                 return true;
2457:             if (!(o instanceof Set))
2458:                 return false;
2459:             Collection<?> c = (Collection<?>) o;
2460:             try {
2461:                 return containsAll(c) && c.containsAll(this);
2462:             } catch (ClassCastException unused)   {
2463:                 return false;
2464:             } catch (NullPointerException unused) {
2465:                 return false;
2466:             }
2467:         }
2468:     public Object[] toArray()     { return toList(this).toArray();  }
2469:     public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }
2470:     }
2471: 
2472:     /**
2473:      * Submaps returned by {@link ConcurrentSkipListMap} submap operations
2474:      * represent a subrange of mappings of their underlying
2475:      * maps. Instances of this class support all methods of their
2476:      * underlying maps, differing in that mappings outside their range are
2477:      * ignored, and attempts to add mappings outside their ranges result
2478:      * in {@link IllegalArgumentException}.  Instances of this class are
2479:      * constructed only using the <tt>subMap</tt>, <tt>headMap</tt>, and
2480:      * <tt>tailMap</tt> methods of their underlying maps.
2481:      *
2482:      * @serial include
2483:      */
2484:     static final class SubMap<K,V> extends AbstractMap<K,V>
2485:         implements ConcurrentNavigableMap<K,V>, Cloneable,
2486:                    java.io.Serializable {
2487:         private static final long serialVersionUID = -7647078645895051609L;
2488: 
2489:         /** Underlying map */
2490:         private final ConcurrentSkipListMap<K,V> m;
2491:         /** lower bound key, or null if from start */
2492:         private final K lo;
2493:         /** upper bound key, or null if to end */
2494:         private final K hi;
2495:         /** inclusion flag for lo */
2496:         private final boolean loInclusive;
2497:         /** inclusion flag for hi */
2498:         private final boolean hiInclusive;
2499:         /** direction */
2500:         private final boolean isDescending;
2501: 
2502:         // Lazily initialized view holders
2503:         private transient KeySet<K> keySetView;
2504:         private transient Set<Map.Entry<K,V>> entrySetView;
2505:         private transient Collection<V> valuesView;
2506: 
2507:         /**
2508:          * Creates a new submap, initializing all fields
2509:          */
2510:         SubMap(ConcurrentSkipListMap<K,V> map,
2511:                K fromKey, boolean fromInclusive,
2512:                K toKey, boolean toInclusive,
2513:                boolean isDescending) {
2514:             if (fromKey != null && toKey != null &&
2515:                 map.compare(fromKey, toKey) > 0)
2516:                 throw new IllegalArgumentException("inconsistent range");
2517:             this.m = map;
2518:             this.lo = fromKey;
2519:             this.hi = toKey;
2520:             this.loInclusive = fromInclusive;
2521:             this.hiInclusive = toInclusive;
2522:             this.isDescending = isDescending;
2523:         }
2524: 
2525:         /* ----------------  Utilities -------------- */
2526: 
2527:         private boolean tooLow(K key) {
2528:             if (lo != null) {
2529:                 int c = m.compare(key, lo);
2530:                 if (c < 0 || (c == 0 && !loInclusive))
2531:                     return true;
2532:             }
2533:             return false;
2534:         }
2535: 
2536:         private boolean tooHigh(K key) {
2537:             if (hi != null) {
2538:                 int c = m.compare(key, hi);
2539:                 if (c > 0 || (c == 0 && !hiInclusive))
2540:                     return true;
2541:             }
2542:             return false;
2543:         }
2544: 
2545:         private boolean inBounds(K key) {
2546:             return !tooLow(key) && !tooHigh(key);
2547:         }
2548: 
2549:         private void checkKeyBounds(K key) throws IllegalArgumentException {
2550:             if (key == null)
2551:                 throw new NullPointerException();
2552:             if (!inBounds(key))
2553:                 throw new IllegalArgumentException("key out of range");
2554:         }
2555: 
2556:         /**
2557:          * Returns true if node key is less than upper bound of range
2558:          */
2559:         private boolean isBeforeEnd(ConcurrentSkipListMap.Node<K,V> n) {
2560:             if (n == null)
2561:                 return false;
2562:             if (hi == null)
2563:                 return true;
2564:             K k = n.key;
2565:             if (k == null) // pass by markers and headers
2566:                 return true;
2567:             int c = m.compare(k, hi);
2568:             if (c > 0 || (c == 0 && !hiInclusive))
2569:                 return false;
2570:             return true;
2571:         }
2572: 
2573:         /**
2574:          * Returns lowest node. This node might not be in range, so
2575:          * most usages need to check bounds
2576:          */
2577:         private ConcurrentSkipListMap.Node<K,V> loNode() {
2578:             if (lo == null)
2579:                 return m.findFirst();
2580:             else if (loInclusive)
2581:                 return m.findNear(lo, m.GT|m.EQ);
2582:             else
2583:                 return m.findNear(lo, m.GT);
2584:         }
2585: 
2586:         /**
2587:          * Returns highest node. This node might not be in range, so
2588:          * most usages need to check bounds
2589:          */
2590:         private ConcurrentSkipListMap.Node<K,V> hiNode() {
2591:             if (hi == null)
2592:                 return m.findLast();
2593:             else if (hiInclusive)
2594:                 return m.findNear(hi, m.LT|m.EQ);
2595:             else
2596:                 return m.findNear(hi, m.LT);
2597:         }
2598: 
2599:         /**
2600:          * Returns lowest absolute key (ignoring directonality)
2601:          */
2602:         private K lowestKey() {
2603:             ConcurrentSkipListMap.Node<K,V> n = loNode();
2604:             if (isBeforeEnd(n))
2605:                 return n.key;
2606:             else
2607:                 throw new NoSuchElementException();
2608:         }
2609: 
2610:         /**
2611:          * Returns highest absolute key (ignoring directonality)
2612:          */
2613:         private K highestKey() {
2614:             ConcurrentSkipListMap.Node<K,V> n = hiNode();
2615:             if (n != null) {
2616:                 K last = n.key;
2617:                 if (inBounds(last))
2618:                     return last;
2619:             }
2620:             throw new NoSuchElementException();
2621:         }
2622: 
2623:         private Map.Entry<K,V> lowestEntry() {
2624:             for (;;) {
2625:                 ConcurrentSkipListMap.Node<K,V> n = loNode();
2626:                 if (!isBeforeEnd(n))
2627:                     return null;
2628:                 Map.Entry<K,V> e = n.createSnapshot();
2629:                 if (e != null)
2630:                     return e;
2631:             }
2632:         }
2633: 
2634:         private Map.Entry<K,V> highestEntry() {
2635:             for (;;) {
2636:                 ConcurrentSkipListMap.Node<K,V> n = hiNode();
2637:                 if (n == null || !inBounds(n.key))
2638:                     return null;
2639:                 Map.Entry<K,V> e = n.createSnapshot();
2640:                 if (e != null)
2641:                     return e;
2642:             }
2643:         }
2644: 
2645:         private Map.Entry<K,V> removeLowest() {
2646:             for (;;) {
2647:                 Node<K,V> n = loNode();
2648:                 if (n == null)
2649:                     return null;
2650:                 K k = n.key;
2651:                 if (!inBounds(k))
2652:                     return null;
2653:                 V v = m.doRemove(k, null);
2654:                 if (v != null)
2655:                     return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
2656:             }
2657:         }
2658: 
2659:         private Map.Entry<K,V> removeHighest() {
2660:             for (;;) {
2661:                 Node<K,V> n = hiNode();
2662:                 if (n == null)
2663:                     return null;
2664:                 K k = n.key;
2665:                 if (!inBounds(k))
2666:                     return null;
2667:                 V v = m.doRemove(k, null);
2668:                 if (v != null)
2669:                     return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
2670:             }
2671:         }
2672: 
2673:         /**
2674:          * Submap version of ConcurrentSkipListMap.getNearEntry
2675:          */
2676:         private Map.Entry<K,V> getNearEntry(K key, int rel) {
2677:             if (isDescending) { // adjust relation for direction
2678:                 if ((rel & m.LT) == 0)
2679:                     rel |= m.LT;
2680:                 else
2681:                     rel &= ~m.LT;
2682:             }
2683:             if (tooLow(key))
2684:                 return ((rel & m.LT) != 0)? null : lowestEntry();
2685:             if (tooHigh(key))
2686:                 return ((rel & m.LT) != 0)? highestEntry() : null;
2687:             for (;;) {
2688:                 Node<K,V> n = m.findNear(key, rel);
2689:                 if (n == null || !inBounds(n.key))
2690:                     return null;
2691:                 K k = n.key;
2692:                 V v = n.getValidValue();
2693:                 if (v != null)
2694:                     return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
2695:             }
2696:         }
2697: 
2698:         // Almost the same as getNearEntry, except for keys
2699:         private K getNearKey(K key, int rel) {
2700:             if (isDescending) { // adjust relation for direction
2701:                 if ((rel & m.LT) == 0)
2702:                     rel |= m.LT;
2703:                 else
2704:                     rel &= ~m.LT;
2705:             }
2706:             if (tooLow(key)) {
2707:                 if ((rel & m.LT) == 0) {
2708:                     ConcurrentSkipListMap.Node<K,V> n = loNode();
2709:                     if (isBeforeEnd(n))
2710:                         return n.key;
2711:                 }
2712:                 return null;
2713:             }
2714:             if (tooHigh(key)) {
2715:                 if ((rel & m.LT) != 0) {
2716:                     ConcurrentSkipListMap.Node<K,V> n = hiNode();
2717:                     if (n != null) {
2718:                         K last = n.key;
2719:                         if (inBounds(last))
2720:                             return last;
2721:                     }
2722:                 }
2723:                 return null;
2724:             }
2725:             for (;;) {
2726:                 Node<K,V> n = m.findNear(key, rel);
2727:                 if (n == null || !inBounds(n.key))
2728:                     return null;
2729:                 K k = n.key;
2730:                 V v = n.getValidValue();
2731:                 if (v != null)
2732:                     return k;
2733:             }
2734:         }
2735: 
2736:         /* ----------------  Map API methods -------------- */
2737: 
2738:         public boolean containsKey(Object key) {
2739:             if (key == null) throw new NullPointerException();
2740:             K k = (K)key;
2741:             return inBounds(k) && m.containsKey(k);
2742:         }
2743: 
2744:         public V get(Object key) {
2745:             if (key == null) throw new NullPointerException();
2746:             K k = (K)key;
2747:             return ((!inBounds(k)) ? null : m.get(k));
2748:         }
2749: 
2750:         public V put(K key, V value) {
2751:             checkKeyBounds(key);
2752:             return m.put(key, value);
2753:         }
2754: 
2755:         public V remove(Object key) {
2756:             K k = (K)key;
2757:             return (!inBounds(k))? null : m.remove(k);
2758:         }
2759: 
2760:         public int size() {
2761:             long count = 0;
2762:             for (ConcurrentSkipListMap.Node<K,V> n = loNode();
2763:                  isBeforeEnd(n);
2764:                  n = n.next) {
2765:                 if (n.getValidValue() != null)
2766:                     ++count;
2767:             }
2768:             return count >= Integer.MAX_VALUE? Integer.MAX_VALUE : (int)count;
2769:         }
2770: 
2771:         public boolean isEmpty() {
2772:             return !isBeforeEnd(loNode());
2773:         }
2774: 
2775:         public boolean containsValue(Object value) {
2776:             if (value == null)
2777:                 throw new NullPointerException();
2778:             for (ConcurrentSkipListMap.Node<K,V> n = loNode();
2779:                  isBeforeEnd(n);
2780:                  n = n.next) {
2781:                 V v = n.getValidValue();
2782:                 if (v != null && value.equals(v))
2783:                     return true;
2784:             }
2785:             return false;
2786:         }
2787: 
2788:         public void clear() {
2789:             for (ConcurrentSkipListMap.Node<K,V> n = loNode();
2790:                  isBeforeEnd(n);
2791:                  n = n.next) {
2792:                 if (n.getValidValue() != null)
2793:                     m.remove(n.key);
2794:             }
2795:         }
2796: 
2797:         /* ----------------  ConcurrentMap API methods -------------- */
2798: 
2799:         public V putIfAbsent(K key, V value) {
2800:             checkKeyBounds(key);
2801:             return m.putIfAbsent(key, value);
2802:         }
2803: 
2804:         public boolean remove(Object key, Object value) {
2805:             K k = (K)key;
2806:             return inBounds(k) && m.remove(k, value);
2807:         }
2808: 
2809:         public boolean replace(K key, V oldValue, V newValue) {
2810:             checkKeyBounds(key);
2811:             return m.replace(key, oldValue, newValue);
2812:         }
2813: 
2814:         public V replace(K key, V value) {
2815:             checkKeyBounds(key);
2816:             return m.replace(key, value);
2817:         }
2818: 
2819:         /* ----------------  SortedMap API methods -------------- */
2820: 
2821:         public Comparator<? super K> comparator() {
2822:             Comparator<? super K> cmp = m.comparator();
2823:         if (isDescending)
2824:         return Collections.reverseOrder(cmp);
2825:         else
2826:         return cmp;
2827:         }
2828: 
2829:         /**
2830:          * Utility to create submaps, where given bounds override
2831:          * unbounded(null) ones and/or are checked against bounded ones.
2832:          */
2833:         private SubMap<K,V> newSubMap(K fromKey,
2834:                                       boolean fromInclusive,
2835:                                       K toKey,
2836:                                       boolean toInclusive) {
2837:             if (isDescending) { // flip senses
2838:                 K tk = fromKey;
2839:                 fromKey = toKey;
2840:                 toKey = tk;
2841:                 boolean ti = fromInclusive;
2842:                 fromInclusive = toInclusive;
2843:                 toInclusive = ti;
2844:             }
2845:             if (lo != null) {
2846:                 if (fromKey == null) {
2847:                     fromKey = lo;
2848:                     fromInclusive = loInclusive;
2849:                 }
2850:                 else {
2851:                     int c = m.compare(fromKey, lo);
2852:                     if (c < 0 || (c == 0 && !loInclusive && fromInclusive))
2853:                         throw new IllegalArgumentException("key out of range");
2854:                 }
2855:             }
2856:             if (hi != null) {
2857:                 if (toKey == null) {
2858:                     toKey = hi;
2859:                     toInclusive = hiInclusive;
2860:                 }
2861:                 else {
2862:                     int c = m.compare(toKey, hi);
2863:                     if (c > 0 || (c == 0 && !hiInclusive && toInclusive))
2864:                         throw new IllegalArgumentException("key out of range");
2865:                 }
2866:             }
2867:             return new SubMap<K,V>(m, fromKey, fromInclusive,
2868:                                    toKey, toInclusive, isDescending);
2869:         }
2870: 
2871:         public SubMap<K,V> subMap(K fromKey,
2872:                                   boolean fromInclusive,
2873:                                   K toKey,
2874:                                   boolean toInclusive) {
2875:             if (fromKey == null || toKey == null)
2876:                 throw new NullPointerException();
2877:             return newSubMap(fromKey, fromInclusive, toKey, toInclusive);
2878:         }
2879: 
2880:         public SubMap<K,V> headMap(K toKey,
2881:                                    boolean inclusive) {
2882:             if (toKey == null)
2883:                 throw new NullPointerException();
2884:             return newSubMap(null, false, toKey, inclusive);
2885:         }
2886: 
2887:         public SubMap<K,V> tailMap(K fromKey,
2888:                                    boolean inclusive) {
2889:             if (fromKey == null)
2890:                 throw new NullPointerException();
2891:             return newSubMap(fromKey, inclusive, null, false);
2892:         }
2893: 
2894:         public SubMap<K,V> subMap(K fromKey, K toKey) {
2895:             return subMap(fromKey, true, toKey, false);
2896:         }
2897: 
2898:         public SubMap<K,V> headMap(K toKey) {
2899:             return headMap(toKey, false);
2900:         }
2901: 
2902:         public SubMap<K,V> tailMap(K fromKey) {
2903:             return tailMap(fromKey, true);
2904:         }
2905: 
2906:         public SubMap<K,V> descendingMap() {
2907:             return new SubMap<K,V>(m, lo, loInclusive,
2908:                                    hi, hiInclusive, !isDescending);
2909:         }
2910: 
2911:         /* ----------------  Relational methods -------------- */
2912: 
2913:         public Map.Entry<K,V> ceilingEntry(K key) {
2914:             return getNearEntry(key, (m.GT|m.EQ));
2915:         }
2916: 
2917:         public K ceilingKey(K key) {
2918:             return getNearKey(key, (m.GT|m.EQ));
2919:         }
2920: 
2921:         public Map.Entry<K,V> lowerEntry(K key) {
2922:             return getNearEntry(key, (m.LT));
2923:         }
2924: 
2925:         public K lowerKey(K key) {
2926:             return getNearKey(key, (m.LT));
2927:         }
2928: 
2929:         public Map.Entry<K,V> floorEntry(K key) {
2930:             return getNearEntry(key, (m.LT|m.EQ));
2931:         }
2932: 
2933:         public K floorKey(K key) {
2934:             return getNearKey(key, (m.LT|m.EQ));
2935:         }
2936: 
2937:         public Map.Entry<K,V> higherEntry(K key) {
2938:             return getNearEntry(key, (m.GT));
2939:         }
2940: 
2941:         public K higherKey(K key) {
2942:             return getNearKey(key, (m.GT));
2943:         }
2944: 
2945:         public K firstKey() {
2946:             return isDescending? highestKey() : lowestKey();
2947:         }
2948: 
2949:         public K lastKey() {
2950:             return isDescending? lowestKey() : highestKey();
2951:         }
2952: 
2953:         public Map.Entry<K,V> firstEntry() {
2954:             return isDescending? highestEntry() : lowestEntry();
2955:         }
2956: 
2957:         public Map.Entry<K,V> lastEntry() {
2958:             return isDescending? lowestEntry() : highestEntry();
2959:         }
2960: 
2961:         public Map.Entry<K,V> pollFirstEntry() {
2962:             return isDescending? removeHighest() : removeLowest();
2963:         }
2964: 
2965:         public Map.Entry<K,V> pollLastEntry() {
2966:             return isDescending? removeLowest() : removeHighest();
2967:         }
2968: 
2969:         /* ---------------- Submap Views -------------- */
2970: 
2971:         public NavigableSet<K> keySet() {
2972:             KeySet<K> ks = keySetView;
2973:             return (ks != null) ? ks : (keySetView = new KeySet(this));
2974:         }
2975: 
2976:         public NavigableSet<K> navigableKeySet() {
2977:             KeySet<K> ks = keySetView;
2978:             return (ks != null) ? ks : (keySetView = new KeySet(this));
2979:         }
2980: 
2981:         public Collection<V> values() {
2982:             Collection<V> vs = valuesView;
2983:             return (vs != null) ? vs : (valuesView = new Values(this));
2984:         }
2985: 
2986:         public Set<Map.Entry<K,V>> entrySet() {
2987:             Set<Map.Entry<K,V>> es = entrySetView;
2988:             return (es != null) ? es : (entrySetView = new EntrySet(this));
2989:         }
2990: 
2991:         public NavigableSet<K> descendingKeySet() {
2992:             return descendingMap().navigableKeySet();
2993:         }
2994: 
2995:         Iterator<K> keyIterator() {
2996:             return new SubMapKeyIterator();
2997:         }
2998: 
2999:         Iterator<V> valueIterator() {
3000:             return new SubMapValueIterator();
3001:         }
3002: 
3003:         Iterator<Map.Entry<K,V>> entryIterator() {
3004:             return new SubMapEntryIterator();
3005:         }
3006: 
3007:         /**
3008:          * Variant of main Iter class to traverse through submaps.
3009:          */
3010:         abstract class SubMapIter<T> implements Iterator<T> {
3011:             /** the last node returned by next() */
3012:             Node<K,V> lastReturned;
3013:             /** the next node to return from next(); */
3014:             Node<K,V> next;
3015:             /** Cache of next value field to maintain weak consistency */
3016:             V nextValue;
3017: 
3018:             SubMapIter() {
3019:                 for (;;) {
3020:                     next = isDescending ? hiNode() : loNode();
3021:                     if (next == null)
3022:                         break;
3023:             Object x = next.value;
3024:                     if (x != null && x != next) {
3025:             if (! inBounds(next.key))
3026:                             next = null;
3027:             else
3028:                 nextValue = (V) x;
3029:                         break;
3030:                     }
3031:                 }
3032:             }
3033: 
3034:             public final boolean hasNext() {
3035:                 return next != null;
3036:             }
3037: 
3038:             final void advance() {
3039:                 if ((lastReturned = next) == null)
3040:                     throw new NoSuchElementException();
3041:                 if (isDescending)
3042:                     descend();
3043:                 else
3044:                     ascend();
3045:             }
3046: 
3047:             private void ascend() {
3048:                 for (;;) {
3049:                     next = next.next;
3050:                     if (next == null)
3051:                         break;
3052:             Object x = next.value;
3053:                     if (x != null && x != next) {
3054:                         if (tooHigh(next.key))
3055:                             next = null;
3056:                         else
3057:                 nextValue = (V) x;
3058:                         break;
3059:                     }
3060:                 }
3061:             }
3062: 
3063:             private void descend() {
3064:                 for (;;) {
3065:                     next = m.findNear(lastReturned.key, LT);
3066:                     if (next == null)
3067:                         break;
3068:             Object x = next.value;
3069:                     if (x != null && x != next) {
3070:                         if (tooLow(next.key))
3071:                             next = null;
3072:             else
3073:                             nextValue = (V) x;
3074:                         break;
3075:                     }
3076:                 }
3077:             }
3078: 
3079:             public void remove() {
3080:                 Node<K,V> l = lastReturned;
3081:                 if (l == null)
3082:                     throw new IllegalStateException();
3083:                 m.remove(l.key);
3084:         lastReturned = null;
3085:             }
3086: 
3087:         }
3088: 
3089:         final class SubMapValueIterator extends SubMapIter<V> {
3090:             public V next() {
3091:                 V v = nextValue;
3092:                 advance();
3093:                 return v;
3094:             }
3095:         }
3096: 
3097:         final class SubMapKeyIterator extends SubMapIter<K> {
3098:             public K next() {
3099:                 Node<K,V> n = next;
3100:                 advance();
3101:                 return n.key;
3102:             }
3103:         }
3104: 
3105:         final class SubMapEntryIterator extends SubMapIter<Map.Entry<K,V>> {
3106:             public Map.Entry<K,V> next() {
3107:                 Node<K,V> n = next;
3108:                 V v = nextValue;
3109:                 advance();
3110:                 return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
3111:             }
3112:         }
3113:     }
3114: }