/*
* Quasar: lightweight threads and actors for the JVM.
* Copyright (c) 2013-2014, Parallel Universe Software Co. All rights reserved.
*
* This program and the accompanying materials are dual-licensed under
* either the terms of the Eclipse Public License v1.0 as published by
* the Eclipse Foundation
*
* or (per the licensee's choosing)
*
* under the terms of the GNU Lesser General Public License version 3.0
* as published by the Free Software Foundation.
*/
/*
* Based on j.u.c.ConcurrentSkipListMap
*/
/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package co.paralleluniverse.concurrent.util;
import co.paralleluniverse.common.util.UtilUnsafe;
import java.util.*;
public class ConcurrentSkipListPriorityQueue<E> extends AbstractQueue<E>
implements Cloneable,
java.io.Serializable {
/*
* This class implements a tree-like two-dimensionally linked skip
* list in which the index levels are represented in separate
* nodes from the base nodes holding data. There are two reasons
* for taking this approach instead of the usual array-based
* structure: 1) Array based implementations seem to encounter
* more complexity and overhead 2) We can use cheaper algorithms
* for the heavily-traversed index lists than can be used for the
* base lists. Here's a picture of some of the basics for a
* possible list with 2 levels of index:
*
* Head nodes Index nodes
* +-+ right +-+ +-+
* |2|---------------->| |--------------------->| |->null
* +-+ +-+ +-+
* | down | |
* v v v
* +-+ +-+ +-+ +-+ +-+ +-+
* |1|----------->| |->| |------>| |----------->| |------>| |->null
* +-+ +-+ +-+ +-+ +-+ +-+
* v | | | | |
* Nodes next v v v v v
* +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
* | |->|A|->|B|->|C|->|D|->|E|->|F|->|G|->|H|->|I|->|J|->|K|->null
* +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
*
* The base lists use a variant of the HM linked ordered set
* algorithm. See Tim Harris, "A pragmatic implementation of
* non-blocking linked lists"
* http://www.cl.cam.ac.uk/~tlh20/publications.html and Maged
* Michael "High Performance Dynamic Lock-Free Hash Tables and
* List-Based Sets"
* http://www.research.ibm.com/people/m/michael/pubs.htm. The
* basic idea in these lists is to mark the "next" pointers of
* deleted nodes when deleting to avoid conflicts with concurrent
* insertions, and when traversing to keep track of triples
* (predecessor, node, successor) in order to detect when and how
* to unlink these deleted nodes.
*
* Rather than using mark-bits to mark list deletions (which can
* be slow and space-intensive using AtomicMarkedReference), nodes
* use direct CAS'able next pointers. On deletion, instead of
* marking a pointer, they splice in another node that can be
* thought of as standing for a marked pointer (indicating this by
* using otherwise impossible field values). Using plain nodes
* acts roughly like "boxed" implementations of marked pointers,
* but uses new nodes only when nodes are deleted, not for every
* link. This requires less space and supports faster
* traversal. Even if marked references were better supported by
* JVMs, traversal using this technique might still be faster
* because any search need only read ahead one more node than
* otherwise required (to check for trailing marker) rather than
* unmasking mark bits or whatever on each read.
*
* This approach maintains the essential property needed in the HM
* algorithm of changing the next-pointer of a deleted node so
* that any other CAS of it will fail, but implements the idea by
* changing the pointer to point to a different node, not by
* marking it. While it would be possible to further squeeze
* space by defining marker nodes not to have key/value fields, it
* isn't worth the extra type-testing overhead. The deletion
* markers are rarely encountered during traversal and are
* normally quickly garbage collected. (Note that this technique
* would not work well in systems without garbage collection.)
*
* In addition to using deletion markers, the lists also use
* nullness of value fields to indicate deletion, in a style
* similar to typical lazy-deletion schemes. If a node's value is
* null, then it is considered logically deleted and ignored even
* though it is still reachable. This maintains proper control of
* concurrent replace vs delete operations -- an attempted replace
* must fail if a delete beat it by nulling field, and a delete
* must return the last non-null value held in the field. (Note:
* Null, rather than some special marker, is used for value fields
* here because it just so happens to mesh with the Map API
* requirement that method get returns null if there is no
* mapping, which allows nodes to remain concurrently readable
* even when deleted. Using any other marker value here would be
* messy at best.)
*
* Here's the sequence of events for a deletion of node n with
* predecessor b and successor f, initially:
*
* +------+ +------+ +------+
* ... | b |------>| n |----->| f | ...
* +------+ +------+ +------+
*
* 1. CAS n's value field from non-null to null.
* From this point on, no public operations encountering
* the node consider this mapping to exist. However, other
* ongoing insertions and deletions might still modify
* n's next pointer.
*
* 2. CAS n's next pointer to point to a new marker node.
* From this point on, no other nodes can be appended to n.
* which avoids deletion errors in CAS-based linked lists.
*
* +------+ +------+ +------+ +------+
* ... | b |------>| n |----->|marker|------>| f | ...
* +------+ +------+ +------+ +------+
*
* 3. CAS b's next pointer over both n and its marker.
* From this point on, no new traversals will encounter n,
* and it can eventually be GCed.
* +------+ +------+
* ... | b |----------------------------------->| f | ...
* +------+ +------+
*
* A failure at step 1 leads to simple retry due to a lost race
* with another operation. Steps 2-3 can fail because some other
* thread noticed during a traversal a node with null value and
* helped out by marking and/or unlinking. This helping-out
* ensures that no thread can become stuck waiting for progress of
* the deleting thread. The use of marker nodes slightly
* complicates helping-out code because traversals must track
* consistent reads of up to four nodes (b, n, marker, f), not
* just (b, n, f), although the next field of a marker is
* immutable, and once a next field is CAS'ed to point to a
* marker, it never again changes, so this requires less care.
*
* Skip lists add indexing to this scheme, so that the base-level
* traversals start close to the locations being found, inserted
* or deleted -- usually base level traversals only traverse a few
* nodes. This doesn't change the basic algorithm except for the
* need to make sure base traversals start at predecessors (here,
* b) that are not (structurally) deleted, otherwise retrying
* after processing the deletion.
*
* Index levels are maintained as lists with volatile next fields,
* using CAS to link and unlink. Races are allowed in index-list
* operations that can (rarely) fail to link in a new index node
* or delete one. (We can't do this of course for data nodes.)
* However, even when this happens, the index lists remain sorted,
* so correctly serve as indices. This can impact performance,
* but since skip lists are probabilistic anyway, the net result
* is that under contention, the effective "p" value may be lower
* than its nominal value. And race windows are kept small enough
* that in practice these failures are rare, even under a lot of
* contention.
*
* The fact that retries (for both base and index lists) are
* relatively cheap due to indexing allows some minor
* simplifications of retry logic. Traversal restarts are
* performed after most "helping-out" CASes. This isn't always
* strictly necessary, but the implicit backoffs tend to help
* reduce other downstream failed CAS's enough to outweigh restart
* cost. This worsens the worst case, but seems to improve even
* highly contended cases.
*
* Unlike most skip-list implementations, index insertion and
* deletion here require a separate traversal pass occuring after
* the base-level action, to add or remove index nodes. This adds
* to single-threaded overhead, but improves contended
* multithreaded performance by narrowing interference windows,
* and allows deletion to ensure that all index nodes will be made
* unreachable upon return from a public remove operation, thus
* avoiding unwanted garbage retention. This is more important
* here than in some other data structures because we cannot null
* out node fields referencing user keys since they might still be
* read by other ongoing traversals.
*
* Indexing uses skip list parameters that maintain good search
* performance while using sparser-than-usual indices: The
* hardwired parameters k=1, p=0.5 (see method randomLevel) mean
* that about one-quarter of the nodes have indices. Of those that
* do, half have one level, a quarter have two, and so on (see
* Pugh's Skip List Cookbook, sec 3.4). The expected total space
* requirement for a map is slightly less than for the current
* implementation of java.util.TreeMap.
*
* Changing the level of the index (i.e, the height of the
* tree-like structure) also uses CAS. The head index has initial
* level/height of one. Creation of an index with height greater
* than the current level adds a level to the head index by
* CAS'ing on a new top-most head. To maintain good performance
* after a lot of removals, deletion methods heuristically try to
* reduce the height if the topmost levels appear to be empty.
* This may encounter races in which it possible (but rare) to
* reduce and "lose" a level just as it is about to contain an
* index (that will then never be encountered). This does no
* structural harm, and in practice appears to be a better option
* than allowing unrestrained growth of levels.
*
* The code for all this is more verbose than you'd like. Most
* operations entail locating an element (or position to insert an
* element). The code to do this can't be nicely factored out
* because subsequent uses require a snapshot of predecessor
* and/or successor and/or value fields which can't be returned
* all at once, at least not without creating yet another object
* to hold them -- creating such little objects is an especially
* bad idea for basic internal search operations because it adds
* to GC overhead. (This is one of the few times I've wished Java
* had macros.) Instead, some traversal code is interleaved within
* insertion and removal operations. The control logic to handle
* all the retry conditions is sometimes twisty. Most search is
* broken into 2 parts. findPredecessor() searches index nodes
* only, returning a base-level predecessor of the key. findNode()
* finishes out the base-level search. Even with this factoring,
* there is a fair amount of near-duplication of code to handle
* variants.
*
* For explanation of algorithms sharing at least a couple of
* features with this one, see Mikhail Fomitchev's thesis
* (http://www.cs.yorku.ca/~mikhail/), Keir Fraser's thesis
* (http://www.cl.cam.ac.uk/users/kaf24/), and Hakan Sundell's
* thesis (http://www.cs.chalmers.se/~phs/).
*
* Given the use of tree-like index nodes, you might wonder why
* this doesn't use some kind of search tree instead, which would
* support somewhat faster search operations. The reason is that
* there are no known efficient lock-free insertion and deletion
* algorithms for search trees. The immutability of the "down"
* links of index nodes (as opposed to mutable "left" fields in
* true trees) makes this tractable using only CAS operations.
*
* Notation guide for local variables
* Node: b, n, f for predecessor, node, successor
* Index: q, r, d for index node, right, down.
* t for another index node
* Head: h
* Levels: j
* Keys: k, key
* Values: v, value
* Comparisons: c
*/
private static final long serialVersionUID = -8627078645895051609L;
/**
* Generates the initial random seed for the cheaper per-instance
* random number generators used in randomLevel.
*/
private static final Random seedGenerator = new Random();
/**
* Special value used to identify base-level header
*/
private static final Object BASE_HEADER = new Object();
/**
* The topmost head index of the skiplist.
*/
private transient volatile HeadIndex<E> head;
/**
* The comparator used to maintain order in this map, or null
* if using natural ordering.
*
* @serial
*/
private final Comparator<? super E> comparator;
/**
* Seed for simple random number generator. Not volatile since it
* doesn't matter too much if different threads don't see updates.
*/
private transient int randomSeed;
/**
* Initializes or resets state. Needed by constructors, clone,
* clear, readObject. and ConcurrentSkipListSet.clone.
* (Note that comparator must be separately initialized.)
*/
final void initialize() {
randomSeed = seedGenerator.nextInt() | 0x0100; // ensure nonzero
head = new HeadIndex<E>(new Node<E>(null, BASE_HEADER, null),
null, null, 1);
}
/**
* compareAndSet head node
*/
private boolean casHead(HeadIndex<E> cmp, HeadIndex<E> val) {
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
}
/* ---------------- Nodes -------------- */
/**
* Nodes hold keys and values, and are singly linked in sorted
* order, possibly with some intervening marker nodes. The list is
* headed by a dummy node accessible as head.node. The value field
* is declared only as Object because it takes special non-V
* values for marker and header nodes.
*/
static final class Node<V> {
final V key;
volatile Object value;
volatile Node<V> next;
Node(V key, Node<V> next) {
this(key, key, next);
}
/**
* Creates a new regular node.
*/
Node(V key, Object value, Node<V> next) {
this.key = key;
this.value = value;
this.next = next;
}
/**
* Creates a new marker node. A marker is distinguished by
* having its value field point to itself. Marker nodes also
* have null keys, a fact that is exploited in a few places,
* but this doesn't distinguish markers from the base-level
* header node (head.node), which also has a null key.
*/
Node(Node<V> next) {
this.key = null;
this.value = this;
this.next = next;
}
/**
* compareAndSet value field
*/
boolean casValue(Object cmp, Object val) {
return UNSAFE.compareAndSwapObject(this, valueOffset, cmp, val);
}
/**
* compareAndSet next field
*/
boolean casNext(Node<V> cmp, Node<V> val) {
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
/**
* Returns true if this node is a marker. This method isn't
* actually called in any current code checking for markers
* because callers will have already read value field and need
* to use that read (not another done here) and so directly
* test if value points to node.
*
* @param n a possibly null reference to a node
* @return true if this node is a marker node
*/
boolean isMarker() {
return value == this;
}
/**
* Returns true if this node is the header of base-level list.
*
* @return true if this node is header node
*/
boolean isBaseHeader() {
return value == BASE_HEADER;
}
/**
* Tries to append a deletion marker to this node.
*
* @param f the assumed current successor of this node
* @return true if successful
*/
boolean appendMarker(Node<V> f) {
return casNext(f, new Node<V>(f));
}
/**
* Helps out a deletion by appending marker or unlinking from
* predecessor. This is called during traversals when value
* field seen to be null.
*
* @param b predecessor
* @param f successor
*/
void helpDelete(Node<V> b, Node<V> f) {
/*
* Rechecking links and then doing only one of the
* help-out stages per call tends to minimize CAS
* interference among helping threads.
*/
if (f == next && this == b.next) {
if (f == null || f.value != f) // not already marked
appendMarker(f);
else
b.casNext(this, f.next);
}
}
/**
* Returns value if this node contains a valid key-value pair,
* else null.
*
* @return this node's value if it isn't a marker or header or
* is deleted, else null.
*/
V getValidValue() {
Object v = value;
if (v == this || v == BASE_HEADER)
return null;
return (V) v;
}
// UNSAFE mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long valueOffset;
private static final long nextOffset;
static {
try {
UNSAFE = UtilUnsafe.getUnsafe();
Class k = Node.class;
valueOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("value"));
nextOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("next"));
} catch (Exception e) {
throw new Error(e);
}
}
}
/* ---------------- Indexing -------------- */
/**
* Index nodes represent the levels of the skip list. Note that
* even though both Nodes and Indexes have forward-pointing
* fields, they have different types and are handled in different
* ways, that can't nicely be captured by placing field in a
* shared abstract class.
*/
static class Index<V> {
final Node<V> node;
final Index<V> down;
volatile Index<V> right;
/**
* Creates index node with given values.
*/
Index(Node<V> node, Index<V> down, Index<V> right) {
this.node = node;
this.down = down;
this.right = right;
}
/**
* compareAndSet right field
*/
final boolean casRight(Index<V> cmp, Index<V> val) {
return UNSAFE.compareAndSwapObject(this, rightOffset, cmp, val);
}
/**
* Returns true if the node this indexes has been deleted.
*
* @return true if indexed node is known to be deleted
*/
final boolean indexesDeletedNode() {
return node.value == null;
}
/**
* Tries to CAS newSucc as successor. To minimize races with
* unlink that may lose this index node, if the node being
* indexed is known to be deleted, it doesn't try to link in.
*
* @param succ the expected current successor
* @param newSucc the new successor
* @return true if successful
*/
final boolean link(Index<V> succ, Index<V> newSucc) {
Node<V> n = node;
newSucc.right = succ;
return n.value != null && casRight(succ, newSucc);
}
/**
* Tries to CAS right field to skip over apparent successor
* succ. Fails (forcing a retraversal by caller) if this node
* is known to be deleted.
*
* @param succ the expected current successor
* @return true if successful
*/
final boolean unlink(Index<V> succ) {
return !indexesDeletedNode() && casRight(succ, succ.right);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long rightOffset;
static {
try {
UNSAFE = UtilUnsafe.getUnsafe();
Class k = Index.class;
rightOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("right"));
} catch (Exception e) {
throw new Error(e);
}
}
}
/* ---------------- Head nodes -------------- */
/**
* Nodes heading each level keep track of their level.
*/
static final class HeadIndex<V> extends Index<V> {
final int level;
HeadIndex(Node<V> node, Index<V> down, Index<V> right, int level) {
super(node, down, right);
this.level = level;
}
}
/* ---------------- Comparison utilities -------------- */
/**
* Represents a key with a comparator as a Comparable.
*
* Because most sorted collections seem to use natural ordering on
* Comparables (Strings, Integers, etc), most internal methods are
* geared to use them. This is generally faster than checking
* per-comparison whether to use comparator or comparable because
* it doesn't require a (Comparable) cast for each comparison.
* (Optimizers can only sometimes remove such redundant checks
* themselves.) When Comparators are used,
* ComparableUsingComparators are created so that they act in the
* same way as natural orderings. This penalizes use of
* Comparators vs Comparables, which seems like the right
* tradeoff.
*/
static final class ComparableUsingComparator<K> implements Comparable<K> {
final K actualKey;
final Comparator<? super K> cmp;
ComparableUsingComparator(K key, Comparator<? super K> cmp) {
this.actualKey = key;
this.cmp = cmp;
}
public int compareTo(K k2) {
return cmp.compare(actualKey, k2);
}
}
/**
* If using comparator, return a ComparableUsingComparator, else
* cast key as Comparable, which may cause ClassCastException,
* which is propagated back to caller.
*/
private Comparable<? super E> comparable(Object key)
throws ClassCastException {
if (key == null)
throw new NullPointerException();
if (comparator != null)
return new ComparableUsingComparator<E>((E) key, comparator);
else
return (Comparable<? super E>) key;
}
/**
* Compares using comparator or natural ordering. Used when the
* ComparableUsingComparator approach doesn't apply.
*/
int compare(E k1, E k2) throws ClassCastException {
Comparator<? super E> cmp = comparator;
if (cmp != null)
return cmp.compare(k1, k2);
else
return ((Comparable<? super E>) k1).compareTo(k2);
}
/* ---------------- Traversal -------------- */
/**
* Returns a base-level node with key strictly less than given key,
* or the base-level header if there is no such node. Also
* unlinks indexes to deleted nodes found along the way. Callers
* rely on this side-effect of clearing indices to deleted nodes.
*
* @param key the key
* @return a predecessor of key
*/
private Node<E> findPredecessor(Comparable<? super E> key) {
if (key == null)
throw new NullPointerException(); // don't postpone errors
for (;;) {
Index<E> q = head;
Index<E> r = q.right;
for (;;) {
if (r != null) {
Node<E> n = r.node;
E k = n.key;
if (n.value == null) {
if (!q.unlink(r))
break; // restart
r = q.right; // reread r
continue;
}
if (key.compareTo(k) >= 0) { // skip "equal" keys
q = r;
r = r.right;
continue;
}
}
Index<E> d = q.down;
if (d != null) {
q = d;
r = d.right;
} else
return q.node;
}
}
}
/**
* Returns node holding key or null if no such, clearing out any
* deleted nodes seen along the way. Repeatedly traverses at
* base-level looking for key starting at predecessor returned
* from findPredecessor, processing base-level deletions as
* encountered. Some callers rely on this side-effect of clearing
* deleted nodes.
*
* Restarts occur, at traversal step centered on node n, if:
*
* (1) After reading n's next field, n is no longer assumed
* predecessor b's current successor, which means that
* we don't have a consistent 3-node snapshot and so cannot
* unlink any subsequent deleted nodes encountered.
*
* (2) n's value field is null, indicating n is deleted, in
* which case we help out an ongoing structural deletion
* before retrying. Even though there are cases where such
* unlinking doesn't require restart, they aren't sorted out
* here because doing so would not usually outweigh cost of
* restarting.
*
* (3) n is a marker or n's predecessor's value field is null,
* indicating (among other possibilities) that
* findPredecessor returned a deleted node. We can't unlink
* the node because we don't know its predecessor, so rely
* on another call to findPredecessor to notice and return
* some earlier predecessor, which it will do. This check is
* only strictly needed at beginning of loop, (and the
* b.value check isn't strictly needed at all) but is done
* each iteration to help avoid contention with other
* threads by callers that will fail to be able to change
* links, and so will retry anyway.
*
* The traversal loops in doPut, doRemove, and findNear all
* include the same three kinds of checks. And specialized
* versions appear in findFirst, and findLast and their
* variants. They can't easily share code because each uses the
* reads of fields held in locals occurring in the orders they
* were performed.
*
* @param key the key
* @return node holding key, or null if no such
*/
Node<E> findNode(Comparable<? super E> key) {
for (;;) {
Node<E> b = findPredecessor(key);
Node<E> n = b.next;
for (;;) {
if (n == null)
return null;
Node<E> f = n.next;
if (n != b.next) // inconsistent read
break;
Object v = n.value;
if (v == null) { // n is deleted
n.helpDelete(b, f);
break;
}
if (v == n || b.value == null) // b is deleted
break;
int c = key.compareTo(n.key);
if (c == 0) {
if (key.equals(n.key)) // allow comapreTo equality
return n;
} else if (c < 0)
return null;
b = n;
n = f;
}
}
}
/**
* Gets value for key using findNode.
*
* @param okey the key
* @return the value, or null if absent
*/
private E doGet(Object okey) {
Comparable<? super E> key = comparable(okey);
/*
* Loop needed here and elsewhere in case value field goes
* null just as it is about to be returned, in which case we
* lost a race with a deletion, so must retry.
*/
for (;;) {
Node<E> n = findNode(key);
if (n == null)
return null;
Object v = n.value;
if (v != null)
return (E) v;
}
}
/* ---------------- Insertion -------------- */
/**
* Main insertion method. Adds element if not present, or
* replaces value if present and onlyIfAbsent is false.
*
* @param kkey the key
* @param value the value that must be associated with key
* @param onlyIfAbsent if should not insert if already present
* @return the old value, or null if newly inserted
*/
private void doPut(E kkey) {
Comparable<? super E> key = comparable(kkey);
for (;;) {
Node<E> b = findPredecessor(key);
Node<E> n = b.next;
for (;;) {
if (n != null) {
Node<E> f = n.next;
if (n != b.next) // inconsistent read
break;
Object v = n.value;
if (v == null) { // n is deleted
n.helpDelete(b, f);
break;
}
if (v == n || b.value == null) // b is deleted
break;
int c = key.compareTo(n.key);
if (c >= 0) {
b = n;
n = f;
continue;
}
// else c < 0; fall through
}
Node<E> z = new Node<E>(kkey, n);
if (!b.casNext(n, z))
break; // restart if lost race to append to b
int level = randomLevel();
if (level > 0)
insertIndex(z, level);
return;
}
}
}
/**
* Returns a random level for inserting a new node.
* Hardwired to k=1, p=0.5, max 31 (see above and
* Pugh's "Skip List Cookbook", sec 3.4).
*
* This uses the simplest of the generators described in George
* Marsaglia's "Xorshift RNGs" paper. This is not a high-quality
* generator but is acceptable here.
*/
private int randomLevel() {
int x = randomSeed;
x ^= x << 13;
x ^= x >>> 17;
randomSeed = x ^= x << 5;
if ((x & 0x80000001) != 0) // test highest and lowest bits
return 0;
int level = 1;
while (((x >>>= 1) & 1) != 0)
++level;
return level;
}
/**
* Creates and adds index nodes for the given node.
*
* @param z the node
* @param level the level of the index
*/
private void insertIndex(Node<E> z, int level) {
HeadIndex<E> h = head;
int max = h.level;
if (level <= max) {
Index<E> idx = null;
for (int i = 1; i <= level; ++i)
idx = new Index<E>(z, idx, null);
addIndex(idx, h, level);
} else { // Add a new level
/*
* To reduce interference by other threads checking for
* empty levels in tryReduceLevel, new levels are added
* with initialized right pointers. Which in turn requires
* keeping levels in an array to access them while
* creating new head index nodes from the opposite
* direction.
*/
level = max + 1;
Index<E>[] idxs = (Index<E>[]) new Index[level + 1];
Index<E> idx = null;
for (int i = 1; i <= level; ++i)
idxs[i] = idx = new Index<E>(z, idx, null);
HeadIndex<E> oldh;
int k;
for (;;) {
oldh = head;
int oldLevel = oldh.level;
if (level <= oldLevel) { // lost race to add level
k = level;
break;
}
HeadIndex<E> newh = oldh;
Node<E> oldbase = oldh.node;
for (int j = oldLevel + 1; j <= level; ++j)
newh = new HeadIndex<E>(oldbase, newh, idxs[j], j);
if (casHead(oldh, newh)) {
k = oldLevel;
break;
}
}
addIndex(idxs[k], oldh, k);
}
}
/**
* Adds given index nodes from given level down to 1.
*
* @param idx the topmost index node being inserted
* @param h the value of head to use to insert. This must be
* snapshotted by callers to provide correct insertion level
* @param indexLevel the level of the index
*/
private void addIndex(Index<E> idx, HeadIndex<E> h, int indexLevel) {
// Track next level to insert in case of retries
int insertionLevel = indexLevel;
Comparable<? super E> key = comparable(idx.node.key);
if (key == null)
throw new NullPointerException();
// Similar to findPredecessor, but adding index nodes along
// path to key.
for (;;) {
int j = h.level;
Index<E> q = h;
Index<E> r = q.right;
Index<E> t = idx;
for (;;) {
if (r != null) {
Node<E> n = r.node;
// compare before deletion check avoids needing recheck
int c = key.compareTo(n.key);
if (n.value == null) {
if (!q.unlink(r))
break;
r = q.right;
continue;
}
if (c > 0) {
q = r;
r = r.right;
continue;
}
}
if (j == insertionLevel) {
// Don't insert index if node already deleted
if (t.indexesDeletedNode()) {
findNode(key); // cleans up
return;
}
if (!q.link(r, t))
break; // restart
if (--insertionLevel == 0) {
// need final deletion check before return
if (t.indexesDeletedNode())
findNode(key);
return;
}
}
if (--j >= insertionLevel && j < indexLevel)
t = t.down;
q = q.down;
r = q.right;
}
}
}
/* ---------------- Deletion -------------- */
/**
* Main deletion method. Locates node, nulls value, appends a
* deletion marker, unlinks predecessor, removes associated index
* nodes, and possibly reduces head index level.
*
* Index nodes are cleared out simply by calling findPredecessor.
* which unlinks indexes to deleted nodes found along path to key,
* which will include the indexes to this node. This is done
* unconditionally. We can't check beforehand whether there are
* index nodes because it might be the case that some or all
* indexes hadn't been inserted yet for this node during initial
* search for it, and we'd like to ensure lack of garbage
* retention, so must call to be sure.
*
* @param okey the key
* @param value if non-null, the value that must be
* associated with key
* @return the node, or null if not found
*/
private E doRemove(Object okey, Object value) {
Comparable<? super E> key = comparable(okey);
for (;;) {
Node<E> b = findPredecessor(key);
Node<E> n = b.next;
for (;;) {
if (n == null)
return null;
Node<E> f = n.next;
if (n != b.next) // inconsistent read
break;
Object v = n.value;
if (v == null) { // n is deleted
n.helpDelete(b, f);
break;
}
if (v == n || b.value == null) // b is deleted
break;
int c = key.compareTo(n.key);
if (c < 0)
return null;
if (c > 0) {
b = n;
n = f;
continue;
}
if (value != null && !value.equals(v))
return null;
if (!n.casValue(v, null))
break;
if (!n.appendMarker(f) || !b.casNext(n, f))
findNode(key); // Retry via findNode
else {
findPredecessor(key); // Clean index
if (head.right == null)
tryReduceLevel();
}
return (E) v;
}
}
}
/**
* Possibly reduce head level if it has no nodes. This method can
* (rarely) make mistakes, in which case levels can disappear even
* though they are about to contain index nodes. This impacts
* performance, not correctness. To minimize mistakes as well as
* to reduce hysteresis, the level is reduced by one only if the
* topmost three levels look empty. Also, if the removed level
* looks non-empty after CAS, we try to change it back quick
* before anyone notices our mistake! (This trick works pretty
* well because this method will practically never make mistakes
* unless current thread stalls immediately before first CAS, in
* which case it is very unlikely to stall again immediately
* afterwards, so will recover.)
*
* We put up with all this rather than just let levels grow
* because otherwise, even a small map that has undergone a large
* number of insertions and removals will have a lot of levels,
* slowing down access more than would an occasional unwanted
* reduction.
*/
private void tryReduceLevel() {
HeadIndex<E> h = head;
HeadIndex<E> d;
HeadIndex<E> e;
if (h.level > 3
&& (d = (HeadIndex<E>) h.down) != null
&& (e = (HeadIndex<E>) d.down) != null
&& e.right == null
&& d.right == null
&& h.right == null
&& casHead(h, d) && // try to set
h.right != null) // recheck
casHead(d, h); // try to backout
}
/* ---------------- Finding and removing first element -------------- */
/**
* Specialized variant of findNode to get first valid node.
*
* @return first node or null if empty
*/
Node<E> findFirst() {
for (;;) {
Node<E> b = head.node;
Node<E> n = b.next;
if (n == null)
return null;
if (n.value != null)
return n;
n.helpDelete(b, n.next);
}
}
/**
* Removes first entry; returns its snapshot.
*
* @return null if empty, else snapshot of first entry
*/
E doRemoveFirst() {
for (;;) {
Node<E> b = head.node;
Node<E> n = b.next;
if (n == null)
return null;
Node<E> f = n.next;
if (n != b.next)
continue;
Object v = n.value;
if (v == null) {
n.helpDelete(b, f);
continue;
}
if (!n.casValue(v, null))
continue;
if (!n.appendMarker(f) || !b.casNext(n, f))
findFirst(); // retry
clearIndexToFirst();
return n.key;
}
}
/**
* Clears out index nodes associated with deleted first entry.
*/
private void clearIndexToFirst() {
for (;;) {
Index<E> q = head;
for (;;) {
Index<E> r = q.right;
if (r != null && r.indexesDeletedNode() && !q.unlink(r))
break;
if ((q = q.down) == null) {
if (head.right == null)
tryReduceLevel();
return;
}
}
}
}
/* ---------------- Finding and removing last element -------------- */
/**
* Specialized version of find to get last valid node.
*
* @return last node or null if empty
*/
Node<E> findLast() {
/*
* findPredecessor can't be used to traverse index level
* because this doesn't use comparisons. So traversals of
* both levels are folded together.
*/
Index<E> q = head;
for (;;) {
Index<E> d, r;
if ((r = q.right) != null) {
if (r.indexesDeletedNode()) {
q.unlink(r);
q = head; // restart
} else
q = r;
} else if ((d = q.down) != null) {
q = d;
} else {
Node<E> b = q.node;
Node<E> n = b.next;
for (;;) {
if (n == null)
return b.isBaseHeader() ? null : b;
Node<E> f = n.next; // inconsistent read
if (n != b.next)
break;
Object v = n.value;
if (v == null) { // n is deleted
n.helpDelete(b, f);
break;
}
if (v == n || b.value == null) // b is deleted
break;
b = n;
n = f;
}
q = head; // restart
}
}
}
/**
* Specialized variant of findPredecessor to get predecessor of last
* valid node. Needed when removing the last entry. It is possible
* that all successors of returned node will have been deleted upon
* return, in which case this method can be retried.
*
* @return likely predecessor of last node
*/
private Node<E> findPredecessorOfLast() {
for (;;) {
Index<E> q = head;
for (;;) {
Index<E> d, r;
if ((r = q.right) != null) {
if (r.indexesDeletedNode()) {
q.unlink(r);
break; // must restart
}
// proceed as far across as possible without overshooting
if (r.node.next != null) {
q = r;
continue;
}
}
if ((d = q.down) != null)
q = d;
else
return q.node;
}
}
}
/**
* Removes last entry; returns its snapshot.
* Specialized variant of doRemove.
*
* @return null if empty, else snapshot of last entry
*/
E doRemoveLastEntry() {
for (;;) {
Node<E> b = findPredecessorOfLast();
Node<E> n = b.next;
if (n == null) {
if (b.isBaseHeader()) // empty
return null;
else
continue; // all b's successors are deleted; retry
}
for (;;) {
Node<E> f = n.next;
if (n != b.next) // inconsistent read
break;
Object v = n.value;
if (v == null) { // n is deleted
n.helpDelete(b, f);
break;
}
if (v == n || b.value == null) // b is deleted
break;
if (f != null) {
b = n;
n = f;
continue;
}
if (!n.casValue(v, null))
break;
E key = n.key;
Comparable<? super E> ck = comparable(key);
if (!n.appendMarker(f) || !b.casNext(n, f))
findNode(ck); // Retry via findNode
else {
findPredecessor(ck); // Clean index
if (head.right == null)
tryReduceLevel();
}
return key;
}
}
}
/* ---------------- Constructors -------------- */
/**
* Constructs a new, empty map, sorted according to the
* {@linkplain Comparable natural ordering} of the keys.
*/
public ConcurrentSkipListPriorityQueue() {
this.comparator = null;
initialize();
}
/**
* Constructs a new, empty map, sorted according to the specified
* comparator.
*
* @param comparator the comparator that will be used to order this map.
* If <tt>null</tt>, the {@linkplain Comparable natural
* ordering} of the keys will be used.
*/
public ConcurrentSkipListPriorityQueue(Comparator<? super E> comparator) {
this.comparator = comparator;
initialize();
}
public ConcurrentSkipListPriorityQueue(Collection<? extends E> m) {
this.comparator = null;
initialize();
addAll(m);
}
/**
* Returns a shallow copy of this <tt>ConcurrentSkipListMap</tt>
* instance. (The keys and values themselves are not cloned.)
*
* @return a shallow copy of this map
*/
@Override
public ConcurrentSkipListPriorityQueue<E> clone() {
ConcurrentSkipListPriorityQueue<E> clone = null;
try {
clone = (ConcurrentSkipListPriorityQueue<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
clone.initialize();
clone.buildFromSorted(this);
return clone;
}
/**
* Streamlined bulk insertion to initialize from elements of
* given sorted map. Call only from constructor or clone
* method.
*/
private void buildFromSorted(ConcurrentSkipListPriorityQueue<E> pq) {
if (pq == null)
throw new NullPointerException();
HeadIndex<E> h = head;
Node<E> basepred = h.node;
// Track the current rightmost node at each level. Uses an
// ArrayList to avoid committing to initial or maximum level.
ArrayList<Index<E>> preds = new ArrayList<Index<E>>();
// initialize
for (int i = 0; i <= h.level; ++i)
preds.add(null);
Index<E> q = h;
for (int i = h.level; i > 0; --i) {
preds.set(i, q);
q = q.down;
}
Iterator<E> it = pq.iterator();
while (it.hasNext()) {
E k = it.next();
int j = randomLevel();
if (j > h.level)
j = h.level + 1;
if (k == null)
throw new NullPointerException();
Node<E> z = new Node<E>(k, null);
basepred.next = z;
basepred = z;
if (j > 0) {
Index<E> idx = null;
for (int i = 1; i <= j; ++i) {
idx = new Index<E>(z, idx, null);
if (i > h.level)
h = new HeadIndex<E>(h.node, h, idx, i);
if (i < preds.size()) {
preds.get(i).right = idx;
preds.set(i, idx);
} else
preds.add(idx);
}
}
}
head = h;
}
/* ---------------- Serialization -------------- */
/**
* Save the state of this map to a stream.
*
* @serialData The key (Object) and value (Object) for each
* key-value mapping represented by the map, followed by
* <tt>null</tt>. The key-value mappings are emitted in key-order
* (as determined by the Comparator, or by the keys' natural
* ordering if no Comparator).
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out the Comparator and any hidden stuff
s.defaultWriteObject();
// Write out keys and values (alternating)
for (Node<E> n = findFirst(); n != null; n = n.next) {
E v = n.getValidValue();
if (v != null)
s.writeObject(n.key);
}
s.writeObject(null);
}
/**
* Reconstitute the map from a stream.
*/
private void readObject(final java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in the Comparator and any hidden stuff
s.defaultReadObject();
// Reset transients
initialize();
/*
* This is nearly identical to buildFromSorted, but is
* distinct because readObject calls can't be nicely adapted
* as the kind of iterator needed by buildFromSorted. (They
* can be, but doing so requires type cheats and/or creation
* of adaptor classes.) It is simpler to just adapt the code.
*/
HeadIndex<E> h = head;
Node<E> basepred = h.node;
ArrayList<Index<E>> preds = new ArrayList<Index<E>>();
for (int i = 0; i <= h.level; ++i)
preds.add(null);
Index<E> q = h;
for (int i = h.level; i > 0; --i) {
preds.set(i, q);
q = q.down;
}
for (;;) {
Object k = s.readObject();
if (k == null)
break;
E key = (E) k;
int j = randomLevel();
if (j > h.level)
j = h.level + 1;
Node<E> z = new Node<E>(key, null);
basepred.next = z;
basepred = z;
if (j > 0) {
Index<E> idx = null;
for (int i = 1; i <= j; ++i) {
idx = new Index<E>(z, idx, null);
if (i > h.level)
h = new HeadIndex<E>(h.node, h, idx, i);
if (i < preds.size()) {
preds.get(i).right = idx;
preds.set(i, idx);
} else
preds.add(idx);
}
}
}
head = h;
}
/* ------ Map API methods ------ */
@Override
public boolean contains(Object key) {
return doGet(key) != null;
}
@Override
public boolean offer(E key) {
if (key == null)
throw new NullPointerException();
doPut(key);
return true;
}
@Override
public E poll() {
return doRemoveFirst();
}
@Override
public E peek() {
Node<E> n = findFirst();
if (n == null)
return null;
return n.key;
}
public E peekLast() {
Node<E> n = findLast();
if (n == null)
return null;
return n.key;
}
@Override
public Iterator<E> iterator() {
return new Iter();
}
@Override
public boolean remove(Object key) {
return doRemove(key, null) != null;
}
/**
* Returns the number of key-value mappings in this map. If this map
* contains more than <tt>Integer.MAX_VALUE</tt> elements, it
* returns <tt>Integer.MAX_VALUE</tt>.
*
* <p>Beware that, unlike in most collections, this method is
* <em>NOT</em> a constant-time operation. Because of the
* asynchronous nature of these maps, determining the current
* number of elements requires traversing them all to count them.
* Additionally, it is possible for the size to change during
* execution of this method, in which case the returned result
* will be inaccurate. Thus, this method is typically not very
* useful in concurrent applications.
*
* @return the number of elements in this map
*/
@Override
public int size() {
long count = 0;
for (Node<E> n = findFirst(); n != null; n = n.next) {
if (n.getValidValue() != null)
++count;
}
return (count >= Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int) count;
}
/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings
*/
@Override
public boolean isEmpty() {
return findFirst() == null;
}
/**
* Removes all of the mappings from this map.
*/
@Override
public void clear() {
initialize();
}
public Comparator<? super E> comparator() {
return comparator;
}
class Iter implements Iterator<E> {
/**
* the last node returned by next()
*/
Node<E> lastReturned;
/**
* the next node to return from next();
*/
Node<E> next;
/**
* Initializes ascending iterator for entire range.
*/
Iter() {
for (;;) {
next = findFirst();
if (next == null)
break;
Object x = next.value;
if (x != null && x != next)
break;
}
}
@Override
public final boolean hasNext() {
return next != null;
}
@Override
public E next() {
Node<E> n = next;
advance();
return n.key;
}
/**
* Advances next to higher entry.
*/
final void advance() {
if (next == null)
throw new NoSuchElementException();
lastReturned = next;
for (;;) {
next = next.next;
if (next == null)
break;
Object x = next.value;
if (x != null && x != next)
break;
}
}
@Override
public void remove() {
Node<E> l = lastReturned;
if (l == null)
throw new IllegalStateException();
// It would not be worth all of the overhead to directly
// unlink from here. Using remove is fast enough.
ConcurrentSkipListPriorityQueue.this.remove(l.key);
lastReturned = null;
}
}
/*
* View classes are static, delegating to a ConcurrentNavigableMap
* to allow use by SubMaps, which outweighs the ugliness of
* needing type-tests for Iterator methods.
*/
static <E> List<E> toList(Collection<E> c) {
// Using size() here would be a pessimization.
List<E> list = new ArrayList<E>(c.size());
for (E e : c)
list.add(e);
return list;
}
@Override
public Object[] toArray() {
return toList(this).toArray();
}
@Override
public <T> T[] toArray(T[] a) {
return toList(this).toArray(a);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long headOffset;
static {
try {
UNSAFE = UtilUnsafe.getUnsafe();
Class k = ConcurrentSkipListPriorityQueue.class;
headOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("head"));
} catch (Exception e) {
throw new Error(e);
}
}
}