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/* |
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* Written by Doug Lea and Martin Buchholz with assistance from members of |
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* JCP JSR-166 Expert Group and released to the public domain, as explained |
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* at http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.lang.invoke.MethodHandles; |
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import java.lang.invoke.VarHandle; |
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import java.util.AbstractCollection; |
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import java.util.Arrays; |
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import java.util.Collection; |
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import java.util.Deque; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Objects; |
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import java.util.Queue; |
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import java.util.Spliterator; |
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import java.util.Spliterators; |
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import java.util.function.Consumer; |
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import java.util.function.Predicate; |
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|
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/** |
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* An unbounded concurrent {@linkplain Deque deque} based on linked nodes. |
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* Concurrent insertion, removal, and access operations execute safely |
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* across multiple threads. |
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* A {@code ConcurrentLinkedDeque} is an appropriate choice when |
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* many threads will share access to a common collection. |
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* Like most other concurrent collection implementations, this class |
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* does not permit the use of {@code null} elements. |
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* |
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* <p>Iterators and spliterators are |
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* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
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* |
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* <p>Beware that, unlike in most collections, the {@code size} method |
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* is <em>NOT</em> a constant-time operation. Because of the |
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* asynchronous nature of these deques, determining the current number |
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* of elements requires a traversal of the elements, and so may report |
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* inaccurate results if this collection is modified during traversal. |
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* |
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* <p>Bulk operations that add, remove, or examine multiple elements, |
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* such as {@link #addAll}, {@link #removeIf} or {@link #forEach}, |
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* are <em>not</em> guaranteed to be performed atomically. |
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* For example, a {@code forEach} traversal concurrent with an {@code |
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* addAll} operation might observe only some of the added elements. |
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* |
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* <p>This class and its iterator implement all of the <em>optional</em> |
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* methods of the {@link Deque} and {@link Iterator} interfaces. |
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* |
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* <p>Memory consistency effects: As with other concurrent collections, |
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* actions in a thread prior to placing an object into a |
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* {@code ConcurrentLinkedDeque} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions subsequent to the access or removal of that element from |
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* the {@code ConcurrentLinkedDeque} in another thread. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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* @author Martin Buchholz |
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* @param <E> the type of elements held in this deque |
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*/ |
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public class ConcurrentLinkedDeque<E> |
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extends AbstractCollection<E> |
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implements Deque<E>, java.io.Serializable { |
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|
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/* |
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* This is an implementation of a concurrent lock-free deque |
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* supporting interior removes but not interior insertions, as |
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* required to support the entire Deque interface. |
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* |
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* We extend the techniques developed for ConcurrentLinkedQueue and |
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* LinkedTransferQueue (see the internal docs for those classes). |
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* Understanding the ConcurrentLinkedQueue implementation is a |
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* prerequisite for understanding the implementation of this class. |
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* |
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* The data structure is a symmetrical doubly-linked "GC-robust" |
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* linked list of nodes. We minimize the number of volatile writes |
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* using two techniques: advancing multiple hops with a single CAS |
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* and mixing volatile and non-volatile writes of the same memory |
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* locations. |
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* |
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* A node contains the expected E ("item") and links to predecessor |
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* ("prev") and successor ("next") nodes: |
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* |
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* class Node<E> { volatile Node<E> prev, next; volatile E item; } |
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* |
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* A node p is considered "live" if it contains a non-null item |
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* (p.item != null). When an item is CASed to null, the item is |
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* atomically logically deleted from the collection. |
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* |
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* At any time, there is precisely one "first" node with a null |
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* prev reference that terminates any chain of prev references |
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* starting at a live node. Similarly there is precisely one |
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* "last" node terminating any chain of next references starting at |
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* a live node. The "first" and "last" nodes may or may not be live. |
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* The "first" and "last" nodes are always mutually reachable. |
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* |
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* A new element is added atomically by CASing the null prev or |
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* next reference in the first or last node to a fresh node |
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* containing the element. The element's node atomically becomes |
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* "live" at that point. |
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* |
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* A node is considered "active" if it is a live node, or the |
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* first or last node. Active nodes cannot be unlinked. |
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* |
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* A "self-link" is a next or prev reference that is the same node: |
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* p.prev == p or p.next == p |
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* Self-links are used in the node unlinking process. Active nodes |
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* never have self-links. |
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* |
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* A node p is active if and only if: |
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* |
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* p.item != null || |
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* (p.prev == null && p.next != p) || |
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* (p.next == null && p.prev != p) |
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* |
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* The deque object has two node references, "head" and "tail". |
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* The head and tail are only approximations to the first and last |
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* nodes of the deque. The first node can always be found by |
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* following prev pointers from head; likewise for tail. However, |
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* it is permissible for head and tail to be referring to deleted |
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* nodes that have been unlinked and so may not be reachable from |
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* any live node. |
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* |
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* There are 3 stages of node deletion; |
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* "logical deletion", "unlinking", and "gc-unlinking". |
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* |
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* 1. "logical deletion" by CASing item to null atomically removes |
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* the element from the collection, and makes the containing node |
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* eligible for unlinking. |
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* |
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* 2. "unlinking" makes a deleted node unreachable from active |
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* nodes, and thus eventually reclaimable by GC. Unlinked nodes |
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* may remain reachable indefinitely from an iterator. |
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* |
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* Physical node unlinking is merely an optimization (albeit a |
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* critical one), and so can be performed at our convenience. At |
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* any time, the set of live nodes maintained by prev and next |
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* links are identical, that is, the live nodes found via next |
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* links from the first node is equal to the elements found via |
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* prev links from the last node. However, this is not true for |
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* nodes that have already been logically deleted - such nodes may |
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* be reachable in one direction only. |
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* |
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* 3. "gc-unlinking" takes unlinking further by making active |
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* nodes unreachable from deleted nodes, making it easier for the |
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* GC to reclaim future deleted nodes. This step makes the data |
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* structure "gc-robust", as first described in detail by Boehm |
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* (http://portal.acm.org/citation.cfm?doid=503272.503282). |
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* |
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* GC-unlinked nodes may remain reachable indefinitely from an |
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* iterator, but unlike unlinked nodes, are never reachable from |
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* head or tail. |
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* |
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* Making the data structure GC-robust will eliminate the risk of |
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* unbounded memory retention with conservative GCs and is likely |
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* to improve performance with generational GCs. |
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* |
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* When a node is dequeued at either end, e.g. via poll(), we would |
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* like to break any references from the node to active nodes. We |
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* develop further the use of self-links that was very effective in |
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* other concurrent collection classes. The idea is to replace |
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* prev and next pointers with special values that are interpreted |
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* to mean off-the-list-at-one-end. These are approximations, but |
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* good enough to preserve the properties we want in our |
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* traversals, e.g. we guarantee that a traversal will never visit |
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* the same element twice, but we don't guarantee whether a |
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* traversal that runs out of elements will be able to see more |
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* elements later after enqueues at that end. Doing gc-unlinking |
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* safely is particularly tricky, since any node can be in use |
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* indefinitely (for example by an iterator). We must ensure that |
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* the nodes pointed at by head/tail never get gc-unlinked, since |
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* head/tail are needed to get "back on track" by other nodes that |
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* are gc-unlinked. gc-unlinking accounts for much of the |
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* implementation complexity. |
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* |
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* Since neither unlinking nor gc-unlinking are necessary for |
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* correctness, there are many implementation choices regarding |
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* frequency (eagerness) of these operations. Since volatile |
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* reads are likely to be much cheaper than CASes, saving CASes by |
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* unlinking multiple adjacent nodes at a time may be a win. |
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* gc-unlinking can be performed rarely and still be effective, |
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* since it is most important that long chains of deleted nodes |
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* are occasionally broken. |
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* |
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* The actual representation we use is that p.next == p means to |
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* goto the first node (which in turn is reached by following prev |
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* pointers from head), and p.next == null && p.prev == p means |
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* that the iteration is at an end and that p is a (static final) |
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* dummy node, NEXT_TERMINATOR, and not the last active node. |
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* Finishing the iteration when encountering such a TERMINATOR is |
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* good enough for read-only traversals, so such traversals can use |
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* p.next == null as the termination condition. When we need to |
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* find the last (active) node, for enqueueing a new node, we need |
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* to check whether we have reached a TERMINATOR node; if so, |
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* restart traversal from tail. |
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* |
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* The implementation is completely directionally symmetrical, |
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* except that most public methods that iterate through the list |
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* follow next pointers, in the "forward" direction. |
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* |
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* We believe (without full proof) that all single-element Deque |
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* operations that operate directly at the two ends of the Deque |
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* (e.g., addFirst, peekLast, pollLast) are linearizable (see |
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* Herlihy and Shavit's book). However, some combinations of |
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* operations are known not to be linearizable. In particular, |
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* when an addFirst(A) is racing with pollFirst() removing B, it |
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* is possible for an observer iterating over the elements to |
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* observe first [A B C] and then [A C], even though no interior |
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* removes are ever performed. Nevertheless, iterators behave |
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* reasonably, providing the "weakly consistent" guarantees. |
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* |
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* Empirically, microbenchmarks suggest that this class adds about |
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* 40% overhead relative to ConcurrentLinkedQueue, which feels as |
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* good as we can hope for. |
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*/ |
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|
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private static final long serialVersionUID = 876323262645176354L; |
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|
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/** |
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* A node from which the first node on list (that is, the unique node p |
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* with p.prev == null && p.next != p) can be reached in O(1) time. |
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* Invariants: |
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* - the first node is always O(1) reachable from head via prev links |
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* - all live nodes are reachable from the first node via succ() |
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* - head != null |
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* - (tmp = head).next != tmp || tmp != head |
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* - head is never gc-unlinked (but may be unlinked) |
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* Non-invariants: |
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* - head.item may or may not be null |
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* - head may not be reachable from the first or last node, or from tail |
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*/ |
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private transient volatile Node<E> head; |
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|
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/** |
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* A node from which the last node on list (that is, the unique node p |
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* with p.next == null && p.prev != p) can be reached in O(1) time. |
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* Invariants: |
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* - the last node is always O(1) reachable from tail via next links |
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* - all live nodes are reachable from the last node via pred() |
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* - tail != null |
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* - tail is never gc-unlinked (but may be unlinked) |
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* Non-invariants: |
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* - tail.item may or may not be null |
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* - tail may not be reachable from the first or last node, or from head |
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*/ |
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private transient volatile Node<E> tail; |
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|
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private static final Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR; |
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|
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@SuppressWarnings("unchecked") |
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Node<E> prevTerminator() { |
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return (Node<E>) PREV_TERMINATOR; |
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} |
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|
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@SuppressWarnings("unchecked") |
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Node<E> nextTerminator() { |
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return (Node<E>) NEXT_TERMINATOR; |
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} |
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|
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static final class Node<E> { |
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volatile Node<E> prev; |
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volatile E item; |
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volatile Node<E> next; |
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} |
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|
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/** |
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* Returns a new node holding item. Uses relaxed write because item |
274 |
* can only be seen after piggy-backing publication via CAS. |
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*/ |
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static <E> Node<E> newNode(E item) { |
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Node<E> node = new Node<E>(); |
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ITEM.set(node, item); |
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return node; |
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} |
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|
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/** |
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* Links e as first element. |
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*/ |
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private void linkFirst(E e) { |
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final Node<E> newNode = newNode(Objects.requireNonNull(e)); |
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|
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restartFromHead: |
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for (;;) |
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for (Node<E> h = head, p = h, q;;) { |
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if ((q = p.prev) != null && |
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(q = (p = q).prev) != null) |
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// Check for head updates every other hop. |
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// If p == q, we are sure to follow head instead. |
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p = (h != (h = head)) ? h : q; |
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else if (p.next == p) // PREV_TERMINATOR |
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continue restartFromHead; |
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else { |
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// p is first node |
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NEXT.set(newNode, p); // CAS piggyback |
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if (PREV.compareAndSet(p, null, newNode)) { |
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// Successful CAS is the linearization point |
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// for e to become an element of this deque, |
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// and for newNode to become "live". |
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if (p != h) // hop two nodes at a time; failure is OK |
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HEAD.weakCompareAndSet(this, h, newNode); |
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return; |
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} |
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// Lost CAS race to another thread; re-read prev |
310 |
} |
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} |
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} |
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|
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/** |
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* Links e as last element. |
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*/ |
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private void linkLast(E e) { |
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final Node<E> newNode = newNode(Objects.requireNonNull(e)); |
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|
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restartFromTail: |
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for (;;) |
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for (Node<E> t = tail, p = t, q;;) { |
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if ((q = p.next) != null && |
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(q = (p = q).next) != null) |
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// Check for tail updates every other hop. |
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// If p == q, we are sure to follow tail instead. |
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p = (t != (t = tail)) ? t : q; |
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else if (p.prev == p) // NEXT_TERMINATOR |
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continue restartFromTail; |
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else { |
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// p is last node |
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PREV.set(newNode, p); // CAS piggyback |
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if (NEXT.compareAndSet(p, null, newNode)) { |
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// Successful CAS is the linearization point |
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// for e to become an element of this deque, |
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// and for newNode to become "live". |
337 |
if (p != t) // hop two nodes at a time; failure is OK |
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TAIL.weakCompareAndSet(this, t, newNode); |
339 |
return; |
340 |
} |
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// Lost CAS race to another thread; re-read next |
342 |
} |
343 |
} |
344 |
} |
345 |
|
346 |
private static final int HOPS = 2; |
347 |
|
348 |
/** |
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* Unlinks non-null node x. |
350 |
*/ |
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void unlink(Node<E> x) { |
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// assert x != null; |
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// assert x.item == null; |
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// assert x != PREV_TERMINATOR; |
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// assert x != NEXT_TERMINATOR; |
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|
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final Node<E> prev = x.prev; |
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final Node<E> next = x.next; |
359 |
if (prev == null) { |
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unlinkFirst(x, next); |
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} else if (next == null) { |
362 |
unlinkLast(x, prev); |
363 |
} else { |
364 |
// Unlink interior node. |
365 |
// |
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// This is the common case, since a series of polls at the |
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// same end will be "interior" removes, except perhaps for |
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// the first one, since end nodes cannot be unlinked. |
369 |
// |
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// At any time, all active nodes are mutually reachable by |
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// following a sequence of either next or prev pointers. |
372 |
// |
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// Our strategy is to find the unique active predecessor |
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// and successor of x. Try to fix up their links so that |
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// they point to each other, leaving x unreachable from |
376 |
// active nodes. If successful, and if x has no live |
377 |
// predecessor/successor, we additionally try to gc-unlink, |
378 |
// leaving active nodes unreachable from x, by rechecking |
379 |
// that the status of predecessor and successor are |
380 |
// unchanged and ensuring that x is not reachable from |
381 |
// tail/head, before setting x's prev/next links to their |
382 |
// logical approximate replacements, self/TERMINATOR. |
383 |
Node<E> activePred, activeSucc; |
384 |
boolean isFirst, isLast; |
385 |
int hops = 1; |
386 |
|
387 |
// Find active predecessor |
388 |
for (Node<E> p = prev; ; ++hops) { |
389 |
if (p.item != null) { |
390 |
activePred = p; |
391 |
isFirst = false; |
392 |
break; |
393 |
} |
394 |
Node<E> q = p.prev; |
395 |
if (q == null) { |
396 |
if (p.next == p) |
397 |
return; |
398 |
activePred = p; |
399 |
isFirst = true; |
400 |
break; |
401 |
} |
402 |
else if (p == q) |
403 |
return; |
404 |
else |
405 |
p = q; |
406 |
} |
407 |
|
408 |
// Find active successor |
409 |
for (Node<E> p = next; ; ++hops) { |
410 |
if (p.item != null) { |
411 |
activeSucc = p; |
412 |
isLast = false; |
413 |
break; |
414 |
} |
415 |
Node<E> q = p.next; |
416 |
if (q == null) { |
417 |
if (p.prev == p) |
418 |
return; |
419 |
activeSucc = p; |
420 |
isLast = true; |
421 |
break; |
422 |
} |
423 |
else if (p == q) |
424 |
return; |
425 |
else |
426 |
p = q; |
427 |
} |
428 |
|
429 |
// TODO: better HOP heuristics |
430 |
if (hops < HOPS |
431 |
// always squeeze out interior deleted nodes |
432 |
&& (isFirst | isLast)) |
433 |
return; |
434 |
|
435 |
// Squeeze out deleted nodes between activePred and |
436 |
// activeSucc, including x. |
437 |
skipDeletedSuccessors(activePred); |
438 |
skipDeletedPredecessors(activeSucc); |
439 |
|
440 |
// Try to gc-unlink, if possible |
441 |
if ((isFirst | isLast) && |
442 |
|
443 |
// Recheck expected state of predecessor and successor |
444 |
(activePred.next == activeSucc) && |
445 |
(activeSucc.prev == activePred) && |
446 |
(isFirst ? activePred.prev == null : activePred.item != null) && |
447 |
(isLast ? activeSucc.next == null : activeSucc.item != null)) { |
448 |
|
449 |
updateHead(); // Ensure x is not reachable from head |
450 |
updateTail(); // Ensure x is not reachable from tail |
451 |
|
452 |
// Finally, actually gc-unlink |
453 |
PREV.setRelease(x, isFirst ? prevTerminator() : x); |
454 |
NEXT.setRelease(x, isLast ? nextTerminator() : x); |
455 |
} |
456 |
} |
457 |
} |
458 |
|
459 |
/** |
460 |
* Unlinks non-null first node. |
461 |
*/ |
462 |
private void unlinkFirst(Node<E> first, Node<E> next) { |
463 |
// assert first != null; |
464 |
// assert next != null; |
465 |
// assert first.item == null; |
466 |
for (Node<E> o = null, p = next, q;;) { |
467 |
if (p.item != null || (q = p.next) == null) { |
468 |
if (o != null && p.prev != p && |
469 |
NEXT.compareAndSet(first, next, p)) { |
470 |
skipDeletedPredecessors(p); |
471 |
if (first.prev == null && |
472 |
(p.next == null || p.item != null) && |
473 |
p.prev == first) { |
474 |
|
475 |
updateHead(); // Ensure o is not reachable from head |
476 |
updateTail(); // Ensure o is not reachable from tail |
477 |
|
478 |
// Finally, actually gc-unlink |
479 |
NEXT.setRelease(o, o); |
480 |
PREV.setRelease(o, prevTerminator()); |
481 |
} |
482 |
} |
483 |
return; |
484 |
} |
485 |
else if (p == q) |
486 |
return; |
487 |
else { |
488 |
o = p; |
489 |
p = q; |
490 |
} |
491 |
} |
492 |
} |
493 |
|
494 |
/** |
495 |
* Unlinks non-null last node. |
496 |
*/ |
497 |
private void unlinkLast(Node<E> last, Node<E> prev) { |
498 |
// assert last != null; |
499 |
// assert prev != null; |
500 |
// assert last.item == null; |
501 |
for (Node<E> o = null, p = prev, q;;) { |
502 |
if (p.item != null || (q = p.prev) == null) { |
503 |
if (o != null && p.next != p && |
504 |
PREV.compareAndSet(last, prev, p)) { |
505 |
skipDeletedSuccessors(p); |
506 |
if (last.next == null && |
507 |
(p.prev == null || p.item != null) && |
508 |
p.next == last) { |
509 |
|
510 |
updateHead(); // Ensure o is not reachable from head |
511 |
updateTail(); // Ensure o is not reachable from tail |
512 |
|
513 |
// Finally, actually gc-unlink |
514 |
PREV.setRelease(o, o); |
515 |
NEXT.setRelease(o, nextTerminator()); |
516 |
} |
517 |
} |
518 |
return; |
519 |
} |
520 |
else if (p == q) |
521 |
return; |
522 |
else { |
523 |
o = p; |
524 |
p = q; |
525 |
} |
526 |
} |
527 |
} |
528 |
|
529 |
/** |
530 |
* Guarantees that any node which was unlinked before a call to |
531 |
* this method will be unreachable from head after it returns. |
532 |
* Does not guarantee to eliminate slack, only that head will |
533 |
* point to a node that was active while this method was running. |
534 |
*/ |
535 |
private final void updateHead() { |
536 |
// Either head already points to an active node, or we keep |
537 |
// trying to cas it to the first node until it does. |
538 |
Node<E> h, p, q; |
539 |
restartFromHead: |
540 |
while ((h = head).item == null && (p = h.prev) != null) { |
541 |
for (;;) { |
542 |
if ((q = p.prev) == null || |
543 |
(q = (p = q).prev) == null) { |
544 |
// It is possible that p is PREV_TERMINATOR, |
545 |
// but if so, the CAS is guaranteed to fail. |
546 |
if (HEAD.compareAndSet(this, h, p)) |
547 |
return; |
548 |
else |
549 |
continue restartFromHead; |
550 |
} |
551 |
else if (h != head) |
552 |
continue restartFromHead; |
553 |
else |
554 |
p = q; |
555 |
} |
556 |
} |
557 |
} |
558 |
|
559 |
/** |
560 |
* Guarantees that any node which was unlinked before a call to |
561 |
* this method will be unreachable from tail after it returns. |
562 |
* Does not guarantee to eliminate slack, only that tail will |
563 |
* point to a node that was active while this method was running. |
564 |
*/ |
565 |
private final void updateTail() { |
566 |
// Either tail already points to an active node, or we keep |
567 |
// trying to cas it to the last node until it does. |
568 |
Node<E> t, p, q; |
569 |
restartFromTail: |
570 |
while ((t = tail).item == null && (p = t.next) != null) { |
571 |
for (;;) { |
572 |
if ((q = p.next) == null || |
573 |
(q = (p = q).next) == null) { |
574 |
// It is possible that p is NEXT_TERMINATOR, |
575 |
// but if so, the CAS is guaranteed to fail. |
576 |
if (TAIL.compareAndSet(this, t, p)) |
577 |
return; |
578 |
else |
579 |
continue restartFromTail; |
580 |
} |
581 |
else if (t != tail) |
582 |
continue restartFromTail; |
583 |
else |
584 |
p = q; |
585 |
} |
586 |
} |
587 |
} |
588 |
|
589 |
private void skipDeletedPredecessors(Node<E> x) { |
590 |
whileActive: |
591 |
do { |
592 |
Node<E> prev = x.prev; |
593 |
// assert prev != null; |
594 |
// assert x != NEXT_TERMINATOR; |
595 |
// assert x != PREV_TERMINATOR; |
596 |
Node<E> p = prev; |
597 |
findActive: |
598 |
for (;;) { |
599 |
if (p.item != null) |
600 |
break findActive; |
601 |
Node<E> q = p.prev; |
602 |
if (q == null) { |
603 |
if (p.next == p) |
604 |
continue whileActive; |
605 |
break findActive; |
606 |
} |
607 |
else if (p == q) |
608 |
continue whileActive; |
609 |
else |
610 |
p = q; |
611 |
} |
612 |
|
613 |
// found active CAS target |
614 |
if (prev == p || PREV.compareAndSet(x, prev, p)) |
615 |
return; |
616 |
|
617 |
} while (x.item != null || x.next == null); |
618 |
} |
619 |
|
620 |
private void skipDeletedSuccessors(Node<E> x) { |
621 |
whileActive: |
622 |
do { |
623 |
Node<E> next = x.next; |
624 |
// assert next != null; |
625 |
// assert x != NEXT_TERMINATOR; |
626 |
// assert x != PREV_TERMINATOR; |
627 |
Node<E> p = next; |
628 |
findActive: |
629 |
for (;;) { |
630 |
if (p.item != null) |
631 |
break findActive; |
632 |
Node<E> q = p.next; |
633 |
if (q == null) { |
634 |
if (p.prev == p) |
635 |
continue whileActive; |
636 |
break findActive; |
637 |
} |
638 |
else if (p == q) |
639 |
continue whileActive; |
640 |
else |
641 |
p = q; |
642 |
} |
643 |
|
644 |
// found active CAS target |
645 |
if (next == p || NEXT.compareAndSet(x, next, p)) |
646 |
return; |
647 |
|
648 |
} while (x.item != null || x.prev == null); |
649 |
} |
650 |
|
651 |
/** |
652 |
* Returns the successor of p, or the first node if p.next has been |
653 |
* linked to self, which will only be true if traversing with a |
654 |
* stale pointer that is now off the list. |
655 |
*/ |
656 |
final Node<E> succ(Node<E> p) { |
657 |
// TODO: should we skip deleted nodes here? |
658 |
if (p == (p = p.next)) |
659 |
p = first(); |
660 |
return p; |
661 |
} |
662 |
|
663 |
/** |
664 |
* Returns the predecessor of p, or the last node if p.prev has been |
665 |
* linked to self, which will only be true if traversing with a |
666 |
* stale pointer that is now off the list. |
667 |
*/ |
668 |
final Node<E> pred(Node<E> p) { |
669 |
if (p == (p = p.prev)) |
670 |
p = last(); |
671 |
return p; |
672 |
} |
673 |
|
674 |
/** |
675 |
* Returns the first node, the unique node p for which: |
676 |
* p.prev == null && p.next != p |
677 |
* The returned node may or may not be logically deleted. |
678 |
* Guarantees that head is set to the returned node. |
679 |
*/ |
680 |
Node<E> first() { |
681 |
restartFromHead: |
682 |
for (;;) |
683 |
for (Node<E> h = head, p = h, q;;) { |
684 |
if ((q = p.prev) != null && |
685 |
(q = (p = q).prev) != null) |
686 |
// Check for head updates every other hop. |
687 |
// If p == q, we are sure to follow head instead. |
688 |
p = (h != (h = head)) ? h : q; |
689 |
else if (p == h |
690 |
// It is possible that p is PREV_TERMINATOR, |
691 |
// but if so, the CAS is guaranteed to fail. |
692 |
|| HEAD.compareAndSet(this, h, p)) |
693 |
return p; |
694 |
else |
695 |
continue restartFromHead; |
696 |
} |
697 |
} |
698 |
|
699 |
/** |
700 |
* Returns the last node, the unique node p for which: |
701 |
* p.next == null && p.prev != p |
702 |
* The returned node may or may not be logically deleted. |
703 |
* Guarantees that tail is set to the returned node. |
704 |
*/ |
705 |
Node<E> last() { |
706 |
restartFromTail: |
707 |
for (;;) |
708 |
for (Node<E> t = tail, p = t, q;;) { |
709 |
if ((q = p.next) != null && |
710 |
(q = (p = q).next) != null) |
711 |
// Check for tail updates every other hop. |
712 |
// If p == q, we are sure to follow tail instead. |
713 |
p = (t != (t = tail)) ? t : q; |
714 |
else if (p == t |
715 |
// It is possible that p is NEXT_TERMINATOR, |
716 |
// but if so, the CAS is guaranteed to fail. |
717 |
|| TAIL.compareAndSet(this, t, p)) |
718 |
return p; |
719 |
else |
720 |
continue restartFromTail; |
721 |
} |
722 |
} |
723 |
|
724 |
// Minor convenience utilities |
725 |
|
726 |
/** |
727 |
* Returns element unless it is null, in which case throws |
728 |
* NoSuchElementException. |
729 |
* |
730 |
* @param v the element |
731 |
* @return the element |
732 |
*/ |
733 |
private E screenNullResult(E v) { |
734 |
if (v == null) |
735 |
throw new NoSuchElementException(); |
736 |
return v; |
737 |
} |
738 |
|
739 |
/** |
740 |
* Constructs an empty deque. |
741 |
*/ |
742 |
public ConcurrentLinkedDeque() { |
743 |
head = tail = new Node<E>(); |
744 |
} |
745 |
|
746 |
/** |
747 |
* Constructs a deque initially containing the elements of |
748 |
* the given collection, added in traversal order of the |
749 |
* collection's iterator. |
750 |
* |
751 |
* @param c the collection of elements to initially contain |
752 |
* @throws NullPointerException if the specified collection or any |
753 |
* of its elements are null |
754 |
*/ |
755 |
public ConcurrentLinkedDeque(Collection<? extends E> c) { |
756 |
// Copy c into a private chain of Nodes |
757 |
Node<E> h = null, t = null; |
758 |
for (E e : c) { |
759 |
Node<E> newNode = newNode(Objects.requireNonNull(e)); |
760 |
if (h == null) |
761 |
h = t = newNode; |
762 |
else { |
763 |
NEXT.set(t, newNode); |
764 |
PREV.set(newNode, t); |
765 |
t = newNode; |
766 |
} |
767 |
} |
768 |
initHeadTail(h, t); |
769 |
} |
770 |
|
771 |
/** |
772 |
* Initializes head and tail, ensuring invariants hold. |
773 |
*/ |
774 |
private void initHeadTail(Node<E> h, Node<E> t) { |
775 |
if (h == t) { |
776 |
if (h == null) |
777 |
h = t = new Node<E>(); |
778 |
else { |
779 |
// Avoid edge case of a single Node with non-null item. |
780 |
Node<E> newNode = new Node<E>(); |
781 |
NEXT.set(t, newNode); |
782 |
PREV.set(newNode, t); |
783 |
t = newNode; |
784 |
} |
785 |
} |
786 |
head = h; |
787 |
tail = t; |
788 |
} |
789 |
|
790 |
/** |
791 |
* Inserts the specified element at the front of this deque. |
792 |
* As the deque is unbounded, this method will never throw |
793 |
* {@link IllegalStateException}. |
794 |
* |
795 |
* @throws NullPointerException if the specified element is null |
796 |
*/ |
797 |
public void addFirst(E e) { |
798 |
linkFirst(e); |
799 |
} |
800 |
|
801 |
/** |
802 |
* Inserts the specified element at the end of this deque. |
803 |
* As the deque is unbounded, this method will never throw |
804 |
* {@link IllegalStateException}. |
805 |
* |
806 |
* <p>This method is equivalent to {@link #add}. |
807 |
* |
808 |
* @throws NullPointerException if the specified element is null |
809 |
*/ |
810 |
public void addLast(E e) { |
811 |
linkLast(e); |
812 |
} |
813 |
|
814 |
/** |
815 |
* Inserts the specified element at the front of this deque. |
816 |
* As the deque is unbounded, this method will never return {@code false}. |
817 |
* |
818 |
* @return {@code true} (as specified by {@link Deque#offerFirst}) |
819 |
* @throws NullPointerException if the specified element is null |
820 |
*/ |
821 |
public boolean offerFirst(E e) { |
822 |
linkFirst(e); |
823 |
return true; |
824 |
} |
825 |
|
826 |
/** |
827 |
* Inserts the specified element at the end of this deque. |
828 |
* As the deque is unbounded, this method will never return {@code false}. |
829 |
* |
830 |
* <p>This method is equivalent to {@link #add}. |
831 |
* |
832 |
* @return {@code true} (as specified by {@link Deque#offerLast}) |
833 |
* @throws NullPointerException if the specified element is null |
834 |
*/ |
835 |
public boolean offerLast(E e) { |
836 |
linkLast(e); |
837 |
return true; |
838 |
} |
839 |
|
840 |
public E peekFirst() { |
841 |
restart: for (;;) { |
842 |
E item; |
843 |
Node<E> first = first(), p = first; |
844 |
while ((item = p.item) == null) { |
845 |
if (p == (p = p.next)) continue restart; |
846 |
if (p == null) |
847 |
break; |
848 |
} |
849 |
// recheck for linearizability |
850 |
if (first.prev != null) continue restart; |
851 |
return item; |
852 |
} |
853 |
} |
854 |
|
855 |
public E peekLast() { |
856 |
restart: for (;;) { |
857 |
E item; |
858 |
Node<E> last = last(), p = last; |
859 |
while ((item = p.item) == null) { |
860 |
if (p == (p = p.prev)) continue restart; |
861 |
if (p == null) |
862 |
break; |
863 |
} |
864 |
// recheck for linearizability |
865 |
if (last.next != null) continue restart; |
866 |
return item; |
867 |
} |
868 |
} |
869 |
|
870 |
/** |
871 |
* @throws NoSuchElementException {@inheritDoc} |
872 |
*/ |
873 |
public E getFirst() { |
874 |
return screenNullResult(peekFirst()); |
875 |
} |
876 |
|
877 |
/** |
878 |
* @throws NoSuchElementException {@inheritDoc} |
879 |
*/ |
880 |
public E getLast() { |
881 |
return screenNullResult(peekLast()); |
882 |
} |
883 |
|
884 |
public E pollFirst() { |
885 |
restart: for (;;) { |
886 |
for (Node<E> first = first(), p = first;;) { |
887 |
final E item; |
888 |
if ((item = p.item) != null) { |
889 |
// recheck for linearizability |
890 |
if (first.prev != null) continue restart; |
891 |
if (ITEM.compareAndSet(p, item, null)) { |
892 |
unlink(p); |
893 |
return item; |
894 |
} |
895 |
} |
896 |
if (p == (p = p.next)) continue restart; |
897 |
if (p == null) { |
898 |
if (first.prev != null) continue restart; |
899 |
return null; |
900 |
} |
901 |
} |
902 |
} |
903 |
} |
904 |
|
905 |
public E pollLast() { |
906 |
restart: for (;;) { |
907 |
for (Node<E> last = last(), p = last;;) { |
908 |
final E item; |
909 |
if ((item = p.item) != null) { |
910 |
// recheck for linearizability |
911 |
if (last.next != null) continue restart; |
912 |
if (ITEM.compareAndSet(p, item, null)) { |
913 |
unlink(p); |
914 |
return item; |
915 |
} |
916 |
} |
917 |
if (p == (p = p.prev)) continue restart; |
918 |
if (p == null) { |
919 |
if (last.next != null) continue restart; |
920 |
return null; |
921 |
} |
922 |
} |
923 |
} |
924 |
} |
925 |
|
926 |
/** |
927 |
* @throws NoSuchElementException {@inheritDoc} |
928 |
*/ |
929 |
public E removeFirst() { |
930 |
return screenNullResult(pollFirst()); |
931 |
} |
932 |
|
933 |
/** |
934 |
* @throws NoSuchElementException {@inheritDoc} |
935 |
*/ |
936 |
public E removeLast() { |
937 |
return screenNullResult(pollLast()); |
938 |
} |
939 |
|
940 |
// *** Queue and stack methods *** |
941 |
|
942 |
/** |
943 |
* Inserts the specified element at the tail of this deque. |
944 |
* As the deque is unbounded, this method will never return {@code false}. |
945 |
* |
946 |
* @return {@code true} (as specified by {@link Queue#offer}) |
947 |
* @throws NullPointerException if the specified element is null |
948 |
*/ |
949 |
public boolean offer(E e) { |
950 |
return offerLast(e); |
951 |
} |
952 |
|
953 |
/** |
954 |
* Inserts the specified element at the tail of this deque. |
955 |
* As the deque is unbounded, this method will never throw |
956 |
* {@link IllegalStateException} or return {@code false}. |
957 |
* |
958 |
* @return {@code true} (as specified by {@link Collection#add}) |
959 |
* @throws NullPointerException if the specified element is null |
960 |
*/ |
961 |
public boolean add(E e) { |
962 |
return offerLast(e); |
963 |
} |
964 |
|
965 |
public E poll() { return pollFirst(); } |
966 |
public E peek() { return peekFirst(); } |
967 |
|
968 |
/** |
969 |
* @throws NoSuchElementException {@inheritDoc} |
970 |
*/ |
971 |
public E remove() { return removeFirst(); } |
972 |
|
973 |
/** |
974 |
* @throws NoSuchElementException {@inheritDoc} |
975 |
*/ |
976 |
public E pop() { return removeFirst(); } |
977 |
|
978 |
/** |
979 |
* @throws NoSuchElementException {@inheritDoc} |
980 |
*/ |
981 |
public E element() { return getFirst(); } |
982 |
|
983 |
/** |
984 |
* @throws NullPointerException {@inheritDoc} |
985 |
*/ |
986 |
public void push(E e) { addFirst(e); } |
987 |
|
988 |
/** |
989 |
* Removes the first occurrence of the specified element from this deque. |
990 |
* If the deque does not contain the element, it is unchanged. |
991 |
* More formally, removes the first element {@code e} such that |
992 |
* {@code o.equals(e)} (if such an element exists). |
993 |
* Returns {@code true} if this deque contained the specified element |
994 |
* (or equivalently, if this deque changed as a result of the call). |
995 |
* |
996 |
* @param o element to be removed from this deque, if present |
997 |
* @return {@code true} if the deque contained the specified element |
998 |
* @throws NullPointerException if the specified element is null |
999 |
*/ |
1000 |
public boolean removeFirstOccurrence(Object o) { |
1001 |
Objects.requireNonNull(o); |
1002 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
1003 |
final E item; |
1004 |
if ((item = p.item) != null |
1005 |
&& o.equals(item) |
1006 |
&& ITEM.compareAndSet(p, item, null)) { |
1007 |
unlink(p); |
1008 |
return true; |
1009 |
} |
1010 |
} |
1011 |
return false; |
1012 |
} |
1013 |
|
1014 |
/** |
1015 |
* Removes the last occurrence of the specified element from this deque. |
1016 |
* If the deque does not contain the element, it is unchanged. |
1017 |
* More formally, removes the last element {@code e} such that |
1018 |
* {@code o.equals(e)} (if such an element exists). |
1019 |
* Returns {@code true} if this deque contained the specified element |
1020 |
* (or equivalently, if this deque changed as a result of the call). |
1021 |
* |
1022 |
* @param o element to be removed from this deque, if present |
1023 |
* @return {@code true} if the deque contained the specified element |
1024 |
* @throws NullPointerException if the specified element is null |
1025 |
*/ |
1026 |
public boolean removeLastOccurrence(Object o) { |
1027 |
Objects.requireNonNull(o); |
1028 |
for (Node<E> p = last(); p != null; p = pred(p)) { |
1029 |
final E item; |
1030 |
if ((item = p.item) != null |
1031 |
&& o.equals(item) |
1032 |
&& ITEM.compareAndSet(p, item, null)) { |
1033 |
unlink(p); |
1034 |
return true; |
1035 |
} |
1036 |
} |
1037 |
return false; |
1038 |
} |
1039 |
|
1040 |
/** |
1041 |
* Returns {@code true} if this deque contains the specified element. |
1042 |
* More formally, returns {@code true} if and only if this deque contains |
1043 |
* at least one element {@code e} such that {@code o.equals(e)}. |
1044 |
* |
1045 |
* @param o element whose presence in this deque is to be tested |
1046 |
* @return {@code true} if this deque contains the specified element |
1047 |
*/ |
1048 |
public boolean contains(Object o) { |
1049 |
if (o != null) { |
1050 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
1051 |
final E item; |
1052 |
if ((item = p.item) != null && o.equals(item)) |
1053 |
return true; |
1054 |
} |
1055 |
} |
1056 |
return false; |
1057 |
} |
1058 |
|
1059 |
/** |
1060 |
* Returns {@code true} if this collection contains no elements. |
1061 |
* |
1062 |
* @return {@code true} if this collection contains no elements |
1063 |
*/ |
1064 |
public boolean isEmpty() { |
1065 |
return peekFirst() == null; |
1066 |
} |
1067 |
|
1068 |
/** |
1069 |
* Returns the number of elements in this deque. If this deque |
1070 |
* contains more than {@code Integer.MAX_VALUE} elements, it |
1071 |
* returns {@code Integer.MAX_VALUE}. |
1072 |
* |
1073 |
* <p>Beware that, unlike in most collections, this method is |
1074 |
* <em>NOT</em> a constant-time operation. Because of the |
1075 |
* asynchronous nature of these deques, determining the current |
1076 |
* number of elements requires traversing them all to count them. |
1077 |
* Additionally, it is possible for the size to change during |
1078 |
* execution of this method, in which case the returned result |
1079 |
* will be inaccurate. Thus, this method is typically not very |
1080 |
* useful in concurrent applications. |
1081 |
* |
1082 |
* @return the number of elements in this deque |
1083 |
*/ |
1084 |
public int size() { |
1085 |
restart: for (;;) { |
1086 |
int count = 0; |
1087 |
for (Node<E> p = first(); p != null;) { |
1088 |
if (p.item != null) |
1089 |
if (++count == Integer.MAX_VALUE) |
1090 |
break; // @see Collection.size() |
1091 |
if (p == (p = p.next)) |
1092 |
continue restart; |
1093 |
} |
1094 |
return count; |
1095 |
} |
1096 |
} |
1097 |
|
1098 |
/** |
1099 |
* Removes the first occurrence of the specified element from this deque. |
1100 |
* If the deque does not contain the element, it is unchanged. |
1101 |
* More formally, removes the first element {@code e} such that |
1102 |
* {@code o.equals(e)} (if such an element exists). |
1103 |
* Returns {@code true} if this deque contained the specified element |
1104 |
* (or equivalently, if this deque changed as a result of the call). |
1105 |
* |
1106 |
* <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}. |
1107 |
* |
1108 |
* @param o element to be removed from this deque, if present |
1109 |
* @return {@code true} if the deque contained the specified element |
1110 |
* @throws NullPointerException if the specified element is null |
1111 |
*/ |
1112 |
public boolean remove(Object o) { |
1113 |
return removeFirstOccurrence(o); |
1114 |
} |
1115 |
|
1116 |
/** |
1117 |
* Appends all of the elements in the specified collection to the end of |
1118 |
* this deque, in the order that they are returned by the specified |
1119 |
* collection's iterator. Attempts to {@code addAll} of a deque to |
1120 |
* itself result in {@code IllegalArgumentException}. |
1121 |
* |
1122 |
* @param c the elements to be inserted into this deque |
1123 |
* @return {@code true} if this deque changed as a result of the call |
1124 |
* @throws NullPointerException if the specified collection or any |
1125 |
* of its elements are null |
1126 |
* @throws IllegalArgumentException if the collection is this deque |
1127 |
*/ |
1128 |
public boolean addAll(Collection<? extends E> c) { |
1129 |
if (c == this) |
1130 |
// As historically specified in AbstractQueue#addAll |
1131 |
throw new IllegalArgumentException(); |
1132 |
|
1133 |
// Copy c into a private chain of Nodes |
1134 |
Node<E> beginningOfTheEnd = null, last = null; |
1135 |
for (E e : c) { |
1136 |
Node<E> newNode = newNode(Objects.requireNonNull(e)); |
1137 |
if (beginningOfTheEnd == null) |
1138 |
beginningOfTheEnd = last = newNode; |
1139 |
else { |
1140 |
NEXT.set(last, newNode); |
1141 |
PREV.set(newNode, last); |
1142 |
last = newNode; |
1143 |
} |
1144 |
} |
1145 |
if (beginningOfTheEnd == null) |
1146 |
return false; |
1147 |
|
1148 |
// Atomically append the chain at the tail of this collection |
1149 |
restartFromTail: |
1150 |
for (;;) |
1151 |
for (Node<E> t = tail, p = t, q;;) { |
1152 |
if ((q = p.next) != null && |
1153 |
(q = (p = q).next) != null) |
1154 |
// Check for tail updates every other hop. |
1155 |
// If p == q, we are sure to follow tail instead. |
1156 |
p = (t != (t = tail)) ? t : q; |
1157 |
else if (p.prev == p) // NEXT_TERMINATOR |
1158 |
continue restartFromTail; |
1159 |
else { |
1160 |
// p is last node |
1161 |
PREV.set(beginningOfTheEnd, p); // CAS piggyback |
1162 |
if (NEXT.compareAndSet(p, null, beginningOfTheEnd)) { |
1163 |
// Successful CAS is the linearization point |
1164 |
// for all elements to be added to this deque. |
1165 |
if (!TAIL.weakCompareAndSet(this, t, last)) { |
1166 |
// Try a little harder to update tail, |
1167 |
// since we may be adding many elements. |
1168 |
t = tail; |
1169 |
if (last.next == null) |
1170 |
TAIL.weakCompareAndSet(this, t, last); |
1171 |
} |
1172 |
return true; |
1173 |
} |
1174 |
// Lost CAS race to another thread; re-read next |
1175 |
} |
1176 |
} |
1177 |
} |
1178 |
|
1179 |
/** |
1180 |
* Removes all of the elements from this deque. |
1181 |
*/ |
1182 |
public void clear() { |
1183 |
while (pollFirst() != null) |
1184 |
; |
1185 |
} |
1186 |
|
1187 |
public String toString() { |
1188 |
String[] a = null; |
1189 |
restart: for (;;) { |
1190 |
int charLength = 0; |
1191 |
int size = 0; |
1192 |
for (Node<E> p = first(); p != null;) { |
1193 |
final E item; |
1194 |
if ((item = p.item) != null) { |
1195 |
if (a == null) |
1196 |
a = new String[4]; |
1197 |
else if (size == a.length) |
1198 |
a = Arrays.copyOf(a, 2 * size); |
1199 |
String s = item.toString(); |
1200 |
a[size++] = s; |
1201 |
charLength += s.length(); |
1202 |
} |
1203 |
if (p == (p = p.next)) |
1204 |
continue restart; |
1205 |
} |
1206 |
|
1207 |
if (size == 0) |
1208 |
return "[]"; |
1209 |
|
1210 |
return Helpers.toString(a, size, charLength); |
1211 |
} |
1212 |
} |
1213 |
|
1214 |
private Object[] toArrayInternal(Object[] a) { |
1215 |
Object[] x = a; |
1216 |
restart: for (;;) { |
1217 |
int size = 0; |
1218 |
for (Node<E> p = first(); p != null;) { |
1219 |
final E item; |
1220 |
if ((item = p.item) != null) { |
1221 |
if (x == null) |
1222 |
x = new Object[4]; |
1223 |
else if (size == x.length) |
1224 |
x = Arrays.copyOf(x, 2 * (size + 4)); |
1225 |
x[size++] = item; |
1226 |
} |
1227 |
if (p == (p = p.next)) |
1228 |
continue restart; |
1229 |
} |
1230 |
if (x == null) |
1231 |
return new Object[0]; |
1232 |
else if (a != null && size <= a.length) { |
1233 |
if (a != x) |
1234 |
System.arraycopy(x, 0, a, 0, size); |
1235 |
if (size < a.length) |
1236 |
a[size] = null; |
1237 |
return a; |
1238 |
} |
1239 |
return (size == x.length) ? x : Arrays.copyOf(x, size); |
1240 |
} |
1241 |
} |
1242 |
|
1243 |
/** |
1244 |
* Returns an array containing all of the elements in this deque, in |
1245 |
* proper sequence (from first to last element). |
1246 |
* |
1247 |
* <p>The returned array will be "safe" in that no references to it are |
1248 |
* maintained by this deque. (In other words, this method must allocate |
1249 |
* a new array). The caller is thus free to modify the returned array. |
1250 |
* |
1251 |
* <p>This method acts as bridge between array-based and collection-based |
1252 |
* APIs. |
1253 |
* |
1254 |
* @return an array containing all of the elements in this deque |
1255 |
*/ |
1256 |
public Object[] toArray() { |
1257 |
return toArrayInternal(null); |
1258 |
} |
1259 |
|
1260 |
/** |
1261 |
* Returns an array containing all of the elements in this deque, |
1262 |
* in proper sequence (from first to last element); the runtime |
1263 |
* type of the returned array is that of the specified array. If |
1264 |
* the deque fits in the specified array, it is returned therein. |
1265 |
* Otherwise, a new array is allocated with the runtime type of |
1266 |
* the specified array and the size of this deque. |
1267 |
* |
1268 |
* <p>If this deque fits in the specified array with room to spare |
1269 |
* (i.e., the array has more elements than this deque), the element in |
1270 |
* the array immediately following the end of the deque is set to |
1271 |
* {@code null}. |
1272 |
* |
1273 |
* <p>Like the {@link #toArray()} method, this method acts as |
1274 |
* bridge between array-based and collection-based APIs. Further, |
1275 |
* this method allows precise control over the runtime type of the |
1276 |
* output array, and may, under certain circumstances, be used to |
1277 |
* save allocation costs. |
1278 |
* |
1279 |
* <p>Suppose {@code x} is a deque known to contain only strings. |
1280 |
* The following code can be used to dump the deque into a newly |
1281 |
* allocated array of {@code String}: |
1282 |
* |
1283 |
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre> |
1284 |
* |
1285 |
* Note that {@code toArray(new Object[0])} is identical in function to |
1286 |
* {@code toArray()}. |
1287 |
* |
1288 |
* @param a the array into which the elements of the deque are to |
1289 |
* be stored, if it is big enough; otherwise, a new array of the |
1290 |
* same runtime type is allocated for this purpose |
1291 |
* @return an array containing all of the elements in this deque |
1292 |
* @throws ArrayStoreException if the runtime type of the specified array |
1293 |
* is not a supertype of the runtime type of every element in |
1294 |
* this deque |
1295 |
* @throws NullPointerException if the specified array is null |
1296 |
*/ |
1297 |
@SuppressWarnings("unchecked") |
1298 |
public <T> T[] toArray(T[] a) { |
1299 |
if (a == null) throw new NullPointerException(); |
1300 |
return (T[]) toArrayInternal(a); |
1301 |
} |
1302 |
|
1303 |
/** |
1304 |
* Returns an iterator over the elements in this deque in proper sequence. |
1305 |
* The elements will be returned in order from first (head) to last (tail). |
1306 |
* |
1307 |
* <p>The returned iterator is |
1308 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
1309 |
* |
1310 |
* @return an iterator over the elements in this deque in proper sequence |
1311 |
*/ |
1312 |
public Iterator<E> iterator() { |
1313 |
return new Itr(); |
1314 |
} |
1315 |
|
1316 |
/** |
1317 |
* Returns an iterator over the elements in this deque in reverse |
1318 |
* sequential order. The elements will be returned in order from |
1319 |
* last (tail) to first (head). |
1320 |
* |
1321 |
* <p>The returned iterator is |
1322 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
1323 |
* |
1324 |
* @return an iterator over the elements in this deque in reverse order |
1325 |
*/ |
1326 |
public Iterator<E> descendingIterator() { |
1327 |
return new DescendingItr(); |
1328 |
} |
1329 |
|
1330 |
private abstract class AbstractItr implements Iterator<E> { |
1331 |
/** |
1332 |
* Next node to return item for. |
1333 |
*/ |
1334 |
private Node<E> nextNode; |
1335 |
|
1336 |
/** |
1337 |
* nextItem holds on to item fields because once we claim |
1338 |
* that an element exists in hasNext(), we must return it in |
1339 |
* the following next() call even if it was in the process of |
1340 |
* being removed when hasNext() was called. |
1341 |
*/ |
1342 |
private E nextItem; |
1343 |
|
1344 |
/** |
1345 |
* Node returned by most recent call to next. Needed by remove. |
1346 |
* Reset to null if this element is deleted by a call to remove. |
1347 |
*/ |
1348 |
private Node<E> lastRet; |
1349 |
|
1350 |
abstract Node<E> startNode(); |
1351 |
abstract Node<E> nextNode(Node<E> p); |
1352 |
|
1353 |
AbstractItr() { |
1354 |
advance(); |
1355 |
} |
1356 |
|
1357 |
/** |
1358 |
* Sets nextNode and nextItem to next valid node, or to null |
1359 |
* if no such. |
1360 |
*/ |
1361 |
private void advance() { |
1362 |
lastRet = nextNode; |
1363 |
|
1364 |
Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode); |
1365 |
for (;; p = nextNode(p)) { |
1366 |
if (p == null) { |
1367 |
// might be at active end or TERMINATOR node; both are OK |
1368 |
nextNode = null; |
1369 |
nextItem = null; |
1370 |
break; |
1371 |
} |
1372 |
final E item; |
1373 |
if ((item = p.item) != null) { |
1374 |
nextNode = p; |
1375 |
nextItem = item; |
1376 |
break; |
1377 |
} |
1378 |
} |
1379 |
} |
1380 |
|
1381 |
public boolean hasNext() { |
1382 |
return nextItem != null; |
1383 |
} |
1384 |
|
1385 |
public E next() { |
1386 |
E item = nextItem; |
1387 |
if (item == null) throw new NoSuchElementException(); |
1388 |
advance(); |
1389 |
return item; |
1390 |
} |
1391 |
|
1392 |
public void remove() { |
1393 |
Node<E> l = lastRet; |
1394 |
if (l == null) throw new IllegalStateException(); |
1395 |
l.item = null; |
1396 |
unlink(l); |
1397 |
lastRet = null; |
1398 |
} |
1399 |
} |
1400 |
|
1401 |
/** Forward iterator */ |
1402 |
private class Itr extends AbstractItr { |
1403 |
Itr() {} // prevent access constructor creation |
1404 |
Node<E> startNode() { return first(); } |
1405 |
Node<E> nextNode(Node<E> p) { return succ(p); } |
1406 |
} |
1407 |
|
1408 |
/** Descending iterator */ |
1409 |
private class DescendingItr extends AbstractItr { |
1410 |
DescendingItr() {} // prevent access constructor creation |
1411 |
Node<E> startNode() { return last(); } |
1412 |
Node<E> nextNode(Node<E> p) { return pred(p); } |
1413 |
} |
1414 |
|
1415 |
/** A customized variant of Spliterators.IteratorSpliterator */ |
1416 |
final class CLDSpliterator implements Spliterator<E> { |
1417 |
static final int MAX_BATCH = 1 << 25; // max batch array size; |
1418 |
Node<E> current; // current node; null until initialized |
1419 |
int batch; // batch size for splits |
1420 |
boolean exhausted; // true when no more nodes |
1421 |
|
1422 |
public Spliterator<E> trySplit() { |
1423 |
Node<E> p, q; |
1424 |
if ((p = current()) == null || (q = p.next) == null) |
1425 |
return null; |
1426 |
int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH); |
1427 |
Object[] a = null; |
1428 |
do { |
1429 |
final E e; |
1430 |
if ((e = p.item) != null) { |
1431 |
if (a == null) |
1432 |
a = new Object[n]; |
1433 |
a[i++] = e; |
1434 |
} |
1435 |
if (p == (p = q)) |
1436 |
p = first(); |
1437 |
} while (p != null && (q = p.next) != null && i < n); |
1438 |
setCurrent(p); |
1439 |
return (i == 0) ? null : |
1440 |
Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED | |
1441 |
Spliterator.NONNULL | |
1442 |
Spliterator.CONCURRENT)); |
1443 |
} |
1444 |
|
1445 |
public void forEachRemaining(Consumer<? super E> action) { |
1446 |
Objects.requireNonNull(action); |
1447 |
Node<E> p; |
1448 |
if ((p = current()) != null) { |
1449 |
current = null; |
1450 |
exhausted = true; |
1451 |
do { |
1452 |
final E e; |
1453 |
if ((e = p.item) != null) |
1454 |
action.accept(e); |
1455 |
if (p == (p = p.next)) |
1456 |
p = first(); |
1457 |
} while (p != null); |
1458 |
} |
1459 |
} |
1460 |
|
1461 |
public boolean tryAdvance(Consumer<? super E> action) { |
1462 |
Objects.requireNonNull(action); |
1463 |
Node<E> p; |
1464 |
if ((p = current()) != null) { |
1465 |
E e; |
1466 |
do { |
1467 |
e = p.item; |
1468 |
if (p == (p = p.next)) |
1469 |
p = first(); |
1470 |
} while (e == null && p != null); |
1471 |
setCurrent(p); |
1472 |
if (e != null) { |
1473 |
action.accept(e); |
1474 |
return true; |
1475 |
} |
1476 |
} |
1477 |
return false; |
1478 |
} |
1479 |
|
1480 |
private void setCurrent(Node<E> p) { |
1481 |
if ((current = p) == null) |
1482 |
exhausted = true; |
1483 |
} |
1484 |
|
1485 |
private Node<E> current() { |
1486 |
Node<E> p; |
1487 |
if ((p = current) == null && !exhausted) |
1488 |
setCurrent(p = first()); |
1489 |
return p; |
1490 |
} |
1491 |
|
1492 |
public long estimateSize() { return Long.MAX_VALUE; } |
1493 |
|
1494 |
public int characteristics() { |
1495 |
return (Spliterator.ORDERED | |
1496 |
Spliterator.NONNULL | |
1497 |
Spliterator.CONCURRENT); |
1498 |
} |
1499 |
} |
1500 |
|
1501 |
/** |
1502 |
* Returns a {@link Spliterator} over the elements in this deque. |
1503 |
* |
1504 |
* <p>The returned spliterator is |
1505 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
1506 |
* |
1507 |
* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, |
1508 |
* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. |
1509 |
* |
1510 |
* @implNote |
1511 |
* The {@code Spliterator} implements {@code trySplit} to permit limited |
1512 |
* parallelism. |
1513 |
* |
1514 |
* @return a {@code Spliterator} over the elements in this deque |
1515 |
* @since 1.8 |
1516 |
*/ |
1517 |
public Spliterator<E> spliterator() { |
1518 |
return new CLDSpliterator(); |
1519 |
} |
1520 |
|
1521 |
/** |
1522 |
* Saves this deque to a stream (that is, serializes it). |
1523 |
* |
1524 |
* @param s the stream |
1525 |
* @throws java.io.IOException if an I/O error occurs |
1526 |
* @serialData All of the elements (each an {@code E}) in |
1527 |
* the proper order, followed by a null |
1528 |
*/ |
1529 |
private void writeObject(java.io.ObjectOutputStream s) |
1530 |
throws java.io.IOException { |
1531 |
|
1532 |
// Write out any hidden stuff |
1533 |
s.defaultWriteObject(); |
1534 |
|
1535 |
// Write out all elements in the proper order. |
1536 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
1537 |
final E item; |
1538 |
if ((item = p.item) != null) |
1539 |
s.writeObject(item); |
1540 |
} |
1541 |
|
1542 |
// Use trailing null as sentinel |
1543 |
s.writeObject(null); |
1544 |
} |
1545 |
|
1546 |
/** |
1547 |
* Reconstitutes this deque from a stream (that is, deserializes it). |
1548 |
* @param s the stream |
1549 |
* @throws ClassNotFoundException if the class of a serialized object |
1550 |
* could not be found |
1551 |
* @throws java.io.IOException if an I/O error occurs |
1552 |
*/ |
1553 |
private void readObject(java.io.ObjectInputStream s) |
1554 |
throws java.io.IOException, ClassNotFoundException { |
1555 |
s.defaultReadObject(); |
1556 |
|
1557 |
// Read in elements until trailing null sentinel found |
1558 |
Node<E> h = null, t = null; |
1559 |
for (Object item; (item = s.readObject()) != null; ) { |
1560 |
@SuppressWarnings("unchecked") |
1561 |
Node<E> newNode = newNode((E) item); |
1562 |
if (h == null) |
1563 |
h = t = newNode; |
1564 |
else { |
1565 |
NEXT.set(t, newNode); |
1566 |
PREV.set(newNode, t); |
1567 |
t = newNode; |
1568 |
} |
1569 |
} |
1570 |
initHeadTail(h, t); |
1571 |
} |
1572 |
|
1573 |
/** |
1574 |
* @throws NullPointerException {@inheritDoc} |
1575 |
*/ |
1576 |
public boolean removeIf(Predicate<? super E> filter) { |
1577 |
Objects.requireNonNull(filter); |
1578 |
return bulkRemove(filter); |
1579 |
} |
1580 |
|
1581 |
/** |
1582 |
* @throws NullPointerException {@inheritDoc} |
1583 |
*/ |
1584 |
public boolean removeAll(Collection<?> c) { |
1585 |
Objects.requireNonNull(c); |
1586 |
return bulkRemove(e -> c.contains(e)); |
1587 |
} |
1588 |
|
1589 |
/** |
1590 |
* @throws NullPointerException {@inheritDoc} |
1591 |
*/ |
1592 |
public boolean retainAll(Collection<?> c) { |
1593 |
Objects.requireNonNull(c); |
1594 |
return bulkRemove(e -> !c.contains(e)); |
1595 |
} |
1596 |
|
1597 |
/** Implementation of bulk remove methods. */ |
1598 |
private boolean bulkRemove(Predicate<? super E> filter) { |
1599 |
boolean removed = false; |
1600 |
for (Node<E> p = first(), succ; p != null; p = succ) { |
1601 |
succ = succ(p); |
1602 |
final E item; |
1603 |
if ((item = p.item) != null |
1604 |
&& filter.test(item) |
1605 |
&& ITEM.compareAndSet(p, item, null)) { |
1606 |
unlink(p); |
1607 |
removed = true; |
1608 |
} |
1609 |
} |
1610 |
return removed; |
1611 |
} |
1612 |
|
1613 |
/** |
1614 |
* @throws NullPointerException {@inheritDoc} |
1615 |
*/ |
1616 |
public void forEach(Consumer<? super E> action) { |
1617 |
Objects.requireNonNull(action); |
1618 |
E item; |
1619 |
for (Node<E> p = first(); p != null; p = succ(p)) |
1620 |
if ((item = p.item) != null) |
1621 |
action.accept(item); |
1622 |
} |
1623 |
|
1624 |
// VarHandle mechanics |
1625 |
private static final VarHandle HEAD; |
1626 |
private static final VarHandle TAIL; |
1627 |
private static final VarHandle PREV; |
1628 |
private static final VarHandle NEXT; |
1629 |
private static final VarHandle ITEM; |
1630 |
static { |
1631 |
PREV_TERMINATOR = new Node<Object>(); |
1632 |
PREV_TERMINATOR.next = PREV_TERMINATOR; |
1633 |
NEXT_TERMINATOR = new Node<Object>(); |
1634 |
NEXT_TERMINATOR.prev = NEXT_TERMINATOR; |
1635 |
try { |
1636 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
1637 |
HEAD = l.findVarHandle(ConcurrentLinkedDeque.class, "head", |
1638 |
Node.class); |
1639 |
TAIL = l.findVarHandle(ConcurrentLinkedDeque.class, "tail", |
1640 |
Node.class); |
1641 |
PREV = l.findVarHandle(Node.class, "prev", Node.class); |
1642 |
NEXT = l.findVarHandle(Node.class, "next", Node.class); |
1643 |
ITEM = l.findVarHandle(Node.class, "item", Object.class); |
1644 |
} catch (ReflectiveOperationException e) { |
1645 |
throw new ExceptionInInitializerError(e); |
1646 |
} |
1647 |
} |
1648 |
} |