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package jsr166y; |
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import java.util.concurrent.*; |
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import java.util.AbstractQueue; |
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import java.util.Collection; |
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import java.util.ConcurrentModificationException; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Queue; |
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import java.util.concurrent.TimeUnit; |
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import java.util.concurrent.locks.LockSupport; |
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/** |
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* An unbounded {@link TransferQueue} based on linked nodes. |
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* This queue orders elements FIFO (first-in-first-out) with respect |
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* situations in which we cannot guarantee to make node s |
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* unreachable in this way: (1) If s is the trailing node of list |
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* (i.e., with null next), then it is pinned as the target node |
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* for appends, so can only be removed later when other nodes are |
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* for appends, so can only be removed later after other nodes are |
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* appended. (2) We cannot necessarily unlink s given a |
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* predecessor node that is matched (including the case of being |
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* cancelled): the predecessor may already be unspliced, in which |
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* When these cases arise, rather than always retraversing the |
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* entire list to find an actual predecessor to unlink (which |
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* won't help for case (1) anyway), we record a conservative |
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* estimate of possible unsplice failures (in "sweepVotes"). We |
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* trigger a full sweep when the estimate exceeds a threshold |
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* indicating the maximum number of estimated removal failures to |
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* tolerate before sweeping through, unlinking cancelled nodes |
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* that were not unlinked upon initial removal. We perform sweeps |
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* by the thread hitting threshold (rather than background threads |
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* or by spreading work to other threads) because in the main |
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* contexts in which removal occurs, the caller is already |
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* timed-out, cancelled, or performing a potentially O(n) |
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* operation (i.e., remove(x)), none of which are time-critical |
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* enough to warrant the overhead that alternatives would impose |
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* on other threads. |
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* estimate of possible unsplice failures (in "sweepVotes"). |
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* We trigger a full sweep when the estimate exceeds a threshold |
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* ("SWEEP_THRESHOLD") indicating the maximum number of estimated |
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* removal failures to tolerate before sweeping through, unlinking |
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* cancelled nodes that were not unlinked upon initial removal. |
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* We perform sweeps by the thread hitting threshold (rather than |
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* background threads or by spreading work to other threads) |
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* because in the main contexts in which removal occurs, the |
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* caller is already timed-out, cancelled, or performing a |
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* potentially O(n) operation (e.g. remove(x)), none of which are |
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* time-critical enough to warrant the overhead that alternatives |
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* would impose on other threads. |
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* |
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* Because the sweepVotes estimate is conservative, and because |
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* nodes become unlinked "naturally" as they fall off the head of |
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} |
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final boolean casItem(Object cmp, Object val) { |
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assert cmp == null || cmp.getClass() != Node.class; |
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// assert cmp == null || cmp.getClass() != Node.class; |
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return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
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} |
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/** |
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* Creates a new node. Uses relaxed write because item can only |
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* be seen if followed by CAS. |
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* Constructs a new node. Uses relaxed write because item can |
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* only be seen after publication via casNext. |
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*/ |
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Node(Object item, boolean isData) { |
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UNSAFE.putObject(this, itemOffset, item); // relaxed write |
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/** |
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* Sets item to self and waiter to null, to avoid garbage |
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* retention after matching or cancelling. Uses relaxed writes |
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* bacause order is already constrained in the only calling |
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* because order is already constrained in the only calling |
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* contexts: item is forgotten only after volatile/atomic |
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* mechanics that extract items. Similarly, clearing waiter |
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* follows either CAS or return from park (if ever parked; |
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* Tries to artificially match a data node -- used by remove. |
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*/ |
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final boolean tryMatchData() { |
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assert isData; |
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// assert isData; |
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Object x = item; |
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if (x != null && x != this && casItem(x, null)) { |
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LockSupport.unpark(waiter); |
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@SuppressWarnings("unchecked") |
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static <E> E cast(Object item) { |
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assert item == null || item.getClass() != Node.class; |
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// assert item == null || item.getClass() != Node.class; |
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return (E) item; |
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} |
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throw new NullPointerException(); |
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Node s = null; // the node to append, if needed |
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retry: for (;;) { // restart on append race |
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retry: |
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for (;;) { // restart on append race |
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for (Node h = head, p = h; p != null;) { // find & match first node |
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boolean isData = p.isData; |
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if (p.casItem(item, e)) { // match |
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for (Node q = p; q != h;) { |
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Node n = q.next; // update by 2 unless singleton |
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if (head == h && casHead(h, n == null? q : n)) { |
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if (head == h && casHead(h, n == null ? q : n)) { |
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h.forgetNext(); |
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break; |
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} // advance and retry |
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for (;;) { |
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Object item = s.item; |
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if (item != e) { // matched |
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assert item != s; |
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// assert item != s; |
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s.forgetContents(); // avoid garbage |
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return this.<E>cast(item); |
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} |
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* Moves to next node after prev, or first node if prev null. |
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*/ |
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private void advance(Node prev) { |
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lastPred = lastRet; |
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lastRet = prev; |
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for (Node p = (prev == null) ? head : succ(prev); |
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p != null; p = succ(p)) { |
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Object item = p.item; |
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if (p.isData) { |
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if (item != null && item != p) { |
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nextItem = LinkedTransferQueue.this.<E>cast(item); |
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nextNode = p; |
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/* |
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* To track and avoid buildup of deleted nodes in the face |
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* of calls to both Queue.remove and Itr.remove, we must |
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* include variants of unsplice and sweep upon each |
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* advance: Upon Itr.remove, we may need to catch up links |
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* from lastPred, and upon other removes, we might need to |
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* skip ahead from stale nodes and unsplice deleted ones |
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* found while advancing. |
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*/ |
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Node r, b; // reset lastPred upon possible deletion of lastRet |
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if ((r = lastRet) != null && !r.isMatched()) |
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lastPred = r; // next lastPred is old lastRet |
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else if ((b = lastPred) == null || b.isMatched()) |
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lastPred = null; // at start of list |
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else { |
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Node s, n; // help with removal of lastPred.next |
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while ((s = b.next) != null && |
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s != b && s.isMatched() && |
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(n = s.next) != null && n != s) |
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b.casNext(s, n); |
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} |
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this.lastRet = prev; |
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for (Node p = prev, s, n;;) { |
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s = (p == null) ? head : p.next; |
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if (s == null) |
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break; |
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else if (s == p) { |
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p = null; |
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continue; |
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} |
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Object item = s.item; |
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if (s.isData) { |
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if (item != null && item != s) { |
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nextItem = LinkedTransferQueue.<E>cast(item); |
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nextNode = s; |
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return; |
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} |
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} |
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else if (item == null) |
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break; |
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// assert s.isMatched(); |
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if (p == null) |
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p = s; |
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else if ((n = s.next) == null) |
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break; |
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else if (s == n) |
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p = null; |
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else |
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p.casNext(s, n); |
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} |
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nextNode = null; |
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nextItem = null; |
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} |
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Itr() { |
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} |
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public final void remove() { |
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Node p = lastRet; |
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if (p == null) throw new IllegalStateException(); |
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if (p.tryMatchData()) |
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unsplice(lastPred, p); |
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final Node lastRet = this.lastRet; |
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if (lastRet == null) |
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throw new IllegalStateException(); |
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this.lastRet = null; |
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if (lastRet.tryMatchData()) |
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unsplice(lastPred, lastRet); |
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} |
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} |
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} |
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/** |
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* Unlinks matched nodes encountered in a traversal from head. |
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* Unlinks matched (typically cancelled) nodes encountered in a |
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* traversal from head. |
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*/ |
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private void sweep() { |
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for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
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if (p == s) // stale |
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p = head; |
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else if (!s.isMatched()) |
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if (!s.isMatched()) |
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// Unmatched nodes are never self-linked |
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p = s; |
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else if ((n = s.next) == null) // trailing node is pinned |
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break; |
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else if (s == n) // stale |
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// No need to also check for p == s, since that implies s == n |
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p = head; |
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else |
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p.casNext(s, n); |
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} |
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* Inserts the specified element at the tail of this queue. |
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* As the queue is unbounded, this method will never return {@code false}. |
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* |
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* @return {@code true} (as specified by |
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* {@link BlockingQueue#offer(Object) BlockingQueue.offer}) |
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* @return {@code true} (as specified by {@link Queue#offer}) |
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* @throws NullPointerException if the specified element is null |
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*/ |
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public boolean offer(E e) { |
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* @return {@code true} if this queue contains no elements |
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*/ |
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public boolean isEmpty() { |
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return firstOfMode(true) == null; |
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for (Node p = head; p != null; p = succ(p)) { |
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if (!p.isMatched()) |
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return !p.isData; |
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} |
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return true; |
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} |
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public boolean hasWaitingConsumer() { |
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} |
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/** |
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* Returns {@code true} if this queue contains the specified element. |
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* More formally, returns {@code true} if and only if this queue contains |
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* at least one element {@code e} such that {@code o.equals(e)}. |
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* |
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* @param o object to be checked for containment in this queue |
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* @return {@code true} if this queue contains the specified element |
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*/ |
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public boolean contains(Object o) { |
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if (o == null) return false; |
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for (Node p = head; p != null; p = succ(p)) { |
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Object item = p.item; |
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if (p.isData) { |
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if (item != null && item != p && o.equals(item)) |
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return true; |
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} |
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else if (item == null) |
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break; |
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} |
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return false; |
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} |
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/** |
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* Always returns {@code Integer.MAX_VALUE} because a |
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* {@code LinkedTransferQueue} is not capacity constrained. |
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* |
