author | kbarrett |
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changeset 50277 | f84ae8aa5d88 |
parent 47216 | 71c04702a3d5 |
child 50301 | fe42de5250f3 |
permissions | -rw-r--r-- |
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/* |
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* Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved. |
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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* |
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* This code is free software; you can redistribute it and/or modify it |
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* under the terms of the GNU General Public License version 2 only, as |
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* published by the Free Software Foundation. Oracle designates this |
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* particular file as subject to the "Classpath" exception as provided |
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* by Oracle in the LICENSE file that accompanied this code. |
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* |
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* This code is distributed in the hope that it will be useful, but WITHOUT |
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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* version 2 for more details (a copy is included in the LICENSE file that |
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* accompanied this code). |
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* |
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* You should have received a copy of the GNU General Public License version |
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* 2 along with this work; if not, write to the Free Software Foundation, |
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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* |
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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* or visit www.oracle.com if you need additional information or have any |
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* questions. |
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*/ |
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package java.lang.ref; |
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import java.util.function.Consumer; |
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import jdk.internal.misc.VM; |
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/** |
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* Reference queues, to which registered reference objects are appended by the |
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* garbage collector after the appropriate reachability changes are detected. |
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* |
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* @author Mark Reinhold |
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* @since 1.2 |
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*/ |
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public class ReferenceQueue<T> { |
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/** |
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* Constructs a new reference-object queue. |
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*/ |
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public ReferenceQueue() { } |
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private static class Null<S> extends ReferenceQueue<S> { |
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boolean enqueue(Reference<? extends S> r) { |
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return false; |
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} |
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} |
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static ReferenceQueue<Object> NULL = new Null<>(); |
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static ReferenceQueue<Object> ENQUEUED = new Null<>(); |
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private static class Lock { }; |
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private Lock lock = new Lock(); |
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private volatile Reference<? extends T> head; |
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private long queueLength = 0; |
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boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */ |
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synchronized (lock) { |
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// Check that since getting the lock this reference hasn't already been |
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// enqueued (and even then removed) |
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ReferenceQueue<?> queue = r.queue; |
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if ((queue == NULL) || (queue == ENQUEUED)) { |
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return false; |
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} |
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assert queue == this; |
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// Self-loop end, so if a FinalReference it remains inactive. |
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r.next = (head == null) ? r : head; |
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head = r; |
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queueLength++; |
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// Update r.queue *after* adding to list, to avoid race |
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// with concurrent enqueued checks and fast-path poll(). |
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// Volatiles ensure ordering. |
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r.queue = ENQUEUED; |
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if (r instanceof FinalReference) { |
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VM.addFinalRefCount(1); |
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} |
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lock.notifyAll(); |
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return true; |
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} |
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} |
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private Reference<? extends T> reallyPoll() { /* Must hold lock */ |
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Reference<? extends T> r = head; |
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if (r != null) { |
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r.queue = NULL; |
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// Update r.queue *before* removing from list, to avoid |
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// race with concurrent enqueued checks and fast-path |
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// poll(). Volatiles ensure ordering. |
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@SuppressWarnings("unchecked") |
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Reference<? extends T> rn = r.next; |
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// Handle self-looped next as end of list designator. |
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head = (rn == r) ? null : rn; |
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// Self-loop next rather than setting to null, so if a |
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// FinalReference it remains inactive. |
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r.next = r; |
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queueLength--; |
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if (r instanceof FinalReference) { |
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VM.addFinalRefCount(-1); |
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} |
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return r; |
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} |
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return null; |
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} |
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/** |
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* Polls this queue to see if a reference object is available. If one is |
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* available without further delay then it is removed from the queue and |
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* returned. Otherwise this method immediately returns {@code null}. |
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* |
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* @return A reference object, if one was immediately available, |
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* otherwise {@code null} |
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*/ |
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public Reference<? extends T> poll() { |
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if (head == null) |
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return null; |
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synchronized (lock) { |
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return reallyPoll(); |
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} |
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} |
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/** |
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* Removes the next reference object in this queue, blocking until either |
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* one becomes available or the given timeout period expires. |
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* |
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* <p> This method does not offer real-time guarantees: It schedules the |
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* timeout as if by invoking the {@link Object#wait(long)} method. |
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* |
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* @param timeout If positive, block for up to {@code timeout} |
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* milliseconds while waiting for a reference to be |
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* added to this queue. If zero, block indefinitely. |
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* |
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* @return A reference object, if one was available within the specified |
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* timeout period, otherwise {@code null} |
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* |
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* @throws IllegalArgumentException |
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* If the value of the timeout argument is negative |
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* |
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* @throws InterruptedException |
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* If the timeout wait is interrupted |
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*/ |
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public Reference<? extends T> remove(long timeout) |
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throws IllegalArgumentException, InterruptedException |
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{ |
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if (timeout < 0) { |
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throw new IllegalArgumentException("Negative timeout value"); |
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} |
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synchronized (lock) { |
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Reference<? extends T> r = reallyPoll(); |
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if (r != null) return r; |
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long start = (timeout == 0) ? 0 : System.nanoTime(); |
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for (;;) { |
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lock.wait(timeout); |
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r = reallyPoll(); |
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if (r != null) return r; |
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if (timeout != 0) { |
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long end = System.nanoTime(); |
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timeout -= (end - start) / 1000_000; |
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if (timeout <= 0) return null; |
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start = end; |
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} |
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} |
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} |
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} |
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/** |
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* Removes the next reference object in this queue, blocking until one |
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* becomes available. |
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* |
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* @return A reference object, blocking until one becomes available |
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* @throws InterruptedException If the wait is interrupted |
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*/ |
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public Reference<? extends T> remove() throws InterruptedException { |
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return remove(0); |
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} |
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/** |
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* Iterate queue and invoke given action with each Reference. |
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* Suitable for diagnostic purposes. |
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* WARNING: any use of this method should make sure to not |
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* retain the referents of iterated references (in case of |
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* FinalReference(s)) so that their life is not prolonged more |
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* than necessary. |
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*/ |
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void forEach(Consumer<? super Reference<? extends T>> action) { |
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for (Reference<? extends T> r = head; r != null;) { |
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action.accept(r); |
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@SuppressWarnings("unchecked") |
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Reference<? extends T> rn = r.next; |
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if (rn == r) { |
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if (r.queue == ENQUEUED) { |
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// still enqueued -> we reached end of chain |
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r = null; |
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} else { |
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// already dequeued: r.queue == NULL; -> |
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// restart from head when overtaken by queue poller(s) |
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r = head; |
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} |
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} else { |
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// next in chain |
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r = rn; |
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} |
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} |
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} |
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} |