content: /**
   * This file is part of Aion-Lightning <aion-lightning.org>.
   *
   *  Aion-Lightning is free software: you can redistribute it and/or modify
   *  it under the terms of the GNU General Public License as published by
   *  the Free Software Foundation, either version 3 of the License, or
   *  (at your option) any later version.
   *
   *  Aion-Lightning is distributed in the hope that it will be useful,
   *  but WITHOUT ANY WARRANTY; without even the implied warranty of
   *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   *  GNU General Public License for more details. *
   *  You should have received a copy of the GNU General Public License
   *  along with Aion-Lightning.
   *  If not, see <http://www.gnu.org/licenses/>.
   */
  
  /*
   * Written by Doug Lea with assistance from members of JCP JSR-166
   * Expert Group and released to the public domain, as explained at
   * http://creativecommons.org/publicdomain/zero/1.0/
   */
  
  package com.aionemu.commons.utils.internal.chmv8;
  
  import java.util.concurrent.RecursiveAction;
  
  /**
   * A {@link ForkJoinTask} with a completion action performed when triggered and
   * there are no remaining pending actions. CountedCompleters are in general more
   * robust in the presence of subtask stalls and blockage than are other forms of
   * ForkJoinTasks, but are less intuitive to program. Uses of CountedCompleter
   * are similar to those of other completion based components (such as
   * {@link java.nio.channels.CompletionHandler}) except that multiple
   * <em>pending</em> completions may be necessary to trigger the
   * {@link #onCompletion} action, not just one. Unless initialized otherwise, the
   * {@link #getPendingCount pending count} starts at zero, but may be
   * (atomically) changed using methods {@link #setPendingCount},
   * {@link #addToPendingCount}, and {@link #compareAndSetPendingCount}. Upon
   * invocation of {@link #tryComplete}, if the pending action count is nonzero,
   * it is decremented; otherwise, the completion action is performed, and if this
   * completer itself has a completer, the process is continued with its
   * completer. As is the case with related synchronization components such as
   * {@link java.util.concurrent.Phaser Phaser} and
   * {@link java.util.concurrent.Semaphore Semaphore}, these methods affect only
   * internal counts; they do not establish any further internal bookkeeping. In
   * particular, the identities of pending tasks are not maintained. As
   * illustrated below, you can create subclasses that do record some or all
   * pending tasks or their results when needed. As illustrated below, utility
   * methods supporting customization of completion traversals are also provided.
   * However, because CountedCompleters provide only basic synchronization
   * mechanisms, it may be useful to create further abstract subclasses that
   * maintain linkages, fields, and additional support methods appropriate for a
   * set of related usages.
   *
   * <p>
   * A concrete CountedCompleter class must define method {@link #compute}, that
   * should in most cases (as illustrated below), invoke {@code tryComplete()}
   * once before returning. The class may also optionally override method
   * {@link #onCompletion} to perform an action upon normal completion, and method
   * {@link #onExceptionalCompletion} to perform an action upon any exception.
   *
   * <p>
   * CountedCompleters most often do not bear results, in which case they are
   * normally declared as {@code CountedCompleter<Void>}, and will always return
   * {@code null} as a result value. In other cases, you should override method
   * {@link #getRawResult} to provide a result from {@code join(), invoke()}, and
   * related methods. In general, this method should return the value of a field
   * (or a function of one or more fields) of the CountedCompleter object that
   * holds the result upon completion. Method {@link #setRawResult} by default
   * plays no role in CountedCompleters. It is possible, but rarely applicable, to
   * override this method to maintain other objects or fields holding result data.
   *
   * <p>
   * A CountedCompleter that does not itself have a completer (i.e., one for which
   * {@link #getCompleter} returns {@code null}) can be used as a regular
   * ForkJoinTask with this added functionality. However, any completer that in
   * turn has another completer serves only as an internal helper for other
   * computations, so its own task status (as reported in methods such as
   * {@link ForkJoinTask#isDone}) is arbitrary; this status changes only upon
   * explicit invocations of {@link #complete}, {@link ForkJoinTask#cancel},
   * {@link ForkJoinTask#completeExceptionally} or upon exceptional completion of
   * method {@code compute}. Upon any exceptional completion, the exception may be
   * relayed to a task's completer (and its completer, and so on), if one exists
   * and it has not otherwise already completed. Similarly, cancelling an internal
   * CountedCompleter has only a local effect on that completer, so is not often
   * useful.
   *
   * <p>
   * <b>Sample Usages.</b>
   *
   * <p>
   * <b>Parallel recursive decomposition.</b> CountedCompleters may be arranged in
   * trees similar to those often used with {@link RecursiveAction}s, although the
   * constructions involved in setting them up typically vary. Here, the completer
   * of each task is its parent in the computation tree. Even though they entail a
   * bit more bookkeeping, CountedCompleters may be better choices when applying a
   * possibly time-consuming operation (that cannot be further subdivided) to each
   * element of an array or collection; especially when the operation takes a
   * significantly different amount of time to complete for some elements than
   * others, either because of intrinsic variation (for example I/O) or auxiliary
   * effects such as garbage collection. Because CountedCompleters provide their
   * own continuations, other threads need not block waiting to perform them.
   *
   * <p>
   * For example, here is an initial version of a class that uses divide-by-two
   * recursive decomposition to divide work into single pieces (leaf tasks). Even
   * when work is split into individual calls, tree-based techniques are usually
   * preferable to directly forking leaf tasks, because they reduce inter-thread
   * communication and improve load balancing. In the recursive case, the second
   * of each pair of subtasks to finish triggers completion of its parent (because
   * no result combination is performed, the default no-op implementation of
   * method {@code onCompletion} is not overridden). A static utility method sets
   * up the base task and invokes it (here, implicitly using the
   * {@link ForkJoinPool#commonPool()}).
   *
   * <pre>
   *  {@code
   * class MyOperation<E> { void apply(E e) { ... }  }
   *
   * class ForEach<E> extends CountedCompleter<Void> {
   *
   *   public static <E> void forEach(E[] array, MyOperation<E> op) {
   *     new ForEach<E>(null, array, op, 0, array.length).invoke();
   *   }
   *
   *   final E[] array; final MyOperation<E> op; final int lo, hi;
   *   ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
   *     super(p);
   *     this.array = array; this.op = op; this.lo = lo; this.hi = hi;
   *   }
   *
   *   public void compute() { // version 1
   *     if (hi - lo >= 2) {
   *       int mid = (lo + hi) >>> 1;
   *       setPendingCount(2); // must set pending count before fork
   *       new ForEach(this, array, op, mid, hi).fork(); // right child
   *       new ForEach(this, array, op, lo, mid).fork(); // left child
   *     }
   *     else if (hi > lo)
   *       op.apply(array[lo]);
   *     tryComplete();
   *   }
   * }}
   * </pre>
   *
   * This design can be improved by noticing that in the recursive case, the task
   * has nothing to do after forking its right task, so can directly invoke its
   * left task before returning. (This is an analog of tail recursion removal.)
   * Also, because the task returns upon executing its left task (rather than
   * falling through to invoke {@code tryComplete}) the pending count is set to
   * one:
   *
   * <pre>
   *  {@code
   * class ForEach<E> ...
   *   public void compute() { // version 2
   *     if (hi - lo >= 2) {
   *       int mid = (lo + hi) >>> 1;
   *       setPendingCount(1); // only one pending
   *       new ForEach(this, array, op, mid, hi).fork(); // right child
   *       new ForEach(this, array, op, lo, mid).compute(); // direct invoke
   *     }
   *     else {
   *       if (hi > lo)
   *         op.apply(array[lo]);
   *       tryComplete();
   *     }
   *   }
   * }
   * </pre>
   *
   * As a further improvement, notice that the left task need not even exist.
   * Instead of creating a new one, we can iterate using the original task, and
   * add a pending count for each fork. Additionally, because no task in this tree
   * implements an {@link #onCompletion} method, {@code tryComplete()} can be
   * replaced with {@link #propagateCompletion}.
   *
   * <pre>
   *  {@code
   * class ForEach<E> ...
   *   public void compute() { // version 3
   *     int l = lo,  h = hi;
   *     while (h - l >= 2) {
   *       int mid = (l + h) >>> 1;
   *       addToPendingCount(1);
   *       new ForEach(this, array, op, mid, h).fork(); // right child
   *       h = mid;
   *     }
   *     if (h > l)
   *       op.apply(array[l]);
   *     propagateCompletion();
   *   }
   * }
   * </pre>
   *
   * Additional improvements of such classes might entail precomputing pending
   * counts so that they can be established in constructors, specializing classes
   * for leaf steps, subdividing by say, four, instead of two per iteration, and
   * using an adaptive threshold instead of always subdividing down to single
   * elements.
   *
   * <p>
   * <b>Searching.</b> A tree of CountedCompleters can search for a value or
   * property in different parts of a data structure, and report a result in an
   * {@link java.util.concurrent.atomic.AtomicReference AtomicReference} as soon
   * as one is found. The others can poll the result to avoid unnecessary work.
   * (You could additionally {@linkplain #cancel cancel} other tasks, but it is
   * usually simpler and more efficient to just let them notice that the result is
   * set and if so skip further processing.) Illustrating again with an array
   * using full partitioning (again, in practice, leaf tasks will almost always
   * process more than one element):
   *
   * <pre>
   *  {@code
   * class Searcher<E> extends CountedCompleter<E> {
   *   final E[] array; final AtomicReference<E> result; final int lo, hi;
   *   Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) {
   *     super(p);
   *     this.array = array; this.result = result; this.lo = lo; this.hi = hi;
   *   }
   *   public E getRawResult() { return result.get(); }
   *   public void compute() { // similar to ForEach version 3
   *     int l = lo,  h = hi;
   *     while (result.get() == null && h >= l) {
   *       if (h - l >= 2) {
   *         int mid = (l + h) >>> 1;
   *         addToPendingCount(1);
   *         new Searcher(this, array, result, mid, h).fork();
   *         h = mid;
   *       }
   *       else {
   *         E x = array[l];
   *         if (matches(x) && result.compareAndSet(null, x))
   *           quietlyCompleteRoot(); // root task is now joinable
   *         break;
   *       }
   *     }
   *     tryComplete(); // normally complete whether or not found
   *   }
   *   boolean matches(E e) { ... } // return true if found
   *
   *   public static <E> E search(E[] array) {
   *       return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke();
   *   }
   * }}
   * </pre>
   *
   * In this example, as well as others in which tasks have no other effects
   * except to compareAndSet a common result, the trailing unconditional
   * invocation of {@code tryComplete} could be made conditional
   * ({@code if (result.get() == null) tryComplete();}) because no further
   * bookkeeping is required to manage completions once the root task completes.
   *
   * <p>
   * <b>Recording subtasks.</b> CountedCompleter tasks that combine results of
   * multiple subtasks usually need to access these results in method
   * {@link #onCompletion}. As illustrated in the following class (that performs a
   * simplified form of map-reduce where mappings and reductions are all of type
   * {@code E}), one way to do this in divide and conquer designs is to have each
   * subtask record its sibling, so that it can be accessed in method
   * {@code onCompletion}. This technique applies to reductions in which the order
   * of combining left and right results does not matter; ordered reductions
   * require explicit left/right designations. Variants of other streamlinings
   * seen in the above examples may also apply.
   *
   * <pre>
   *  {@code
   * class MyMapper<E> { E apply(E v) {  ...  } }
   * class MyReducer<E> { E apply(E x, E y) {  ...  } }
   * class MapReducer<E> extends CountedCompleter<E> {
   *   final E[] array; final MyMapper<E> mapper;
   *   final MyReducer<E> reducer; final int lo, hi;
   *   MapReducer<E> sibling;
   *   E result;
   *   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
   *              MyReducer<E> reducer, int lo, int hi) {
   *     super(p);
   *     this.array = array; this.mapper = mapper;
   *     this.reducer = reducer; this.lo = lo; this.hi = hi;
   *   }
   *   public void compute() {
   *     if (hi - lo >= 2) {
   *       int mid = (lo + hi) >>> 1;
   *       MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
   *       MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
   *       left.sibling = right;
   *       right.sibling = left;
   *       setPendingCount(1); // only right is pending
   *       right.fork();
   *       left.compute();     // directly execute left
   *     }
   *     else {
   *       if (hi > lo)
   *           result = mapper.apply(array[lo]);
   *       tryComplete();
   *     }
   *   }
   *   public void onCompletion(CountedCompleter<?> caller) {
   *     if (caller != this) {
   *       MapReducer<E> child = (MapReducer<E>)caller;
   *       MapReducer<E> sib = child.sibling;
   *       if (sib == null || sib.result == null)
   *         result = child.result;
   *       else
   *         result = reducer.apply(child.result, sib.result);
   *     }
   *   }
   *   public E getRawResult() { return result; }
   *
   *   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
   *     return new MapReducer<E>(null, array, mapper, reducer,
   *                              0, array.length).invoke();
   *   }
   * }}
   * </pre>
   *
   * Here, method {@code onCompletion} takes a form common to many completion
   * designs that combine results. This callback-style method is triggered once
   * per task, in either of the two different contexts in which the pending count
   * is, or becomes, zero: (1) by a task itself, if its pending count is zero upon
   * invocation of {@code
   * tryComplete}, or (2) by any of its subtasks when they complete and decrement
   * the pending count to zero. The {@code caller} argument distinguishes cases.
   * Most often, when the caller is {@code this}, no action is necessary.
   * Otherwise the caller argument can be used (usually via a cast) to supply a
   * value (and/or links to other values) to be combined. Assuming proper use of
   * pending counts, the actions inside {@code onCompletion} occur (once) upon
   * completion of a task and its subtasks. No additional synchronization is
   * required within this method to ensure thread safety of accesses to fields of
   * this task or other completed tasks.
   *
   * <p>
   * <b>Completion Traversals</b>. If using {@code onCompletion} to process
   * completions is inapplicable or inconvenient, you can use methods
   * {@link #firstComplete} and {@link #nextComplete} to create custom traversals.
   * For example, to define a MapReducer that only splits out right-hand tasks in
   * the form of the third ForEach example, the completions must cooperatively
   * reduce along unexhausted subtask links, which can be done as follows:
   *
   * <pre>
   *  {@code
   * class MapReducer<E> extends CountedCompleter<E> { // version 2
   *   final E[] array; final MyMapper<E> mapper;
   *   final MyReducer<E> reducer; final int lo, hi;
   *   MapReducer<E> forks, next; // record subtask forks in list
   *   E result;
   *   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
   *              MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) {
   *     super(p);
   *     this.array = array; this.mapper = mapper;
   *     this.reducer = reducer; this.lo = lo; this.hi = hi;
   *     this.next = next;
   *   }
   *   public void compute() {
   *     int l = lo,  h = hi;
   *     while (h - l >= 2) {
   *       int mid = (l + h) >>> 1;
   *       addToPendingCount(1);
   *       (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork;
   *       h = mid;
   *     }
   *     if (h > l)
   *       result = mapper.apply(array[l]);
   *     // process completions by reducing along and advancing subtask links
   *     for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
   *       for (MapReducer t = (MapReducer)c, s = t.forks;  s != null; s = t.forks = s.next)
   *         t.result = reducer.apply(t.result, s.result);
   *     }
   *   }
   *   public E getRawResult() { return result; }
   *
   *   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
   *     return new MapReducer<E>(null, array, mapper, reducer,
   *                              0, array.length, null).invoke();
   *   }
   * }}
   * </pre>
   *
   * <p>
   * <b>Triggers.</b> Some CountedCompleters are themselves never forked, but
   * instead serve as bits of plumbing in other designs; including those in which
   * the completion of one of more async tasks triggers another async task. For
   * example:
   *
   * <pre>
   *  {@code
   * class HeaderBuilder extends CountedCompleter<...> { ... }
   * class BodyBuilder extends CountedCompleter<...> { ... }
   * class PacketSender extends CountedCompleter<...> {
   *   PacketSender(...) { super(null, 1); ... } // trigger on second completion
   *   public void compute() { } // never called
   *   public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
   * }
   * // sample use:
   * PacketSender p = new PacketSender();
   * new HeaderBuilder(p, ...).fork();
   * new BodyBuilder(p, ...).fork();
   * }
   * </pre>
   *
   * @since 1.8
   * @author Doug Lea
   */
  public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
  	private static final long serialVersionUID = 5232453752276485070L;
  
  	/** This task's completer, or null if none */
  	final CountedCompleter<?> completer;
  	/** The number of pending tasks until completion */
  	volatile int pending;
  
  	/**
  	 * Creates a new CountedCompleter with the given completer and initial
  	 * pending count.
  	 *
  	 * @param completer
  	 *            this task's completer, or {@code null} if none
  	 * @param initialPendingCount
  	 *            the initial pending count
  	 */
  	protected CountedCompleter(CountedCompleter<?> completer, int initialPendingCount) {
  		this.completer = completer;
  		this.pending = initialPendingCount;
  	}
  
  	/**
  	 * Creates a new CountedCompleter with the given completer and an initial
  	 * pending count of zero.
  	 *
  	 * @param completer
  	 *            this task's completer, or {@code null} if none
  	 */
  	protected CountedCompleter(CountedCompleter<?> completer) {
  		this.completer = completer;
  	}
  
  	/**
  	 * Creates a new CountedCompleter with no completer and an initial pending
  	 * count of zero.
  	 */
  	protected CountedCompleter() {
  		this.completer = null;
  	}
  
  	/**
  	 * The main computation performed by this task.
  	 */
  	public abstract void compute();
  
  	/**
  	 * Performs an action when method {@link #tryComplete} is invoked and the
  	 * pending count is zero, or when the unconditional method {@link #complete}
  	 * is invoked. By default, this method does nothing. You can distinguish
  	 * cases by checking the identity of the given caller argument. If not equal
  	 * to {@code
  	 * this}, then it is typically a subtask that may contain results (and/or
  	 * links to other results) to combine.
  	 *
  	 * @param caller
  	 *            the task invoking this method (which may be this task itself)
  	 */
  	public void onCompletion(CountedCompleter<?> caller) {
  	}
  
  	/**
  	 * Performs an action when method {@link #completeExceptionally} is invoked
  	 * or method {@link #compute} throws an exception, and this task has not
  	 * otherwise already completed normally. On entry to this method, this task
  	 * {@link ForkJoinTask#isCompletedAbnormally}. The return value of this
  	 * method controls further propagation: If {@code true} and this task has a
  	 * completer, then this completer is also completed exceptionally. The
  	 * default implementation of this method does nothing except return
  	 * {@code true}.
  	 *
  	 * @param ex
  	 *            the exception
  	 * @param caller
  	 *            the task invoking this method (which may be this task itself)
  	 * @return true if this exception should be propagated to this task's
  	 *         completer, if one exists
  	 */
  	public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
  		return true;
  	}
  
  	/**
  	 * Returns the completer established in this task's constructor, or
  	 * {@code null} if none.
  	 *
  	 * @return the completer
  	 */
  	public final CountedCompleter<?> getCompleter() {
  		return completer;
  	}
  
  	/**
  	 * Returns the current pending count.
  	 *
  	 * @return the current pending count
  	 */
  	public final int getPendingCount() {
  		return pending;
  	}
  
  	/**
  	 * Sets the pending count to the given value.
  	 *
  	 * @param count
  	 *            the count
  	 */
  	public final void setPendingCount(int count) {
  		pending = count;
  	}
  
  	/**
  	 * Adds (atomically) the given value to the pending count.
  	 *
  	 * @param delta
  	 *            the value to add
  	 */
  	public final void addToPendingCount(int delta) {
  		int c; // note: can replace with intrinsic in jdk8
  		do {
  		} while (!U.compareAndSwapInt(this, PENDING, c = pending, c + delta));
  	}
  
  	/**
  	 * Sets (atomically) the pending count to the given count only if it
  	 * currently holds the given expected value.
  	 *
  	 * @param expected
  	 *            the expected value
  	 * @param count
  	 *            the new value
  	 * @return true if successful
  	 */
  	public final boolean compareAndSetPendingCount(int expected, int count) {
  		return U.compareAndSwapInt(this, PENDING, expected, count);
  	}
  
  	/**
  	 * If the pending count is nonzero, (atomically) decrements it.
  	 *
  	 * @return the initial (undecremented) pending count holding on entry to
  	 *         this method
  	 */
  	public final int decrementPendingCountUnlessZero() {
  		int c;
  		do {
  		} while ((c = pending) != 0 && !U.compareAndSwapInt(this, PENDING, c, c - 1));
  		return c;
  	}
  
  	/**
  	 * Returns the root of the current computation; i.e., this task if it has no
  	 * completer, else its completer's root.
  	 *
  	 * @return the root of the current computation
  	 */
  	public final CountedCompleter<?> getRoot() {
  		CountedCompleter<?> a = this, p;
  		while ((p = a.completer) != null)
  			a = p;
  		return a;
  	}
  
  	/**
  	 * If the pending count is nonzero, decrements the count; otherwise invokes
  	 * {@link #onCompletion} and then similarly tries to complete this task's
  	 * completer, if one exists, else marks this task as complete.
  	 */
  	public final void tryComplete() {
  		CountedCompleter<?> a = this, s = a;
  		for (int c;;) {
  			if ((c = a.pending) == 0) {
  				a.onCompletion(s);
  				if ((a = (s = a).completer) == null) {
  					s.quietlyComplete();
  					return;
  				}
  			} else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
  				return;
  		}
  	}
  
  	/**
  	 * Equivalent to {@link #tryComplete} but does not invoke
  	 * {@link #onCompletion} along the completion path: If the pending count is
  	 * nonzero, decrements the count; otherwise, similarly tries to complete
  	 * this task's completer, if one exists, else marks this task as complete.
  	 * This method may be useful in cases where {@code onCompletion} should not,
  	 * or need not, be invoked for each completer in a computation.
  	 */
  	public final void propagateCompletion() {
  		CountedCompleter<?> a = this, s = a;
  		for (int c;;) {
  			if ((c = a.pending) == 0) {
  				if ((a = (s = a).completer) == null) {
  					s.quietlyComplete();
  					return;
  				}
  			} else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
  				return;
  		}
  	}
  
  	/**
  	 * Regardless of pending count, invokes {@link #onCompletion}, marks this
  	 * task as complete and further triggers {@link #tryComplete} on this task's
  	 * completer, if one exists. The given rawResult is used as an argument to
  	 * {@link #setRawResult} before invoking {@link #onCompletion} or marking
  	 * this task as complete; its value is meaningful only for classes
  	 * overriding {@code setRawResult}.
  	 *
  	 * <p>
  	 * This method may be useful when forcing completion as soon as any one
  	 * (versus all) of several subtask results are obtained. However, in the
  	 * common (and recommended) case in which {@code
  	 * setRawResult} is not overridden, this effect can be obtained more simply
  	 * using {@code quietlyCompleteRoot();}.
  	 *
  	 * @param rawResult
  	 *            the raw result
  	 */
  	@Override
  	public void complete(T rawResult) {
  		CountedCompleter<?> p;
  		setRawResult(rawResult);
  		onCompletion(this);
  		quietlyComplete();
  		if ((p = completer) != null)
  			p.tryComplete();
  	}
  
  	/**
  	 * If this task's pending count is zero, returns this task; otherwise
  	 * decrements its pending count and returns {@code
  	 * null}. This method is designed to be used with {@link #nextComplete} in
  	 * completion traversal loops.
  	 *
  	 * @return this task, if pending count was zero, else {@code null}
  	 */
  	public final CountedCompleter<?> firstComplete() {
  		for (int c;;) {
  			if ((c = pending) == 0)
  				return this;
  			else if (U.compareAndSwapInt(this, PENDING, c, c - 1))
  				return null;
  		}
  	}
  
  	/**
  	 * If this task does not have a completer, invokes
  	 * {@link ForkJoinTask#quietlyComplete} and returns {@code null}. Or, if
  	 * this task's pending count is non-zero, decrements its pending count and
  	 * returns {@code null}. Otherwise, returns the completer. This method can
  	 * be used as part of a completion traversal loop for homogeneous task
  	 * hierarchies:
  	 *
  	 * <pre>
  	 *  {@code
  	 * for (CountedCompleter<?> c = firstComplete();
  	 *      c != null;
  	 *      c = c.nextComplete()) {
  	 *   // ... process c ...
  	 * }}
  	 * </pre>
  	 *
  	 * @return the completer, or {@code null} if none
  	 */
  	public final CountedCompleter<?> nextComplete() {
  		CountedCompleter<?> p;
  		if ((p = completer) != null)
  			return p.firstComplete();
  		else {
  			quietlyComplete();
  			return null;
  		}
  	}
  
  	/**
  	 * Equivalent to {@code getRoot().quietlyComplete()}.
  	 */
  	public final void quietlyCompleteRoot() {
  		for (CountedCompleter<?> a = this, p;;) {
  			if ((p = a.completer) == null) {
  				a.quietlyComplete();
  				return;
  			}
  			a = p;
  		}
  	}
  
  	/**
  	 * Supports ForkJoinTask exception propagation.
  	 */
  	@Override
  	void internalPropagateException(Throwable ex) {
  		CountedCompleter<?> a = this, s = a;
  		while (a.onExceptionalCompletion(ex, s) && (a = (s = a).completer) != null && a.status >= 0)
  			a.recordExceptionalCompletion(ex);
  	}
  
  	/**
  	 * Implements execution conventions for CountedCompleters.
  	 */
  	@Override
  	protected final boolean exec() {
  		compute();
  		return false;
  	}
  
  	/**
  	 * Returns the result of the computation. By default returns {@code null},
  	 * which is appropriate for {@code Void} actions, but in other cases should
  	 * be overridden, almost always to return a field or function of a field
  	 * that holds the result upon completion.
  	 *
  	 * @return the result of the computation
  	 */
  	@Override
  	public T getRawResult() {
  		return null;
  	}
  
  	/**
  	 * A method that result-bearing CountedCompleters may optionally use to help
  	 * maintain result data. By default, does nothing. Overrides are not
  	 * recommended. However, if this method is overridden to update existing
  	 * objects or fields, then it must in general be defined to be thread-safe.
  	 */
  	@Override
  	protected void setRawResult(T t) {
  	}
  
  	// Unsafe mechanics
  	private static final sun.misc.Unsafe U;
  	private static final long PENDING;
  	static {
  		try {
  			U = getUnsafe();
  			PENDING = U.objectFieldOffset(CountedCompleter.class.getDeclaredField("pending"));
  		} catch (Exception e) {
  			throw new Error(e);
  		}
  	}
  
  	/**
  	 * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
  	 * Replace with a simple call to Unsafe.getUnsafe when integrating into a
  	 * jdk.
  	 *
  	 * @return a sun.misc.Unsafe
  	 */
  	private static sun.misc.Unsafe getUnsafe() {
  		try {
  			return sun.misc.Unsafe.getUnsafe();
  		} catch (SecurityException tryReflectionInstead) {
  		}
  		try {
  			return java.security.AccessController
  					.doPrivileged(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
  						@Override
  						public sun.misc.Unsafe run() throws Exception {
  							Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
  							for (java.lang.reflect.Field f : k.getDeclaredFields()) {
  								f.setAccessible(true);
  								Object x = f.get(null);
  								if (k.isInstance(x))
  									return k.cast(x);
  							}
  							throw new NoSuchFieldError("the Unsafe");
  						}
  					});
  		} catch (java.security.PrivilegedActionException e) {
  			throw new RuntimeException("Could not initialize intrinsics", e.getCause());
  		}
  	}
  }
  