java/11 : java.base/java/util/concurrent/CountedCompleter.java

CountedCompleter
https://openjdk.java.net/
GPLv2 + Classpath Exception
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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 java.util.concurrent;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;

A 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 CompletionHandler) except that multiple pending completions may be necessary to trigger the completion action onCompletion(CountedCompleter), not just one. Unless initialized otherwise, the pending
count starts at zero, but may be (atomically) changed using methods setPendingCount, addToPendingCount, and compareAndSetPendingCount. Upon invocation of 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 Phaser and 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. A concrete CountedCompleter class must define method compute, that should in most cases (as illustrated below), invoke tryComplete() once before returning. The class may also optionally override method onCompletion(CountedCompleter) to perform an action upon normal completion, and method onExceptionalCompletion(Throwable, CountedCompleter) to perform an action upon any exception. 
CountedCompleters most often do not bear results, in which case they are normally declared as CountedCompleter<Void>, and will always return null as a result value. In other cases, you should override method getRawResult to provide a result from 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 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. 
A CountedCompleter that does not itself have a completer (i.e., one for which getCompleter returns 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 ForkJoinTask.isDone) is arbitrary; this status changes only upon explicit invocations of complete, ForkJoinTask.cancel, ForkJoinTask.completeExceptionally(Throwable) or upon exceptional completion of method 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. 
Sample Usages.
Parallel recursive decomposition. CountedCompleters may be arranged in trees similar to those often used with RecursiveActions, 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 tasks need not block waiting to perform them. 
For example, here is an initial version of a utility method 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 their parent (because no result combination is performed, the default no-op implementation of method onCompletion is not overridden). The utility method sets up the root task and invokes it (here, implicitly using the ForkJoinPool.commonPool()). It is straightforward and reliable (but not optimal) to always set the pending count to the number of child tasks and call 
tryComplete() immediately before returning. 
 
public static <E> void forEach(E[] array, Consumer<E> action) {
  class Task extends CountedCompleter<Void> {
    final int lo, hi;
    Task(Task parent, int lo, int hi) {
      super(parent); this.lo = lo; this.hi = hi;
    }
    public void compute() {
      if (hi - lo >= 2) {
        int mid = (lo + hi) >>> 1;
        // must set pending count before fork
        setPendingCount(2);
        new Task(this, mid, hi).fork(); // right child
        new Task(this, lo, mid).fork(); // left child
      }
      else if (hi > lo)
        action.accept(array[lo]);
      tryComplete();
    }
  }
  new Task(null, 0, array.length).invoke();
 }
 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, when the last action in a task is to fork or invoke a subtask (a "tail call"), the call to 
tryComplete() can be optimized away, at the cost of making the pending count look "off by one".  
    public void compute() {
      if (hi - lo >= 2) {
        int mid = (lo + hi) >>> 1;
        setPendingCount(1); // looks off by one, but correct!
        new Task(this, mid, hi).fork(); // right child
        new Task(this, lo, mid).compute(); // direct invoke
      } else {
        if (hi > lo)
          action.accept(array[lo]);
        tryComplete();
      }
    }
 As a further optimization, notice that the left task need not even exist. Instead of creating a new one, we can continue using the original task, and add a pending count for each fork. Additionally, because no task in this tree implements an onCompletion(CountedCompleter) method, tryComplete can be replaced with propagateCompletion.  
    public void compute() {
      int n = hi - lo;
      for (; n >= 2; n /= 2) {
        addToPendingCount(1);
        new Task(this, lo + n/2, lo + n).fork();
      }
      if (n > 0)
        action.accept(array[lo]);
      propagateCompletion();
    }
When pending counts can be precomputed, they can be established in
the constructor:
 
public static <E> void forEach(E[] array, Consumer<E> action) {
  class Task extends CountedCompleter<Void> {
    final int lo, hi;
    Task(Task parent, int lo, int hi) {
      super(parent, 31 - Integer.numberOfLeadingZeros(hi - lo));
      this.lo = lo; this.hi = hi;
    }
    public void compute() {
      for (int n = hi - lo; n >= 2; n /= 2)
        new Task(this, lo + n/2, lo + n).fork();
      action.accept(array[lo]);
      propagateCompletion();
    }
  }
  if (array.length > 0)
    new Task(null, 0, array.length).invoke();
 }
Additional optimizations of such classes might entail 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.
Searching. A tree of CountedCompleters can search for a value or property in different parts of a data structure, and report a result in an AtomicReference as soon as one is found. The others can poll the result to avoid unnecessary work. (You could additionally 
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): 
 
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();
  }
 }
 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 tryComplete could be made conditional (if (result.get() == null) tryComplete();) because no further bookkeeping is required to manage completions once the root task completes. Recording subtasks. CountedCompleter tasks that combine results of multiple subtasks usually need to access these results in method onCompletion(CountedCompleter). As illustrated in the following class (that performs a simplified form of map-reduce where mappings and reductions are all of type 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 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. 
 
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();
  }
 }
 Here, method 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 
tryComplete, or (2) by any of its subtasks when they complete and decrement the pending count to zero. The caller argument distinguishes cases. Most often, when the caller is 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 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. Completion Traversals. If using onCompletion to process completions is inapplicable or inconvenient, you can use methods firstComplete and 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: 
 
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();
  }
 }
Triggers. Some CountedCompleters are themselves never
forked, but instead serve as bits of plumbing in other designs;
including those in which the completion of one or more async tasks
triggers another async task. For example:
 
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();
Author: Doug Lea
Since: 1.8/**
 * 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 completion action {@link #onCompletion(CountedCompleter)},
 * not just one.
 * Unless initialized otherwise, the {@linkplain #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 Phaser} and {@link 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(CountedCompleter)}
 * to perform an action upon normal completion, and method
 * {@link #onExceptionalCompletion(Throwable, CountedCompleter)} 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(Throwable)} 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 tasks need not block waiting to perform them.
 *
 * <p>For example, here is an initial version of a utility method 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 their parent
 * (because no result combination is performed, the default no-op
 * implementation of method {@code onCompletion} is not overridden).
 * The utility method sets up the root task and invokes it (here,
 * implicitly using the {@link ForkJoinPool#commonPool()}).  It is
 * straightforward and reliable (but not optimal) to always set the
 * pending count to the number of child tasks and call {@code
 * tryComplete()} immediately before returning.
 *
 * <pre> {@code
 * public static <E> void forEach(E[] array, Consumer<E> action) {
 *   class Task extends CountedCompleter<Void> {
 *     final int lo, hi;
 *     Task(Task parent, int lo, int hi) {
 *       super(parent); this.lo = lo; this.hi = hi;
 *     }
 *
 *     public void compute() {
 *       if (hi - lo >= 2) {
 *         int mid = (lo + hi) >>> 1;
 *         // must set pending count before fork
 *         setPendingCount(2);
 *         new Task(this, mid, hi).fork(); // right child
 *         new Task(this, lo, mid).fork(); // left child
 *       }
 *       else if (hi > lo)
 *         action.accept(array[lo]);
 *       tryComplete();
 *     }
 *   }
 *   new Task(null, 0, array.length).invoke();
 * }}</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, when the last action in a task
 * is to fork or invoke a subtask (a "tail call"), the call to {@code
 * tryComplete()} can be optimized away, at the cost of making the
 * pending count look "off by one".
 *
 * <pre> {@code
 *     public void compute() {
 *       if (hi - lo >= 2) {
 *         int mid = (lo + hi) >>> 1;
 *         setPendingCount(1); // looks off by one, but correct!
 *         new Task(this, mid, hi).fork(); // right child
 *         new Task(this, lo, mid).compute(); // direct invoke
 *       } else {
 *         if (hi > lo)
 *           action.accept(array[lo]);
 *         tryComplete();
 *       }
 *     }}</pre>
 *
 * As a further optimization, notice that the left task need not even exist.
 * Instead of creating a new one, we can continue using the original task,
 * and add a pending count for each fork.  Additionally, because no task
 * in this tree implements an {@link #onCompletion(CountedCompleter)} method,
 * {@code tryComplete} can be replaced with {@link #propagateCompletion}.
 *
 * <pre> {@code
 *     public void compute() {
 *       int n = hi - lo;
 *       for (; n >= 2; n /= 2) {
 *         addToPendingCount(1);
 *         new Task(this, lo + n/2, lo + n).fork();
 *       }
 *       if (n > 0)
 *         action.accept(array[lo]);
 *       propagateCompletion();
 *     }}</pre>
 *
 * When pending counts can be precomputed, they can be established in
 * the constructor:
 *
 * <pre> {@code
 * public static <E> void forEach(E[] array, Consumer<E> action) {
 *   class Task extends CountedCompleter<Void> {
 *     final int lo, hi;
 *     Task(Task parent, int lo, int hi) {
 *       super(parent, 31 - Integer.numberOfLeadingZeros(hi - lo));
 *       this.lo = lo; this.hi = hi;
 *     }
 *
 *     public void compute() {
 *       for (int n = hi - lo; n >= 2; n /= 2)
 *         new Task(this, lo + n/2, lo + n).fork();
 *       action.accept(array[lo]);
 *       propagateCompletion();
 *     }
 *   }
 *   if (array.length > 0)
 *     new Task(null, 0, array.length).invoke();
 * }}</pre>
 *
 * Additional optimizations of such classes might entail 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 {@code 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(CountedCompleter)}. 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 or 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 /** This task's completer, or null if none */
    final CountedCompleter<?> completer;
    The number of pending tasks until completion /** The number of pending tasks until completion */
    volatile int pending;

    Creates a new CountedCompleter with the given completer
and initial pending count.
Params: completer – this task's completer, or null if none
initialPendingCount – the initial pending count/**
     * 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.
Params: completer – this task's completer, or null if none/**
     * 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.
/**
     * 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.
/**
     * The main computation performed by this task.
     */
    public abstract void compute();

    Performs an action when method tryComplete is invoked and the pending count is zero, or when the unconditional method 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 
this, then it is typically a subtask that may contain results (and/or links to other results) to combine. 
Params: caller – the task invoking this method (which may
be this task itself)/**
     * 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 ForkJoinTask.completeExceptionally(Throwable) is invoked or method compute throws an exception, and this task has not already otherwise completed normally. On entry to this method, this task ForkJoinTask.isCompletedAbnormally. The return value of this method controls further propagation: If true and this task has a completer that has not completed, then that completer is also completed exceptionally, with the same exception as this completer. The default implementation of this method does nothing except return true. 
Params: ex – the exception
caller – the task invoking this method (which may
be this task itself)
Returns: true if this exception should be propagated to this task's completer, if one exists/**
     * Performs an action when method {@link
     * #completeExceptionally(Throwable)} is invoked or method {@link
     * #compute} throws an exception, and this task has not already
     * otherwise 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 that has not completed, then that
     * completer is also completed exceptionally, with the same
     * exception as this completer.  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 {@code 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 null if none. 
Returns: the completer/**
     * 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.
Returns: the current pending count/**
     * Returns the current pending count.
     *
     * @return the current pending count
     */
    public final int getPendingCount() {
        return pending;
    }

    Sets the pending count to the given value.
Params: count – the count/**
     * 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.
Params: delta – the value to add/**
     * Adds (atomically) the given value to the pending count.
     *
     * @param delta the value to add
     */
    public final void addToPendingCount(int delta) {
        PENDING.getAndAdd(this, delta);
    }

    Sets (atomically) the pending count to the given count only if
it currently holds the given expected value.
Params: expected – the expected value
count – the new value
Returns: true if successful/**
     * 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 {@code true} if successful
     */
    public final boolean compareAndSetPendingCount(int expected, int count) {
        return PENDING.compareAndSet(this, expected, count);
    }

    If the pending count is nonzero, (atomically) decrements it.
Returns: the initial (undecremented) pending count holding on entry
to this method/**
     * 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 &&
                     !PENDING.weakCompareAndSet(this, 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.
Returns: the root of the current computation/**
     * 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 onCompletion(CountedCompleter) and then similarly tries to complete this task's completer, if one exists, else marks this task as complete. /**
     * If the pending count is nonzero, decrements the count;
     * otherwise invokes {@link #onCompletion(CountedCompleter)}
     * 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 (PENDING.weakCompareAndSet(a, c, c - 1))
                return;
        }
    }

    Equivalent to tryComplete but does not invoke onCompletion(CountedCompleter) 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 onCompletion should not, or need not, be invoked for each completer in a computation. /**
     * Equivalent to {@link #tryComplete} but does not invoke {@link
     * #onCompletion(CountedCompleter)} 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;
        for (int c;;) {
            if ((c = a.pending) == 0) {
                if ((a = (s = a).completer) == null) {
                    s.quietlyComplete();
                    return;
                }
            }
            else if (PENDING.weakCompareAndSet(a, c, c - 1))
                return;
        }
    }

    Regardless of pending count, invokes onCompletion(CountedCompleter), marks this task as complete and further triggers tryComplete on this task's completer, if one exists. The given rawResult is used as an argument to setRawResult before invoking onCompletion(CountedCompleter) or marking this task as complete; its value is meaningful only for classes overriding setRawResult. This method does not modify the pending count. 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 
setRawResult is not overridden, this effect can be obtained more simply using quietlyCompleteRoot(). 
Params: rawResult – the raw result/**
     * Regardless of pending count, invokes
     * {@link #onCompletion(CountedCompleter)}, 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(CountedCompleter)} or marking this task
     * as complete; its value is meaningful only for classes
     * overriding {@code setRawResult}.  This method does not modify
     * the pending count.
     *
     * <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 {@link #quietlyCompleteRoot()}.
     *
     * @param rawResult the raw result
     */
    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 null. This method is designed to be used with nextComplete in completion traversal loops. 
Returns: this task, if pending count was zero, else null/**
     * 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 (PENDING.weakCompareAndSet(this, c, c - 1))
                return null;
        }
    }

    If this task does not have a completer, invokes ForkJoinTask.quietlyComplete and returns null. Or, if the completer's pending count is non-zero, decrements that pending count and returns null. Otherwise, returns the completer. This method can be used as part of a completion traversal loop for homogeneous task hierarchies:  
for (CountedCompleter<?> c = firstComplete();
     c != null;
     c = c.nextComplete()) {
  // ... process c ...
 }
Returns: the completer, or null if none/**
     * If this task does not have a completer, invokes {@link
     * ForkJoinTask#quietlyComplete} and returns {@code null}.  Or, if
     * the completer's pending count is non-zero, decrements that
     * 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 getRoot().quietlyComplete(). /**
     * Equivalent to {@code getRoot().quietlyComplete()}.
     */
    public final void quietlyCompleteRoot() {
        for (CountedCompleter<?> a = this, p;;) {
            if ((p = a.completer) == null) {
                a.quietlyComplete();
                return;
            }
            a = p;
        }
    }

    If this task has not completed, attempts to process at most the
given number of other unprocessed tasks for which this task is
on the completion path, if any are known to exist.
Params: maxTasks – the maximum number of tasks to process.  If
                less than or equal to zero, then no tasks are
                processed./**
     * If this task has not completed, attempts to process at most the
     * given number of other unprocessed tasks for which this task is
     * on the completion path, if any are known to exist.
     *
     * @param maxTasks the maximum number of tasks to process.  If
     *                 less than or equal to zero, then no tasks are
     *                 processed.
     */
    public final void helpComplete(int maxTasks) {
        Thread t; ForkJoinWorkerThread wt;
        if (maxTasks > 0 && status >= 0) {
            if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
                (wt = (ForkJoinWorkerThread)t).pool.
                    helpComplete(wt.workQueue, this, maxTasks);
            else
                ForkJoinPool.common.externalHelpComplete(this, maxTasks);
        }
    }

    Supports ForkJoinTask exception propagation.
/**
     * Supports ForkJoinTask exception propagation.
     */
    void internalPropagateException(Throwable ex) {
        CountedCompleter<?> a = this, s = a;
        while (a.onExceptionalCompletion(ex, s) &&
               (a = (s = a).completer) != null && a.status >= 0 &&
               isExceptionalStatus(a.recordExceptionalCompletion(ex)))
            ;
    }

    Implements execution conventions for CountedCompleters.
/**
     * Implements execution conventions for CountedCompleters.
     */
    protected final boolean exec() {
        compute();
        return false;
    }

    Returns the result of the computation. By default, returns null, which is appropriate for 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. 
Returns: the result of the computation/**
     * 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
     */
    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.
/**
     * 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.
     */
    protected void setRawResult(T t) { }

    // VarHandle mechanics
    private static final VarHandle PENDING;
    static {
        try {
            MethodHandles.Lookup l = MethodHandles.lookup();
            PENDING = l.findVarHandle(CountedCompleter.class, "pending", int.class);

        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(e);
        }
    }
}
Params:	ex – the exception caller – the task invoking this method (which may be this task itself)
Returns:	`true` if this exception should be propagated to this task's completer, if one exists
Params:	expected – the expected value count – the new value
Returns:	`true` if successful
/

java/ 11/ java.base/java/util/concurrent/CountedCompleter.java
