alluxio-master-2.x/core/common/src/main/java/alluxio/concurrent/jsr/CompletableFuture.java
/*
 * 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 alluxio.concurrent.jsr;

import java.util.Objects;
import java.util.concurrent.CancellationException;
import java.util.concurrent.CompletionException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.LockSupport;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;

/**
 * A {@link Future} that may be explicitly completed (setting its value and status), and may be used
 * as a {@link CompletionStage}, supporting dependent functions and actions that trigger upon its
 * completion.
 *
 * <p>
 * When two or more threads attempt to {@link #complete complete}, {@link #completeExceptionally
 * completeExceptionally}, or {@link #cancel cancel} a CompletableFuture, only one of them succeeds.
 *
 * <p>
 * In addition to these and related methods for directly manipulating status and results,
 * CompletableFuture implements interface {@link CompletionStage} with the following policies:
 * <ul>
 *
 * <li>Actions supplied for dependent completions of <em>non-async</em> methods may be performed by
 * the thread that completes the current CompletableFuture, or by any other caller of a completion
 * method.
 *
 * <li>All <em>async</em> methods without an explicit Executor argument are performed using the
 * {@link ForkJoinPool#commonPool()} (unless it does not support a parallelism level of at least
 * two, in which case, a new Thread is created to run each task). This may be overridden for
 * non-static methods in subclasses by defining method {@link #defaultExecutor()}. To simplify
 * monitoring, debugging, and tracking, all generated asynchronous tasks are instances of the marker
 * interface {@link AsynchronousCompletionTask}. Operations with time-delays can use adapter methods
 * defined in this class, for example: {@code supplyAsync(supplier, delayedExecutor(timeout,
 * timeUnit))}. To support methods with delays and timeouts, this class maintains at most one daemon
 * thread for triggering and cancelling actions, not for running them.
 *
 * <li>All CompletionStage methods are implemented independently of other public methods, so the
 * behavior of one method is not impacted by overrides of others in subclasses.
 *
 * <li>All CompletionStage methods return CompletableFutures. To restrict usages to only those
 * methods defined in interface CompletionStage, use method {@link #minimalCompletionStage}. Or to
 * ensure only that clients do not themselves modify a future, use method {@link #copy}.
 * </ul>
 *
 * <p>
 * CompletableFuture also implements {@link Future} with the following policies:
 * <ul>
 *
 * <li>Since (unlike {@link FutureTask}) this class has no direct control over the computation that
 * causes it to be completed, cancellation is treated as just another form of exceptional
 * completion. Method {@link #cancel cancel} has the same effect as
 * {@code completeExceptionally(new CancellationException())}. Method
 * {@link #isCompletedExceptionally} can be used to determine if a CompletableFuture completed in
 * any exceptional fashion.
 *
 * <li>In case of exceptional completion with a CompletionException, methods {@link #get()} and
 * {@link #get(long, TimeUnit)} throw an {@link ExecutionException} with the same cause as held in
 * the corresponding CompletionException. To simplify usage in most contexts, this class also
 * defines methods {@link #join()} and {@link #getNow} that instead throw the CompletionException
 * directly in these cases.
 * </ul>
 *
 * <p>
 * Arguments used to pass a completion result (that is, for parameters of type {@code T}) for
 * methods accepting them may be null, but passing a null value for any other parameter will result
 * in a {@link NullPointerException} being thrown.
 *
 * <p>
 * Subclasses of this class should normally override the "virtual constructor" method
 * {@link #newIncompleteFuture}, which establishes the concrete type returned by CompletionStage
 * methods. For example, here is a class that substitutes a different default Executor and disables
 * the {@code obtrude} methods:
 *
 * <pre>
 *  {@code
 * class MyCompletableFuture<T> extends CompletableFuture<T> {
 *   static final Executor myExecutor = ...;
 *   public MyCompletableFuture() { }
 *   public <U> CompletableFuture<U> newIncompleteFuture() {
 *     return new MyCompletableFuture<U>(); }
 *   public Executor defaultExecutor() {
 *     return myExecutor; }
 *   public void obtrudeValue(T value) {
 *     throw new UnsupportedOperationException(); }
 *   public void obtrudeException(Throwable ex) {
 *     throw new UnsupportedOperationException(); }
 * }}
 * </pre>
 *
 * @author Doug Lea
 * @param <T> The result type returned by this future's {@code join} and {@code get} methods
 * @since 1.8
 */
public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
  // CVS rev. 1.212
  /*
   * Overview:
   *
   * A CompletableFuture may have dependent completion actions, collected in a linked stack. It
   * atomically completes by CASing a result field, and then pops off and runs those actions. This
   * applies across normal vs exceptional outcomes, sync vs async actions, binary triggers, and
   * various forms of completions.
   *
   * Non-nullness of volatile field "result" indicates done. It may be set directly if known to be
   * thread-confined, else via CAS. An AltResult is used to box null as a result, as well as to hold
   * exceptions. Using a single field makes completion simple to detect and trigger. Result encoding
   * and decoding is straightforward but tedious and adds to the sprawl of trapping and associating
   * exceptions with targets. Minor simplifications rely on (static) NIL (to box null results) being
   * the only AltResult with a null exception field, so we don't usually need explicit comparisons.
   * Even though some of the generics casts are unchecked (see SuppressWarnings annotations), they
   * are placed to be appropriate even if checked.
   *
   * Dependent actions are represented by Completion objects linked as Treiber stacks headed by
   * field "stack". There are Completion classes for each kind of action, grouped into: -
   * single-input (UniCompletion), - two-input (BiCompletion), - projected (BiCompletions using
   * exactly one of two inputs), - shared (CoCompletion, used by the second of two sources), -
   * zero-input source actions, - Signallers that unblock waiters. Class Completion extends
   * ForkJoinTask to enable async execution (adding no space overhead because we exploit its "tag"
   * methods to maintain claims). It is also declared as Runnable to allow usage with arbitrary
   * executors.
   *
   * Support for each kind of CompletionStage relies on a separate class, along with two
   * CompletableFuture methods:
   *
   * * A Completion class with name X corresponding to function, prefaced with "Uni", "Bi", or "Or".
   * Each class contains fields for source(s), actions, and dependent. They are boringly similar,
   * differing from others only with respect to underlying functional forms. We do this so that
   * users don't encounter layers of adapters in common usages.
   *
   * * Boolean CompletableFuture method x(...) (for example biApply) takes all of the arguments
   * needed to check that an action is triggerable, and then either runs the action or arranges its
   * async execution by executing its Completion argument, if present. The method returns true if
   * known to be complete.
   *
   * * Completion method tryFire(int mode) invokes the associated x method with its held arguments,
   * and on success cleans up. The mode argument allows tryFire to be called twice (SYNC, then
   * ASYNC); the first to screen and trap exceptions while arranging to execute, and the second when
   * called from a task. (A few classes are not used async so take slightly different forms.) The
   * claim() callback suppresses function invocation if already claimed by another thread.
   *
   * * Some classes (for example UniApply) have separate handling code for when known to be
   * thread-confined ("now" methods) and for when shared (in tryFire), for efficiency.
   *
   * * CompletableFuture method xStage(...) is called from a public stage method of
   * CompletableFuture f. It screens user arguments and invokes and/or creates the stage object. If
   * not async and already triggerable, the action is run immediately. Otherwise a Completion c is
   * created, and submitted to the executor if triggerable, or pushed onto f's stack if not.
   * Completion actions are started via c.tryFire. We recheck after pushing to a source future's
   * stack to cover possible races if the source completes while pushing. Classes with two inputs
   * (for example BiApply) deal with races across both while pushing actions. The second completion
   * is a CoCompletion pointing to the first, shared so that at most one performs the action. The
   * multiple-arity methods allOf does this pairwise to form trees of completions. Method anyOf is
   * handled differently from allOf because completion of any source should trigger a cleanStack of
   * other sources. Each AnyOf completion can reach others via a shared array.
   *
   * Note that the generic type parameters of methods vary according to whether "this" is a source,
   * dependent, or completion.
   *
   * Method postComplete is called upon completion unless the target is guaranteed not to be
   * observable (i.e., not yet returned or linked). Multiple threads can call postComplete, which
   * atomically pops each dependent action, and tries to trigger it via method tryFire, in NESTED
   * mode. Triggering can propagate recursively, so NESTED mode returns its completed dependent (if
   * one exists) for further processing by its caller (see method postFire).
   *
   * Blocking methods get() and join() rely on Signaller Completions that wake up waiting threads.
   * The mechanics are similar to Treiber stack wait-nodes used in FutureTask, Phaser, and
   * SynchronousQueue. See their internal documentation for algorithmic details.
   *
   * Without precautions, CompletableFutures would be prone to garbage accumulation as chains of
   * Completions build up, each pointing back to its sources. So we null out fields as soon as
   * possible. The screening checks needed anyway harmlessly ignore null arguments that may have
   * been obtained during races with threads nulling out fields. We also try to unlink non-isLive
   * (fired or cancelled) Completions from stacks that might otherwise never be popped: Method
   * cleanStack always unlinks non isLive completions from the head of stack; others may
   * occasionally remain if racing with other cancellations or removals.
   *
   * Completion fields need not be declared as final or volatile because they are only visible to
   * other threads upon safe publication.
   */

  /** The encoding of the null value. */
  static final AltResult NIL = new AltResult(null);
  // Modes for Completion.tryFire. Signedness matters.
  static final int SYNC = 0;
  static final int ASYNC = 1;
  static final int NESTED = -1;
  private static final boolean USE_COMMON_POOL = (ForkJoinPool.getCommonPoolParallelism() > 1);
  /**
   * Default executor -- ForkJoinPool.commonPool() unless it cannot support parallelism.
   */
  private static final Executor ASYNC_POOL =
      USE_COMMON_POOL ? ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();

  /* ------------- Encoding and decoding outcomes -------------- */
  // Unsafe mechanics
  private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
  private static final long RESULT;
  private static final long STACK;
  private static final long NEXT;

  static {
    try {
      RESULT = U.objectFieldOffset(CompletableFuture.class.getDeclaredField("result"));
      STACK = U.objectFieldOffset(CompletableFuture.class.getDeclaredField("stack"));
      NEXT = U.objectFieldOffset(Completion.class.getDeclaredField("next"));
    } catch (Exception e) {
      throw new ExceptionInInitializerError(e);
    }

    // Reduce the risk of rare disastrous classloading in first call to
    // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
    @SuppressWarnings("unused")
    Class<?> ensureLoaded = LockSupport.class;
  }

  volatile Object result; // Either the result or boxed AltResult
  volatile Completion stack; // Top of Treiber stack of dependent actions

  /**
   * Creates a new incomplete CompletableFuture.
   */
  public CompletableFuture() {}

  /**
   * Creates a new complete CompletableFuture with given encoded result.
   */
  CompletableFuture(Object r) {
    this.result = r;
  }

  /**
   * Returns the encoding of the given (non-null) exception as a wrapped CompletionException unless
   * it is one already.
   */
  static AltResult encodeThrowable(Throwable x) {
    return new AltResult((x instanceof CompletionException) ? x : new CompletionException(x));
  }

  /**
   * Returns the encoding of the given (non-null) exception as a wrapped CompletionException unless
   * it is one already. May return the given Object r (which must have been the result of a source
   * future) if it is equivalent, i.e. if this is a simple relay of an existing CompletionException.
   */
  static Object encodeThrowable(Throwable x, Object r) {
    if (!(x instanceof CompletionException))
      x = new CompletionException(x);
    else if (r instanceof AltResult && x == ((AltResult) r).ex)
      return r;
    return new AltResult(x);
  }

  /**
   * Returns the encoding of a copied outcome; if exceptional, rewraps as a CompletionException,
   * else returns argument.
   */
  static Object encodeRelay(Object r) {
    Throwable x;
    if (r instanceof AltResult && (x = ((AltResult) r).ex) != null
        && !(x instanceof CompletionException))
      r = new AltResult(new CompletionException(x));
    return r;
  }

  /**
   * Reports result using Future.get conventions.
   */
  private static Object reportGet(Object r) throws InterruptedException, ExecutionException {
    if (r == null) // by convention below, null means interrupted
      throw new InterruptedException();
    if (r instanceof AltResult) {
      Throwable x, cause;
      if ((x = ((AltResult) r).ex) == null)
        return null;
      if (x instanceof CancellationException)
        throw (CancellationException) x;
      if ((x instanceof CompletionException) && (cause = x.getCause()) != null)
        x = cause;
      throw new ExecutionException(x);
    }
    return r;
  }

  /**
   * Decodes outcome to return result or throw unchecked exception.
   */
  private static Object reportJoin(Object r) {
    if (r instanceof AltResult) {
      Throwable x;
      if ((x = ((AltResult) r).ex) == null)
        return null;
      if (x instanceof CancellationException)
        throw (CancellationException) x;
      if (x instanceof CompletionException)
        throw (CompletionException) x;
      throw new CompletionException(x);
    }
    return r;
  }

  /* ------------- Async task preliminaries -------------- */

  /**
   * Null-checks user executor argument, and translates uses of commonPool to ASYNC_POOL in case
   * parallelism disabled.
   */
  static Executor screenExecutor(Executor e) {
    if (!USE_COMMON_POOL && e == ForkJoinPool.commonPool())
      return ASYNC_POOL;
    return Objects.requireNonNull(e);
  }

  static void lazySetNext(Completion c, Completion next) {
    U.putOrderedObject(c, NEXT, next);
  }

  static boolean casNext(Completion c, Completion cmp, Completion val) {
    return U.compareAndSwapObject(c, NEXT, cmp, val);
  }

  private static <U, T extends U> CompletableFuture<U> uniCopyStage(CompletableFuture<T> src) {
    Object r;
    CompletableFuture<U> d = src.newIncompleteFuture();
    if ((r = src.result) != null)
      d.result = encodeRelay(r);
    else
      src.unipush(new UniRelay<U, T>(d, src));
    return d;
  }

  /** Recursively constructs a tree of completions. */
  static CompletableFuture<Void> andTree(CompletableFuture<?>[] cfs, int lo, int hi) {
    CompletableFuture<Void> d = new CompletableFuture<Void>();
    if (lo > hi) // empty
      d.result = NIL;
    else {
      CompletableFuture<?> a, b;
      Object r, s, z;
      Throwable x;
      int mid = (lo + hi) >>> 1;
      if ((a = (lo == mid ? cfs[lo] : andTree(cfs, lo, mid))) == null
          || (b = (lo == hi ? a : (hi == mid + 1) ? cfs[hi] : andTree(cfs, mid + 1, hi))) == null)
        throw new NullPointerException();
      if ((r = a.result) == null || (s = b.result) == null)
        a.bipush(b, new BiRelay(d, a, b));
      else if ((r instanceof AltResult && (x = ((AltResult) (z = r)).ex) != null)
          || (s instanceof AltResult && (x = ((AltResult) (z = s)).ex) != null))
        d.result = encodeThrowable(x, z);
      else
        d.result = NIL;
    }
    return d;
  }

  static <U> CompletableFuture<U> asyncSupplyStage(Executor e, Supplier<U> f) {
    Objects.requireNonNull(f);
    CompletableFuture<U> d = new CompletableFuture<U>();
    e.execute(new AsyncSupply<U>(d, f));
    return d;
  }

  static CompletableFuture<Void> asyncRunStage(Executor e, Runnable f) {
    Objects.requireNonNull(f);
    CompletableFuture<Void> d = new CompletableFuture<Void>();
    e.execute(new AsyncRun(d, f));
    return d;
  }

  /**
   * Returns a new CompletableFuture that is asynchronously completed by a task running in the
   * {@link ForkJoinPool#commonPool()} with the value obtained by calling the given Supplier.
   *
   * @param supplier a function returning the value to be used to complete the returned
   *        CompletableFuture
   * @param <U> the function's return type
   * @return the new CompletableFuture
   */
  public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {
    return asyncSupplyStage(ASYNC_POOL, supplier);
  }

  /* ------------- Base Completion classes and operations -------------- */

  /**
   * Returns a new CompletableFuture that is asynchronously completed by a task running in the given
   * executor with the value obtained by calling the given Supplier.
   *
   * @param supplier a function returning the value to be used to complete the returned
   *        CompletableFuture
   * @param executor the executor to use for asynchronous execution
   * @param <U> the function's return type
   * @return the new CompletableFuture
   */
  public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor) {
    return asyncSupplyStage(screenExecutor(executor), supplier);
  }

  /**
   * Returns a new CompletableFuture that is asynchronously completed by a task running in the
   * {@link ForkJoinPool#commonPool()} after it runs the given action.
   *
   * @param runnable the action to run before completing the returned CompletableFuture
   * @return the new CompletableFuture
   */
  public static CompletableFuture<Void> runAsync(Runnable runnable) {
    return asyncRunStage(ASYNC_POOL, runnable);
  }

  /**
   * Returns a new CompletableFuture that is asynchronously completed by a task running in the given
   * executor after it runs the given action.
   *
   * @param runnable the action to run before completing the returned CompletableFuture
   * @param executor the executor to use for asynchronous execution
   * @return the new CompletableFuture
   */
  public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor) {
    return asyncRunStage(screenExecutor(executor), runnable);
  }

  /**
   * Returns a new CompletableFuture that is already completed with the given value.
   *
   * @param value the value
   * @param <U> the type of the value
   * @return the completed CompletableFuture
   */
  public static <U> CompletableFuture<U> completedFuture(U value) {
    return new CompletableFuture<U>((value == null) ? NIL : value);
  }

  /**
   * Returns a new CompletableFuture that is completed when all of the given CompletableFutures
   * complete. If any of the given CompletableFutures complete exceptionally, then the returned
   * CompletableFuture also does so, with a CompletionException holding this exception as its cause.
   * Otherwise, the results, if any, of the given CompletableFutures are not reflected in the
   * returned CompletableFuture, but may be obtained by inspecting them individually. If no
   * CompletableFutures are provided, returns a CompletableFuture completed with the value
   * {@code null}.
   *
   * <p>
   * Among the applications of this method is to await completion of a set of independent
   * CompletableFutures before continuing a program, as in: {@code CompletableFuture.allOf(c1, c2,
   * c3).join();}.
   *
   * @param cfs the CompletableFutures
   * @return a new CompletableFuture that is completed when all of the given CompletableFutures
   *         complete
   * @throws NullPointerException if the array or any of its elements are {@code null}
   */
  public static CompletableFuture<Void> allOf(CompletableFuture<?>... cfs) {
    return andTree(cfs, 0, cfs.length - 1);
  }

  /* ------------- One-input Completions -------------- */

  /**
   * Returns a new CompletableFuture that is completed when any of the given CompletableFutures
   * complete, with the same result. Otherwise, if it completed exceptionally, the returned
   * CompletableFuture also does so, with a CompletionException holding this exception as its cause.
   * If no CompletableFutures are provided, returns an incomplete CompletableFuture.
   *
   * @param cfs the CompletableFutures
   * @return a new CompletableFuture that is completed with the result or exception of any of the
   *         given CompletableFutures when one completes
   * @throws NullPointerException if the array or any of its elements are {@code null}
   */
  public static CompletableFuture<Object> anyOf(CompletableFuture<?>... cfs) {
    int n;
    Object r;
    if ((n = cfs.length) <= 1)
      return (n == 0) ? new CompletableFuture<Object>()
          : (CompletableFuture<Object>) uniCopyStage(cfs[0]);
    for (CompletableFuture<?> cf : cfs)
      if ((r = cf.result) != null)
        return new CompletableFuture<Object>(encodeRelay(r));
    cfs = cfs.clone();
    CompletableFuture<Object> d = new CompletableFuture<Object>();
    for (CompletableFuture<?> cf : cfs)
      cf.unipush(new AnyOf(d, cf, cfs));
    // If d was completed while we were adding completions, we should
    // clean the stack of any sources that may have had completions
    // pushed on their stack after d was completed.
    if (d.result != null)
      for (int i = 0, len = cfs.length; i < len; i++)
        if (cfs[i].result != null)
          for (i++; i < len; i++)
            if (cfs[i].result == null)
              cfs[i].cleanStack();
    return d;
  }

  /**
   * Returns a new Executor that submits a task to the given base executor after the given delay (or
   * no delay if non-positive). Each delay commences upon invocation of the returned executor's
   * {@code execute} method.
   *
   * @param delay how long to delay, in units of {@code unit}
   * @param unit a {@code TimeUnit} determining how to interpret the {@code delay} parameter
   * @param executor the base executor
   * @return the new delayed executor
   * @since 9
   */
  public static Executor delayedExecutor(long delay, TimeUnit unit, Executor executor) {
    if (unit == null || executor == null)
      throw new NullPointerException();
    return new DelayedExecutor(delay, unit, executor);
  }

  /**
   * Returns a new Executor that submits a task to the default executor after the given delay (or no
   * delay if non-positive). Each delay commences upon invocation of the returned executor's
   * {@code execute} method.
   *
   * @param delay how long to delay, in units of {@code unit}
   * @param unit a {@code TimeUnit} determining how to interpret the {@code delay} parameter
   * @return the new delayed executor
   * @since 9
   */
  public static Executor delayedExecutor(long delay, TimeUnit unit) {
    return new DelayedExecutor(delay, Objects.requireNonNull(unit), ASYNC_POOL);
  }

  /**
   * Returns a new CompletionStage that is already completed with the given value and supports only
   * those methods in interface {@link CompletionStage}.
   *
   * @param value the value
   * @param <U> the type of the value
   * @return the completed CompletionStage
   * @since 9
   */
  public static <U> CompletionStage<U> completedStage(U value) {
    return new MinimalStage<U>((value == null) ? NIL : value);
  }

  /**
   * Returns a new CompletableFuture that is already completed exceptionally with the given
   * exception.
   *
   * @param ex the exception
   * @param <U> the type of the value
   * @return the exceptionally completed CompletableFuture
   * @since 9
   */
  public static <U> CompletableFuture<U> failedFuture(Throwable ex) {
    return new CompletableFuture<U>(new AltResult(Objects.requireNonNull(ex)));
  }

  /**
   * Returns a new CompletionStage that is already completed exceptionally with the given exception
   * and supports only those methods in interface {@link CompletionStage}.
   *
   * @param ex the exception
   * @param <U> the type of the value
   * @return the exceptionally completed CompletionStage
   * @since 9
   */
  public static <U> CompletionStage<U> failedStage(Throwable ex) {
    return new MinimalStage<U>(new AltResult(Objects.requireNonNull(ex)));
  }

  final boolean internalComplete(Object r) { // CAS from null to r
    return U.compareAndSwapObject(this, RESULT, null, r);
  }

  final boolean casStack(Completion cmp, Completion val) {
    return U.compareAndSwapObject(this, STACK, cmp, val);
  }

  /** Returns true if successfully pushed c onto stack. */
  final boolean tryPushStack(Completion c) {
    Completion h = stack;
    lazySetNext(c, h);
    return U.compareAndSwapObject(this, STACK, h, c);
  }

  /** Unconditionally pushes c onto stack, retrying if necessary. */
  final void pushStack(Completion c) {
    do {
    } while (!tryPushStack(c));
  }

  /** Completes with the null value, unless already completed. */
  final boolean completeNull() {
    return U.compareAndSwapObject(this, RESULT, null, NIL);
  }

  /** Returns the encoding of the given non-exceptional value. */
  final Object encodeValue(T t) {
    return (t == null) ? NIL : t;
  }

  /** Completes with a non-exceptional result, unless already completed. */
  final boolean completeValue(T t) {
    return U.compareAndSwapObject(this, RESULT, null, (t == null) ? NIL : t);
  }

  /** Completes with an exceptional result, unless already completed. */
  final boolean completeThrowable(Throwable x) {
    return U.compareAndSwapObject(this, RESULT, null, encodeThrowable(x));
  }

  /**
   * Completes with the given (non-null) exceptional result as a wrapped CompletionException unless
   * it is one already, unless already completed. May complete with the given Object r (which must
   * have been the result of a source future) if it is equivalent, i.e. if this is a simple
   * propagation of an existing CompletionException.
   */
  final boolean completeThrowable(Throwable x, Object r) {
    return U.compareAndSwapObject(this, RESULT, null, encodeThrowable(x, r));
  }

  /**
   * Returns the encoding of the given arguments: if the exception is non-null, encodes as
   * AltResult. Otherwise uses the given value, boxed as NIL if null.
   */
  Object encodeOutcome(T t, Throwable x) {
    return (x == null) ? (t == null) ? NIL : t : encodeThrowable(x);
  }

  /**
   * Completes with r or a copy of r, unless already completed. If exceptional, r is first coerced
   * to a CompletionException.
   */
  final boolean completeRelay(Object r) {
    return U.compareAndSwapObject(this, RESULT, null, encodeRelay(r));
  }

  /**
   * Pops and tries to trigger all reachable dependents. Call only when known to be done.
   */
  final void postComplete() {
    /*
     * On each step, variable f holds current dependents to pop and run. It is extended along only
     * one path at a time, pushing others to avoid unbounded recursion.
     */
    CompletableFuture<?> f = this;
    Completion h;
    while ((h = f.stack) != null || (f != this && (h = (f = this).stack) != null)) {
      CompletableFuture<?> d;
      Completion t;
      if (f.casStack(h, t = h.next)) {
        if (t != null) {
          if (f != this) {
            pushStack(h);
            continue;
          }
          casNext(h, t, null); // try to detach
        }
        f = (d = h.tryFire(NESTED)) == null ? this : d;
      }
    }
  }

  /** Traverses stack and unlinks one or more dead Completions, if found. */
  final void cleanStack() {
    boolean unlinked = false;
    Completion p;
    while ((p = stack) != null && !p.isLive()) // ensure head of stack live
      unlinked = casStack(p, p.next);
    if (p != null && !unlinked) { // try to unlink first nonlive
      for (Completion q = p.next; q != null;) {
        Completion s = q.next;
        if (q.isLive()) {
          p = q;
          q = s;
        } else {
          casNext(p, q, s);
          break;
        }
      }
    }
  }

  /**
   * Pushes the given completion unless it completes while trying. Caller should first check that
   * result is null.
   */
  final void unipush(Completion c) {
    if (c != null) {
      while (!tryPushStack(c)) {
        if (result != null) {
          lazySetNext(c, null);
          break;
        }
      }
      if (result != null)
        c.tryFire(SYNC);
    }
  }

  /**
   * Post-processing by dependent after successful UniCompletion tryFire. Tries to clean stack of
   * source a, and then either runs postComplete or returns this to caller, depending on mode.
   */
  final CompletableFuture<T> postFire(CompletableFuture<?> a, int mode) {
    if (a != null && a.stack != null) {
      Object r;
      if ((r = a.result) == null)
        a.cleanStack();
      if (mode >= 0 && (r != null || a.result != null))
        a.postComplete();
    }
    if (result != null && stack != null) {
      if (mode < 0)
        return this;
      else
        postComplete();
    }
    return null;
  }

  private <V> CompletableFuture<V> uniApplyStage(Executor e, Function<? super T, ? extends V> f) {
    Objects.requireNonNull(f);
    Object r;
    if ((r = result) != null)
      return uniApplyNow(r, e, f);
    CompletableFuture<V> d = newIncompleteFuture();
    unipush(new UniApply<T, V>(e, d, this, f));
    return d;
  }

  private <V> CompletableFuture<V> uniApplyNow(Object r, Executor e,
      Function<? super T, ? extends V> f) {
    Throwable x;
    CompletableFuture<V> d = newIncompleteFuture();
    if (r instanceof AltResult) {
      if ((x = ((AltResult) r).ex) != null) {
        d.result = encodeThrowable(x, r);
        return d;
      }
      r = null;
    }
    try {
      if (e != null) {
        e.execute(new UniApply<T, V>(null, d, this, f));
      } else {
        @SuppressWarnings("unchecked")
        T t = (T) r;
        d.result = d.encodeValue(f.apply(t));
      }
    } catch (Throwable ex) {
      d.result = encodeThrowable(ex);
    }
    return d;
  }

  private CompletableFuture<Void> uniAcceptStage(Executor e, Consumer<? super T> f) {
    Objects.requireNonNull(f);
    Object r;
    if ((r = result) != null)
      return uniAcceptNow(r, e, f);
    CompletableFuture<Void> d = newIncompleteFuture();
    unipush(new UniAccept<T>(e, d, this, f));
    return d;
  }

  private CompletableFuture<Void> uniAcceptNow(Object r, Executor e, Consumer<? super T> f) {
    Throwable x;
    CompletableFuture<Void> d = newIncompleteFuture();
    if (r instanceof AltResult) {
      if ((x = ((AltResult) r).ex) != null) {
        d.result = encodeThrowable(x, r);
        return d;
      }
      r = null;
    }
    try {
      if (e != null) {
        e.execute(new UniAccept<T>(null, d, this, f));
      } else {
        @SuppressWarnings("unchecked")
        T t = (T) r;
        f.accept(t);
        d.result = NIL;
      }
    } catch (Throwable ex) {
      d.result = encodeThrowable(ex);
    }
    return d;
  }

  private CompletableFuture<Void> uniRunStage(Executor e, Runnable f) {
    Objects.requireNonNull(f);
    Object r;
    if ((r = result) != null)
      return uniRunNow(r, e, f);
    CompletableFuture<Void> d = newIncompleteFuture();
    unipush(new UniRun<T>(e, d, this, f));
    return d;
  }

  /* ------------- Two-input Completions -------------- */

  private CompletableFuture<Void> uniRunNow(Object r, Executor e, Runnable f) {
    Throwable x;
    CompletableFuture<Void> d = newIncompleteFuture();
    if (r instanceof AltResult && (x = ((AltResult) r).ex) != null)
      d.result = encodeThrowable(x, r);
    else
      try {
        if (e != null) {
          e.execute(new UniRun<T>(null, d, this, f));
        } else {
          f.run();
          d.result = NIL;
        }
      } catch (Throwable ex) {
        d.result = encodeThrowable(ex);
      }
    return d;
  }

  final boolean uniWhenComplete(Object r, BiConsumer<? super T, ? super Throwable> f,
      UniWhenComplete<T> c) {
    T t;
    Throwable x = null;
    if (result == null) {
      try {
        if (c != null && !c.claim())
          return false;
        if (r instanceof AltResult) {
          x = ((AltResult) r).ex;
          t = null;
        } else {
          @SuppressWarnings("unchecked")
          T tr = (T) r;
          t = tr;
        }
        f.accept(t, x);
        if (x == null) {
          internalComplete(r);
          return true;
        }
      } catch (Throwable ex) {
        if (x == null) {
          x = ex;
        } else if (x != ex) {
          x.addSuppressed(ex);
        }
      }
      completeThrowable(x, r);
    }
    return true;
  }

  private CompletableFuture<T> uniWhenCompleteStage(Executor e,
      BiConsumer<? super T, ? super Throwable> f) {
    Objects.requireNonNull(f);
    CompletableFuture<T> d = newIncompleteFuture();
    Object r;
    if ((r = result) == null)
      unipush(new UniWhenComplete<T>(e, d, this, f));
    else if (e == null)
      d.uniWhenComplete(r, f, null);
    else {
      try {
        e.execute(new UniWhenComplete<T>(null, d, this, f));
      } catch (Throwable ex) {
        d.result = encodeThrowable(ex);
      }
    }
    return d;
  }

  final <S> boolean uniHandle(Object r, BiFunction<? super S, Throwable, ? extends T> f,
      UniHandle<S, T> c) {
    S s;
    Throwable x;
    if (result == null) {
      try {
        if (c != null && !c.claim())
          return false;
        if (r instanceof AltResult) {
          x = ((AltResult) r).ex;
          s = null;
        } else {
          x = null;
          @SuppressWarnings("unchecked")
          S ss = (S) r;
          s = ss;
        }
        completeValue(f.apply(s, x));
      } catch (Throwable ex) {
        completeThrowable(ex);
      }
    }
    return true;
  }

  private <V> CompletableFuture<V> uniHandleStage(Executor e,
      BiFunction<? super T, Throwable, ? extends V> f) {
    Objects.requireNonNull(f);
    CompletableFuture<V> d = newIncompleteFuture();
    Object r;
    if ((r = result) == null)
      unipush(new UniHandle<T, V>(e, d, this, f));
    else if (e == null)
      d.uniHandle(r, f, null);
    else {
      try {
        e.execute(new UniHandle<T, V>(null, d, this, f));
      } catch (Throwable ex) {
        d.result = encodeThrowable(ex);
      }
    }
    return d;
  }

  final boolean uniExceptionally(Object r, Function<? super Throwable, ? extends T> f,
      UniExceptionally<T> c) {
    Throwable x;
    if (result == null) {
      try {
        if (r instanceof AltResult && (x = ((AltResult) r).ex) != null) {
          if (c != null && !c.claim())
            return false;
          completeValue(f.apply(x));
        } else
          internalComplete(r);
      } catch (Throwable ex) {
        completeThrowable(ex);
      }
    }
    return true;
  }

  private CompletableFuture<T> uniExceptionallyStage(Function<Throwable, ? extends T> f) {
    Objects.requireNonNull(f);
    CompletableFuture<T> d = newIncompleteFuture();
    Object r;
    if ((r = result) == null)
      unipush(new UniExceptionally<T>(d, this, f));
    else
      d.uniExceptionally(r, f, null);
    return d;
  }

  private MinimalStage<T> uniAsMinimalStage() {
    Object r;
    if ((r = result) != null)
      return new MinimalStage<T>(encodeRelay(r));
    MinimalStage<T> d = new MinimalStage<T>();
    unipush(new UniRelay<T, T>(d, this));
    return d;
  }

  private <V> CompletableFuture<V> uniComposeStage(Executor e,
      Function<? super T, ? extends CompletionStage<V>> f) {
    Objects.requireNonNull(f);
    CompletableFuture<V> d = newIncompleteFuture();
    Object r, s;
    Throwable x;
    if ((r = result) == null)
      unipush(new UniCompose<T, V>(e, d, this, f));
    else if (e == null) {
      if (r instanceof AltResult) {
        if ((x = ((AltResult) r).ex) != null) {
          d.result = encodeThrowable(x, r);
          return d;
        }
        r = null;
      }
      try {
        @SuppressWarnings("unchecked")
        T t = (T) r;
        CompletableFuture<V> g = f.apply(t).toCompletableFuture();
        if ((s = g.result) != null)
          d.result = encodeRelay(s);
        else {
          g.unipush(new UniRelay<V, V>(d, g));
        }
      } catch (Throwable ex) {
        d.result = encodeThrowable(ex);
      }
    } else
      try {
        e.execute(new UniCompose<T, V>(null, d, this, f));
      } catch (Throwable ex) {
        d.result = encodeThrowable(ex);
      }
    return d;
  }

  /**
   * Pushes completion to this and b unless both done. Caller should first check that either result
   * or b.result is null.
   */
  final void bipush(CompletableFuture<?> b, BiCompletion<?, ?, ?> c) {
    if (c != null) {
      while (result == null) {
        if (tryPushStack(c)) {
          if (b.result == null)
            b.unipush(new CoCompletion(c));
          else if (result != null)
            c.tryFire(SYNC);
          return;
        }
      }
      b.unipush(c);
    }
  }

  /** Post-processing after successful BiCompletion tryFire. */
  final CompletableFuture<T> postFire(CompletableFuture<?> a, CompletableFuture<?> b, int mode) {
    if (b != null && b.stack != null) { // clean second source
      Object r;
      if ((r = b.result) == null)
        b.cleanStack();
      if (mode >= 0 && (r != null || b.result != null))
        b.postComplete();
    }
    return postFire(a, mode);
  }

  final <R, S> boolean biApply(Object r, Object s, BiFunction<? super R, ? super S, ? extends T> f,
      BiApply<R, S, T> c) {
    Throwable x;
    tryComplete: if (result == null) {
      if (r instanceof AltResult) {
        if ((x = ((AltResult) r).ex) != null) {
          completeThrowable(x, r);
          break tryComplete;
        }
        r = null;
      }
      if (s instanceof AltResult) {
        if ((x = ((AltResult) s).ex) != null) {
          completeThrowable(x, s);
          break tryComplete;
        }
        s = null;
      }
      try {
        if (c != null && !c.claim())
          return false;
        @SuppressWarnings("unchecked")
        R rr = (R) r;
        @SuppressWarnings("unchecked")
        S ss = (S) s;
        completeValue(f.apply(rr, ss));
      } catch (Throwable ex) {
        completeThrowable(ex);
      }
    }
    return true;
  }

  private <U, V> CompletableFuture<V> biApplyStage(Executor e, CompletionStage<U> o,
      BiFunction<? super T, ? super U, ? extends V> f) {
    CompletableFuture<U> b;
    Object r, s;
    if (f == null || (b = o.toCompletableFuture()) == null)
      throw new NullPointerException();
    CompletableFuture<V> d = newIncompleteFuture();
    if ((r = result) == null || (s = b.result) == null)
      bipush(b, new BiApply<T, U, V>(e, d, this, b, f));
    else if (e == null)
      d.biApply(r, s, f, null);
    else
      try {
        e.execute(new BiApply<T, U, V>(null, d, this, b, f));
      } catch (Throwable ex) {
        d.result = encodeThrowable(ex);
      }
    return d;
  }

  final <R, S> boolean biAccept(Object r, Object s, BiConsumer<? super R, ? super S> f,
      BiAccept<R, S> c) {
    Throwable x;
    tryComplete: if (result == null) {
      if (r instanceof AltResult) {
        if ((x = ((AltResult) r).ex) != null) {
          completeThrowable(x, r);
          break tryComplete;
        }
        r = null;
      }
      if (s instanceof AltResult) {
        if ((x = ((AltResult) s).ex) != null) {
          completeThrowable(x, s);
          break tryComplete;
        }
        s = null;
      }
      try {
        if (c != null && !c.claim())
          return false;
        @SuppressWarnings("unchecked")
        R rr = (R) r;
        @SuppressWarnings("unchecked")
        S ss = (S) s;
        f.accept(rr, ss);
        completeNull();
      } catch (Throwable ex) {
        completeThrowable(ex);
      }
    }
    return true;
  }

  private <U> CompletableFuture<Void> biAcceptStage(Executor e, CompletionStage<U> o,
      BiConsumer<? super T, ? super U> f) {
    CompletableFuture<U> b;
    Object r, s;
    if (f == null || (b = o.toCompletableFuture()) == null)
      throw new NullPointerException();
    CompletableFuture<Void> d = newIncompleteFuture();
    if ((r = result) == null || (s = b.result) == null)
      bipush(b, new BiAccept<T, U>(e, d, this, b, f));
    else if (e == null)
      d.biAccept(r, s, f, null);
    else
      try {
        e.execute(new BiAccept<T, U>(null, d, this, b, f));
      } catch (Throwable ex) {
        d.result = encodeThrowable(ex);
      }
    return d;
  }

  /* ------------- Projected (Ored) BiCompletions -------------- */

  final boolean biRun(Object r, Object s, Runnable f, BiRun<?, ?> c) {
    Throwable x;
    Object z;
    if (result == null) {
      if ((r instanceof AltResult && (x = ((AltResult) (z = r)).ex) != null)
          || (s instanceof AltResult && (x = ((AltResult) (z = s)).ex) != null))
        completeThrowable(x, z);
      else
        try {
          if (c != null && !c.claim())
            return false;
          f.run();
          completeNull();
        } catch (Throwable ex) {
          completeThrowable(ex);
        }
    }
    return true;
  }

  private CompletableFuture<Void> biRunStage(Executor e, CompletionStage<?> o, Runnable f) {
    CompletableFuture<?> b;
    Object r, s;
    if (f == null || (b = o.toCompletableFuture()) == null)
      throw new NullPointerException();
    CompletableFuture<Void> d = newIncompleteFuture();
    if ((r = result) == null || (s = b.result) == null)
      bipush(b, new BiRun(e, d, this, b, f));
    else if (e == null)
      d.biRun(r, s, f, null);
    else
      try {
        e.execute(new BiRun(null, d, this, b, f));
      } catch (Throwable ex) {
        d.result = encodeThrowable(ex);
      }
    return d;
  }

  /**
   * Pushes completion to this and b unless either done. Caller should first check that result and
   * b.result are both null.
   */
  final void orpush(CompletableFuture<?> b, BiCompletion<?, ?, ?> c) {
    if (c != null) {
      while (!tryPushStack(c)) {
        if (result != null) {
          lazySetNext(c, null);
          break;
        }
      }
      if (result != null)
        c.tryFire(SYNC);
      else
        b.unipush(new CoCompletion(c));
    }
  }

  private <U extends T, V> CompletableFuture<V> orApplyStage(Executor e, CompletionStage<U> o,
      Function<? super T, ? extends V> f) {
    CompletableFuture<U> b;
    if (f == null || (b = o.toCompletableFuture()) == null)
      throw new NullPointerException();

    Object r;
    CompletableFuture<? extends T> z;
    if ((r = (z = this).result) != null || (r = (z = b).result) != null)
      return z.uniApplyNow(r, e, f);

    CompletableFuture<V> d = newIncompleteFuture();
    orpush(b, new OrApply<T, U, V>(e, d, this, b, f));
    return d;
  }

  private <U extends T> CompletableFuture<Void> orAcceptStage(Executor e, CompletionStage<U> o,
      Consumer<? super T> f) {
    CompletableFuture<U> b;
    if (f == null || (b = o.toCompletableFuture()) == null)
      throw new NullPointerException();

    Object r;
    CompletableFuture<? extends T> z;
    if ((r = (z = this).result) != null || (r = (z = b).result) != null)
      return z.uniAcceptNow(r, e, f);

    CompletableFuture<Void> d = newIncompleteFuture();
    orpush(b, new OrAccept<T, U>(e, d, this, b, f));
    return d;
  }

  private CompletableFuture<Void> orRunStage(Executor e, CompletionStage<?> o, Runnable f) {
    CompletableFuture<?> b;
    if (f == null || (b = o.toCompletableFuture()) == null)
      throw new NullPointerException();

    Object r;
    CompletableFuture<?> z;
    if ((r = (z = this).result) != null || (r = (z = b).result) != null)
      return z.uniRunNow(r, e, f);

    CompletableFuture<Void> d = newIncompleteFuture();
    orpush(b, new OrRun(e, d, this, b, f));
    return d;
  }

  /**
   * Returns raw result after waiting, or null if interruptible and interrupted.
   */
  private Object waitingGet(boolean interruptible) {
    Signaller q = null;
    boolean queued = false;
    Object r;
    while ((r = result) == null) {
      if (q == null) {
        q = new Signaller(interruptible, 0L, 0L);
        if (Thread.currentThread() instanceof ForkJoinWorkerThread)
          ForkJoinPool.helpAsyncBlocker(defaultExecutor(), q);
      } else if (!queued)
        queued = tryPushStack(q);
      else {
        try {
          ForkJoinPool.managedBlock(q);
        } catch (InterruptedException ie) { // currently cannot happen
          q.interrupted = true;
        }
        if (q.interrupted && interruptible)
          break;
      }
    }
    if (q != null && queued) {
      q.thread = null;
      if (!interruptible && q.interrupted)
        Thread.currentThread().interrupt();
      if (r == null)
        cleanStack();
    }
    if (r != null || (r = result) != null)
      postComplete();
    return r;
  }

  /**
   * Returns raw result after waiting, or null if interrupted, or throws TimeoutException on
   * timeout.
   */
  private Object timedGet(long nanos) throws TimeoutException {
    if (Thread.interrupted())
      return null;
    if (nanos > 0L) {
      long d = System.nanoTime() + nanos;
      long deadline = (d == 0L) ? 1L : d; // avoid 0
      Signaller q = null;
      boolean queued = false;
      Object r;
      while ((r = result) == null) { // similar to untimed
        if (q == null) {
          q = new Signaller(true, nanos, deadline);
          if (Thread.currentThread() instanceof ForkJoinWorkerThread)
            ForkJoinPool.helpAsyncBlocker(defaultExecutor(), q);
        } else if (!queued)
          queued = tryPushStack(q);
        else if (q.nanos <= 0L)
          break;
        else {
          try {
            ForkJoinPool.managedBlock(q);
          } catch (InterruptedException ie) {
            q.interrupted = true;
          }
          if (q.interrupted)
            break;
        }
      }
      if (q != null && queued) {
        q.thread = null;
        if (r == null)
          cleanStack();
      }
      if (r != null || (r = result) != null)
        postComplete();
      if (r != null || (q != null && q.interrupted))
        return r;
    }
    throw new TimeoutException();
  }

  /* ------------- Zero-input Async forms -------------- */

  /**
   * Returns {@code true} if completed in any fashion: normally, exceptionally, or via cancellation.
   *
   * @return {@code true} if completed
   */
  public boolean isDone() {
    return result != null;
  }

  /**
   * Waits if necessary for this future to complete, and then returns its result.
   *
   * @return the result value
   * @throws CancellationException if this future was cancelled
   * @throws ExecutionException if this future completed exceptionally
   * @throws InterruptedException if the current thread was interrupted while waiting
   */
  @SuppressWarnings("unchecked")
  public T get() throws InterruptedException, ExecutionException {
    Object r;
    if ((r = result) == null)
      r = waitingGet(true);
    return (T) reportGet(r);
  }

  /**
   * Waits if necessary for at most the given time for this future to complete, and then returns its
   * result, if available.
   *
   * @param timeout the maximum time to wait
   * @param unit the time unit of the timeout argument
   * @return the result value
   * @throws CancellationException if this future was cancelled
   * @throws ExecutionException if this future completed exceptionally
   * @throws InterruptedException if the current thread was interrupted while waiting
   * @throws TimeoutException if the wait timed out
   */
  public T get(long timeout, TimeUnit unit)
      throws InterruptedException, ExecutionException, TimeoutException {
    long nanos = unit.toNanos(timeout);
    Object r;
    if ((r = result) == null)
      r = timedGet(nanos);
    return (T) reportGet(r);
  }

  /**
   * Returns the result value when complete, or throws an (unchecked) exception if completed
   * exceptionally. To better conform with the use of common functional forms, if a computation
   * involved in the completion of this CompletableFuture threw an exception, this method throws an
   * (unchecked) {@link CompletionException} with the underlying exception as its cause.
   *
   * @return the result value
   * @throws CancellationException if the computation was cancelled
   * @throws CompletionException if this future completed exceptionally or a completion computation
   *         threw an exception
   */
  @SuppressWarnings("unchecked")
  public T join() {
    Object r;
    if ((r = result) == null)
      r = waitingGet(false);
    return (T) reportJoin(r);
  }

  /* ------------- Signallers -------------- */

  /**
   * Returns the result value (or throws any encountered exception) if completed, else returns the
   * given valueIfAbsent.
   *
   * @param valueIfAbsent the value to return if not completed
   * @return the result value, if completed, else the given valueIfAbsent
   * @throws CancellationException if the computation was cancelled
   * @throws CompletionException if this future completed exceptionally or a completion computation
   *         threw an exception
   */
  @SuppressWarnings("unchecked")
  public T getNow(T valueIfAbsent) {
    Object r;
    return ((r = result) == null) ? valueIfAbsent : (T) reportJoin(r);
  }

  /**
   * If not already completed, sets the value returned by {@link #get()} and related methods to the
   * given value.
   *
   * @param value the result value
   * @return {@code true} if this invocation caused this CompletableFuture to transition to a
   *         completed state, else {@code false}
   */
  public boolean complete(T value) {
    boolean triggered = completeValue(value);
    postComplete();
    return triggered;
  }

  /**
   * If not already completed, causes invocations of {@link #get()} and related methods to throw the
   * given exception.
   *
   * @param ex the exception
   * @return {@code true} if this invocation caused this CompletableFuture to transition to a
   *         completed state, else {@code false}
   */
  public boolean completeExceptionally(Throwable ex) {
    boolean triggered = internalComplete(new AltResult(Objects.requireNonNull(ex)));
    postComplete();
    return triggered;
  }

  /* ------------- public methods -------------- */

  public <U> CompletableFuture<U> thenApply(Function<? super T, ? extends U> fn) {
    return uniApplyStage(null, fn);
  }

  public <U> CompletableFuture<U> thenApplyAsync(Function<? super T, ? extends U> fn) {
    return uniApplyStage(defaultExecutor(), fn);
  }

  public <U> CompletableFuture<U> thenApplyAsync(Function<? super T, ? extends U> fn,
      Executor executor) {
    return uniApplyStage(screenExecutor(executor), fn);
  }

  public CompletableFuture<Void> thenAccept(Consumer<? super T> action) {
    return uniAcceptStage(null, action);
  }

  public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action) {
    return uniAcceptStage(defaultExecutor(), action);
  }

  public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action, Executor executor) {
    return uniAcceptStage(screenExecutor(executor), action);
  }

  public CompletableFuture<Void> thenRun(Runnable action) {
    return uniRunStage(null, action);
  }

  public CompletableFuture<Void> thenRunAsync(Runnable action) {
    return uniRunStage(defaultExecutor(), action);
  }

  public CompletableFuture<Void> thenRunAsync(Runnable action, Executor executor) {
    return uniRunStage(screenExecutor(executor), action);
  }

  public <U, V> CompletableFuture<V> thenCombine(CompletionStage<? extends U> other,
      BiFunction<? super T, ? super U, ? extends V> fn) {
    return biApplyStage(null, other, fn);
  }

  public <U, V> CompletableFuture<V> thenCombineAsync(CompletionStage<? extends U> other,
      BiFunction<? super T, ? super U, ? extends V> fn) {
    return biApplyStage(defaultExecutor(), other, fn);
  }

  public <U, V> CompletableFuture<V> thenCombineAsync(CompletionStage<? extends U> other,
      BiFunction<? super T, ? super U, ? extends V> fn, Executor executor) {
    return biApplyStage(screenExecutor(executor), other, fn);
  }

  public <U> CompletableFuture<Void> thenAcceptBoth(CompletionStage<? extends U> other,
      BiConsumer<? super T, ? super U> action) {
    return biAcceptStage(null, other, action);
  }

  public <U> CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other,
      BiConsumer<? super T, ? super U> action) {
    return biAcceptStage(defaultExecutor(), other, action);
  }

  public <U> CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other,
      BiConsumer<? super T, ? super U> action, Executor executor) {
    return biAcceptStage(screenExecutor(executor), other, action);
  }

  public CompletableFuture<Void> runAfterBoth(CompletionStage<?> other, Runnable action) {
    return biRunStage(null, other, action);
  }

  public CompletableFuture<Void> runAfterBothAsync(CompletionStage<?> other, Runnable action) {
    return biRunStage(defaultExecutor(), other, action);
  }

  public CompletableFuture<Void> runAfterBothAsync(CompletionStage<?> other, Runnable action,
      Executor executor) {
    return biRunStage(screenExecutor(executor), other, action);
  }

  public <U> CompletableFuture<U> applyToEither(CompletionStage<? extends T> other,
      Function<? super T, U> fn) {
    return orApplyStage(null, other, fn);
  }

  public <U> CompletableFuture<U> applyToEitherAsync(CompletionStage<? extends T> other,
      Function<? super T, U> fn) {
    return orApplyStage(defaultExecutor(), other, fn);
  }

  public <U> CompletableFuture<U> applyToEitherAsync(CompletionStage<? extends T> other,
      Function<? super T, U> fn, Executor executor) {
    return orApplyStage(screenExecutor(executor), other, fn);
  }

  public CompletableFuture<Void> acceptEither(CompletionStage<? extends T> other,
      Consumer<? super T> action) {
    return orAcceptStage(null, other, action);
  }

  public CompletableFuture<Void> acceptEitherAsync(CompletionStage<? extends T> other,
      Consumer<? super T> action) {
    return orAcceptStage(defaultExecutor(), other, action);
  }

  public CompletableFuture<Void> acceptEitherAsync(CompletionStage<? extends T> other,
      Consumer<? super T> action, Executor executor) {
    return orAcceptStage(screenExecutor(executor), other, action);
  }

  public CompletableFuture<Void> runAfterEither(CompletionStage<?> other, Runnable action) {
    return orRunStage(null, other, action);
  }

  public CompletableFuture<Void> runAfterEitherAsync(CompletionStage<?> other, Runnable action) {
    return orRunStage(defaultExecutor(), other, action);
  }

  public CompletableFuture<Void> runAfterEitherAsync(CompletionStage<?> other, Runnable action,
      Executor executor) {
    return orRunStage(screenExecutor(executor), other, action);
  }

  public <U> CompletableFuture<U> thenCompose(
      Function<? super T, ? extends CompletionStage<U>> fn) {
    return uniComposeStage(null, fn);
  }

  public <U> CompletableFuture<U> thenComposeAsync(
      Function<? super T, ? extends CompletionStage<U>> fn) {
    return uniComposeStage(defaultExecutor(), fn);
  }

  public <U> CompletableFuture<U> thenComposeAsync(
      Function<? super T, ? extends CompletionStage<U>> fn, Executor executor) {
    return uniComposeStage(screenExecutor(executor), fn);
  }

  public CompletableFuture<T> whenComplete(BiConsumer<? super T, ? super Throwable> action) {
    return uniWhenCompleteStage(null, action);
  }

  public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action) {
    return uniWhenCompleteStage(defaultExecutor(), action);
  }

  public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action,
      Executor executor) {
    return uniWhenCompleteStage(screenExecutor(executor), action);
  }

  public <U> CompletableFuture<U> handle(BiFunction<? super T, Throwable, ? extends U> fn) {
    return uniHandleStage(null, fn);
  }

  public <U> CompletableFuture<U> handleAsync(BiFunction<? super T, Throwable, ? extends U> fn) {
    return uniHandleStage(defaultExecutor(), fn);
  }

  public <U> CompletableFuture<U> handleAsync(BiFunction<? super T, Throwable, ? extends U> fn,
      Executor executor) {
    return uniHandleStage(screenExecutor(executor), fn);
  }

  /**
   * Returns this CompletableFuture.
   *
   * @return this CompletableFuture
   */
  public CompletableFuture<T> toCompletableFuture() {
    return this;
  }

  /**
   * Returns a new CompletableFuture that is completed when this CompletableFuture completes, with
   * the result of the given function of the exception triggering this CompletableFuture's
   * completion when it completes exceptionally; otherwise, if this CompletableFuture completes
   * normally, then the returned CompletableFuture also completes normally with the same value.
   * Note: More flexible versions of this functionality are available using methods
   * {@code whenComplete} and {@code handle}.
   *
   * @param fn the function to use to compute the value of the returned CompletableFuture if this
   *        CompletableFuture completed exceptionally
   * @return the new CompletableFuture
   */
  public CompletableFuture<T> exceptionally(Function<Throwable, ? extends T> fn) {
    return uniExceptionallyStage(fn);
  }

  /**
   * If not already completed, completes this CompletableFuture with a
   * {@link CancellationException}. Dependent CompletableFutures that have not already completed
   * will also complete exceptionally, with a {@link CompletionException} caused by this
   * {@code CancellationException}.
   *
   * @param mayInterruptIfRunning this value has no effect in this implementation because interrupts
   *        are not used to control processing.
   *
   * @return {@code true} if this task is now cancelled
   */
  public boolean cancel(boolean mayInterruptIfRunning) {
    boolean cancelled =
        (result == null) && internalComplete(new AltResult(new CancellationException()));
    postComplete();
    return cancelled || isCancelled();
  }

  /**
   * Returns {@code true} if this CompletableFuture was cancelled before it completed normally.
   *
   * @return {@code true} if this CompletableFuture was cancelled before it completed normally
   */
  public boolean isCancelled() {
    Object r;
    return ((r = result) instanceof AltResult)
        && (((AltResult) r).ex instanceof CancellationException);
  }

  /**
   * Returns {@code true} if this CompletableFuture completed exceptionally, in any way. Possible
   * causes include cancellation, explicit invocation of {@code
   * completeExceptionally}, and abrupt termination of a CompletionStage action.
   *
   * @return {@code true} if this CompletableFuture completed exceptionally
   */
  public boolean isCompletedExceptionally() {
    Object r;
    return ((r = result) instanceof AltResult) && r != NIL;
  }

  /**
   * Forcibly sets or resets the value subsequently returned by method {@link #get()} and related
   * methods, whether or not already completed. This method is designed for use only in error
   * recovery actions, and even in such situations may result in ongoing dependent completions using
   * established versus overwritten outcomes.
   *
   * @param value the completion value
   */
  public void obtrudeValue(T value) {
    result = (value == null) ? NIL : value;
    postComplete();
  }

  /**
   * Forcibly causes subsequent invocations of method {@link #get()} and related methods to throw
   * the given exception, whether or not already completed. This method is designed for use only in
   * error recovery actions, and even in such situations may result in ongoing dependent completions
   * using established versus overwritten outcomes.
   *
   * @param ex the exception
   * @throws NullPointerException if the exception is null
   */
  public void obtrudeException(Throwable ex) {
    result = new AltResult(Objects.requireNonNull(ex));
    postComplete();
  }

  /**
   * Returns the estimated number of CompletableFutures whose completions are awaiting completion of
   * this CompletableFuture. This method is designed for use in monitoring system state, not for
   * synchronization control.
   *
   * @return the number of dependent CompletableFutures
   */
  public int getNumberOfDependents() {
    int count = 0;
    for (Completion p = stack; p != null; p = p.next)
      ++count;
    return count;
  }

  /**
   * Returns a string identifying this CompletableFuture, as well as its completion state. The
   * state, in brackets, contains the String {@code "Completed Normally"} or the String {@code
   * "Completed Exceptionally"}, or the String {@code "Not
   * completed"} followed by the number of CompletableFutures dependent upon its completion, if any.
   *
   * @return a string identifying this CompletableFuture, as well as its state
   */
  public String toString() {
    Object r = result;
    int count = 0; // avoid call to getNumberOfDependents in case disabled
    for (Completion p = stack; p != null; p = p.next)
      ++count;
    return super.toString() + ((r == null)
        ? ((count == 0) ? "[Not completed]" : "[Not completed, " + count + " dependents]")
        : (((r instanceof AltResult) && ((AltResult) r).ex != null)
            ? "[Completed exceptionally: " + ((AltResult) r).ex + "]"
            : "[Completed normally]"));
  }

  /**
   * Returns a new incomplete CompletableFuture of the type to be returned by a CompletionStage
   * method. Subclasses should normally override this method to return an instance of the same class
   * as this CompletableFuture. The default implementation returns an instance of class
   * CompletableFuture.
   *
   * @param <U> the type of the value
   * @return a new CompletableFuture
   * @since 9
   */
  public <U> CompletableFuture<U> newIncompleteFuture() {
    return new CompletableFuture<U>();
  }

  /**
   * Returns the default Executor used for async methods that do not specify an Executor. This class
   * uses the {@link ForkJoinPool#commonPool()} if it supports more than one parallel thread, or
   * else an Executor using one thread per async task. This method may be overridden in subclasses
   * to return an Executor that provides at least one independent thread.
   *
   * @return the executor
   * @since 9
   */
  public Executor defaultExecutor() {
    return ASYNC_POOL;
  }

  /**
   * Returns a new CompletableFuture that is completed normally with the same value as this
   * CompletableFuture when it completes normally. If this CompletableFuture completes
   * exceptionally, then the returned CompletableFuture completes exceptionally with a
   * CompletionException with this exception as cause. The behavior is equivalent to
   * {@code thenApply(x -> x)}. This method may be useful as a form of "defensive copying", to
   * prevent clients from completing, while still being able to arrange dependent actions.
   *
   * @return the new CompletableFuture
   * @since 9
   */
  public CompletableFuture<T> copy() {
    return uniCopyStage(this);
  }

  /**
   * Returns a new CompletionStage that is completed normally with the same value as this
   * CompletableFuture when it completes normally, and cannot be independently completed or
   * otherwise used in ways not defined by the methods of interface {@link CompletionStage}. If this
   * CompletableFuture completes exceptionally, then the returned CompletionStage completes
   * exceptionally with a CompletionException with this exception as cause.
   *
   * <p>
   * Unless overridden by a subclass, a new non-minimal CompletableFuture with all methods available
   * can be obtained from a minimal CompletionStage via {@link #toCompletableFuture()}. For example,
   * completion of a minimal stage can be awaited by
   *
   * <pre>
   *  {@code minimalStage.toCompletableFuture().join(); }
   * </pre>
   *
   * @return the new CompletionStage
   * @since 9
   */
  public CompletionStage<T> minimalCompletionStage() {
    return uniAsMinimalStage();
  }

  /**
   * Completes this CompletableFuture with the result of the given Supplier function invoked from an
   * asynchronous task using the given executor.
   *
   * @param supplier a function returning the value to be used to complete this CompletableFuture
   * @param executor the executor to use for asynchronous execution
   * @return this CompletableFuture
   * @since 9
   */
  public CompletableFuture<T> completeAsync(Supplier<? extends T> supplier, Executor executor) {
    if (supplier == null || executor == null)
      throw new NullPointerException();
    executor.execute(new AsyncSupply<T>(this, supplier));
    return this;
  }

  /**
   * Completes this CompletableFuture with the result of the given Supplier function invoked from an
   * asynchronous task using the default executor.
   *
   * @param supplier a function returning the value to be used to complete this CompletableFuture
   * @return this CompletableFuture
   * @since 9
   */
  public CompletableFuture<T> completeAsync(Supplier<? extends T> supplier) {
    return completeAsync(supplier, defaultExecutor());
  }

  // not in interface CompletionStage

  /**
   * Exceptionally completes this CompletableFuture with a {@link TimeoutException} if not otherwise
   * completed before the given timeout.
   *
   * @param timeout how long to wait before completing exceptionally with a TimeoutException, in
   *        units of {@code unit}
   * @param unit a {@code TimeUnit} determining how to interpret the {@code timeout} parameter
   * @return this CompletableFuture
   * @since 9
   */
  public CompletableFuture<T> orTimeout(long timeout, TimeUnit unit) {
    Objects.requireNonNull(unit);
    if (result == null)
      whenComplete(new Canceller(Delayer.delay(new Timeout(this), timeout, unit)));
    return this;
  }


  /* ------------- Arbitrary-arity constructions -------------- */

  /**
   * Completes this CompletableFuture with the given value if not otherwise completed before the
   * given timeout.
   *
   * @param value the value to use upon timeout
   * @param timeout how long to wait before completing normally with the given value, in units of
   *        {@code unit}
   * @param unit a {@code TimeUnit} determining how to interpret the {@code timeout} parameter
   * @return this CompletableFuture
   * @since 9
   */
  public CompletableFuture<T> completeOnTimeout(T value, long timeout, TimeUnit unit) {
    Objects.requireNonNull(unit);
    if (result == null)
      whenComplete(
          new Canceller(Delayer.delay(new DelayedCompleter<T>(this, value), timeout, unit)));
    return this;
  }

  /**
   * A marker interface identifying asynchronous tasks produced by {@code async} methods. This may
   * be useful for monitoring, debugging, and tracking asynchronous activities.
   *
   * @since 1.8
   */
  public static interface AsynchronousCompletionTask {
  }

  /* ------------- Control and status methods -------------- */

  static final class AltResult { // See above
    final Throwable ex; // null only for NIL

    AltResult(Throwable x) {
      this.ex = x;
    }
  }

  /** Fallback if ForkJoinPool.commonPool() cannot support parallelism */
  static final class ThreadPerTaskExecutor implements Executor {
    public void execute(Runnable r) {
      new Thread(r).start();
    }
  }

  @SuppressWarnings("serial")
  abstract static class Completion extends ForkJoinTask<Void>
      implements Runnable, AsynchronousCompletionTask {
    volatile Completion next; // Treiber stack link

    /**
     * Performs completion action if triggered, returning a dependent that may need propagation, if
     * one exists.
     *
     * @param mode SYNC, ASYNC, or NESTED
     */
    abstract CompletableFuture<?> tryFire(int mode);

    /** Returns true if possibly still triggerable. Used by cleanStack. */
    abstract boolean isLive();

    public final void run() {
      tryFire(ASYNC);
    }

    public final boolean exec() {
      tryFire(ASYNC);
      return false;
    }

    public final Void getRawResult() {
      return null;
    }

    public final void setRawResult(Void v) {}
  }

  /** A Completion with a source, dependent, and executor. */
  @SuppressWarnings("serial")
  abstract static class UniCompletion<T, V> extends Completion {
    Executor executor; // executor to use (null if none)
    CompletableFuture<V> dep; // the dependent to complete
    CompletableFuture<T> src; // source for action

    UniCompletion(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src) {
      this.executor = executor;
      this.dep = dep;
      this.src = src;
    }

    /**
     * Returns true if action can be run. Call only when known to be triggerable. Uses FJ tag bit to
     * ensure that only one thread claims ownership. If async, starts as task -- a later call to
     * tryFire will run action.
     */
    final boolean claim() {
      Executor e = executor;
      if (compareAndSetForkJoinTaskTag((short) 0, (short) 1)) {
        if (e == null)
          return true;
        executor = null; // disable
        e.execute(this);
      }
      return false;
    }

    final boolean isLive() {
      return dep != null;
    }
  }

  @SuppressWarnings("serial")
  static final class UniApply<T, V> extends UniCompletion<T, V> {
    Function<? super T, ? extends V> fn;

    UniApply(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src,
        Function<? super T, ? extends V> fn) {
      super(executor, dep, src);
      this.fn = fn;
    }

    final CompletableFuture<V> tryFire(int mode) {
      CompletableFuture<V> d;
      CompletableFuture<T> a;
      Object r;
      Throwable x;
      Function<? super T, ? extends V> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null)
        return null;
      tryComplete: if (d.result == null) {
        if (r instanceof AltResult) {
          if ((x = ((AltResult) r).ex) != null) {
            d.completeThrowable(x, r);
            break tryComplete;
          }
          r = null;
        }
        try {
          if (mode <= 0 && !claim())
            return null;
          else {
            @SuppressWarnings("unchecked")
            T t = (T) r;
            d.completeValue(f.apply(t));
          }
        } catch (Throwable ex) {
          d.completeThrowable(ex);
        }
      }
      dep = null;
      src = null;
      fn = null;
      return d.postFire(a, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class UniAccept<T> extends UniCompletion<T, Void> {
    Consumer<? super T> fn;

    UniAccept(Executor executor, CompletableFuture<Void> dep, CompletableFuture<T> src,
        Consumer<? super T> fn) {
      super(executor, dep, src);
      this.fn = fn;
    }

    final CompletableFuture<Void> tryFire(int mode) {
      CompletableFuture<Void> d;
      CompletableFuture<T> a;
      Object r;
      Throwable x;
      Consumer<? super T> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null)
        return null;
      tryComplete: if (d.result == null) {
        if (r instanceof AltResult) {
          if ((x = ((AltResult) r).ex) != null) {
            d.completeThrowable(x, r);
            break tryComplete;
          }
          r = null;
        }
        try {
          if (mode <= 0 && !claim())
            return null;
          else {
            @SuppressWarnings("unchecked")
            T t = (T) r;
            f.accept(t);
            d.completeNull();
          }
        } catch (Throwable ex) {
          d.completeThrowable(ex);
        }
      }
      dep = null;
      src = null;
      fn = null;
      return d.postFire(a, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class UniRun<T> extends UniCompletion<T, Void> {
    Runnable fn;

    UniRun(Executor executor, CompletableFuture<Void> dep, CompletableFuture<T> src, Runnable fn) {
      super(executor, dep, src);
      this.fn = fn;
    }

    final CompletableFuture<Void> tryFire(int mode) {
      CompletableFuture<Void> d;
      CompletableFuture<T> a;
      Object r;
      Throwable x;
      Runnable f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null)
        return null;
      if (d.result == null) {
        if (r instanceof AltResult && (x = ((AltResult) r).ex) != null)
          d.completeThrowable(x, r);
        else
          try {
            if (mode <= 0 && !claim())
              return null;
            else {
              f.run();
              d.completeNull();
            }
          } catch (Throwable ex) {
            d.completeThrowable(ex);
          }
      }
      dep = null;
      src = null;
      fn = null;
      return d.postFire(a, mode);
    }
  }

  // jdk9 additions

  @SuppressWarnings("serial")
  static final class UniWhenComplete<T> extends UniCompletion<T, T> {
    BiConsumer<? super T, ? super Throwable> fn;

    UniWhenComplete(Executor executor, CompletableFuture<T> dep, CompletableFuture<T> src,
        BiConsumer<? super T, ? super Throwable> fn) {
      super(executor, dep, src);
      this.fn = fn;
    }

    final CompletableFuture<T> tryFire(int mode) {
      CompletableFuture<T> d;
      CompletableFuture<T> a;
      Object r;
      BiConsumer<? super T, ? super Throwable> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null
          || !d.uniWhenComplete(r, f, mode > 0 ? null : this))
        return null;
      dep = null;
      src = null;
      fn = null;
      return d.postFire(a, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class UniHandle<T, V> extends UniCompletion<T, V> {
    BiFunction<? super T, Throwable, ? extends V> fn;

    UniHandle(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src,
        BiFunction<? super T, Throwable, ? extends V> fn) {
      super(executor, dep, src);
      this.fn = fn;
    }

    final CompletableFuture<V> tryFire(int mode) {
      CompletableFuture<V> d;
      CompletableFuture<T> a;
      Object r;
      BiFunction<? super T, Throwable, ? extends V> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null
          || !d.uniHandle(r, f, mode > 0 ? null : this))
        return null;
      dep = null;
      src = null;
      fn = null;
      return d.postFire(a, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class UniExceptionally<T> extends UniCompletion<T, T> {
    Function<? super Throwable, ? extends T> fn;

    UniExceptionally(CompletableFuture<T> dep, CompletableFuture<T> src,
        Function<? super Throwable, ? extends T> fn) {
      super(null, dep, src);
      this.fn = fn;
    }

    final CompletableFuture<T> tryFire(int mode) { // never ASYNC
      // assert mode != ASYNC;
      CompletableFuture<T> d;
      CompletableFuture<T> a;
      Object r;
      Function<? super Throwable, ? extends T> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null
          || !d.uniExceptionally(r, f, this))
        return null;
      dep = null;
      src = null;
      fn = null;
      return d.postFire(a, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class UniRelay<U, T extends U> extends UniCompletion<T, U> {
    UniRelay(CompletableFuture<U> dep, CompletableFuture<T> src) {
      super(null, dep, src);
    }

    final CompletableFuture<U> tryFire(int mode) {
      CompletableFuture<U> d;
      CompletableFuture<T> a;
      Object r;
      if ((d = dep) == null || (a = src) == null || (r = a.result) == null)
        return null;
      if (d.result == null)
        d.completeRelay(r);
      src = null;
      dep = null;
      return d.postFire(a, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class UniCompose<T, V> extends UniCompletion<T, V> {
    Function<? super T, ? extends CompletionStage<V>> fn;

    UniCompose(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src,
        Function<? super T, ? extends CompletionStage<V>> fn) {
      super(executor, dep, src);
      this.fn = fn;
    }

    final CompletableFuture<V> tryFire(int mode) {
      CompletableFuture<V> d;
      CompletableFuture<T> a;
      Function<? super T, ? extends CompletionStage<V>> f;
      Object r;
      Throwable x;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null)
        return null;
      tryComplete: if (d.result == null) {
        if (r instanceof AltResult) {
          if ((x = ((AltResult) r).ex) != null) {
            d.completeThrowable(x, r);
            break tryComplete;
          }
          r = null;
        }
        try {
          if (mode <= 0 && !claim())
            return null;
          @SuppressWarnings("unchecked")
          T t = (T) r;
          CompletableFuture<V> g = f.apply(t).toCompletableFuture();
          if ((r = g.result) != null)
            d.completeRelay(r);
          else {
            g.unipush(new UniRelay<V, V>(d, g));
            if (d.result == null)
              return null;
          }
        } catch (Throwable ex) {
          d.completeThrowable(ex);
        }
      }
      dep = null;
      src = null;
      fn = null;
      return d.postFire(a, mode);
    }
  }

  /** A Completion for an action with two sources */
  @SuppressWarnings("serial")
  abstract static class BiCompletion<T, U, V> extends UniCompletion<T, V> {
    CompletableFuture<U> snd; // second source for action

    BiCompletion(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src,
        CompletableFuture<U> snd) {
      super(executor, dep, src);
      this.snd = snd;
    }
  }

  /** A Completion delegating to a BiCompletion */
  @SuppressWarnings("serial")
  static final class CoCompletion extends Completion {
    BiCompletion<?, ?, ?> base;

    CoCompletion(BiCompletion<?, ?, ?> base) {
      this.base = base;
    }

    final CompletableFuture<?> tryFire(int mode) {
      BiCompletion<?, ?, ?> c;
      CompletableFuture<?> d;
      if ((c = base) == null || (d = c.tryFire(mode)) == null)
        return null;
      base = null; // detach
      return d;
    }

    final boolean isLive() {
      BiCompletion<?, ?, ?> c;
      return (c = base) != null
          // && c.isLive()
          && c.dep != null;
    }
  }

  @SuppressWarnings("serial")
  static final class BiApply<T, U, V> extends BiCompletion<T, U, V> {
    BiFunction<? super T, ? super U, ? extends V> fn;

    BiApply(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src,
        CompletableFuture<U> snd, BiFunction<? super T, ? super U, ? extends V> fn) {
      super(executor, dep, src, snd);
      this.fn = fn;
    }

    final CompletableFuture<V> tryFire(int mode) {
      CompletableFuture<V> d;
      CompletableFuture<T> a;
      CompletableFuture<U> b;
      Object r, s;
      BiFunction<? super T, ? super U, ? extends V> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null
          || (b = snd) == null || (s = b.result) == null
          || !d.biApply(r, s, f, mode > 0 ? null : this))
        return null;
      dep = null;
      src = null;
      snd = null;
      fn = null;
      return d.postFire(a, b, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class BiAccept<T, U> extends BiCompletion<T, U, Void> {
    BiConsumer<? super T, ? super U> fn;

    BiAccept(Executor executor, CompletableFuture<Void> dep, CompletableFuture<T> src,
        CompletableFuture<U> snd, BiConsumer<? super T, ? super U> fn) {
      super(executor, dep, src, snd);
      this.fn = fn;
    }

    final CompletableFuture<Void> tryFire(int mode) {
      CompletableFuture<Void> d;
      CompletableFuture<T> a;
      CompletableFuture<U> b;
      Object r, s;
      BiConsumer<? super T, ? super U> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null
          || (b = snd) == null || (s = b.result) == null
          || !d.biAccept(r, s, f, mode > 0 ? null : this))
        return null;
      dep = null;
      src = null;
      snd = null;
      fn = null;
      return d.postFire(a, b, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class BiRun<T, U> extends BiCompletion<T, U, Void> {
    Runnable fn;

    BiRun(Executor executor, CompletableFuture<Void> dep, CompletableFuture<T> src,
        CompletableFuture<U> snd, Runnable fn) {
      super(executor, dep, src, snd);
      this.fn = fn;
    }

    final CompletableFuture<Void> tryFire(int mode) {
      CompletableFuture<Void> d;
      CompletableFuture<T> a;
      CompletableFuture<U> b;
      Object r, s;
      Runnable f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (r = a.result) == null
          || (b = snd) == null || (s = b.result) == null
          || !d.biRun(r, s, f, mode > 0 ? null : this))
        return null;
      dep = null;
      src = null;
      snd = null;
      fn = null;
      return d.postFire(a, b, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class BiRelay<T, U> extends BiCompletion<T, U, Void> { // for And
    BiRelay(CompletableFuture<Void> dep, CompletableFuture<T> src, CompletableFuture<U> snd) {
      super(null, dep, src, snd);
    }

    final CompletableFuture<Void> tryFire(int mode) {
      CompletableFuture<Void> d;
      CompletableFuture<T> a;
      CompletableFuture<U> b;
      Object r, s, z;
      Throwable x;
      if ((d = dep) == null || (a = src) == null || (r = a.result) == null || (b = snd) == null
          || (s = b.result) == null)
        return null;
      if (d.result == null) {
        if ((r instanceof AltResult && (x = ((AltResult) (z = r)).ex) != null)
            || (s instanceof AltResult && (x = ((AltResult) (z = s)).ex) != null))
          d.completeThrowable(x, z);
        else
          d.completeNull();
      }
      src = null;
      snd = null;
      dep = null;
      return d.postFire(a, b, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class OrApply<T, U extends T, V> extends BiCompletion<T, U, V> {
    Function<? super T, ? extends V> fn;

    OrApply(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src,
        CompletableFuture<U> snd, Function<? super T, ? extends V> fn) {
      super(executor, dep, src, snd);
      this.fn = fn;
    }

    final CompletableFuture<V> tryFire(int mode) {
      CompletableFuture<V> d;
      CompletableFuture<T> a;
      CompletableFuture<U> b;
      Object r;
      Throwable x;
      Function<? super T, ? extends V> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (b = snd) == null
          || ((r = a.result) == null && (r = b.result) == null))
        return null;
      tryComplete: if (d.result == null) {
        try {
          if (mode <= 0 && !claim())
            return null;
          if (r instanceof AltResult) {
            if ((x = ((AltResult) r).ex) != null) {
              d.completeThrowable(x, r);
              break tryComplete;
            }
            r = null;
          }
          @SuppressWarnings("unchecked")
          T t = (T) r;
          d.completeValue(f.apply(t));
        } catch (Throwable ex) {
          d.completeThrowable(ex);
        }
      }
      dep = null;
      src = null;
      snd = null;
      fn = null;
      return d.postFire(a, b, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class OrAccept<T, U extends T> extends BiCompletion<T, U, Void> {
    Consumer<? super T> fn;

    OrAccept(Executor executor, CompletableFuture<Void> dep, CompletableFuture<T> src,
        CompletableFuture<U> snd, Consumer<? super T> fn) {
      super(executor, dep, src, snd);
      this.fn = fn;
    }

    final CompletableFuture<Void> tryFire(int mode) {
      CompletableFuture<Void> d;
      CompletableFuture<T> a;
      CompletableFuture<U> b;
      Object r;
      Throwable x;
      Consumer<? super T> f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (b = snd) == null
          || ((r = a.result) == null && (r = b.result) == null))
        return null;
      tryComplete: if (d.result == null) {
        try {
          if (mode <= 0 && !claim())
            return null;
          if (r instanceof AltResult) {
            if ((x = ((AltResult) r).ex) != null) {
              d.completeThrowable(x, r);
              break tryComplete;
            }
            r = null;
          }
          @SuppressWarnings("unchecked")
          T t = (T) r;
          f.accept(t);
          d.completeNull();
        } catch (Throwable ex) {
          d.completeThrowable(ex);
        }
      }
      dep = null;
      src = null;
      snd = null;
      fn = null;
      return d.postFire(a, b, mode);
    }
  }

  @SuppressWarnings("serial")
  static final class OrRun<T, U> extends BiCompletion<T, U, Void> {
    Runnable fn;

    OrRun(Executor executor, CompletableFuture<Void> dep, CompletableFuture<T> src,
        CompletableFuture<U> snd, Runnable fn) {
      super(executor, dep, src, snd);
      this.fn = fn;
    }

    final CompletableFuture<Void> tryFire(int mode) {
      CompletableFuture<Void> d;
      CompletableFuture<T> a;
      CompletableFuture<U> b;
      Object r;
      Throwable x;
      Runnable f;
      if ((d = dep) == null || (f = fn) == null || (a = src) == null || (b = snd) == null
          || ((r = a.result) == null && (r = b.result) == null))
        return null;
      if (d.result == null) {
        try {
          if (mode <= 0 && !claim())
            return null;
          else if (r instanceof AltResult && (x = ((AltResult) r).ex) != null)
            d.completeThrowable(x, r);
          else {
            f.run();
            d.completeNull();
          }
        } catch (Throwable ex) {
          d.completeThrowable(ex);
        }
      }
      dep = null;
      src = null;
      snd = null;
      fn = null;
      return d.postFire(a, b, mode);
    }
  }

  // Little class-ified lambdas to better support monitoring

  /** Completion for an anyOf input future. */
  @SuppressWarnings("serial")
  static class AnyOf extends Completion {
    CompletableFuture<Object> dep;
    CompletableFuture<?> src;
    CompletableFuture<?>[] srcs;

    AnyOf(CompletableFuture<Object> dep, CompletableFuture<?> src, CompletableFuture<?>[] srcs) {
      this.dep = dep;
      this.src = src;
      this.srcs = srcs;
    }

    final CompletableFuture<Object> tryFire(int mode) {
      // assert mode != ASYNC;
      CompletableFuture<Object> d;
      CompletableFuture<?> a;
      CompletableFuture<?>[] as;
      Object r;
      if ((d = dep) == null || (a = src) == null || (r = a.result) == null || (as = srcs) == null)
        return null;
      dep = null;
      src = null;
      srcs = null;
      if (d.completeRelay(r)) {
        for (CompletableFuture<?> b : as)
          if (b != a)
            b.cleanStack();
        if (mode < 0)
          return d;
        else
          d.postComplete();
      }
      return null;
    }

    final boolean isLive() {
      CompletableFuture<Object> d;
      return (d = dep) != null && d.result == null;
    }
  }

  @SuppressWarnings("serial")
  static final class AsyncSupply<T> extends ForkJoinTask<Void>
      implements Runnable, AsynchronousCompletionTask {
    CompletableFuture<T> dep;
    Supplier<? extends T> fn;

    AsyncSupply(CompletableFuture<T> dep, Supplier<? extends T> fn) {
      this.dep = dep;
      this.fn = fn;
    }

    public final Void getRawResult() {
      return null;
    }

    public final void setRawResult(Void v) {}

    public final boolean exec() {
      run();
      return false;
    }

    public void run() {
      CompletableFuture<T> d;
      Supplier<? extends T> f;
      if ((d = dep) != null && (f = fn) != null) {
        dep = null;
        fn = null;
        if (d.result == null) {
          try {
            d.completeValue(f.get());
          } catch (Throwable ex) {
            d.completeThrowable(ex);
          }
        }
        d.postComplete();
      }
    }
  }

  @SuppressWarnings("serial")
  static final class AsyncRun extends ForkJoinTask<Void>
      implements Runnable, AsynchronousCompletionTask {
    CompletableFuture<Void> dep;
    Runnable fn;

    AsyncRun(CompletableFuture<Void> dep, Runnable fn) {
      this.dep = dep;
      this.fn = fn;
    }

    public final Void getRawResult() {
      return null;
    }

    public final void setRawResult(Void v) {}

    public final boolean exec() {
      run();
      return false;
    }

    public void run() {
      CompletableFuture<Void> d;
      Runnable f;
      if ((d = dep) != null && (f = fn) != null) {
        dep = null;
        fn = null;
        if (d.result == null) {
          try {
            f.run();
            d.completeNull();
          } catch (Throwable ex) {
            d.completeThrowable(ex);
          }
        }
        d.postComplete();
      }
    }
  }

  /**
   * Completion for recording and releasing a waiting thread. This class implements ManagedBlocker
   * to avoid starvation when blocking actions pile up in ForkJoinPools.
   */
  @SuppressWarnings("serial")
  static final class Signaller extends Completion implements ForkJoinPool.ManagedBlocker {
    final long deadline; // non-zero if timed
    final boolean interruptible;
    long nanos; // remaining wait time if timed
    boolean interrupted;
    volatile Thread thread;

    Signaller(boolean interruptible, long nanos, long deadline) {
      this.thread = Thread.currentThread();
      this.interruptible = interruptible;
      this.nanos = nanos;
      this.deadline = deadline;
    }

    final CompletableFuture<?> tryFire(int ignore) {
      Thread w; // no need to atomically claim
      if ((w = thread) != null) {
        thread = null;
        LockSupport.unpark(w);
      }
      return null;
    }

    public boolean isReleasable() {
      if (Thread.interrupted())
        interrupted = true;
      return ((interrupted && interruptible)
          || (deadline != 0L && (nanos <= 0L || (nanos = deadline - System.nanoTime()) <= 0L))
          || thread == null);
    }

    public boolean block() {
      while (!isReleasable()) {
        if (deadline == 0L)
          LockSupport.park(this);
        else
          LockSupport.parkNanos(this, nanos);
      }
      return true;
    }

    final boolean isLive() {
      return thread != null;
    }
  }

  /**
   * Singleton delay scheduler, used only for starting and cancelling tasks.
   */
  static final class Delayer {
    static final ScheduledThreadPoolExecutor delayer;

    static {
      (delayer = new ScheduledThreadPoolExecutor(1, new DaemonThreadFactory()))
          .setRemoveOnCancelPolicy(true);
    }

    static ScheduledFuture<?> delay(Runnable command, long delay, TimeUnit unit) {
      return delayer.schedule(command, delay, unit);
    }

    static final class DaemonThreadFactory implements ThreadFactory {
      public Thread newThread(Runnable r) {
        Thread t = new Thread(r);
        t.setDaemon(true);
        t.setName("CompletableFutureDelayScheduler");
        return t;
      }
    }
  }

  static final class DelayedExecutor implements Executor {
    final long delay;
    final TimeUnit unit;
    final Executor executor;

    DelayedExecutor(long delay, TimeUnit unit, Executor executor) {
      this.delay = delay;
      this.unit = unit;
      this.executor = executor;
    }

    public void execute(Runnable r) {
      Delayer.delay(new TaskSubmitter(executor, r), delay, unit);
    }
  }

  /** Action to submit user task */
  static final class TaskSubmitter implements Runnable {
    final Executor executor;
    final Runnable action;

    TaskSubmitter(Executor executor, Runnable action) {
      this.executor = executor;
      this.action = action;
    }

    public void run() {
      executor.execute(action);
    }
  }

  /** Action to completeExceptionally on timeout */
  static final class Timeout implements Runnable {
    final CompletableFuture<?> f;

    Timeout(CompletableFuture<?> f) {
      this.f = f;
    }

    public void run() {
      if (f != null && !f.isDone())
        f.completeExceptionally(new TimeoutException());
    }
  }

  /** Action to complete on timeout */
  static final class DelayedCompleter<U> implements Runnable {
    final CompletableFuture<U> f;
    final U u;

    DelayedCompleter(CompletableFuture<U> f, U u) {
      this.f = f;
      this.u = u;
    }

    public void run() {
      if (f != null)
        f.complete(u);
    }
  }

  /** Action to cancel unneeded timeouts */
  static final class Canceller implements BiConsumer<Object, Throwable> {
    final Future<?> f;

    Canceller(Future<?> f) {
      this.f = f;
    }

    public void accept(Object ignore, Throwable ex) {
      if (ex == null && f != null && !f.isDone())
        f.cancel(false);
    }
  }

  /**
   * A subclass that just throws UOE for most non-CompletionStage methods.
   */
  static final class MinimalStage<T> extends CompletableFuture<T> {
    MinimalStage() {}

    MinimalStage(Object r) {
      super(r);
    }

    @Override
    public <U> CompletableFuture<U> newIncompleteFuture() {
      return new MinimalStage<U>();
    }

    @Override
    public T get() {
      throw new UnsupportedOperationException();
    }

    @Override
    public T get(long timeout, TimeUnit unit) {
      throw new UnsupportedOperationException();
    }

    @Override
    public T getNow(T valueIfAbsent) {
      throw new UnsupportedOperationException();
    }

    @Override
    public T join() {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean complete(T value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean completeExceptionally(Throwable ex) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean cancel(boolean mayInterruptIfRunning) {
      throw new UnsupportedOperationException();
    }

    @Override
    public void obtrudeValue(T value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public void obtrudeException(Throwable ex) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean isDone() {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean isCancelled() {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean isCompletedExceptionally() {
      throw new UnsupportedOperationException();
    }

    @Override
    public int getNumberOfDependents() {
      throw new UnsupportedOperationException();
    }

    @Override
    public CompletableFuture<T> completeAsync(Supplier<? extends T> supplier, Executor executor) {
      throw new UnsupportedOperationException();
    }

    @Override
    public CompletableFuture<T> completeAsync(Supplier<? extends T> supplier) {
      throw new UnsupportedOperationException();
    }

    @Override
    public CompletableFuture<T> orTimeout(long timeout, TimeUnit unit) {
      throw new UnsupportedOperationException();
    }

    @Override
    public CompletableFuture<T> completeOnTimeout(T value, long timeout, TimeUnit unit) {
      throw new UnsupportedOperationException();
    }

    @Override
    public CompletableFuture<T> toCompletableFuture() {
      Object r;
      if ((r = result) != null)
        return new CompletableFuture<T>(encodeRelay(r));
      else {
        CompletableFuture<T> d = new CompletableFuture<T>();
        unipush(new UniRelay<T, T>(d, this));
        return d;
      }
    }
  }
}
