/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
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 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;

/**
 * A reusable synchronization barrier, similar in functionality to
 * {@link CyclicBarrier} and {@link CountDownLatch} but supporting
 * more flexible usage.
 *
 * <p><b>Registration.</b> Unlike the case for other barriers, the
 * number of parties <em>registered</em> to synchronize on a phaser
 * may vary over time. Tasks may be registered at any time (using
 * methods {@link #register}, {@link #bulkRegister}, or forms of
 * constructors establishing initial numbers of parties), and
 * optionally deregistered upon any arrival (using {@link
 * #arriveAndDeregister}). As is the case with most basic
 * synchronization constructs, registration and deregistration affect
 * only internal counts; they do not establish any further internal
 * bookkeeping, so tasks cannot query whether they are registered.
 * (However, you can introduce such bookkeeping by subclassing this
 * class.)
 *
 * <p><b>Synchronization.</b> Like a {@code CyclicBarrier}, a {@code
 * Phaser} may be repeatedly awaited. Method {@link
 * #arriveAndAwaitAdvance} has effect analogous to {@link
 * java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each
 * generation of a phaser has an associated phase number. The phase
 * number starts at zero, and advances when all parties arrive at the
 * phaser, wrapping around to zero after reaching {@code
 * Integer.MAX_VALUE}. The use of phase numbers enables independent
 * control of actions upon arrival at a phaser and upon awaiting
 * others, via two kinds of methods that may be invoked by any
 * registered party:
 *
 * <ul>
 *
 * <li><b>Arrival.</b> Methods {@link #arrive} and
 * {@link #arriveAndDeregister} record arrival. These methods
 * do not block, but return an associated <em>arrival phase
 * number</em>; that is, the phase number of the phaser to which
 * the arrival applied. When the final party for a given phase
 * arrives, an optional action is performed and the phase
 * advances. These actions are performed by the party
 * triggering a phase advance, and are arranged by overriding
 * method {@link #onAdvance(int, int)}, which also controls
 * termination. Overriding this method is similar to, but more
 * flexible than, providing a barrier action to a {@code
 * CyclicBarrier}.
 *
 * <li><b>Waiting.</b> Method {@link #awaitAdvance} requires an
 * argument indicating an arrival phase number, and returns when
 * the phaser advances to (or is already at) a different phase.
 * Unlike similar constructions using {@code CyclicBarrier},
 * method {@code awaitAdvance} continues to wait even if the
 * waiting thread is interrupted. Interruptible and timeout
 * versions are also available, but exceptions encountered while
 * tasks wait interruptibly or with timeout do not change the
 * state of the phaser. If necessary, you can perform any
 * associated recovery within handlers of those exceptions,
 * often after invoking {@code forceTermination}. Phasers may
 * also be used by tasks executing in a {@link ForkJoinPool}.
 * Progress is ensured if the pool's parallelismLevel can
 * accommodate the maximum number of simultaneously blocked
 * parties.
 *
 * </ul>
 *
 * <p><b>Termination.</b> A phaser may enter a <em>termination</em>
 * state, that may be checked using method {@link #isTerminated}. Upon
 * termination, all synchronization methods immediately return without
 * waiting for advance, as indicated by a negative return value.
 * Similarly, attempts to register upon termination have no effect.
 * Termination is triggered when an invocation of {@code onAdvance}
 * returns {@code true}. The default implementation returns {@code
 * true} if a deregistration has caused the number of registered
 * parties to become zero. As illustrated below, when phasers control
 * actions with a fixed number of iterations, it is often convenient
 * to override this method to cause termination when the current phase
 * number reaches a threshold. Method {@link #forceTermination} is
 * also available to abruptly release waiting threads and allow them
 * to terminate.
 *
 * <p><b>Tiering.</b> Phasers may be <em>tiered</em> (i.e.,
 * constructed in tree structures) to reduce contention. Phasers with
 * large numbers of parties that would otherwise experience heavy
 * synchronization contention costs may instead be set up so that
 * groups of sub-phasers share a common parent. This may greatly
 * increase throughput even though it incurs greater per-operation
 * overhead.
 *
 * <p>In a tree of tiered phasers, registration and deregistration of
 * child phasers with their parent are managed automatically.
 * Whenever the number of registered parties of a child phaser becomes
 * non-zero (as established in the {@link #Phaser(Phaser,int)}
 * constructor, {@link #register}, or {@link #bulkRegister}), the
 * child phaser is registered with its parent. Whenever the number of
 * registered parties becomes zero as the result of an invocation of
 * {@link #arriveAndDeregister}, the child phaser is deregistered
 * from its parent.
 *
 * <p><b>Monitoring.</b> While synchronization methods may be invoked
 * only by registered parties, the current state of a phaser may be
 * monitored by any caller. At any given moment there are {@link
 * #getRegisteredParties} parties in total, of which {@link
 * #getArrivedParties} have arrived at the current phase ({@link
 * #getPhase}). When the remaining ({@link #getUnarrivedParties})
 * parties arrive, the phase advances. The values returned by these
 * methods may reflect transient states and so are not in general
 * useful for synchronization control. Method {@link #toString}
 * returns snapshots of these state queries in a form convenient for
 * informal monitoring.
 *
 * <p><b>Sample usages:</b>
 *
 * <p>A {@code Phaser} may be used instead of a {@code CountDownLatch}
 * to control a one-shot action serving a variable number of parties.
 * The typical idiom is for the method setting this up to first
 * register, then start all the actions, then deregister, as in:
 *
 * <pre> {@code
 * void runTasks(List<Runnable> tasks) {
 * Phaser startingGate = new Phaser(1); // "1" to register self
 * // create and start threads
 * for (Runnable task : tasks) {
 * startingGate.register();
 * new Thread(() -> {
 * startingGate.arriveAndAwaitAdvance();
 * task.run();
 * }).start();
 * }
 *
 * // deregister self to allow threads to proceed
 * startingGate.arriveAndDeregister();
 * }}</pre>
 *
 * <p>One way to cause a set of threads to repeatedly perform actions
 * for a given number of iterations is to override {@code onAdvance}:
 *
 * <pre> {@code
 * void startTasks(List<Runnable> tasks, int iterations) {
 * Phaser phaser = new Phaser() {
 * protected boolean onAdvance(int phase, int registeredParties) {
 * return phase >= iterations - 1 || registeredParties == 0;
 * }
 * };
 * phaser.register();
 * for (Runnable task : tasks) {
 * phaser.register();
 * new Thread(() -> {
 * do {
 * task.run();
 * phaser.arriveAndAwaitAdvance();
 * } while (!phaser.isTerminated());
 * }).start();
 * }
 * // allow threads to proceed; don't wait for them
 * phaser.arriveAndDeregister();
 * }}</pre>
 *
 * If the main task must later await termination, it
 * may re-register and then execute a similar loop:
 * <pre> {@code
 * // ...
 * phaser.register();
 * while (!phaser.isTerminated())
 * phaser.arriveAndAwaitAdvance();}</pre>
 *
 * <p>Related constructions may be used to await particular phase numbers
 * in contexts where you are sure that the phase will never wrap around
 * {@code Integer.MAX_VALUE}. For example:
 *
 * <pre> {@code
 * void awaitPhase(Phaser phaser, int phase) {
 * int p = phaser.register(); // assumes caller not already registered
 * while (p < phase) {
 * if (phaser.isTerminated())
 * // ... deal with unexpected termination
 * else
 * p = phaser.arriveAndAwaitAdvance();
 * }
 * phaser.arriveAndDeregister();
 * }}</pre>
 *
 * <p>To create a set of {@code n} tasks using a tree of phasers, you
 * could use code of the following form, assuming a Task class with a
 * constructor accepting a {@code Phaser} that it registers with upon
 * construction. After invocation of {@code build(new Task[n], 0, n,
 * new Phaser())}, these tasks could then be started, for example by
 * submitting to a pool:
 *
 * <pre> {@code
 * void build(Task[] tasks, int lo, int hi, Phaser ph) {
 * if (hi - lo > TASKS_PER_PHASER) {
 * for (int i = lo; i < hi; i += TASKS_PER_PHASER) {
 * int j = Math.min(i + TASKS_PER_PHASER, hi);
 * build(tasks, i, j, new Phaser(ph));
 * }
 * } else {
 * for (int i = lo; i < hi; ++i)
 * tasks[i] = new Task(ph);
 * // assumes new Task(ph) performs ph.register()
 * }
 * }}</pre>
 *
 * The best value of {@code TASKS_PER_PHASER} depends mainly on
 * expected synchronization rates. A value as low as four may
 * be appropriate for extremely small per-phase task bodies (thus
 * high rates), or up to hundreds for extremely large ones.
 *
 * <p><b>Implementation notes</b>: This implementation restricts the
 * maximum number of parties to 65535. Attempts to register additional
 * parties result in {@code IllegalStateException}. However, you can and
 * should create tiered phasers to accommodate arbitrarily large sets
 * of participants.
 *
 * @since 1.7
 * @author Doug Lea
 */
public class Phaser {
 /*
 * This class implements an extension of X10 "clocks". Thanks to
 * Vijay Saraswat for the idea, and to Vivek Sarkar for
 * enhancements to extend functionality.
 */

 /**
 * Primary state representation, holding four bit-fields:
 *
 * unarrived -- the number of parties yet to hit barrier (bits 0-15)
 * parties -- the number of parties to wait (bits 16-31)
 * phase -- the generation of the barrier (bits 32-62)
 * terminated -- set if barrier is terminated (bit 63 / sign)
 *
 * Except that a phaser with no registered parties is
 * distinguished by the otherwise illegal state of having zero
 * parties and one unarrived parties (encoded as EMPTY below).
 *
 * To efficiently maintain atomicity, these values are packed into
 * a single (atomic) long. Good performance relies on keeping
 * state decoding and encoding simple, and keeping race windows
 * short.
 *
 * All state updates are performed via CAS except initial
 * registration of a sub-phaser (i.e., one with a non-null
 * parent). In this (relatively rare) case, we use built-in
 * synchronization to lock while first registering with its
 * parent.
 *
 * The phase of a subphaser is allowed to lag that of its
 * ancestors until it is actually accessed -- see method
 * reconcileState.
 */
 private volatile long state;

 private static final int MAX_PARTIES = 0xffff;
 private static final int MAX_PHASE = Integer.MAX_VALUE;
 private static final int PARTIES_SHIFT = 16;
 private static final int PHASE_SHIFT = 32;
 private static final int UNARRIVED_MASK = 0xffff; // to mask ints
 private static final long PARTIES_MASK = 0xffff0000L; // to mask longs
 private static final long COUNTS_MASK = 0xffffffffL;
 private static final long TERMINATION_BIT = 1L << 63;

 // some special values
 private static final int ONE_ARRIVAL = 1;
 private static final int ONE_PARTY = 1 << PARTIES_SHIFT;
 private static final int ONE_DEREGISTER = ONE_ARRIVAL|ONE_PARTY;
 private static final int EMPTY = 1;

 // The following unpacking methods are usually manually inlined

 private static int unarrivedOf(long s) {
 int counts = (int)s;
 return (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
 }

 private static int partiesOf(long s) {
 return (int)s >>> PARTIES_SHIFT;
 }

 private static int phaseOf(long s) {
 return (int)(s >>> PHASE_SHIFT);
 }

 private static int arrivedOf(long s) {
 int counts = (int)s;
 return (counts == EMPTY) ? 0 :
 (counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK);
 }

 /**
 * The parent of this phaser, or null if none.
 */
 private final Phaser parent;

 /**
 * The root of phaser tree. Equals this if not in a tree.
 */
 private final Phaser root;

 /**
 * Heads of Treiber stacks for waiting threads. To eliminate
 * contention when releasing some threads while adding others, we
 * use two of them, alternating across even and odd phases.
 * Subphasers share queues with root to speed up releases.
 */
 private final AtomicReference<QNode> evenQ;
 private final AtomicReference<QNode> oddQ;

 /**
 * Returns message string for bounds exceptions on arrival.
 */
 private String badArrive(long s) {
 return "Attempted arrival of unregistered party for " +
 stateToString(s);
 }

 /**
 * Returns message string for bounds exceptions on registration.
 */
 private String badRegister(long s) {
 return "Attempt to register more than " +
 MAX_PARTIES + " parties for " + stateToString(s);
 }

 /**
 * Main implementation for methods arrive and arriveAndDeregister.
 * Manually tuned to speed up and minimize race windows for the
 * common case of just decrementing unarrived field.
 *
 * @param adjust value to subtract from state;
 * ONE_ARRIVAL for arrive,
 * ONE_DEREGISTER for arriveAndDeregister
 */
 private int doArrive(int adjust) {
 final Phaser root = this.root;
 for (;;) {
 long s = (root == this) ? state : reconcileState();
 int phase = (int)(s >>> PHASE_SHIFT);
 if (phase < 0)
 return phase;
 int counts = (int)s;
 int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
 if (unarrived <= 0)
 throw new IllegalStateException(badArrive(s));
 if (STATE.compareAndSet(this, s, s-=adjust)) {
 if (unarrived == 1) {
 long n = s & PARTIES_MASK; // base of next state
 int nextUnarrived = (int)n >>> PARTIES_SHIFT;
 if (root == this) {
 if (onAdvance(phase, nextUnarrived))
 n |= TERMINATION_BIT;
 else if (nextUnarrived == 0)
 n |= EMPTY;
 else
 n |= nextUnarrived;
 int nextPhase = (phase + 1) & MAX_PHASE;
 n |= (long)nextPhase << PHASE_SHIFT;
 STATE.compareAndSet(this, s, n);
 releaseWaiters(phase);
 }
 else if (nextUnarrived == 0) { // propagate deregistration
 phase = parent.doArrive(ONE_DEREGISTER);
 STATE.compareAndSet(this, s, s | EMPTY);
 }
 else
 phase = parent.doArrive(ONE_ARRIVAL);
 }
 return phase;
 }
 }
 }

 /**
 * Implementation of register, bulkRegister.
 *
 * @param registrations number to add to both parties and
 * unarrived fields. Must be greater than zero.
 */
 private int doRegister(int registrations) {
 // adjustment to state
 long adjust = ((long)registrations << PARTIES_SHIFT) | registrations;
 final Phaser parent = this.parent;
 int phase;
 for (;;) {
 long s = (parent == null) ? state : reconcileState();
 int counts = (int)s;
 int parties = counts >>> PARTIES_SHIFT;
 int unarrived = counts & UNARRIVED_MASK;
 if (registrations > MAX_PARTIES - parties)
 throw new IllegalStateException(badRegister(s));
 phase = (int)(s >>> PHASE_SHIFT);
 if (phase < 0)
 break;
 if (counts != EMPTY) { // not 1st registration
 if (parent == null || reconcileState() == s) {
 if (unarrived == 0) // wait out advance
 root.internalAwaitAdvance(phase, null);
 else if (STATE.compareAndSet(this, s, s + adjust))
 break;
 }
 }
 else if (parent == null) { // 1st root registration
 long next = ((long)phase << PHASE_SHIFT) | adjust;
 if (STATE.compareAndSet(this, s, next))
 break;
 }
 else {
 synchronized (this) { // 1st sub registration
 if (state == s) { // recheck under lock
 phase = parent.doRegister(1);
 if (phase < 0)
 break;
 // finish registration whenever parent registration
 // succeeded, even when racing with termination,
 // since these are part of the same "transaction".
 while (!STATE.weakCompareAndSet
 (this, s,
 ((long)phase << PHASE_SHIFT) | adjust)) {
 s = state;
 phase = (int)(root.state >>> PHASE_SHIFT);
 // assert (int)s == EMPTY;
 }
 break;
 }
 }
 }
 }
 return phase;
 }

 /**
 * Resolves lagged phase propagation from root if necessary.
 * Reconciliation normally occurs when root has advanced but
 * subphasers have not yet done so, in which case they must finish
 * their own advance by setting unarrived to parties (or if
 * parties is zero, resetting to unregistered EMPTY state).
 *
 * @return reconciled state
 */
 private long reconcileState() {
 final Phaser root = this.root;
 long s = state;
 if (root != this) {
 int phase, p;
 // CAS to root phase with current parties, tripping unarrived
 while ((phase = (int)(root.state >>> PHASE_SHIFT)) !=
 (int)(s >>> PHASE_SHIFT) &&
 !STATE.weakCompareAndSet
 (this, s,
 s = (((long)phase << PHASE_SHIFT) |
 ((phase < 0) ? (s & COUNTS_MASK) :
 (((p = (int)s >>> PARTIES_SHIFT) == 0) ? EMPTY :
 ((s & PARTIES_MASK) | p))))))
 s = state;
 }
 return s;
 }

 /**
 * Creates a new phaser with no initially registered parties, no
 * parent, and initial phase number 0. Any thread using this
 * phaser will need to first register for it.
 */
 public Phaser() {
 this(null, 0);
 }

 /**
 * Creates a new phaser with the given number of registered
 * unarrived parties, no parent, and initial phase number 0.
 *
 * @param parties the number of parties required to advance to the
 * next phase
 * @throws IllegalArgumentException if parties less than zero
 * or greater than the maximum number of parties supported
 */
 public Phaser(int parties) {
 this(null, parties);
 }

 /**
 * Equivalent to {@link #Phaser(Phaser, int) Phaser(parent, 0)}.
 *
 * @param parent the parent phaser
 */
 public Phaser(Phaser parent) {
 this(parent, 0);
 }

 /**
 * Creates a new phaser with the given parent and number of
 * registered unarrived parties. When the given parent is non-null
 * and the given number of parties is greater than zero, this
 * child phaser is registered with its parent.
 *
 * @param parent the parent phaser
 * @param parties the number of parties required to advance to the
 * next phase
 * @throws IllegalArgumentException if parties less than zero
 * or greater than the maximum number of parties supported
 */
 public Phaser(Phaser parent, int parties) {
 if (parties >>> PARTIES_SHIFT != 0)
 throw new IllegalArgumentException("Illegal number of parties");
 int phase = 0;
 this.parent = parent;
 if (parent != null) {
 final Phaser root = parent.root;
 this.root = root;
 this.evenQ = root.evenQ;
 this.oddQ = root.oddQ;
 if (parties != 0)
 phase = parent.doRegister(1);
 }
 else {
 this.root = this;
 this.evenQ = new AtomicReference<QNode>();
 this.oddQ = new AtomicReference<QNode>();
 }
 this.state = (parties == 0) ? (long)EMPTY :
 ((long)phase << PHASE_SHIFT) |
 ((long)parties << PARTIES_SHIFT) |
 ((long)parties);
 }

 /**
 * Adds a new unarrived party to this phaser. If an ongoing
 * invocation of {@link #onAdvance} is in progress, this method
 * may await its completion before returning. If this phaser has
 * a parent, and this phaser previously had no registered parties,
 * this child phaser is also registered with its parent. If
 * this phaser is terminated, the attempt to register has
 * no effect, and a negative value is returned.
 *
 * @return the arrival phase number to which this registration
 * applied. If this value is negative, then this phaser has
 * terminated, in which case registration has no effect.
 * @throws IllegalStateException if attempting to register more
 * than the maximum supported number of parties
 */
 public int register() {
 return doRegister(1);
 }

 /**
 * Adds the given number of new unarrived parties to this phaser.
 * If an ongoing invocation of {@link #onAdvance} is in progress,
 * this method may await its completion before returning. If this
 * phaser has a parent, and the given number of parties is greater
 * than zero, and this phaser previously had no registered
 * parties, this child phaser is also registered with its parent.
 * If this phaser is terminated, the attempt to register has no
 * effect, and a negative value is returned.
 *
 * @param parties the number of additional parties required to
 * advance to the next phase
 * @return the arrival phase number to which this registration
 * applied. If this value is negative, then this phaser has
 * terminated, in which case registration has no effect.
 * @throws IllegalStateException if attempting to register more
 * than the maximum supported number of parties
 * @throws IllegalArgumentException if {@code parties < 0}
 */
 public int bulkRegister(int parties) {
 if (parties < 0)
 throw new IllegalArgumentException();
 if (parties == 0)
 return getPhase();
 return doRegister(parties);
 }

 /**
 * Arrives at this phaser, without waiting for others to arrive.
 *
 * <p>It is a usage error for an unregistered party to invoke this
 * method. However, this error may result in an {@code
 * IllegalStateException} only upon some subsequent operation on
 * this phaser, if ever.
 *
 * @return the arrival phase number, or a negative value if terminated
 * @throws IllegalStateException if not terminated and the number
 * of unarrived parties would become negative
 */
 public int arrive() {
 return doArrive(ONE_ARRIVAL);
 }

 /**
 * Arrives at this phaser and deregisters from it without waiting
 * for others to arrive. Deregistration reduces the number of
 * parties required to advance in future phases. If this phaser
 * has a parent, and deregistration causes this phaser to have
 * zero parties, this phaser is also deregistered from its parent.
 *
 * <p>It is a usage error for an unregistered party to invoke this
 * method. However, this error may result in an {@code
 * IllegalStateException} only upon some subsequent operation on
 * this phaser, if ever.
 *
 * @return the arrival phase number, or a negative value if terminated
 * @throws IllegalStateException if not terminated and the number
 * of registered or unarrived parties would become negative
 */
 public int arriveAndDeregister() {
 return doArrive(ONE_DEREGISTER);
 }

 /**
 * Arrives at this phaser and awaits others. Equivalent in effect
 * to {@code awaitAdvance(arrive())}. If you need to await with
 * interruption or timeout, you can arrange this with an analogous
 * construction using one of the other forms of the {@code
 * awaitAdvance} method. If instead you need to deregister upon
 * arrival, use {@code awaitAdvance(arriveAndDeregister())}.
 *
 * <p>It is a usage error for an unregistered party to invoke this
 * method. However, this error may result in an {@code
 * IllegalStateException} only upon some subsequent operation on
 * this phaser, if ever.
 *
 * @return the arrival phase number, or the (negative)
 * {@linkplain #getPhase() current phase} if terminated
 * @throws IllegalStateException if not terminated and the number
 * of unarrived parties would become negative
 */
 public int arriveAndAwaitAdvance() {
 // Specialization of doArrive+awaitAdvance eliminating some reads/paths
 final Phaser root = this.root;
 for (;;) {
 long s = (root == this) ? state : reconcileState();
 int phase = (int)(s >>> PHASE_SHIFT);
 if (phase < 0)
 return phase;
 int counts = (int)s;
 int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
 if (unarrived <= 0)
 throw new IllegalStateException(badArrive(s));
 if (STATE.compareAndSet(this, s, s -= ONE_ARRIVAL)) {
 if (unarrived > 1)
 return root.internalAwaitAdvance(phase, null);
 if (root != this)
 return parent.arriveAndAwaitAdvance();
 long n = s & PARTIES_MASK; // base of next state
 int nextUnarrived = (int)n >>> PARTIES_SHIFT;
 if (onAdvance(phase, nextUnarrived))
 n |= TERMINATION_BIT;
 else if (nextUnarrived == 0)
 n |= EMPTY;
 else
 n |= nextUnarrived;
 int nextPhase = (phase + 1) & MAX_PHASE;
 n |= (long)nextPhase << PHASE_SHIFT;
 if (!STATE.compareAndSet(this, s, n))
 return (int)(state >>> PHASE_SHIFT); // terminated
 releaseWaiters(phase);
 return nextPhase;
 }
 }
 }

 /**
 * Awaits the phase of this phaser to advance from the given phase
 * value, returning immediately if the current phase is not equal
 * to the given phase value or this phaser is terminated.
 *
 * @param phase an arrival phase number, or negative value if
 * terminated; this argument is normally the value returned by a
 * previous call to {@code arrive} or {@code arriveAndDeregister}.
 * @return the next arrival phase number, or the argument if it is
 * negative, or the (negative) {@linkplain #getPhase() current phase}
 * if terminated
 */
 public int awaitAdvance(int phase) {
 final Phaser root = this.root;
 long s = (root == this) ? state : reconcileState();
 int p = (int)(s >>> PHASE_SHIFT);
 if (phase < 0)
 return phase;
 if (p == phase)
 return root.internalAwaitAdvance(phase, null);
 return p;
 }

 /**
 * Awaits the phase of this phaser to advance from the given phase
 * value, throwing {@code InterruptedException} if interrupted
 * while waiting, or returning immediately if the current phase is
 * not equal to the given phase value or this phaser is
 * terminated.
 *
 * @param phase an arrival phase number, or negative value if
 * terminated; this argument is normally the value returned by a
 * previous call to {@code arrive} or {@code arriveAndDeregister}.
 * @return the next arrival phase number, or the argument if it is
 * negative, or the (negative) {@linkplain #getPhase() current phase}
 * if terminated
 * @throws InterruptedException if thread interrupted while waiting
 */
 public int awaitAdvanceInterruptibly(int phase)
 throws InterruptedException {
 final Phaser root = this.root;
 long s = (root == this) ? state : reconcileState();
 int p = (int)(s >>> PHASE_SHIFT);
 if (phase < 0)
 return phase;
 if (p == phase) {
 QNode node = new QNode(this, phase, true, false, 0L);
 p = root.internalAwaitAdvance(phase, node);
 if (node.wasInterrupted)
 throw new InterruptedException();
 }
 return p;
 }

 /**
 * Awaits the phase of this phaser to advance from the given phase
 * value or the given timeout to elapse, throwing {@code
 * InterruptedException} if interrupted while waiting, or
 * returning immediately if the current phase is not equal to the
 * given phase value or this phaser is terminated.
 *
 * @param phase an arrival phase number, or negative value if
 * terminated; this argument is normally the value returned by a
 * previous call to {@code arrive} or {@code arriveAndDeregister}.
 * @param timeout how long to wait before giving up, in units of
 * {@code unit}
 * @param unit a {@code TimeUnit} determining how to interpret the
 * {@code timeout} parameter
 * @return the next arrival phase number, or the argument if it is
 * negative, or the (negative) {@linkplain #getPhase() current phase}
 * if terminated
 * @throws InterruptedException if thread interrupted while waiting
 * @throws TimeoutException if timed out while waiting
 */
 public int awaitAdvanceInterruptibly(int phase,
 long timeout, TimeUnit unit)
 throws InterruptedException, TimeoutException {
 long nanos = unit.toNanos(timeout);
 final Phaser root = this.root;
 long s = (root == this) ? state : reconcileState();
 int p = (int)(s >>> PHASE_SHIFT);
 if (phase < 0)
 return phase;
 if (p == phase) {
 QNode node = new QNode(this, phase, true, true, nanos);
 p = root.internalAwaitAdvance(phase, node);
 if (node.wasInterrupted)
 throw new InterruptedException();
 else if (p == phase)
 throw new TimeoutException();
 }
 return p;
 }

 /**
 * Forces this phaser to enter termination state. Counts of
 * registered parties are unaffected. If this phaser is a member
 * of a tiered set of phasers, then all of the phasers in the set
 * are terminated. If this phaser is already terminated, this
 * method has no effect. This method may be useful for
 * coordinating recovery after one or more tasks encounter
 * unexpected exceptions.
 */
 public void forceTermination() {
 // Only need to change root state
 final Phaser root = this.root;
 long s;
 while ((s = root.state) >= 0) {
 if (STATE.compareAndSet(root, s, s | TERMINATION_BIT)) {
 // signal all threads
 releaseWaiters(0); // Waiters on evenQ
 releaseWaiters(1); // Waiters on oddQ
 return;
 }
 }
 }

 /**
 * Returns the current phase number. The maximum phase number is
 * {@code Integer.MAX_VALUE}, after which it restarts at
 * zero. Upon termination, the phase number is negative,
 * in which case the prevailing phase prior to termination
 * may be obtained via {@code getPhase() + Integer.MIN_VALUE}.
 *
 * @return the phase number, or a negative value if terminated
 */
 public final int getPhase() {
 return (int)(root.state >>> PHASE_SHIFT);
 }

 /**
 * Returns the number of parties registered at this phaser.
 *
 * @return the number of parties
 */
 public int getRegisteredParties() {
 return partiesOf(state);
 }

 /**
 * Returns the number of registered parties that have arrived at
 * the current phase of this phaser. If this phaser has terminated,
 * the returned value is meaningless and arbitrary.
 *
 * @return the number of arrived parties
 */
 public int getArrivedParties() {
 return arrivedOf(reconcileState());
 }

 /**
 * Returns the number of registered parties that have not yet
 * arrived at the current phase of this phaser. If this phaser has
 * terminated, the returned value is meaningless and arbitrary.
 *
 * @return the number of unarrived parties
 */
 public int getUnarrivedParties() {
 return unarrivedOf(reconcileState());
 }

 /**
 * Returns the parent of this phaser, or {@code null} if none.
 *
 * @return the parent of this phaser, or {@code null} if none
 */
 public Phaser getParent() {
 return parent;
 }

 /**
 * Returns the root ancestor of this phaser, which is the same as
 * this phaser if it has no parent.
 *
 * @return the root ancestor of this phaser
 */
 public Phaser getRoot() {
 return root;
 }

 /**
 * Returns {@code true} if this phaser has been terminated.
 *
 * @return {@code true} if this phaser has been terminated
 */
 public boolean isTerminated() {
 return root.state < 0L;
 }

 /**
 * Overridable method to perform an action upon impending phase
 * advance, and to control termination. This method is invoked
 * upon arrival of the party advancing this phaser (when all other
 * waiting parties are dormant). If this method returns {@code
 * true}, this phaser will be set to a final termination state
 * upon advance, and subsequent calls to {@link #isTerminated}
 * will return true. Any (unchecked) Exception or Error thrown by
 * an invocation of this method is propagated to the party
 * attempting to advance this phaser, in which case no advance
 * occurs.
 *
 * <p>The arguments to this method provide the state of the phaser
 * prevailing for the current transition. The effects of invoking
 * arrival, registration, and waiting methods on this phaser from
 * within {@code onAdvance} are unspecified and should not be
 * relied on.
 *
 * <p>If this phaser is a member of a tiered set of phasers, then
 * {@code onAdvance} is invoked only for its root phaser on each
 * advance.
 *
 * <p>To support the most common use cases, the default
 * implementation of this method returns {@code true} when the
 * number of registered parties has become zero as the result of a
 * party invoking {@code arriveAndDeregister}. You can disable
 * this behavior, thus enabling continuation upon future
 * registrations, by overriding this method to always return
 * {@code false}:
 *
 * <pre> {@code
 * Phaser phaser = new Phaser() {
 * protected boolean onAdvance(int phase, int parties) { return false; }
 * }}</pre>
 *
 * @param phase the current phase number on entry to this method,
 * before this phaser is advanced
 * @param registeredParties the current number of registered parties
 * @return {@code true} if this phaser should terminate
 */
 protected boolean onAdvance(int phase, int registeredParties) {
 return registeredParties == 0;
 }

 /**
 * Returns a string identifying this phaser, as well as its
 * state. The state, in brackets, includes the String {@code
 * "phase = "} followed by the phase number, {@code "parties = "}
 * followed by the number of registered parties, and {@code
 * "arrived = "} followed by the number of arrived parties.
 *
 * @return a string identifying this phaser, as well as its state
 */
 public String toString() {
 return stateToString(reconcileState());
 }

 /**
 * Implementation of toString and string-based error messages.
 */
 private String stateToString(long s) {
 return super.toString() +
 "[phase = " + phaseOf(s) +
 " parties = " + partiesOf(s) +
 " arrived = " + arrivedOf(s) + "]";
 }

 // Waiting mechanics

 /**
 * Removes and signals threads from queue for phase.
 */
 private void releaseWaiters(int phase) {
 QNode q; // first element of queue
 Thread t; // its thread
 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
 while ((q = head.get()) != null &&
 q.phase != (int)(root.state >>> PHASE_SHIFT)) {
 if (head.compareAndSet(q, q.next) &&
 (t = q.thread) != null) {
 q.thread = null;
 LockSupport.unpark(t);
 }
 }
 }

 /**
 * Variant of releaseWaiters that additionally tries to remove any
 * nodes no longer waiting for advance due to timeout or
 * interrupt. Currently, nodes are removed only if they are at
 * head of queue, which suffices to reduce memory footprint in
 * most usages.
 *
 * @return current phase on exit
 */
 private int abortWait(int phase) {
 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
 for (;;) {
 Thread t;
 QNode q = head.get();
 int p = (int)(root.state >>> PHASE_SHIFT);
 if (q == null || ((t = q.thread) != null && q.phase == p))
 return p;
 if (head.compareAndSet(q, q.next) && t != null) {
 q.thread = null;
 LockSupport.unpark(t);
 }
 }
 }

 /** The number of CPUs, for spin control */
 private static final int NCPU = Runtime.getRuntime().availableProcessors();

 /**
 * The number of times to spin before blocking while waiting for
 * advance, per arrival while waiting. On multiprocessors, fully
 * blocking and waking up a large number of threads all at once is
 * usually a very slow process, so we use rechargeable spins to
 * avoid it when threads regularly arrive: When a thread in
 * internalAwaitAdvance notices another arrival before blocking,
 * and there appear to be enough CPUs available, it spins
 * SPINS_PER_ARRIVAL more times before blocking. The value trades
 * off good-citizenship vs big unnecessary slowdowns.
 */
 static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8;

 /**
 * Possibly blocks and waits for phase to advance unless aborted.
 * Call only on root phaser.
 *
 * @param phase current phase
 * @param node if non-null, the wait node to track interrupt and timeout;
 * if null, denotes noninterruptible wait
 * @return current phase
 */
 private int internalAwaitAdvance(int phase, QNode node) {
 // assert root == this;
 releaseWaiters(phase-1); // ensure old queue clean
 boolean queued = false; // true when node is enqueued
 int lastUnarrived = 0; // to increase spins upon change
 int spins = SPINS_PER_ARRIVAL;
 long s;
 int p;
 while ((p = (int)((s = state) >>> PHASE_SHIFT)) == phase) {
 if (node == null) { // spinning in noninterruptible mode
 int unarrived = (int)s & UNARRIVED_MASK;
 if (unarrived != lastUnarrived &&
 (lastUnarrived = unarrived) < NCPU)
 spins += SPINS_PER_ARRIVAL;
 boolean interrupted = Thread.interrupted();
 if (interrupted || --spins < 0) { // need node to record intr
 node = new QNode(this, phase, false, false, 0L);
 node.wasInterrupted = interrupted;
 }
 else
 Thread.onSpinWait();
 }
 else if (node.isReleasable()) // done or aborted
 break;
 else if (!queued) { // push onto queue
 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
 QNode q = node.next = head.get();
 if ((q == null || q.phase == phase) &&
 (int)(state >>> PHASE_SHIFT) == phase) // avoid stale enq
 queued = head.compareAndSet(q, node);
 }
 else {
 try {
 ForkJoinPool.managedBlock(node);
 } catch (InterruptedException cantHappen) {
 node.wasInterrupted = true;
 }
 }
 }

 if (node != null) {
 if (node.thread != null)
 node.thread = null; // avoid need for unpark()
 if (node.wasInterrupted && !node.interruptible)
 Thread.currentThread().interrupt();
 if (p == phase && (p = (int)(state >>> PHASE_SHIFT)) == phase)
 return abortWait(phase); // possibly clean up on abort
 }
 releaseWaiters(phase);
 return p;
 }

 /**
 * Wait nodes for Treiber stack representing wait queue.
 */
 static final class QNode implements ForkJoinPool.ManagedBlocker {
 final Phaser phaser;
 final int phase;
 final boolean interruptible;
 final boolean timed;
 boolean wasInterrupted;
 long nanos;
 final long deadline;
 volatile Thread thread; // nulled to cancel wait
 QNode next;

 QNode(Phaser phaser, int phase, boolean interruptible,
 boolean timed, long nanos) {
 this.phaser = phaser;
 this.phase = phase;
 this.interruptible = interruptible;
 this.nanos = nanos;
 this.timed = timed;
 this.deadline = timed ? System.nanoTime() + nanos : 0L;
 thread = Thread.currentThread();
 }

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

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

 // VarHandle mechanics
 private static final VarHandle STATE;
 static {
 try {
 MethodHandles.Lookup l = MethodHandles.lookup();
 STATE = l.findVarHandle(Phaser.class, "state", long.class);
 } catch (ReflectiveOperationException 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
 Class<?> ensureLoaded = LockSupport.class;
 }
}