/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

package java.util.concurrent;

import java.lang.invoke.MethodHandles;

import java.lang.invoke.VarHandle;

import java.util.concurrent.atomic.AtomicReference;

import java.util.concurrent.locks.LockSupport;

/**

 * A reusable synchronization barrier, similar in functionality to

 * {@link java.util.concurrent.CyclicBarrier CyclicBarrier} and

 * {@link java.util.concurrent.CountDownLatch 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;

    }