/*
 * Hunt - A refined core library for D programming language.
 *
 * Copyright (C) 2018-2019 HuntLabs
 *
 * Website: https://www.huntlabs.net/
 *
 * Licensed under the Apache-2.0 License.
 *
 */

module hunt.concurrency.AbstractQueuedSynchronizer;

import hunt.concurrency.AbstractOwnableSynchronizer;
import hunt.concurrency.atomic.AtomicHelper;

import hunt.collection.ArrayList;
import hunt.collection.Collection;
import hunt.util.DateTime;
import hunt.Exceptions;

import core.thread;
import core.sync.mutex;
import core.sync.condition;

import std.conv;
import std.datetime;


/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues.  This class is designed to
 * be a useful basis for most kinds of synchronizers that rely on a
 * single atomic {@code int} value to represent state. Subclasses
 * must define the protected methods that change this state, and which
 * define what that state means in terms of this object being acquired
 * or released.  Given these, the other methods in this class carry
 * out all queuing and blocking mechanics. Subclasses can maintain
 * other state fields, but only the atomically updated {@code int}
 * value manipulated using methods {@link #getState}, {@link
 * #setState} and {@link #compareAndSetState} is tracked with respect
 * to synchronization.
 *
 * <p>Subclasses should be defined as non-internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class.  Class
 * {@code AbstractQueuedSynchronizer} does not implement any
 * synchronization interface.  Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as
 * appropriate by concrete locks and related synchronizers to
 * implement their methods.
 *
 * <p>This class supports either or both a default <em>exclusive</em>
 * mode and a <em>shared</em> mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. This class
 * does not &quot;understand&quot; these differences except in the
 * mechanical sense that when a shared mode acquire succeeds, the next
 * waiting thread (if one exists) must also determine whether it can
 * acquire as well. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 *
 * <p>This class defines a nested {@link ConditionObject} class that
 * can be used as a {@link Condition} implementation by subclasses
 * supporting exclusive mode for which method {@link
 * #isHeldExclusively} reports whether synchronization is exclusively
 * held with respect to the current thread, method {@link #release}
 * invoked with the current {@link #getState} value fully releases
 * this object, and {@link #acquire}, given this saved state value,
 * eventually restores this object to its previous acquired state.  No
 * {@code AbstractQueuedSynchronizer} method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.  The
 * behavior of {@link ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *
 * <p>This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an {@code AbstractQueuedSynchronizer} for their
 * synchronization mechanics.
 *
 * <p>Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a {@code readObject} method that restores this to a known
 * initial state upon deserialization.
 *
 * <h3>Usage</h3>
 *
 * <p>To use this class as the basis of a synchronizer, redefine the
 * following methods, as applicable, by inspecting and/or modifying
 * the synchronization state using {@link #getState}, {@link
 * #setState} and/or {@link #compareAndSetState}:
 *
 * <ul>
 * <li>{@link #tryAcquire}
 * <li>{@link #tryRelease}
 * <li>{@link #tryAcquireShared}
 * <li>{@link #tryReleaseShared}
 * <li>{@link #isHeldExclusively}
 * </ul>
 *
 * Each of these methods by default throws {@link
 * UnsupportedOperationException}.  Implementations of these methods
 * must be internally thread-safe, and should in general be short and
 * not block. Defining these methods is the <em>only</em> supported
 * means of using this class. All other methods are declared
 * {@code final} because they cannot be independently varied.
 *
 * <p>You may also find the inherited methods from {@link
 * AbstractOwnableSynchronizer} useful to keep track of the thread
 * owning an exclusive synchronizer.  You are encouraged to use them
 * -- this enables monitoring and diagnostic tools to assist users in
 * determining which threads hold locks.
 *
 * <p>Even though this class is based on an internal FIFO queue, it
 * does not automatically enforce FIFO acquisition policies.  The core
 * of exclusive synchronization takes the form:
 *
 * <pre>
 * Acquire:
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        <em>possibly block current thread</em>;
 *     }
 *
 * Release:
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 * </pre>
 *
 * (Shared mode is similar but may involve cascading signals.)
 *
 * <p id="barging">Because checks in acquire are invoked before
 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
 * others that are blocked and queued.  However, you can, if desired,
 * define {@code tryAcquire} and/or {@code tryAcquireShared} to
 * disable barging by internally invoking one or more of the inspection
 * methods, thereby providing a <em>fair</em> FIFO acquisition order.
 * In particular, most fair synchronizers can define {@code tryAcquire}
 * to return {@code false} if {@link #hasQueuedPredecessors} (a method
 * specifically designed to be used by fair synchronizers) returns
 * {@code true}.  Other variations are possible.
 *
 * <p>Throughput and scalability are generally highest for the
 * default barging (also known as <em>greedy</em>,
 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
 * While this is not guaranteed to be fair or starvation-free, earlier
 * queued threads are allowed to recontend before later queued
 * threads, and each recontention has an unbiased chance to succeed
 * against incoming threads.  Also, while acquires do not
 * &quot;spin&quot; in the usual sense, they may perform multiple
 * invocations of {@code tryAcquire} interspersed with other
 * computations before blocking.  This gives most of the benefits of
 * spins when exclusive synchronization is only briefly held, without
 * most of the liabilities when it isn't. If so desired, you can
 * augment this by preceding calls to acquire methods with
 * "fast-path" checks, possibly prechecking {@link #hasContended}
 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 * is likely not to be contended.
 *
 * <p>This class provides an efficient and scalable basis for
 * synchronization in part by specializing its range of use to
 * synchronizers that can rely on {@code int} state, acquire, and
 * release parameters, and an internal FIFO wait queue. When this does
 * not suffice, you can build synchronizers from a lower level using
 * {@link hunt.concurrency.atomic atomic} classes, your own custom
 * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 * support.
 *
 * <h3>Usage Examples</h3>
 *
 * <p>Here is a non-reentrant mutual exclusion lock class that uses
 * the value zero to represent the unlocked state, and one to
 * represent the locked state. While a non-reentrant lock
 * does not strictly require recording of the current owner
 * thread, this class does so anyway to make usage easier to monitor.
 * It also supports conditions and exposes some instrumentation methods:
 *
 * <pre> {@code
 * class Mutex implements Lock, java.io.Serializable {
 *
 *   // Our internal helper class
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     // Acquires the lock if state is zero
 *     bool tryAcquire(int acquires) {
 *       assert acquires == 1; // Otherwise unused
 *       if (compareAndSetState(0, 1)) {
 *         setExclusiveOwnerThread(Thread.getThis());
 *         return true;
 *       }
 *       return false;
 *     }
 *
 *     // Releases the lock by setting state to zero
 *     protected bool tryRelease(int releases) {
 *       assert releases == 1; // Otherwise unused
 *       if (!isHeldExclusively())
 *         throw new IllegalMonitorStateException();
 *       setExclusiveOwnerThread(null);
 *       setState(0);
 *       return true;
 *     }
 *
 *     // Reports whether in locked state
 *     bool isLocked() {
 *       return getState() != 0;
 *     }
 *
 *     bool isHeldExclusively() {
 *       // a data race, but safe due to out-of-thin-air guarantees
 *       return getExclusiveOwnerThread() == Thread.getThis();
 *     }
 *
 *     // Provides a Condition
 *     Condition newCondition() {
 *       return new ConditionObject();
 *     }
 *
 *     // Deserializes properly
 *     private void readObject(ObjectInputStream s)
 *         throws IOException, ClassNotFoundException {
 *       s.defaultReadObject();
 *       setState(0); // reset to unlocked state
 *     }
 *   }
 *
 *   // The sync object does all the hard work. We just forward to it.
 *   private final Sync sync = new Sync();
 *
 *   void lock()              { sync.acquire(1); }
 *   bool tryLock()        { return sync.tryAcquire(1); }
 *   void unlock()            { sync.release(1); }
 *   Condition newCondition() { return sync.newCondition(); }
 *   bool isLocked()       { return sync.isLocked(); }
 *   bool isHeldByCurrentThread() {
 *     return sync.isHeldExclusively();
 *   }
 *   bool hasQueuedThreads() {
 *     return sync.hasQueuedThreads();
 *   }
 *   void lockInterruptibly() {
 *     sync.acquireInterruptibly(1);
 *   }
 *   bool tryLock(Duration timeout)
 *       {
 *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 *   }
 * }}</pre>
 *
 * <p>Here is a latch class that is like a
 * {@link hunt.concurrency.CountDownLatch CountDownLatch}
 * except that it only requires a single {@code signal} to
 * fire. Because a latch is non-exclusive, it uses the {@code shared}
 * acquire and release methods.
 *
 * <pre> {@code
 * class BooleanLatch {
 *
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     bool isSignalled() { return getState() != 0; }
 *
 *     protected int tryAcquireShared(int ignore) {
 *       return isSignalled() ? 1 : -1;
 *     }
 *
 *     protected bool tryReleaseShared(int ignore) {
 *       setState(1);
 *       return true;
 *     }
 *   }
 *
 *   private final Sync sync = new Sync();
 *   bool isSignalled() { return sync.isSignalled(); }
 *   void signal()         { sync.releaseShared(1); }
 *   void await() {
 *     sync.acquireSharedInterruptibly(1);
 *   }
 * }}</pre>
 *
 * @author Doug Lea
 */
abstract class AbstractQueuedSynchronizer : AbstractOwnableSynchronizer {


    /**
     * Creates a new {@code AbstractQueuedSynchronizer} instance
     * with initial synchronization state of zero.
     */
    protected this() { }

    /**
     * Head of the wait queue, lazily initialized.  Except for
     * initialization, it is modified only via method setHead.  Note:
     * If head exists, its waitStatus is guaranteed not to be
     * CANCELLED.
     */
    private Node head;

    /**
     * Tail of the wait queue, lazily initialized.  Modified only via
     * method enq to add new wait node.
     */
    private Node tail;

    /**
     * The synchronization state.
     */
    private int state;

    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a {@code volatile} read.
     * @return current state value
     */
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a {@code volatile} write.
     * @param newState the new state value
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * Atomically sets synchronization state to the given updated
     * value if the current state value equals the expected value.
     * This operation has memory semantics of a {@code volatile} read
     * and write.
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that the actual
     *         value was not equal to the expected value.
     */
    protected final bool compareAndSetState(int expect, int update) {
        return AtomicHelper.compareAndSet(state, expect, update);
    }

    // Queuing utilities

    /**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices
     * to improve responsiveness with very short timeouts.
     */
    enum long SPIN_FOR_TIMEOUT_THRESHOLD = 1000L;

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     */
    private Node enq(Node node) {
        for (;;) {
            Node oldTail = tail;
            if (oldTail !is null) {
                node.setPrevRelaxed(oldTail);
                if (compareAndSetTail(oldTail, node)) {
                    oldTail.next = node;
                    return oldTail;
                }
            } else {
                initializeSyncQueue();
            }
        }
    }

    /**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        Node node = new Node(mode);

        for (;;) {
            Node oldTail = tail;
            if (oldTail !is null) {
                node.setPrevRelaxed(oldTail);
                if (compareAndSetTail(oldTail, node)) {
                    oldTail.next = node;
                    return node;
                }
            } else {
                initializeSyncQueue();
            }
        }
    }

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by
     * acquire methods.  Also nulls out unused fields for sake of GC
     * and to suppress unnecessary signals and traversals.
     *
     * @param node the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

    /**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            node.compareAndSetWaitStatus(ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s is null || s.waitStatus > 0) {
            s = null;
            for (Node p = tail; p != node && p !is null; p = p.prev)
                if (p.waitStatus <= 0)
                    s = p;
        }
        implementationMissing(false);
        // if (s !is null)
        //     LockSupport.unpark(s.thread);
    }

    /**
     * Release action for shared mode -- signals successor and ensures
     * propagation. (Note: For exclusive mode, release just amounts
     * to calling unparkSuccessor of head if it needs signal.)
     */
    private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (;;) {
            Node h = head;
            if (h !is null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!h.compareAndSetWaitStatus(Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !h.compareAndSetWaitStatus(0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

    /**
     * Sets head of queue, and checks if successor may be waiting
     * in shared mode, if so propagating if either propagate > 0 or
     * PROPAGATE status was set.
     *
     * @param node the node
     * @param propagate the return value from a tryAcquireShared
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        setHead(node);
        /*
         * Try to signal next queued node if:
         *   Propagation was indicated by caller,
         *     or was recorded (as h.waitStatus either before
         *     or after setHead) by a previous operation
         *     (note: this uses sign-check of waitStatus because
         *      PROPAGATE status may transition to SIGNAL.)
         * and
         *   The next node is waiting in shared mode,
         *     or we don't know, because it appears null
         *
         * The conservatism in both of these checks may cause
         * unnecessary wake-ups, but only when there are multiple
         * racing acquires/releases, so most need signals now or soon
         * anyway.
         */
        if (propagate > 0 || h is null || h.waitStatus < 0 ||
            (h = head) is null || h.waitStatus < 0) {
            Node s = node.next;
            if (s is null || s.isShared())
                doReleaseShared();
        }
    }

    // Utilities for various versions of acquire

    /**
     * Cancels an ongoing attempt to acquire.
     *
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node is null)
            return;

        node.thread = null;

        // Skip cancelled predecessors
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary, although with
        // a possibility that a cancelled node may transiently remain
        // reachable.
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves.
        if (node == tail && compareAndSetTail(node, pred)) {
            pred.compareAndSetNext(predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            int ws;
            if (pred != head &&
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && pred.compareAndSetWaitStatus(ws, Node.SIGNAL))) &&
                pred.thread !is null) {
                Node next = node.next;
                if (next !is null && next.waitStatus <= 0)
                    pred.compareAndSetNext(predNext, next);
            } else {
                unparkSuccessor(node);
            }

            node.next = node; // help GC
        }
    }

    /**
     * Checks and updates status for a node that failed to acquire.
     * Returns true if thread should block. This is the main signal
     * control in all acquire loops.  Requires that pred == node.prev.
     *
     * @param pred node's predecessor holding status
     * @param node the node
     * @return {@code true} if thread should block
     */
    private static bool shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
            pred.compareAndSetWaitStatus(ws, Node.SIGNAL);
        }
        return false;
    }

    /**
     * Convenience method to interrupt current thread.
     */
    static void selfInterrupt() {
        // Thread.getThis().interrupt();
        implementationMissing(false);
    }

    /**
     * Convenience method to park and then check if interrupted.
     *
     * @return {@code true} if interrupted
     */
    private final bool parkAndCheckInterrupt() {
        // LockSupport.park(this);
        // return Thread.interrupted();
        implementationMissing(false);
        return true;
    }

    /*
     * Various flavors of acquire, varying in exclusive/shared and
     * control modes.  Each is mostly the same, but annoyingly
     * different.  Only a little bit of factoring is possible due to
     * interactions of exception mechanics (including ensuring that we
     * cancel if tryAcquire throws exception) and other control, at
     * least not without hurting performance too much.
     */

    /**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     *
     * @param node the node
     * @param arg the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final bool acquireQueued(Node node, int arg) {
        bool interrupted = false;
        try {
            for (;;) {
                Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node))
                    interrupted |= parkAndCheckInterrupt();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            if (interrupted)
                selfInterrupt();
            throw t;
        }
    }

    /**
     * Acquires in exclusive interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireInterruptibly(int arg) {
        Node node = addWaiter(Node.EXCLUSIVE);
        try {
            for (;;) {
                Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }

    /**
     * Acquires in exclusive timed mode.
     *
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private bool doAcquireNanos(int arg, long nanosTimeout) {
        if (nanosTimeout <= 0L)
            return false;
        long deadline = Clock.currStdTime() + nanosTimeout;
        Node node = addWaiter(Node.EXCLUSIVE);
        try {
            for (;;) {
                Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return true;
                }
                nanosTimeout = deadline - Clock.currStdTime();
                if (nanosTimeout <= 0L) {
                    cancelAcquire(node);
                    return false;
                }
                implementationMissing(false);
                // if (shouldParkAfterFailedAcquire(p, node) &&
                //     nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
                //     LockSupport.parkNanos(this, nanosTimeout);
                // if (Thread.interrupted())
                //     throw new InterruptedException();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }

    /**
     * Acquires in shared uninterruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireShared(int arg) {
        Node node = addWaiter(Node.SHARED);
        bool interrupted = false;
        try {
            for (;;) {
                Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node))
                    interrupted |= parkAndCheckInterrupt();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            throw t;
        } finally {
            if (interrupted)
                selfInterrupt();
        }
    }

    /**
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg) {
        Node node = addWaiter(Node.SHARED);
        try {
            for (;;) {
                Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }

    /**
     * Acquires in shared timed mode.
     *
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private bool doAcquireSharedNanos(int arg, long nanosTimeout) {
        if (nanosTimeout <= 0L)
            return false;
        long deadline = Clock.currStdTime() + nanosTimeout;
        Node node = addWaiter(Node.SHARED);
        try {
            for (;;) {
                Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        return true;
                    }
                }
                nanosTimeout = deadline - Clock.currStdTime();
                if (nanosTimeout <= 0L) {
                    cancelAcquire(node);
                    return false;
                }
                implementationMissing(false);
                // if (shouldParkAfterFailedAcquire(p, node) &&
                //     nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
                //     LockSupport.parkNanos(this, nanosTimeout);
                // if (Thread.interrupted())
                //     throw new InterruptedException();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }

    // Main exported methods

    /**
     * Attempts to acquire in exclusive mode. This method should query
     * if the state of the object permits it to be acquired in the
     * exclusive mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread. This can be used
     * to implement method {@link Lock#tryLock()}.
     *
     * <p>The default
     * implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *        passed to an acquire method, or is the value saved on entry
     *        to a condition wait.  The value is otherwise uninterpreted
     *        and can represent anything you like.
     * @return {@code true} if successful. Upon success, this object has
     *         been acquired.
     * @throws IllegalMonitorStateException if acquiring would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected bool tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in exclusive
     * mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *        passed to a release method, or the current state value upon
     *        entry to a condition wait.  The value is otherwise
     *        uninterpreted and can represent anything you like.
     * @return {@code true} if this object is now in a fully released
     *         state, so that any waiting threads may attempt to acquire;
     *         and {@code false} otherwise.
     * @throws IllegalMonitorStateException if releasing would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected bool tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to acquire in shared mode. This method should query if
     * the state of the object permits it to be acquired in the shared
     * mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *        passed to an acquire method, or is the value saved on entry
     *        to a condition wait.  The value is otherwise uninterpreted
     *        and can represent anything you like.
     * @return a negative value on failure; zero if acquisition in shared
     *         mode succeeded but no subsequent shared-mode acquire can
     *         succeed; and a positive value if acquisition in shared
     *         mode succeeded and subsequent shared-mode acquires might
     *         also succeed, in which case a subsequent waiting thread
     *         must check availability. (Support for three different
     *         return values enables this method to be used in contexts
     *         where acquires only sometimes act exclusively.)  Upon
     *         success, this object has been acquired.
     * @throws IllegalMonitorStateException if acquiring would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in shared mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *        passed to a release method, or the current state value upon
     *        entry to a condition wait.  The value is otherwise
     *        uninterpreted and can represent anything you like.
     * @return {@code true} if this release of shared mode may permit a
     *         waiting acquire (shared or exclusive) to succeed; and
     *         {@code false} otherwise
     * @throws IllegalMonitorStateException if releasing would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected bool tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Returns {@code true} if synchronization is held exclusively with
     * respect to the current (calling) thread.  This method is invoked
     * upon each call to a {@link ConditionObject} method.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}. This method is invoked
     * internally only within {@link ConditionObject} methods, so need
     * not be defined if conditions are not used.
     *
     * @return {@code true} if synchronization is held exclusively;
     *         {@code false} otherwise
     * @throws UnsupportedOperationException if conditions are not supported
     */
    protected bool isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

    /**
     * Acquires in exclusive mode, ignoring interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     */
    final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    /**
     * Acquires in exclusive mode, aborting if interrupted.
     * Implemented by first checking interrupt status, then invoking
     * at least once {@link #tryAcquire}, returning on
     * success.  Otherwise the thread is queued, possibly repeatedly
     * blocking and unblocking, invoking {@link #tryAcquire}
     * until success or the thread is interrupted.  This method can be
     * used to implement method {@link Lock#lockInterruptibly}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    final void acquireInterruptibly(int arg) {
        // if (Thread.interrupted())
        //     throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }

    /**
     * Attempts to acquire in exclusive mode, aborting if interrupted,
     * and failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquire}, returning on success.  Otherwise, the thread is
     * queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquire} until success or the thread is interrupted
     * or the timeout elapses.  This method can be used to implement
     * method {@link Lock#tryLock(long, TimeUnit)}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */
    final bool tryAcquireNanos(int arg, long nanosTimeout) {
        // if (Thread.interrupted())
        //     throw new InterruptedException();
        return tryAcquire(arg) ||
            doAcquireNanos(arg, nanosTimeout);
    }

    /**
     * Releases in exclusive mode.  Implemented by unblocking one or
     * more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryRelease} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    final bool release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h !is null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    /**
     * Acquires in shared mode, ignoring interrupts.  Implemented by
     * first invoking at least once {@link #tryAcquireShared},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquireShared} until success.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquireShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     */
    final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }

    /**
     * Acquires in shared mode, aborting if interrupted.  Implemented
     * by first checking interrupt status, then invoking at least once
     * {@link #tryAcquireShared}, returning on success.  Otherwise the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted.
     * @param arg the acquire argument.
     * This value is conveyed to {@link #tryAcquireShared} but is
     * otherwise uninterpreted and can represent anything
     * you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    final void acquireSharedInterruptibly(int arg) {
        // if (Thread.interrupted())
        //     throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

    /**
     * Attempts to acquire in shared mode, aborting if interrupted, and
     * failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquireShared}, returning on success.  Otherwise, the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted or the timeout elapses.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquireShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */
    final bool tryAcquireSharedNanos(int arg, long nanosTimeout) {
        // if (Thread.interrupted())
        //     throw new InterruptedException();
        return tryAcquireShared(arg) >= 0 ||
            doAcquireSharedNanos(arg, nanosTimeout);
    }

    /**
     * Releases in shared mode.  Implemented by unblocking one or more
     * threads if {@link #tryReleaseShared} returns true.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryReleaseShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @return the value returned from {@link #tryReleaseShared}
     */
    final bool releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

    // Queue inspection methods

    /**
     * Queries whether any threads are waiting to acquire. Note that
     * because cancellations due to interrupts and timeouts may occur
     * at any time, a {@code true} return does not guarantee that any
     * other thread will ever acquire.
     *
     * @return {@code true} if there may be other threads waiting to acquire
     */
    final bool hasQueuedThreads() {
        for (Node p = tail, h = head; p != h && p !is null; p = p.prev)
            if (p.waitStatus <= 0)
                return true;
        return false;
    }

    /**
     * Queries whether any threads have ever contended to acquire this
     * synchronizer; that is, if an acquire method has ever blocked.
     *
     * <p>In this implementation, this operation returns in
     * constant time.
     *
     * @return {@code true} if there has ever been contention
     */
    final bool hasContended() {
        return head !is null;
    }

    /**
     * Returns the first (longest-waiting) thread in the queue, or
     * {@code null} if no threads are currently queued.
     *
     * <p>In this implementation, this operation normally returns in
     * constant time, but may iterate upon contention if other threads are
     * concurrently modifying the queue.
     *
     * @return the first (longest-waiting) thread in the queue, or
     *         {@code null} if no threads are currently queued
     */
    final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }

    /**
     * Version of getFirstQueuedThread called when fastpath fails.
     */
    private Thread fullGetFirstQueuedThread() {
        /*
         * The first node is normally head.next. Try to get its
         * thread field, ensuring consistent reads: If thread
         * field is nulled out or s.prev is no longer head, then
         * some other thread(s) concurrently performed setHead in
         * between some of our reads. We try this twice before
         * resorting to traversal.
         */
        Node h, s;
        Thread st;
        if (((h = head) !is null && (s = h.next) !is null &&
             s.prev == head && (st = s.thread) !is null) ||
            ((h = head) !is null && (s = h.next) !is null &&
             s.prev == head && (st = s.thread) !is null))
            return st;

        /*
         * Head's next field might not have been set yet, or may have
         * been unset after setHead. So we must check to see if tail
         * is actually first node. If not, we continue on, safely
         * traversing from tail back to head to find first,
         * guaranteeing termination.
         */

        Thread firstThread = null;
        for (Node p = tail; p !is null && p != head; p = p.prev) {
            Thread t = p.thread;
            if (t !is null)
                firstThread = t;
        }
        return firstThread;
    }

    /**
     * Returns true if the given thread is currently queued.
     *
     * <p>This implementation traverses the queue to determine
     * presence of the given thread.
     *
     * @param thread the thread
     * @return {@code true} if the given thread is on the queue
     * @throws NullPointerException if the thread is null
     */
    final bool isQueued(Thread thread) {
        if (thread is null)
            throw new NullPointerException();
        for (Node p = tail; p !is null; p = p.prev)
            if (p.thread == thread)
                return true;
        return false;
    }

    /**
     * Returns {@code true} if the apparent first queued thread, if one
     * exists, is waiting in exclusive mode.  If this method returns
     * {@code true}, and the current thread is attempting to acquire in
     * shared mode (that is, this method is invoked from {@link
     * #tryAcquireShared}) then it is guaranteed that the current thread
     * is not the first queued thread.  Used only as a heuristic in
     * ReentrantReadWriteLock.
     */
    final bool apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) !is null &&
            (s = h.next)  !is null &&
            !s.isShared()         &&
            s.thread !is null;
    }

    /**
     * Queries whether any threads have been waiting to acquire longer
     * than the current thread.
     *
     * <p>An invocation of this method is equivalent to (but may be
     * more efficient than):
     * <pre> {@code
     * getFirstQueuedThread() != Thread.getThis()
     *   && hasQueuedThreads()}</pre>
     *
     * <p>Note that because cancellations due to interrupts and
     * timeouts may occur at any time, a {@code true} return does not
     * guarantee that some other thread will acquire before the current
     * thread.  Likewise, it is possible for another thread to win a
     * race to enqueue after this method has returned {@code false},
     * due to the queue being empty.
     *
     * <p>This method is designed to be used by a fair synchronizer to
     * avoid <a href="AbstractQueuedSynchronizer.html#barging">barging</a>.
     * Such a synchronizer's {@link #tryAcquire} method should return
     * {@code false}, and its {@link #tryAcquireShared} method should
     * return a negative value, if this method returns {@code true}
     * (unless this is a reentrant acquire).  For example, the {@code
     * tryAcquire} method for a fair, reentrant, exclusive mode
     * synchronizer might look like this:
     *
     * <pre> {@code
     * protected bool tryAcquire(int arg) {
     *   if (isHeldExclusively()) {
     *     // A reentrant acquire; increment hold count
     *     return true;
     *   } else if (hasQueuedPredecessors()) {
     *     return false;
     *   } else {
     *     // try to acquire normally
     *   }
     * }}</pre>
     *
     * @return {@code true} if there is a queued thread preceding the
     *         current thread, and {@code false} if the current thread
     *         is at the head of the queue or the queue is empty
     */
    final bool hasQueuedPredecessors() {
        Node h, s;
        if ((h = head) !is null) {
            if ((s = h.next) is null || s.waitStatus > 0) {
                s = null; // traverse in case of concurrent cancellation
                for (Node p = tail; p != h && p !is null; p = p.prev) {
                    if (p.waitStatus <= 0)
                        s = p;
                }
            }
            if (s !is null && s.thread != Thread.getThis())
                return true;
        }
        return false;
    }

    // Instrumentation and monitoring methods

    /**
     * Returns an estimate of the number of threads waiting to
     * acquire.  The value is only an estimate because the number of
     * threads may change dynamically while this method traverses
     * internal data structures.  This method is designed for use in
     * monitoring system state, not for synchronization control.
     *
     * @return the estimated number of threads waiting to acquire
     */
    final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p !is null; p = p.prev) {
            if (p.thread !is null)
                ++n;
        }
        return n;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate.  The elements of the
     * returned collection are in no particular order.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     *
     * @return the collection of threads
     */
    final Collection!(Thread) getQueuedThreads() {
        ArrayList!(Thread) list = new ArrayList!(Thread)();
        for (Node p = tail; p !is null; p = p.prev) {
            Thread t = p.thread;
            if (t !is null)
                list.add(t);
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in exclusive mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to an exclusive acquire.
     *
     * @return the collection of threads
     */
    final Collection!(Thread) getExclusiveQueuedThreads() {
        ArrayList!(Thread) list = new ArrayList!(Thread)();
        for (Node p = tail; p !is null; p = p.prev) {
            if (!p.isShared()) {
                Thread t = p.thread;
                if (t !is null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in shared mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to a shared acquire.
     *
     * @return the collection of threads
     */
    final Collection!(Thread) getSharedQueuedThreads() {
        ArrayList!(Thread) list = new ArrayList!(Thread)();
        for (Node p = tail; p !is null; p = p.prev) {
            if (p.isShared()) {
                Thread t = p.thread;
                if (t !is null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a string identifying this synchronizer, as well as its state.
     * The state, in brackets, includes the string {@code "State ="}
     * followed by the current value of {@link #getState}, and either
     * {@code "nonempty"} or {@code "empty"} depending on whether the
     * queue is empty.
     *
     * @return a string identifying this synchronizer, as well as its state
     */
    override string toString() {
        return super.toString()
            ~ "[State = " ~ getState().to!string() ~ ", "
            ~ (hasQueuedThreads() ? "non" : "") ~ "empty queue]";
    }


    // Internal support methods for Conditions

    /**
     * Returns true if a node, always one that was initially placed on
     * a condition queue, is now waiting to reacquire on sync queue.
     * @param node the node
     * @return true if is reacquiring
     */
    final bool isOnSyncQueue(Node node) {
        if (node.waitStatus == Node.CONDITION || node.prev is null)
            return false;
        if (node.next !is null) // If has successor, it must be on queue
            return true;
        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
        return findNodeFromTail(node);
    }

    /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     * @return true if present
     */
    private bool findNodeFromTail(Node node) {
        // We check for node first, since it's likely to be at or near tail.
        // tail is known to be non-null, so we could re-order to "save"
        // one null check, but we leave it this way to help the VM.
        for (Node p = tail;;) {
            if (p == node)
                return true;
            if (p is null)
                return false;
            p = p.prev;
        }
    }

    /**
     * Transfers a node from a condition queue onto sync queue.
     * Returns true if successful.
     * @param node the node
     * @return true if successfully transferred (else the node was
     * cancelled before signal)
     */
    final bool transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!node.compareAndSetWaitStatus(Node.CONDITION, 0))
            return false;

        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        implementationMissing(false);
        // if (ws > 0 || !p.compareAndSetWaitStatus(ws, Node.SIGNAL))
        //     LockSupport.unpark(node.thread);
        return true;
    }

    /**
     * Transfers node, if necessary, to sync queue after a cancelled wait.
     * Returns true if thread was cancelled before being signalled.
     *
     * @param node the node
     * @return true if cancelled before the node was signalled
     */
    final bool transferAfterCancelledWait(Node node) {
        if (node.compareAndSetWaitStatus(Node.CONDITION, 0)) {
            enq(node);
            return true;
        }
        /*
         * If we lost out to a signal(), then we can't proceed
         * until it finishes its enq().  Cancelling during an
         * incomplete transfer is both rare and transient, so just
         * spin.
         */
        while (!isOnSyncQueue(node))
            Thread.yield();
        return false;
    }

    /**
     * Invokes release with current state value; returns saved state.
     * Cancels node and throws exception on failure.
     * @param node the condition node for this wait
     * @return previous sync state
     */
    final int fullyRelease(Node node) {
        try {
            int savedState = getState();
            if (release(savedState))
                return savedState;
            throw new IllegalMonitorStateException();
        } catch (Throwable t) {
            node.waitStatus = Node.CANCELLED;
            throw t;
        }
    }

    // Instrumentation methods for conditions

    /**
     * Queries whether the given ConditionObject
     * uses this synchronizer as its lock.
     *
     * @param condition the condition
     * @return {@code true} if owned
     * @throws NullPointerException if the condition is null
     */
    // final bool owns(ConditionObject condition) {
    //     return condition.isOwnedBy(this);
    // }

    /**
     * Queries whether any threads are waiting on the given condition
     * associated with this synchronizer. Note that because timeouts
     * and interrupts may occur at any time, a {@code true} return
     * does not guarantee that a future {@code signal} will awaken
     * any threads.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @param condition the condition
     * @return {@code true} if there are any waiting threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    // final bool hasWaiters(ConditionObject condition) {
    //     if (!owns(condition))
    //         throw new IllegalArgumentException("Not owner");
    //     return condition.hasWaiters();
    // }

    /**
     * Returns an estimate of the number of threads waiting on the
     * given condition associated with this synchronizer. Note that
     * because timeouts and interrupts may occur at any time, the
     * estimate serves only as an upper bound on the actual number of
     * waiters.  This method is designed for use in monitoring system
     * state, not for synchronization control.
     *
     * @param condition the condition
     * @return the estimated number of waiting threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    // final int getWaitQueueLength(ConditionObject condition) {
    //     if (!owns(condition))
    //         throw new IllegalArgumentException("Not owner");
    //     return condition.getWaitQueueLength();
    // }

    /**
     * Returns a collection containing those threads that may be
     * waiting on the given condition associated with this
     * synchronizer.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate. The elements of the
     * returned collection are in no particular order.
     *
     * @param condition the condition
     * @return the collection of threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    // final Collection!(Thread) getWaitingThreads(ConditionObject condition) {
    //     if (!owns(condition))
    //         throw new IllegalArgumentException("Not owner");
    //     return condition.getWaitingThreads();
    // }

    /**
     * Condition implementation for a {@link AbstractQueuedSynchronizer}
     * serving as the basis of a {@link Lock} implementation.
     *
     * <p>Method documentation for this class describes mechanics,
     * not behavioral specifications from the point of view of Lock
     * and Condition users. Exported versions of this class will in
     * general need to be accompanied by documentation describing
     * condition semantics that rely on those of the associated
     * {@code AbstractQueuedSynchronizer}.
     *
     * <p>This class is Serializable, but all fields are transient,
     * so deserialized conditions have no waiters.
     */
    // class ConditionObject : Condition {
    //     /** First node of condition queue. */
    //     private Node firstWaiter;
    //     /** Last node of condition queue. */
    //     private Node lastWaiter;

    //     /**
    //      * Creates a new {@code ConditionObject} instance.
    //      */
    //     this() { }

    //     // Internal methods

    //     /**
    //      * Adds a new waiter to wait queue.
    //      * @return its new wait node
    //      */
    //     private Node addConditionWaiter() {
    //         if (!isHeldExclusively())
    //             throw new IllegalMonitorStateException();
    //         Node t = lastWaiter;
    //         // If lastWaiter is cancelled, clean out.
    //         if (t !is null && t.waitStatus != Node.CONDITION) {
    //             unlinkCancelledWaiters();
    //             t = lastWaiter;
    //         }

    //         Node node = new Node(Node.CONDITION);

    //         if (t is null)
    //             firstWaiter = node;
    //         else
    //             t.nextWaiter = node;
    //         lastWaiter = node;
    //         return node;
    //     }

    //     /**
    //      * Removes and transfers nodes until hit non-cancelled one or
    //      * null. Split out from signal in part to encourage compilers
    //      * to inline the case of no waiters.
    //      * @param first (non-null) the first node on condition queue
    //      */
    //     private void doSignal(Node first) {
    //         do {
    //             if ( (firstWaiter = first.nextWaiter) is null)
    //                 lastWaiter = null;
    //             first.nextWaiter = null;
    //         } while (!transferForSignal(first) &&
    //                  (first = firstWaiter) !is null);
    //     }

    //     /**
    //      * Removes and transfers all nodes.
    //      * @param first (non-null) the first node on condition queue
    //      */
    //     private void doSignalAll(Node first) {
    //         lastWaiter = firstWaiter = null;
    //         do {
    //             Node next = first.nextWaiter;
    //             first.nextWaiter = null;
    //             transferForSignal(first);
    //             first = next;
    //         } while (first !is null);
    //     }

    //     /**
    //      * Unlinks cancelled waiter nodes from condition queue.
    //      * Called only while holding lock. This is called when
    //      * cancellation occurred during condition wait, and upon
    //      * insertion of a new waiter when lastWaiter is seen to have
    //      * been cancelled. This method is needed to avoid garbage
    //      * retention in the absence of signals. So even though it may
    //      * require a full traversal, it comes into play only when
    //      * timeouts or cancellations occur in the absence of
    //      * signals. It traverses all nodes rather than stopping at a
    //      * particular target to unlink all pointers to garbage nodes
    //      * without requiring many re-traversals during cancellation
    //      * storms.
    //      */
    //     private void unlinkCancelledWaiters() {
    //         Node t = firstWaiter;
    //         Node trail = null;
    //         while (t !is null) {
    //             Node next = t.nextWaiter;
    //             if (t.waitStatus != Node.CONDITION) {
    //                 t.nextWaiter = null;
    //                 if (trail is null)
    //                     firstWaiter = next;
    //                 else
    //                     trail.nextWaiter = next;
    //                 if (next is null)
    //                     lastWaiter = trail;
    //             }
    //             else
    //                 trail = t;
    //             t = next;
    //         }
    //     }

    //     // methods

    //     /**
    //      * Moves the longest-waiting thread, if one exists, from the
    //      * wait queue for this condition to the wait queue for the
    //      * owning lock.
    //      *
    //      * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
    //      *         returns {@code false}
    //      */
    //     final void signal() {
    //         if (!isHeldExclusively())
    //             throw new IllegalMonitorStateException();
    //         Node first = firstWaiter;
    //         if (first !is null)
    //             doSignal(first);
    //     }

    //     /**
    //      * Moves all threads from the wait queue for this condition to
    //      * the wait queue for the owning lock.
    //      *
    //      * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
    //      *         returns {@code false}
    //      */
    //     final void signalAll() {
    //         if (!isHeldExclusively())
    //             throw new IllegalMonitorStateException();
    //         Node first = firstWaiter;
    //         if (first !is null)
    //             doSignalAll(first);
    //     }

    //     /**
    //      * Implements uninterruptible condition wait.
    //      * <ol>
    //      * <li>Save lock state returned by {@link #getState}.
    //      * <li>Invoke {@link #release} with saved state as argument,
    //      *     throwing IllegalMonitorStateException if it fails.
    //      * <li>Block until signalled.
    //      * <li>Reacquire by invoking specialized version of
    //      *     {@link #acquire} with saved state as argument.
    //      * </ol>
    //      */
    //     final void awaitUninterruptibly() {
    //         Node node = addConditionWaiter();
    //         int savedState = fullyRelease(node);
    //         bool interrupted = false;
    //         while (!isOnSyncQueue(node)) {
    //             implementationMissing(false);
    //             // LockSupport.park(this);
    //             // if (Thread.interrupted())
    //             //     interrupted = true;
    //         }
    //         if (acquireQueued(node, savedState) || interrupted)
    //             selfInterrupt();
    //     }

    //     /*
    //      * For interruptible waits, we need to track whether to throw
    //      * InterruptedException, if interrupted while blocked on
    //      * condition, versus reinterrupt current thread, if
    //      * interrupted while blocked waiting to re-acquire.
    //      */

    //     /** Mode meaning to reinterrupt on exit from wait */
    //     private enum int REINTERRUPT =  1;
    //     /** Mode meaning to throw InterruptedException on exit from wait */
    //     private enum int THROW_IE    = -1;

    //     /**
    //      * Checks for interrupt, returning THROW_IE if interrupted
    //      * before signalled, REINTERRUPT if after signalled, or
    //      * 0 if not interrupted.
    //      */
    //     private int checkInterruptWhileWaiting(Node node) {
    //         // return Thread.interrupted() ?
    //         //     (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
    //         //     0;
    //         // TODO: Tasks pending completion -@zxp at 10/14/2018, 3:41:57 PM
    //         // 
    //         return 0;
    //     }

    //     /**
    //      * Throws InterruptedException, reinterrupts current thread, or
    //      * does nothing, depending on mode.
    //      */
    //     private void reportInterruptAfterWait(int interruptMode) {
    //         if (interruptMode == THROW_IE)
    //             throw new InterruptedException();
    //         else if (interruptMode == REINTERRUPT)
    //             selfInterrupt();
    //     }

    //     /**
    //      * Implements interruptible condition wait.
    //      * <ol>
    //      * <li>If current thread is interrupted, throw InterruptedException.
    //      * <li>Save lock state returned by {@link #getState}.
    //      * <li>Invoke {@link #release} with saved state as argument,
    //      *     throwing IllegalMonitorStateException if it fails.
    //      * <li>Block until signalled or interrupted.
    //      * <li>Reacquire by invoking specialized version of
    //      *     {@link #acquire} with saved state as argument.
    //      * <li>If interrupted while blocked in step 4, throw InterruptedException.
    //      * </ol>
    //      */
    //     final void await() {
    //         // if (Thread.interrupted())
    //         //     throw new InterruptedException();
    //         Node node = addConditionWaiter();
    //         int savedState = fullyRelease(node);
    //         int interruptMode = 0;
    //         while (!isOnSyncQueue(node)) {
    //             // LockSupport.park(this);
    //             implementationMissing(false);
    //             if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
    //                 break;
    //         }
    //         if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
    //             interruptMode = REINTERRUPT;
    //         if (node.nextWaiter !is null) // clean up if cancelled
    //             unlinkCancelledWaiters();
    //         if (interruptMode != 0)
    //             reportInterruptAfterWait(interruptMode);
    //     }

    //     /**
    //      * Implements timed condition wait.
    //      * <ol>
    //      * <li>If current thread is interrupted, throw InterruptedException.
    //      * <li>Save lock state returned by {@link #getState}.
    //      * <li>Invoke {@link #release} with saved state as argument,
    //      *     throwing IllegalMonitorStateException if it fails.
    //      * <li>Block until signalled, interrupted, or timed out.
    //      * <li>Reacquire by invoking specialized version of
    //      *     {@link #acquire} with saved state as argument.
    //      * <li>If interrupted while blocked in step 4, throw InterruptedException.
    //      * </ol>
    //      */
    //     final long awaitNanos(long nanosTimeout) {
    //         // if (Thread.interrupted())
    //         //     throw new InterruptedException();

    //         // We don't check for nanosTimeout <= 0L here, to allow
    //         // awaitNanos(0) as a way to "yield the lock".
    //         long deadline = Clock.currStdTime() + nanosTimeout;
    //         long initialNanos = nanosTimeout;
    //         Node node = addConditionWaiter();
    //         int savedState = fullyRelease(node);
    //         int interruptMode = 0;
    //         while (!isOnSyncQueue(node)) {
    //             if (nanosTimeout <= 0L) {
    //                 transferAfterCancelledWait(node);
    //                 break;
    //             }
    //             // TODO: Tasks pending completion -@zxp at 10/14/2018, 3:39:04 PM
    //             // 
    //             implementationMissing(false);
    //             // if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
    //             //     LockSupport.parkNanos(this, nanosTimeout);
    //             if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
    //                 break;
    //             nanosTimeout = deadline - Clock.currStdTime();
    //         }
    //         if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
    //             interruptMode = REINTERRUPT;
    //         if (node.nextWaiter !is null)
    //             unlinkCancelledWaiters();
    //         if (interruptMode != 0)
    //             reportInterruptAfterWait(interruptMode);
    //         long remaining = deadline - Clock.currStdTime(); // avoid overflow
    //         return (remaining <= initialNanos) ? remaining : long.min;
    //     }

    //     /**
    //      * Implements absolute timed condition wait.
    //      * <ol>
    //      * <li>If current thread is interrupted, throw InterruptedException.
    //      * <li>Save lock state returned by {@link #getState}.
    //      * <li>Invoke {@link #release} with saved state as argument,
    //      *     throwing IllegalMonitorStateException if it fails.
    //      * <li>Block until signalled, interrupted, or timed out.
    //      * <li>Reacquire by invoking specialized version of
    //      *     {@link #acquire} with saved state as argument.
    //      * <li>If interrupted while blocked in step 4, throw InterruptedException.
    //      * <li>If timed out while blocked in step 4, return false, else true.
    //      * </ol>
    //      */
    //     final bool awaitUntil(SysTime deadline) {
    //         long abstime = deadline.stdTime();
    //         // if (Thread.interrupted())
    //         //     throw new InterruptedException();
    //         Node node = addConditionWaiter();
    //         int savedState = fullyRelease(node);
    //         bool timedout = false;
    //         int interruptMode = 0;
    //         while (!isOnSyncQueue(node)) {
    //             if (Clock.currStdTime() >= abstime) {
    //                 timedout = transferAfterCancelledWait(node);
    //                 break;
    //             }
    //             // LockSupport.parkUntil(this, abstime);
    //             // TODO: Tasks pending completion -@zxp at 10/14/2018, 3:38:12 PM
    //             // 
    //             implementationMissing(false);
    //             if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
    //                 break;
    //         }
    //         if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
    //             interruptMode = REINTERRUPT;
    //         if (node.nextWaiter !is null)
    //             unlinkCancelledWaiters();
    //         if (interruptMode != 0)
    //             reportInterruptAfterWait(interruptMode);
    //         return !timedout;
    //     }

    //     /**
    //      * Implements timed condition wait.
    //      * <ol>
    //      * <li>If current thread is interrupted, throw InterruptedException.
    //      * <li>Save lock state returned by {@link #getState}.
    //      * <li>Invoke {@link #release} with saved state as argument,
    //      *     throwing IllegalMonitorStateException if it fails.
    //      * <li>Block until signalled, interrupted, or timed out.
    //      * <li>Reacquire by invoking specialized version of
    //      *     {@link #acquire} with saved state as argument.
    //      * <li>If interrupted while blocked in step 4, throw InterruptedException.
    //      * <li>If timed out while blocked in step 4, return false, else true.
    //      * </ol>
    //      */
    //     final bool await(Duration time) {
    //         long nanosTimeout = time.total!(TimeUnit.HectoNanosecond)();
    //         // if (Thread.interrupted())
    //         //     throw new InterruptedException();
    //         // We don't check for nanosTimeout <= 0L here, to allow
    //         // await(0, unit) as a way to "yield the lock".
    //         long deadline = Clock.currStdTime() + nanosTimeout;
    //         Node node = addConditionWaiter();
    //         int savedState = fullyRelease(node);
    //         bool timedout = false;
    //         int interruptMode = 0;
    //         while (!isOnSyncQueue(node)) {
    //             if (nanosTimeout <= 0L) {
    //                 timedout = transferAfterCancelledWait(node);
    //                 break;
    //             }
    //             // TODO: Tasks pending completion -@zxp at 10/14/2018, 3:37:19 PM
    //             // 
    //             implementationMissing(false);
    //             // if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
    //             //     LockSupport.parkNanos(this, nanosTimeout);
    //             if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
    //                 break;
    //             nanosTimeout = deadline - Clock.currStdTime();
    //         }
    //         if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
    //             interruptMode = REINTERRUPT;
    //         if (node.nextWaiter !is null)
    //             unlinkCancelledWaiters();
    //         if (interruptMode != 0)
    //             reportInterruptAfterWait(interruptMode);
    //         return !timedout;
    //     }

    //     //  support for instrumentation

    //     /**
    //      * Returns true if this condition was created by the given
    //      * synchronization object.
    //      *
    //      * @return {@code true} if owned
    //      */
    //     final bool isOwnedBy(AbstractQueuedSynchronizer sync) {
    //         return sync == this.outer; // AbstractQueuedSynchronizer.this;
    //     }

    //     /**
    //      * Queries whether any threads are waiting on this condition.
    //      * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
    //      *
    //      * @return {@code true} if there are any waiting threads
    //      * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
    //      *         returns {@code false}
    //      */
    //     protected final bool hasWaiters() {
    //         if (!isHeldExclusively())
    //             throw new IllegalMonitorStateException();
    //         for (Node w = firstWaiter; w !is null; w = w.nextWaiter) {
    //             if (w.waitStatus == Node.CONDITION)
    //                 return true;
    //         }
    //         return false;
    //     }

    //     /**
    //      * Returns an estimate of the number of threads waiting on
    //      * this condition.
    //      * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
    //      *
    //      * @return the estimated number of waiting threads
    //      * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
    //      *         returns {@code false}
    //      */
    //     protected final int getWaitQueueLength() {
    //         if (!isHeldExclusively())
    //             throw new IllegalMonitorStateException();
    //         int n = 0;
    //         for (Node w = firstWaiter; w !is null; w = w.nextWaiter) {
    //             if (w.waitStatus == Node.CONDITION)
    //                 ++n;
    //         }
    //         return n;
    //     }

    //     /**
    //      * Returns a collection containing those threads that may be
    //      * waiting on this Condition.
    //      * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
    //      *
    //      * @return the collection of threads
    //      * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
    //      *         returns {@code false}
    //      */
    //     protected final Collection!Thread getWaitingThreads() {
    //         if (!isHeldExclusively())
    //             throw new IllegalMonitorStateException();
    //         ArrayList!Thread list = new ArrayList!Thread();
    //         for (Node w = firstWaiter; w !is null; w = w.nextWaiter) {
    //             if (w.waitStatus == Node.CONDITION) {
    //                 Thread t = w.thread;
    //                 if (t !is null)
    //                     list.add(t);
    //             }
    //         }
    //         return list;
    //     }
    // }

    /**
     * Initializes head and tail fields on first contention.
     */
    private final void initializeSyncQueue() {
        Node h;
        if (AtomicHelper.compareAndSet(head, null, (h = new Node())))
            tail = h;
    }

    /**
     * CASes tail field.
     */
    private final bool compareAndSetTail(Node expect, Node update) {
        return AtomicHelper.compareAndSet(tail, expect, update);
    }
}



/**
 * Wait queue node class.
 *
 * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
 * Hagersten) lock queue. CLH locks are normally used for
 * spinlocks.  We instead use them for blocking synchronizers, but
 * use the same basic tactic of holding some of the control
 * information about a thread in the predecessor of its node.  A
 * "status" field in each node keeps track of whether a thread
 * should block.  A node is signalled when its predecessor
 * releases.  Each node of the queue otherwise serves as a
 * specific-notification-style monitor holding a single waiting
 * thread. The status field does NOT control whether threads are
 * granted locks etc though.  A thread may try to acquire if it is
 * first in the queue. But being first does not guarantee success;
 * it only gives the right to contend.  So the currently released
 * contender thread may need to rewait.
 *
 * <p>To enqueue into a CLH lock, you atomically splice it in as new
 * tail. To dequeue, you just set the head field.
 * <pre>
 *      +------+  prev +-----+       +-----+
 * head |      | <---- |     | <---- |     |  tail
 *      +------+       +-----+       +-----+
 * </pre>
 *
 * <p>Insertion into a CLH queue requires only a single atomic
 * operation on "tail", so there is a simple atomic point of
 * demarcation from unqueued to queued. Similarly, dequeuing
 * involves only updating the "head". However, it takes a bit
 * more work for nodes to determine who their successors are,
 * in part to deal with possible cancellation due to timeouts
 * and interrupts.
 *
 * <p>The "prev" links (not used in original CLH locks), are mainly
 * needed to handle cancellation. If a node is cancelled, its
 * successor is (normally) relinked to a non-cancelled
 * predecessor. For explanation of similar mechanics in the case
 * of spin locks, see the papers by Scott and Scherer at
 * http://www.cs.rochester.edu/u/scott/synchronization/
 *
 * <p>We also use "next" links to implement blocking mechanics.
 * The thread id for each node is kept in its own node, so a
 * predecessor signals the next node to wake up by traversing
 * next link to determine which thread it is.  Determination of
 * successor must avoid races with newly queued nodes to set
 * the "next" fields of their predecessors.  This is solved
 * when necessary by checking backwards from the atomically
 * updated "tail" when a node's successor appears to be null.
 * (Or, said differently, the next-links are an optimization
 * so that we don't usually need a backward scan.)
 *
 * <p>Cancellation introduces some conservatism to the basic
 * algorithms.  Since we must poll for cancellation of other
 * nodes, we can miss noticing whether a cancelled node is
 * ahead or behind us. This is dealt with by always unparking
 * successors upon cancellation, allowing them to stabilize on
 * a new predecessor, unless we can identify an uncancelled
 * predecessor who will carry this responsibility.
 *
 * <p>CLH queues need a dummy header node to get started. But
 * we don't create them on construction, because it would be wasted
 * effort if there is never contention. Instead, the node
 * is constructed and head and tail pointers are set upon first
 * contention.
 *
 * <p>Threads waiting on Conditions use the same nodes, but
 * use an additional link. Conditions only need to link nodes
 * in simple (non-concurrent) linked queues because they are
 * only accessed when exclusively held.  Upon await, a node is
 * inserted into a condition queue.  Upon signal, the node is
 * transferred to the main queue.  A special value of status
 * field is used to mark which queue a node is on.
 *
 * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
 * Scherer and Michael Scott, along with members of JSR-166
 * expert group, for helpful ideas, discussions, and critiques
 * on the design of this class.
 */
private final class Node {
    /** Marker to indicate a node is waiting in shared mode */
    __gshared Node SHARED; // = new Node();
    /** Marker to indicate a node is waiting in exclusive mode */
    __gshared Node EXCLUSIVE = null;

    /** waitStatus value to indicate thread has cancelled. */
    enum int CANCELLED =  1;
    /** waitStatus value to indicate successor's thread needs unparking. */
    enum int SIGNAL    = -1;
    /** waitStatus value to indicate thread is waiting on condition. */
    enum int CONDITION = -2;
    /**
     * waitStatus value to indicate the next acquireShared should
     * unconditionally propagate.
     */
    enum int PROPAGATE = -3;

    /**
     * Status field, taking on only the values:
     *   SIGNAL:     The successor of this node is (or will soon be)
     *               blocked (via park), so the current node must
     *               unpark its successor when it releases or
     *               cancels. To avoid races, acquire methods must
     *               first indicate they need a signal,
     *               then retry the atomic acquire, and then,
     *               on failure, block.
     *   CANCELLED:  This node is cancelled due to timeout or interrupt.
     *               Nodes never leave this state. In particular,
     *               a thread with cancelled node never again blocks.
     *   CONDITION:  This node is currently on a condition queue.
     *               It will not be used as a sync queue node
     *               until transferred, at which time the status
     *               will be set to 0. (Use of this value here has
     *               nothing to do with the other uses of the
     *               field, but simplifies mechanics.)
     *   PROPAGATE:  A releaseShared should be propagated to other
     *               nodes. This is set (for head node only) in
     *               doReleaseShared to ensure propagation
     *               continues, even if other operations have
     *               since intervened.
     *   0:          None of the above
     *
     * The values are arranged numerically to simplify use.
     * Non-negative values mean that a node doesn't need to
     * signal. So, most code doesn't need to check for particular
     * values, just for sign.
     *
     * The field is initialized to 0 for normal sync nodes, and
     * CONDITION for condition nodes.  It is modified using CAS
     * (or when possible, unconditional writes).
     */
    int waitStatus;

    /**
     * Link to predecessor node that current node/thread relies on
     * for checking waitStatus. Assigned during enqueuing, and nulled
     * out (for sake of GC) only upon dequeuing.  Also, upon
     * cancellation of a predecessor, we short-circuit while
     * finding a non-cancelled one, which will always exist
     * because the head node is never cancelled: A node becomes
     * head only as a result of successful acquire. A
     * cancelled thread never succeeds in acquiring, and a thread only
     * cancels itself, not any other node.
     */
    Node prev;

    /**
     * Link to the successor node that the current node/thread
     * unparks upon release. Assigned during enqueuing, adjusted
     * when bypassing cancelled predecessors, and nulled out (for
     * sake of GC) when dequeued.  The enq operation does not
     * assign next field of a predecessor until after attachment,
     * so seeing a null next field does not necessarily mean that
     * node is at end of queue. However, if a next field appears
     * to be null, we can scan prev's from the tail to
     * double-check.  The next field of cancelled nodes is set to
     * point to the node itself instead of null, to make life
     * easier for isOnSyncQueue.
     */
    Node next;

    /**
     * The thread that enqueued this node.  Initialized on
     * construction and nulled out after use.
     */
    Thread thread;

    /**
     * Link to next node waiting on condition, or the special
     * value SHARED.  Because condition queues are accessed only
     * when holding in exclusive mode, we just need a simple
     * linked queue to hold nodes while they are waiting on
     * conditions. They are then transferred to the queue to
     * re-acquire. And because conditions can only be exclusive,
     * we save a field by using special value to indicate shared
     * mode.
     */
    Node nextWaiter;

    /**
     * Returns true if node is waiting in shared mode.
     */
    final bool isShared() {
        return nextWaiter == SHARED;
    }

    /**
     * Returns previous node, or throws NullPointerException if null.
     * Use when predecessor cannot be null.  The null check could
     * be elided, but is present to help the VM.
     *
     * @return the predecessor of this node
     */
    Node predecessor() {
        Node p = prev;
        if (p is null)
            throw new NullPointerException();
        else
            return p;
    }

    shared static this() {
        SHARED = new Node();
    }

    /** Establishes initial head or SHARED marker. */
    this() {}

    /** Constructor used by addWaiter. */
    this(Node nextWaiter) {
        this.nextWaiter = nextWaiter;
        thread = Thread.getThis();
    }

    /** Constructor used by addConditionWaiter. */
    this(int waitStatus) {
        this.waitStatus = waitStatus;
        thread = Thread.getThis();
    }

    /** CASes waitStatus field. */
    final bool compareAndSetWaitStatus(int expect, int update) {
        return AtomicHelper.compareAndSet(waitStatus, expect, update);
    }

    /** CASes next field. */
    final bool compareAndSetNext(Node expect, Node update) {
        return AtomicHelper.compareAndSet(next, expect, update);
    }

    final void setPrevRelaxed(Node p) {
        this.prev = p;
    }
}