/*
    * Copyright 2012 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.util;
  
  import static io.netty.util.internal.ObjectUtil.checkInRange;
  import static io.netty.util.internal.ObjectUtil.checkPositive;
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  
  import io.netty.util.concurrent.ImmediateExecutor;
  import io.netty.util.internal.MathUtil;
  import io.netty.util.internal.PlatformDependent;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.util.Collections;
  import java.util.HashSet;
  import java.util.Queue;
  import java.util.Set;
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.Executor;
  import java.util.concurrent.Executors;
  import java.util.concurrent.RejectedExecutionException;
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
  import java.util.concurrent.atomic.AtomicLong;
  
  import static io.netty.util.internal.StringUtil.simpleClassName;
  
  /**
   * A {@link Timer} optimized for approximated I/O timeout scheduling.
   *
   * <h3>Tick Duration</h3>
   *
   * As described with 'approximated', this timer does not execute the scheduled
   * {@link TimerTask} on time.  {@link HashedWheelTimer}, on every tick, will
   * check if there are any {@link TimerTask}s behind the schedule and execute
   * them.
   * <p>
   * You can increase or decrease the accuracy of the execution timing by
   * specifying smaller or larger tick duration in the constructor.  In most
   * network applications, I/O timeout does not need to be accurate.  Therefore,
   * the default tick duration is 100 milliseconds and you will not need to try
   * different configurations in most cases.
   *
   * <h3>Ticks per Wheel (Wheel Size)</h3>
   *
   * {@link HashedWheelTimer} maintains a data structure called 'wheel'.
   * To put simply, a wheel is a hash table of {@link TimerTask}s whose hash
   * function is 'dead line of the task'.  The default number of ticks per wheel
   * (i.e. the size of the wheel) is 512.  You could specify a larger value
   * if you are going to schedule a lot of timeouts.
   *
   * <h3>Do not create many instances.</h3>
   *
   * {@link HashedWheelTimer} creates a new thread whenever it is instantiated and
   * started.  Therefore, you should make sure to create only one instance and
   * share it across your application.  One of the common mistakes, that makes
   * your application unresponsive, is to create a new instance for every connection.
   *
   * <h3>Implementation Details</h3>
   *
   * {@link HashedWheelTimer} is based on
   * <a href="https://cseweb.ucsd.edu/users/varghese/">George Varghese</a> and
   * Tony Lauck's paper,
   * <a href="https://cseweb.ucsd.edu/users/varghese/PAPERS/twheel.ps.Z">'Hashed
   * and Hierarchical Timing Wheels: data structures to efficiently implement a
   * timer facility'</a>.  More comprehensive slides are located
   * <a href="https://www.cse.wustl.edu/~cdgill/courses/cs6874/TimingWheels.ppt">here</a>.
   */
  public class HashedWheelTimer implements Timer {
  
      static final InternalLogger logger =
              InternalLoggerFactory.getInstance(HashedWheelTimer.class);
  
      private static final AtomicInteger INSTANCE_COUNTER = new AtomicInteger();
      private static final AtomicBoolean WARNED_TOO_MANY_INSTANCES = new AtomicBoolean();
      private static final int INSTANCE_COUNT_LIMIT = 64;
      private static final long MILLISECOND_NANOS = TimeUnit.MILLISECONDS.toNanos(1);
      private static final ResourceLeakDetector<HashedWheelTimer> leakDetector = ResourceLeakDetectorFactory.instance()
              .newResourceLeakDetector(HashedWheelTimer.class, 1);
  
      private static final AtomicIntegerFieldUpdater<HashedWheelTimer> WORKER_STATE_UPDATER =
              AtomicIntegerFieldUpdater.newUpdater(HashedWheelTimer.class, "workerState");
 
     private final ResourceLeakTracker<HashedWheelTimer> leak;
     private final Worker worker = new Worker();
     private final Thread workerThread;
 
     public static final int WORKER_STATE_INIT = 0;
     public static final int WORKER_STATE_STARTED = 1;
     public static final int WORKER_STATE_SHUTDOWN = 2;
     @SuppressWarnings({"unused", "FieldMayBeFinal"})
     private volatile int workerState; // 0 - init, 1 - started, 2 - shut down
 
     private final long tickDuration;
     private final HashedWheelBucket[] wheel;
     private final int mask;
     private final CountDownLatch startTimeInitialized = new CountDownLatch(1);
     private final Queue<HashedWheelTimeout> timeouts = PlatformDependent.newMpscQueue();
     private final Queue<HashedWheelTimeout> cancelledTimeouts = PlatformDependent.newMpscQueue();
     private final AtomicLong pendingTimeouts = new AtomicLong(0);
     private final long maxPendingTimeouts;
     private final Executor taskExecutor;
 
     private volatile long startTime;
 
     /**
      * Creates a new timer with the default thread factory
      * ({@link Executors#defaultThreadFactory()}), default tick duration, and
      * default number of ticks per wheel.
      */
     public HashedWheelTimer() {
         this(Executors.defaultThreadFactory());
     }
 
     /**
      * Creates a new timer with the default thread factory
      * ({@link Executors#defaultThreadFactory()}) and default number of ticks
      * per wheel.
      *
      * @param tickDuration the duration between tick
      * @param unit         the time unit of the {@code tickDuration}
      * @throws NullPointerException     if {@code unit} is {@code null}
      * @throws IllegalArgumentException if {@code tickDuration} is &lt;= 0
      */
     public HashedWheelTimer(long tickDuration, TimeUnit unit) {
         this(Executors.defaultThreadFactory(), tickDuration, unit);
     }
 
     /**
      * Creates a new timer with the default thread factory
      * ({@link Executors#defaultThreadFactory()}).
      *
      * @param tickDuration  the duration between tick
      * @param unit          the time unit of the {@code tickDuration}
      * @param ticksPerWheel the size of the wheel
      * @throws NullPointerException     if {@code unit} is {@code null}
      * @throws IllegalArgumentException if either of {@code tickDuration} and {@code ticksPerWheel} is &lt;= 0
      */
     public HashedWheelTimer(long tickDuration, TimeUnit unit, int ticksPerWheel) {
         this(Executors.defaultThreadFactory(), tickDuration, unit, ticksPerWheel);
     }
 
     /**
      * Creates a new timer with the default tick duration and default number of
      * ticks per wheel.
      *
      * @param threadFactory a {@link ThreadFactory} that creates a
      *                      background {@link Thread} which is dedicated to
      *                      {@link TimerTask} execution.
      * @throws NullPointerException if {@code threadFactory} is {@code null}
      */
     public HashedWheelTimer(ThreadFactory threadFactory) {
         this(threadFactory, 100, TimeUnit.MILLISECONDS);
     }
 
     /**
      * Creates a new timer with the default number of ticks per wheel.
      *
      * @param threadFactory a {@link ThreadFactory} that creates a
      *                      background {@link Thread} which is dedicated to
      *                      {@link TimerTask} execution.
      * @param tickDuration  the duration between tick
      * @param unit          the time unit of the {@code tickDuration}
      * @throws NullPointerException     if either of {@code threadFactory} and {@code unit} is {@code null}
      * @throws IllegalArgumentException if {@code tickDuration} is &lt;= 0
      */
     public HashedWheelTimer(
             ThreadFactory threadFactory, long tickDuration, TimeUnit unit) {
         this(threadFactory, tickDuration, unit, 512);
     }
 
     /**
      * Creates a new timer.
      *
      * @param threadFactory a {@link ThreadFactory} that creates a
      *                      background {@link Thread} which is dedicated to
      *                      {@link TimerTask} execution.
      * @param tickDuration  the duration between tick
      * @param unit          the time unit of the {@code tickDuration}
      * @param ticksPerWheel the size of the wheel
      * @throws NullPointerException     if either of {@code threadFactory} and {@code unit} is {@code null}
      * @throws IllegalArgumentException if either of {@code tickDuration} and {@code ticksPerWheel} is &lt;= 0
      */
     public HashedWheelTimer(
             ThreadFactory threadFactory,
             long tickDuration, TimeUnit unit, int ticksPerWheel) {
         this(threadFactory, tickDuration, unit, ticksPerWheel, true);
     }
 
     /**
      * Creates a new timer.
      *
      * @param threadFactory a {@link ThreadFactory} that creates a
      *                      background {@link Thread} which is dedicated to
      *                      {@link TimerTask} execution.
      * @param tickDuration  the duration between tick
      * @param unit          the time unit of the {@code tickDuration}
      * @param ticksPerWheel the size of the wheel
      * @param leakDetection {@code true} if leak detection should be enabled always,
      *                      if false it will only be enabled if the worker thread is not
      *                      a daemon thread.
      * @throws NullPointerException     if either of {@code threadFactory} and {@code unit} is {@code null}
      * @throws IllegalArgumentException if either of {@code tickDuration} and {@code ticksPerWheel} is &lt;= 0
      */
     public HashedWheelTimer(
             ThreadFactory threadFactory,
             long tickDuration, TimeUnit unit, int ticksPerWheel, boolean leakDetection) {
         this(threadFactory, tickDuration, unit, ticksPerWheel, leakDetection, -1);
     }
 
     /**
      * Creates a new timer.
      *
      * @param threadFactory        a {@link ThreadFactory} that creates a
      *                             background {@link Thread} which is dedicated to
      *                             {@link TimerTask} execution.
      * @param tickDuration         the duration between tick
      * @param unit                 the time unit of the {@code tickDuration}
      * @param ticksPerWheel        the size of the wheel
      * @param leakDetection        {@code true} if leak detection should be enabled always,
      *                             if false it will only be enabled if the worker thread is not
      *                             a daemon thread.
      * @param  maxPendingTimeouts  The maximum number of pending timeouts after which call to
      *                             {@code newTimeout} will result in
      *                             {@link java.util.concurrent.RejectedExecutionException}
      *                             being thrown. No maximum pending timeouts limit is assumed if
      *                             this value is 0 or negative.
      * @throws NullPointerException     if either of {@code threadFactory} and {@code unit} is {@code null}
      * @throws IllegalArgumentException if either of {@code tickDuration} and {@code ticksPerWheel} is &lt;= 0
      */
     public HashedWheelTimer(
             ThreadFactory threadFactory,
             long tickDuration, TimeUnit unit, int ticksPerWheel, boolean leakDetection,
             long maxPendingTimeouts) {
         this(threadFactory, tickDuration, unit, ticksPerWheel, leakDetection,
                 maxPendingTimeouts, ImmediateExecutor.INSTANCE);
     }
     /**
      * Creates a new timer.
      *
      * @param threadFactory        a {@link ThreadFactory} that creates a
      *                             background {@link Thread} which is dedicated to
      *                             {@link TimerTask} execution.
      * @param tickDuration         the duration between tick
      * @param unit                 the time unit of the {@code tickDuration}
      * @param ticksPerWheel        the size of the wheel
      * @param leakDetection        {@code true} if leak detection should be enabled always,
      *                             if false it will only be enabled if the worker thread is not
      *                             a daemon thread.
      * @param maxPendingTimeouts   The maximum number of pending timeouts after which call to
      *                             {@code newTimeout} will result in
      *                             {@link java.util.concurrent.RejectedExecutionException}
      *                             being thrown. No maximum pending timeouts limit is assumed if
      *                             this value is 0 or negative.
      * @param taskExecutor         The {@link Executor} that is used to execute the submitted {@link TimerTask}s.
      *                             The caller is responsible to shutdown the {@link Executor} once it is not needed
      *                             anymore.
      * @throws NullPointerException     if either of {@code threadFactory} and {@code unit} is {@code null}
      * @throws IllegalArgumentException if either of {@code tickDuration} and {@code ticksPerWheel} is &lt;= 0
      */
     public HashedWheelTimer(
             ThreadFactory threadFactory,
             long tickDuration, TimeUnit unit, int ticksPerWheel, boolean leakDetection,
             long maxPendingTimeouts, Executor taskExecutor) {
 
         checkNotNull(threadFactory, "threadFactory");
         checkNotNull(unit, "unit");
         checkPositive(tickDuration, "tickDuration");
         checkPositive(ticksPerWheel, "ticksPerWheel");
         this.taskExecutor = checkNotNull(taskExecutor, "taskExecutor");
 
         // Normalize ticksPerWheel to power of two and initialize the wheel.
         wheel = createWheel(ticksPerWheel);
         mask = wheel.length - 1;
 
         // Convert tickDuration to nanos.
         long duration = unit.toNanos(tickDuration);
 
         // Prevent overflow.
         if (duration >= Long.MAX_VALUE / wheel.length) {
             throw new IllegalArgumentException(String.format(
                     "tickDuration: %d (expected: 0 < tickDuration in nanos < %d",
                     tickDuration, Long.MAX_VALUE / wheel.length));
         }
 
         if (duration < MILLISECOND_NANOS) {
             logger.warn("Configured tickDuration {} smaller than {}, using 1ms.",
                         tickDuration, MILLISECOND_NANOS);
             this.tickDuration = MILLISECOND_NANOS;
         } else {
             this.tickDuration = duration;
         }
 
         workerThread = threadFactory.newThread(worker);
 
         leak = leakDetection || !workerThread.isDaemon() ? leakDetector.track(this) : null;
 
         this.maxPendingTimeouts = maxPendingTimeouts;
 
         if (INSTANCE_COUNTER.incrementAndGet() > INSTANCE_COUNT_LIMIT &&
             WARNED_TOO_MANY_INSTANCES.compareAndSet(false, true)) {
             reportTooManyInstances();
         }
     }
 
     @Override
     protected void finalize() throws Throwable {
         try {
             super.finalize();
         } finally {
             // This object is going to be GCed and it is assumed the ship has sailed to do a proper shutdown. If
             // we have not yet shutdown then we want to make sure we decrement the active instance count.
             if (WORKER_STATE_UPDATER.getAndSet(this, WORKER_STATE_SHUTDOWN) != WORKER_STATE_SHUTDOWN) {
                 INSTANCE_COUNTER.decrementAndGet();
             }
         }
     }
 
     private static HashedWheelBucket[] createWheel(int ticksPerWheel) {
         ticksPerWheel = MathUtil.findNextPositivePowerOfTwo(ticksPerWheel);
 
         HashedWheelBucket[] wheel = new HashedWheelBucket[ticksPerWheel];
         for (int i = 0; i < wheel.length; i ++) {
             wheel[i] = new HashedWheelBucket();
         }
         return wheel;
     }
 
     /**
      * Starts the background thread explicitly.  The background thread will
      * start automatically on demand even if you did not call this method.
      *
      * @throws IllegalStateException if this timer has been
      *                               {@linkplain #stop() stopped} already
      */
     public void start() {
         switch (WORKER_STATE_UPDATER.get(this)) {
             case WORKER_STATE_INIT:
                 if (WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_INIT, WORKER_STATE_STARTED)) {
                     workerThread.start();
                 }
                 break;
             case WORKER_STATE_STARTED:
                 break;
             case WORKER_STATE_SHUTDOWN:
                 throw new IllegalStateException("cannot be started once stopped");
             default:
                 throw new Error("Invalid WorkerState");
         }
 
         // Wait until the startTime is initialized by the worker.
         while (startTime == 0) {
             try {
                 startTimeInitialized.await();
             } catch (InterruptedException ignore) {
                 // Ignore - it will be ready very soon.
             }
         }
     }
 
     @Override
     public Set<Timeout> stop() {
         if (Thread.currentThread() == workerThread) {
             throw new IllegalStateException(
                     HashedWheelTimer.class.getSimpleName() +
                             ".stop() cannot be called from " +
                             TimerTask.class.getSimpleName());
         }
 
         if (!WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_STARTED, WORKER_STATE_SHUTDOWN)) {
             // workerState can be 0 or 2 at this moment - let it always be 2.
             if (WORKER_STATE_UPDATER.getAndSet(this, WORKER_STATE_SHUTDOWN) != WORKER_STATE_SHUTDOWN) {
                 INSTANCE_COUNTER.decrementAndGet();
                 if (leak != null) {
                     boolean closed = leak.close(this);
                     assert closed;
                 }
             }
 
             return Collections.emptySet();
         }
 
         try {
             boolean interrupted = false;
             while (workerThread.isAlive()) {
                 workerThread.interrupt();
                 try {
                     workerThread.join(100);
                 } catch (InterruptedException ignored) {
                     interrupted = true;
                 }
             }
 
             if (interrupted) {
                 Thread.currentThread().interrupt();
             }
         } finally {
             INSTANCE_COUNTER.decrementAndGet();
             if (leak != null) {
                 boolean closed = leak.close(this);
                 assert closed;
             }
         }
         Set<Timeout> unprocessed = worker.unprocessedTimeouts();
         Set<Timeout> cancelled = new HashSet<Timeout>(unprocessed.size());
         for (Timeout timeout : unprocessed) {
             if (timeout.cancel()) {
                 cancelled.add(timeout);
             }
         }
         return cancelled;
     }
 
     @Override
     public Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {
         checkNotNull(task, "task");
         checkNotNull(unit, "unit");
 
         long pendingTimeoutsCount = pendingTimeouts.incrementAndGet();
 
         if (maxPendingTimeouts > 0 && pendingTimeoutsCount > maxPendingTimeouts) {
             pendingTimeouts.decrementAndGet();
             throw new RejectedExecutionException("Number of pending timeouts ("
                 + pendingTimeoutsCount + ") is greater than or equal to maximum allowed pending "
                 + "timeouts (" + maxPendingTimeouts + ")");
         }
 
         start();
 
         // Add the timeout to the timeout queue which will be processed on the next tick.
         // During processing all the queued HashedWheelTimeouts will be added to the correct HashedWheelBucket.
         long deadline = System.nanoTime() + unit.toNanos(delay) - startTime;
 
         // Guard against overflow.
         if (delay > 0 && deadline < 0) {
             deadline = Long.MAX_VALUE;
         }
         HashedWheelTimeout timeout = new HashedWheelTimeout(this, task, deadline);
         timeouts.add(timeout);
         return timeout;
     }
 
     /**
      * Returns the number of pending timeouts of this {@link Timer}.
      */
     public long pendingTimeouts() {
         return pendingTimeouts.get();
     }
 
     private static void reportTooManyInstances() {
         if (logger.isErrorEnabled()) {
             String resourceType = simpleClassName(HashedWheelTimer.class);
             logger.error("You are creating too many " + resourceType + " instances. " +
                     resourceType + " is a shared resource that must be reused across the JVM, " +
                     "so that only a few instances are created.");
         }
     }
 
     private final class Worker implements Runnable {
         private final Set<Timeout> unprocessedTimeouts = new HashSet<Timeout>();
 
         private long tick;
 
         @Override
         public void run() {
             // Initialize the startTime.
             startTime = System.nanoTime();
             if (startTime == 0) {
                 // We use 0 as an indicator for the uninitialized value here, so make sure it's not 0 when initialized.
                 startTime = 1;
             }
 
             // Notify the other threads waiting for the initialization at start().
             startTimeInitialized.countDown();
 
             do {
                 final long deadline = waitForNextTick();
                 if (deadline > 0) {
                     int idx = (int) (tick & mask);
                     processCancelledTasks();
                     HashedWheelBucket bucket =
                             wheel[idx];
                     transferTimeoutsToBuckets();
                     bucket.expireTimeouts(deadline);
                     tick++;
                 }
             } while (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_STARTED);
 
             // Fill the unprocessedTimeouts so we can return them from stop() method.
             for (HashedWheelBucket bucket: wheel) {
                 bucket.clearTimeouts(unprocessedTimeouts);
             }
             for (;;) {
                 HashedWheelTimeout timeout = timeouts.poll();
                 if (timeout == null) {
                     break;
                 }
                 if (!timeout.isCancelled()) {
                     unprocessedTimeouts.add(timeout);
                 }
             }
             processCancelledTasks();
         }
 
         private void transferTimeoutsToBuckets() {
             // transfer only max. 100000 timeouts per tick to prevent a thread to stale the workerThread when it just
             // adds new timeouts in a loop.
             for (int i = 0; i < 100000; i++) {
                 HashedWheelTimeout timeout = timeouts.poll();
                 if (timeout == null) {
                     // all processed
                     break;
                 }
                 if (timeout.state() == HashedWheelTimeout.ST_CANCELLED) {
                     // Was cancelled in the meantime.
                     continue;
                 }
 
                 long calculated = timeout.deadline / tickDuration;
                 timeout.remainingRounds = (calculated - tick) / wheel.length;
 
                 final long ticks = Math.max(calculated, tick); // Ensure we don't schedule for past.
                 int stopIndex = (int) (ticks & mask);
 
                 HashedWheelBucket bucket = wheel[stopIndex];
                 bucket.addTimeout(timeout);
             }
         }
 
         private void processCancelledTasks() {
             for (;;) {
                 HashedWheelTimeout timeout = cancelledTimeouts.poll();
                 if (timeout == null) {
                     // all processed
                     break;
                 }
                 try {
                     timeout.remove();
                 } catch (Throwable t) {
                     if (logger.isWarnEnabled()) {
                         logger.warn("An exception was thrown while process a cancellation task", t);
                     }
                 }
             }
         }
 
         /**
          * calculate goal nanoTime from startTime and current tick number,
          * then wait until that goal has been reached.
          * @return Long.MIN_VALUE if received a shutdown request,
          * current time otherwise (with Long.MIN_VALUE changed by +1)
          */
         private long waitForNextTick() {
             long deadline = tickDuration * (tick + 1);
 
             for (;;) {
                 final long currentTime = System.nanoTime() - startTime;
                 long sleepTimeMs = (deadline - currentTime + 999999) / 1000000;
 
                 if (sleepTimeMs <= 0) {
                     if (currentTime == Long.MIN_VALUE) {
                         return -Long.MAX_VALUE;
                     } else {
                         return currentTime;
                     }
                 }
 
                 // Check if we run on windows, as if thats the case we will need
                 // to round the sleepTime as workaround for a bug that only affect
                 // the JVM if it runs on windows.
                 //
                 // See https://github.com/netty/netty/issues/356
                 if (PlatformDependent.isWindows()) {
                     sleepTimeMs = sleepTimeMs / 10 * 10;
                     if (sleepTimeMs == 0) {
                         sleepTimeMs = 1;
                     }
                 }
 
                 try {
                     Thread.sleep(sleepTimeMs);
                 } catch (InterruptedException ignored) {
                     if (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_SHUTDOWN) {
                         return Long.MIN_VALUE;
                     }
                 }
             }
         }
 
         public Set<Timeout> unprocessedTimeouts() {
             return Collections.unmodifiableSet(unprocessedTimeouts);
         }
     }
 
     private static final class HashedWheelTimeout implements Timeout, Runnable {
 
         private static final int ST_INIT = 0;
         private static final int ST_CANCELLED = 1;
         private static final int ST_EXPIRED = 2;
         private static final AtomicIntegerFieldUpdater<HashedWheelTimeout> STATE_UPDATER =
                 AtomicIntegerFieldUpdater.newUpdater(HashedWheelTimeout.class, "state");
 
         private final HashedWheelTimer timer;
         private final TimerTask task;
         private final long deadline;
 
         @SuppressWarnings({"unused", "FieldMayBeFinal", "RedundantFieldInitialization" })
         private volatile int state = ST_INIT;
 
         // remainingRounds will be calculated and set by Worker.transferTimeoutsToBuckets() before the
         // HashedWheelTimeout will be added to the correct HashedWheelBucket.
         long remainingRounds;
 
         // This will be used to chain timeouts in HashedWheelTimerBucket via a double-linked-list.
         // As only the workerThread will act on it there is no need for synchronization / volatile.
         HashedWheelTimeout next;
         HashedWheelTimeout prev;
 
         // The bucket to which the timeout was added
         HashedWheelBucket bucket;
 
         HashedWheelTimeout(HashedWheelTimer timer, TimerTask task, long deadline) {
             this.timer = timer;
             this.task = task;
             this.deadline = deadline;
         }
 
         @Override
         public Timer timer() {
             return timer;
         }
 
         @Override
         public TimerTask task() {
             return task;
         }
 
         @Override
         public boolean cancel() {
             // only update the state it will be removed from HashedWheelBucket on next tick.
             if (!compareAndSetState(ST_INIT, ST_CANCELLED)) {
                 return false;
             }
             // If a task should be canceled we put this to another queue which will be processed on each tick.
             // So this means that we will have a GC latency of max. 1 tick duration which is good enough. This way
             // we can make again use of our MpscLinkedQueue and so minimize the locking / overhead as much as possible.
             timer.cancelledTimeouts.add(this);
             return true;
         }
 
         void remove() {
             HashedWheelBucket bucket = this.bucket;
             if (bucket != null) {
                 bucket.remove(this);
             }
             timer.pendingTimeouts.decrementAndGet();
         }
 
         public boolean compareAndSetState(int expected, int state) {
             return STATE_UPDATER.compareAndSet(this, expected, state);
         }
 
         public int state() {
             return state;
         }
 
         @Override
         public boolean isCancelled() {
             return state() == ST_CANCELLED;
         }
 
         @Override
         public boolean isExpired() {
             return state() == ST_EXPIRED;
         }
 
         public void expire() {
             if (!compareAndSetState(ST_INIT, ST_EXPIRED)) {
                 return;
             }
 
             try {
                 remove();
                 timer.taskExecutor.execute(this);
             } catch (Throwable t) {
                 if (logger.isWarnEnabled()) {
                     logger.warn("An exception was thrown while submit " + TimerTask.class.getSimpleName()
                             + " for execution.", t);
                 }
             }
         }
 
         @Override
         public void run() {
             try {
                 task.run(this);
             } catch (Throwable t) {
                 if (logger.isWarnEnabled()) {
                     logger.warn("An exception was thrown by " + TimerTask.class.getSimpleName() + '.', t);
                 }
             }
         }
 
         @Override
         public String toString() {
             final long currentTime = System.nanoTime();
             long remaining = deadline - currentTime + timer.startTime;
 
             StringBuilder buf = new StringBuilder(192)
                .append(simpleClassName(this))
                .append('(')
                .append("deadline: ");
             if (remaining > 0) {
                 buf.append(remaining)
                    .append(" ns later");
             } else if (remaining < 0) {
                 buf.append(-remaining)
                    .append(" ns ago");
             } else {
                 buf.append("now");
             }
 
             if (isCancelled()) {
                 buf.append(", cancelled");
             }
 
             return buf.append(", task: ")
                       .append(task())
                       .append(')')
                       .toString();
         }
     }
 
     /**
      * Bucket that stores HashedWheelTimeouts. These are stored in a linked-list like datastructure to allow easy
      * removal of HashedWheelTimeouts in the middle. Also the HashedWheelTimeout act as nodes themself and so no
      * extra object creation is needed.
      */
     private static final class HashedWheelBucket {
         // Used for the linked-list datastructure
         private HashedWheelTimeout head;
         private HashedWheelTimeout tail;
 
         /**
          * Add {@link HashedWheelTimeout} to this bucket.
          */
         public void addTimeout(HashedWheelTimeout timeout) {
             assert timeout.bucket == null;
             timeout.bucket = this;
             if (head == null) {
                 head = tail = timeout;
             } else {
                 tail.next = timeout;
                 timeout.prev = tail;
                 tail = timeout;
             }
         }
 
         /**
          * Expire all {@link HashedWheelTimeout}s for the given {@code deadline}.
          */
         public void expireTimeouts(long deadline) {
             HashedWheelTimeout timeout = head;
 
             // process all timeouts
             while (timeout != null) {
                 HashedWheelTimeout next = timeout.next;
                 if (timeout.remainingRounds <= 0) {
                     if (timeout.deadline <= deadline) {
                         timeout.expire();
                     } else {
                         // The timeout was placed into a wrong slot. This should never happen.
                         throw new IllegalStateException(String.format(
                                 "timeout.deadline (%d) > deadline (%d)", timeout.deadline, deadline));
                     }
                 } else if (!timeout.isCancelled()) {
                     timeout.remainingRounds --;
                 }
                 timeout = next;
             }
         }
 
         public HashedWheelTimeout remove(HashedWheelTimeout timeout) {
             HashedWheelTimeout next = timeout.next;
             // remove timeout that was either processed or cancelled by updating the linked-list
             if (timeout.prev != null) {
                 timeout.prev.next = next;
             }
             if (timeout.next != null) {
                 timeout.next.prev = timeout.prev;
             }
 
             if (timeout == head) {
                 // if timeout is also the tail we need to adjust the entry too
                 if (timeout == tail) {
                     tail = null;
                     head = null;
                 } else {
                     head = next;
                 }
             } else if (timeout == tail) {
                 // if the timeout is the tail modify the tail to be the prev node.
                 tail = timeout.prev;
             }
             // null out prev, next and bucket to allow for GC.
             timeout.prev = null;
             timeout.next = null;
             timeout.bucket = null;
             return next;
         }
 
         /**
          * Clear this bucket and return all not expired / cancelled {@link Timeout}s.
          */
         public void clearTimeouts(Set<Timeout> set) {
             for (;;) {
                 HashedWheelTimeout timeout = pollTimeout();
                 if (timeout == null) {
                     return;
                 }
                 if (timeout.isExpired() || timeout.isCancelled()) {
                     continue;
                 }
                 set.add(timeout);
             }
         }
 
         private HashedWheelTimeout pollTimeout() {
             HashedWheelTimeout head = this.head;
             if (head == null) {
                 return null;
             }
             HashedWheelTimeout next = head.next;
             if (next == null) {
                 tail = this.head =  null;
             } else {
                 this.head = next;
                 next.prev = null;
             }
 
             // null out prev and next to allow for GC.
             head.next = null;
             head.prev = null;
             head.bucket = null;
             return head;
         }
     }
 }