/*
    * 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.netty5.util.concurrent;
  
  import io.netty5.util.internal.ThreadExecutorMap;
  import io.netty5.util.internal.logging.InternalLogger;
  import io.netty5.util.internal.logging.InternalLoggerFactory;
  import org.jetbrains.annotations.Async.Execute;
  import org.jetbrains.annotations.Async.Schedule;
  
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  import java.util.Queue;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.Executors;
  import java.util.concurrent.LinkedBlockingQueue;
  import java.util.concurrent.RejectedExecutionException;
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicBoolean;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Single-thread singleton {@link EventExecutor}.  It starts the thread automatically and stops it when there is no
   * task pending in the task queue for 1 second.  Please note it is not scalable to schedule large number of tasks to
   * this executor; use a dedicated executor.
   */
  public final class GlobalEventExecutor extends AbstractScheduledEventExecutor implements OrderedEventExecutor {
  
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(GlobalEventExecutor.class);
  
      private static final long SCHEDULE_QUIET_PERIOD_INTERVAL = TimeUnit.SECONDS.toNanos(1);
  
      private final RunnableScheduledFutureAdapter<Void> quietPeriodTask;
      public static final GlobalEventExecutor INSTANCE;
  
      static {
          INSTANCE = new GlobalEventExecutor();
      }
  
      private final BlockingQueue<Runnable> taskQueue = new LinkedBlockingQueue<>();
  
      // because the GlobalEventExecutor is a singleton, tasks submitted to it can come from arbitrary threads and this
      // can trigger the creation of a thread from arbitrary thread groups; for this reason, the thread factory must not
      // be sticky about its thread group
      // visible for testing
      final ThreadFactory threadFactory;
      private final TaskRunner taskRunner = new TaskRunner();
      private final AtomicBoolean started = new AtomicBoolean();
      volatile Thread thread;
  
      private final Future<Void> terminationFuture = DefaultPromise.<Void>newFailedPromise(
              this, new UnsupportedOperationException()).asFuture();
  
      private GlobalEventExecutor() {
          threadFactory = ThreadExecutorMap.apply(new DefaultThreadFactory(
                  DefaultThreadFactory.toPoolName(getClass()), false, Thread.NORM_PRIORITY, null), this);
          quietPeriodTask = new RunnableScheduledFutureAdapter<>(
                  this, newPromise(), Executors.callable(() -> {
              // NOOP
          }, null),
                  // note: the getCurrentTimeNanos() call here only works because this is a final class, otherwise
                  // the method could be overridden leading to unsafe initialization here!
                  deadlineNanos(getCurrentTimeNanos(), SCHEDULE_QUIET_PERIOD_INTERVAL),
                  -SCHEDULE_QUIET_PERIOD_INTERVAL);
  
          scheduledTaskQueue().add(quietPeriodTask);
      }
  
      /**
       * Take the next {@link Runnable} from the task queue and so will block if no task is currently present.
       *
       * @return {@code null} if the executor thread has been interrupted or waken up.
       */
      private Runnable takeTask() {
          BlockingQueue<Runnable> taskQueue = this.taskQueue;
          for (;;) {
              RunnableScheduledFuture<?> scheduledTask = peekScheduledTask();
              if (scheduledTask == null) {
                  Runnable task = null;
                  try {
                      task = taskQueue.take();
                  } catch (InterruptedException e) {
                      // Ignore
                  }
                 return task;
             } else {
                 long delayNanos = scheduledTask.delayNanos();
                 Runnable task = null;
                 if (delayNanos > 0) {
                     try {
                         task = taskQueue.poll(delayNanos, TimeUnit.NANOSECONDS);
                     } catch (InterruptedException e) {
                         // Waken up.
                         return null;
                     }
                 }
                 if (task == null) {
                     // We need to fetch the scheduled tasks now as otherwise there may be a chance that
                     // scheduled tasks are never executed if there is always one task in the taskQueue.
                     // This is for example true for the read task of OIO Transport
                     // See https://github.com/netty/netty/issues/1614
                     fetchFromScheduledTaskQueue();
                     task = taskQueue.poll();
                 }
 
                 if (task != null) {
                     return task;
                 }
             }
         }
     }
 
     private void fetchFromScheduledTaskQueue() {
         long nanoTime = getCurrentTimeNanos();
         Runnable scheduledTask = pollScheduledTask(nanoTime);
         while (scheduledTask != null) {
             taskQueue.add(scheduledTask);
             scheduledTask = pollScheduledTask(nanoTime);
         }
     }
 
     /**
      * Return the number of tasks that are pending for processing.
      */
     public int pendingTasks() {
         return taskQueue.size();
     }
 
     /**
      * Add a task to the task queue, or throws a {@link RejectedExecutionException} if this instance was shutdown
      * before.
      */
     private void addTask(Runnable task) {
         requireNonNull(task, "task");
         taskQueue.add(task);
     }
 
     @Override
     public boolean inEventLoop(Thread thread) {
         return thread == this.thread;
     }
 
     @Override
     public Future<Void> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
         return terminationFuture();
     }
 
     @Override
     public Future<Void> terminationFuture() {
         return terminationFuture;
     }
 
     @Override
     public boolean isShuttingDown() {
         return false;
     }
 
     @Override
     public boolean isShutdown() {
         return false;
     }
 
     @Override
     public boolean isTerminated() {
         return false;
     }
 
     @Override
     public boolean awaitTermination(long timeout, TimeUnit unit) {
         return false;
     }
 
     /**
      * Waits until the worker thread of this executor has no tasks left in its task queue and terminates itself.
      * Because a new worker thread will be started again when a new task is submitted, this operation is only useful
      * when you want to ensure that the worker thread is terminated <strong>after</strong> your application is shut
      * down and there's no chance of submitting a new task afterwards.
      *
      * @return {@code true} if and only if the worker thread has been terminated
      */
     public boolean awaitInactivity(long timeout, TimeUnit unit) throws InterruptedException {
         requireNonNull(unit, "unit");
 
         final Thread thread = this.thread;
         if (thread == null) {
             throw new IllegalStateException("thread was not started");
         }
         thread.join(unit.toMillis(timeout));
         return !thread.isAlive();
     }
 
     @Override
     public void execute(@Schedule Runnable task) {
         requireNonNull(task, "task");
 
         addTask(task);
         if (!inEventLoop()) {
             startThread();
         }
     }
 
     private void startThread() {
         if (started.compareAndSet(false, true)) {
             final Thread t = threadFactory.newThread(taskRunner);
             // Set to null to ensure we not create classloader leaks by holds a strong reference to the inherited
             // classloader.
             // See:
             // - https://github.com/netty/netty/issues/7290
             // - https://bugs.openjdk.java.net/browse/JDK-7008595
             AccessController.doPrivileged((PrivilegedAction<Void>) () -> {
                 t.setContextClassLoader(null);
                 return null;
             });
 
             // Set the thread before starting it as otherwise inEventLoop() may return false and so produce
             // an assert error.
             // See https://github.com/netty/netty/issues/4357
             thread = t;
             t.start();
         }
     }
 
     final class TaskRunner implements Runnable {
         @Override
         public void run() {
             for (;;) {
                 Runnable task = takeTask();
                 if (task != null) {
                     try {
                         runTask(task);
                     } catch (Throwable t) {
                         logger.warn("Unexpected exception from the global event executor: ", t);
                     }
 
                     if (task != quietPeriodTask) {
                         continue;
                     }
                 }
 
                 Queue<RunnableScheduledFutureNode<?>> scheduledTaskQueue = scheduledTaskQueue();
                 // Terminate if there is no task in the queue (except the noop task).
                 if (taskQueue.isEmpty() && scheduledTaskQueue.size() <= 1) {
                     // Mark the current thread as stopped.
                     // The following CAS must always success and must be uncontended,
                     // because only one thread should be running at the same time.
                     boolean stopped = started.compareAndSet(true, false);
                     assert stopped;
 
                     // Do not check scheduledTaskQueue because it is not thread-safe and can only be mutated from a
                     // TaskRunner actively running tasks.
                     if (taskQueue.isEmpty()) {
                         // A) No new task was added and thus there's nothing to handle
                         //    -> safe to terminate because there's nothing left to do
                         // B) A new thread started and handled all the new tasks.
                         //    -> safe to terminate the new thread will take care the rest
                         break;
                     }
 
                     // There are pending tasks added again.
                     if (!started.compareAndSet(false, true)) {
                         // startThread() started a new thread and set 'started' to true.
                         // -> terminate this thread so that the new thread reads from taskQueue exclusively.
                         break;
                     }
 
                     // New tasks were added, but this worker was faster to set 'started' to true.
                     // i.e. a new worker thread was not started by startThread().
                     // -> keep this thread alive to handle the newly added entries.
                 }
             }
         }
 
         private void runTask(@Execute Runnable task) {
             task.run();
         }
     }
 }