/*
    * Copyright 2016 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.PlatformDependent;
  import io.netty5.util.internal.UnstableApi;
  
  import java.util.Iterator;
  import java.util.Queue;
  import java.util.concurrent.Callable;
  import java.util.concurrent.RejectedExecutionException;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicInteger;
  
  import static io.netty5.util.internal.ObjectUtil.checkPositive;
  import static java.util.Objects.requireNonNull;
  
  /**
   * {@link EventExecutorGroup} which will preserve {@link Runnable} execution order but makes no guarantees about what
   * {@link EventExecutor} (and therefore {@link Thread}) will be used to execute the {@link Runnable}s.
   *
   * <p>The {@link EventExecutorGroup#next()} for the wrapped {@link EventExecutorGroup} must <strong>NOT</strong> return
   * executors of type {@link OrderedEventExecutor}.
   */
  @UnstableApi
  public final class NonStickyEventExecutorGroup implements EventExecutorGroup {
      private final EventExecutorGroup group;
      private final int maxTaskExecutePerRun;
  
      /**
       * Creates a new instance. Be aware that the given {@link EventExecutorGroup} <strong>MUST NOT</strong> contain
       * any {@link OrderedEventExecutor}s.
       */
      public NonStickyEventExecutorGroup(EventExecutorGroup group) {
          this(group, 1024);
      }
  
      /**
       * Creates a new instance. Be aware that the given {@link EventExecutorGroup} <strong>MUST NOT</strong> contain
       * any {@link OrderedEventExecutor}s.
       */
      public NonStickyEventExecutorGroup(EventExecutorGroup group, int maxTaskExecutePerRun) {
          this.group = verify(group);
          this.maxTaskExecutePerRun = checkPositive(maxTaskExecutePerRun, "maxTaskExecutePerRun");
      }
  
      private static EventExecutorGroup verify(EventExecutorGroup group) {
          Iterator<EventExecutor> executors = requireNonNull(group, "group").iterator();
          while (executors.hasNext()) {
              EventExecutor executor = executors.next();
              if (executor instanceof OrderedEventExecutor) {
                  throw new IllegalArgumentException("EventExecutorGroup " + group
                          + " contains OrderedEventExecutors: " + executor);
              }
          }
          return group;
      }
  
      private NonStickyOrderedEventExecutor newExecutor(EventExecutor executor) {
          return new NonStickyOrderedEventExecutor(executor, maxTaskExecutePerRun);
      }
  
      @Override
      public boolean isShuttingDown() {
          return group.isShuttingDown();
      }
  
      @Override
      public Future<Void> shutdownGracefully() {
          return group.shutdownGracefully();
      }
  
      @Override
      public Future<Void> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
          return group.shutdownGracefully(quietPeriod, timeout, unit);
      }
  
      @Override
      public Future<Void> terminationFuture() {
          return group.terminationFuture();
      }
  
      @Override
      public EventExecutor next() {
          return newExecutor(group.next());
      }
 
     @Override
     public Iterator<EventExecutor> iterator() {
         final Iterator<EventExecutor> itr = group.iterator();
         return new Iterator<>() {
             @Override
             public boolean hasNext() {
                 return itr.hasNext();
             }
 
             @Override
             public EventExecutor next() {
                 return newExecutor(itr.next());
             }
 
             @Override
             public void remove() {
                 itr.remove();
             }
         };
     }
 
     @Override
     public Future<Void> submit(Runnable task) {
         return group.submit(task);
     }
 
     @Override
     public <T> Future<T> submit(Runnable task, T result) {
         return group.submit(task, result);
     }
 
     @Override
     public <T> Future<T> submit(Callable<T> task) {
         return group.submit(task);
     }
 
     @Override
     public Future<Void> schedule(Runnable task, long delay, TimeUnit unit) {
         return group.schedule(task, delay, unit);
     }
 
     @Override
     public <V> Future<V> schedule(Callable<V> task, long delay, TimeUnit unit) {
         return group.schedule(task, delay, unit);
     }
 
     @Override
     public Future<Void> scheduleAtFixedRate(Runnable task, long initialDelay, long period, TimeUnit unit) {
         return group.scheduleAtFixedRate(task, initialDelay, period, unit);
     }
 
     @Override
     public Future<Void> scheduleWithFixedDelay(Runnable task, long initialDelay, long delay, TimeUnit unit) {
         return group.scheduleWithFixedDelay(task, initialDelay, delay, unit);
     }
 
     @Override
     public boolean isShutdown() {
         return group.isShutdown();
     }
 
     @Override
     public boolean isTerminated() {
         return group.isTerminated();
     }
 
     @Override
     public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
         return group.awaitTermination(timeout, unit);
     }
 
     @Override
     public void execute(Runnable task) {
         group.execute(task);
     }
 
     private static final class NonStickyOrderedEventExecutor extends AbstractEventExecutor
             implements Runnable, OrderedEventExecutor {
         private final EventExecutor executor;
         private final Queue<Runnable> tasks = PlatformDependent.newMpscQueue();
 
         private static final int NONE = 0;
         private static final int SUBMITTED = 1;
         private static final int RUNNING = 2;
 
         private final AtomicInteger state = new AtomicInteger();
         private final int maxTaskExecutePerRun;
 
         NonStickyOrderedEventExecutor(EventExecutor executor, int maxTaskExecutePerRun) {
             this.executor = executor;
             this.maxTaskExecutePerRun = maxTaskExecutePerRun;
         }
 
         @Override
         public void run() {
             if (!state.compareAndSet(SUBMITTED, RUNNING)) {
                 return;
             }
             for (;;) {
                 int i = 0;
                 try {
                     for (; i < maxTaskExecutePerRun; i++) {
                         Runnable task = tasks.poll();
                         if (task == null) {
                             break;
                         }
                         safeExecute(task);
                     }
                 } finally {
                     if (i == maxTaskExecutePerRun) {
                         try {
                             state.set(SUBMITTED);
                             executor.execute(this);
                             return; // done
                         } catch (Throwable ignore) {
                             // Reset the state back to running as we will keep on executing tasks.
                             state.set(RUNNING);
                             // if an error happened we should just ignore it and let the loop run again as there is not
                             // much else we can do. Most likely this was triggered by a full task queue. In this case
                             // we just will run more tasks and try again later.
                         }
                     } else {
                         state.set(NONE);
                         // After setting the state to NONE, look at the tasks queue one more time.
                         // If it is empty, then we can return from this method.
                         // Otherwise, it means the producer thread has called execute(Runnable)
                         // and enqueued a task in between the tasks.poll() above and the state.set(NONE) here.
                         // There are two possible scenarios when this happen
                         //
                         // 1. The producer thread sees state == NONE, hence the compareAndSet(NONE, SUBMITTED)
                         //    is successfully setting the state to SUBMITTED. This mean the producer
                         //    will call / has called executor.execute(this). In this case, we can just return.
                         // 2. The producer thread don't see the state change, hence the compareAndSet(NONE, SUBMITTED)
                         //    returns false. In this case, the producer thread won't call executor.execute.
                         //    In this case, we need to change the state to RUNNING and keeps running.
                         //
                         // The above cases can be distinguished by performing a
                         // compareAndSet(NONE, RUNNING). If it returns "false", it is case 1; otherwise it is case 2.
                         if (tasks.isEmpty() || !state.compareAndSet(NONE, RUNNING)) {
                             return; // done
                         }
                     }
                 }
             }
         }
 
         @Override
         public boolean inEventLoop(Thread thread) {
             return false;
         }
 
         @Override
         public boolean isShuttingDown() {
             return executor.isShutdown();
         }
 
         @Override
         public Future<Void> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
             return executor.shutdownGracefully(quietPeriod, timeout, unit);
         }
 
         @Override
         public Future<Void> terminationFuture() {
             return executor.terminationFuture();
         }
 
         @Override
         public boolean isShutdown() {
             return executor.isShutdown();
         }
 
         @Override
         public boolean isTerminated() {
             return executor.isTerminated();
         }
 
         @Override
         public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
             return executor.awaitTermination(timeout, unit);
         }
 
         @Override
         public void execute(Runnable task) {
             if (!tasks.offer(task)) {
                 throw new RejectedExecutionException();
             }
             if (state.compareAndSet(NONE, SUBMITTED)) {
                 // Actually it could happen that the runnable was picked up in between but we not care to much and just
                 // execute ourself. At worst this will be a NOOP when run() is called.
                 executor.execute(this);
             }
         }
 
         @Override
         public Future<Void> schedule(Runnable task, long delay,
                                      TimeUnit unit) {
             throw new UnsupportedOperationException();
         }
 
         @Override
         public <V> Future<V> schedule(Callable<V> task, long delay, TimeUnit unit) {
             throw new UnsupportedOperationException();
         }
 
         @Override
         public Future<Void> scheduleAtFixedRate(
                 Runnable task, long initialDelay, long period, TimeUnit unit) {
             throw new UnsupportedOperationException();
         }
 
         @Override
         public Future<Void> scheduleWithFixedDelay(
                 Runnable task, long initialDelay, long delay, TimeUnit unit) {
             throw new UnsupportedOperationException();
         }
     }
 }