/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF 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
 *
 *    http://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 org.apache.pekko.dispatch

import java.util
import java.util.concurrent.{ Callable, Executor, ExecutorService, Future, ThreadFactory, TimeUnit }
import java.util.concurrent.atomic.{ AtomicBoolean, AtomicInteger }

import org.apache.pekko.annotation.InternalApi

/**
 * A virtualized executor service that creates a new virtual thread for each task.
 * Will shut down the underlying executor service when this executor is being shutdown.
 *
 * INTERNAL API
 */
@InternalApi
final class VirtualizedExecutorService(
    vtFactory: ThreadFactory,
    underlying: ExecutorService,
    loadMetricsProvider: Executor => Boolean,
    cascadeShutdown: Boolean)
    extends ExecutorService with LoadMetrics {
  require(VirtualThreadSupport.isSupported, "Virtual thread is not supported.")
  require(vtFactory != null, "Virtual thread factory must not be null")
  require(loadMetricsProvider != null, "Load metrics provider must not be null")

  private final val UnderlyingRunning = 0
  private final val UnderlyingShutdown = 1
  private final val UnderlyingShutdownNow = 2

  private val executor = VirtualThreadSupport.newThreadPerTaskExecutor(vtFactory)
  private val underlyingShutdownScheduled = new AtomicBoolean(false)
  private val underlyingShutdownNowRequested = new AtomicBoolean(false)
  private val underlyingShutdownState = new AtomicInteger(UnderlyingRunning)

  override def atFullThrottle(): Boolean = loadMetricsProvider(this)

  override def shutdown(): Unit = {
    executor.shutdown()
    shutdownUnderlyingWhenVirtualThreadsHaveTerminated()
  }

  override def shutdownNow(): util.List[Runnable] = {
    underlyingShutdownNowRequested.set(true)
    val result = executor.shutdownNow()
    shutdownUnderlyingWhenVirtualThreadsHaveTerminated()
    result
  }

  override def isShutdown: Boolean = executor.isShutdown

  override def isTerminated: Boolean = {
    if (cascadeShutdown) {
      if (executor.isTerminated) {
        shutdownUnderlying()
      }
      executor.isTerminated && ((underlying eq null) || underlying.isTerminated)
    } else {
      executor.isTerminated
    }
  }

  override def awaitTermination(timeout: Long, unit: TimeUnit): Boolean = {
    if (cascadeShutdown) {
      val timeoutNanos = unit.toNanos(timeout)
      val deadline = System.nanoTime() + timeoutNanos
      executor.awaitTermination(timeout, unit) && {
        shutdownUnderlying()
        (underlying eq null) || {
          val remainingNanos = deadline - System.nanoTime()
          if (remainingNanos <= 0L) underlying.isTerminated
          else underlying.awaitTermination(remainingNanos, TimeUnit.NANOSECONDS)
        }
      }
    } else {
      executor.awaitTermination(timeout, unit)
    }
  }

  private def shutdownUnderlyingWhenVirtualThreadsHaveTerminated(): Unit = {
    if (cascadeShutdown && (underlying ne null)) {
      if (executor.isTerminated) {
        shutdownUnderlying()
      } else if (underlyingShutdownScheduled.compareAndSet(false, true)) {
        VirtualThreadSupport.startVirtualThread(
          "pekko-virtualized-executor-shutdown",
          new Runnable {
            override def run(): Unit = {
              var interrupted = false
              try {
                while (!executor.isTerminated) {
                  try executor.awaitTermination(Long.MaxValue, TimeUnit.NANOSECONDS)
                  catch {
                    case _: InterruptedException => interrupted = true
                  }
                }
              } finally {
                shutdownUnderlying()
                if (interrupted) {
                  Thread.currentThread().interrupt()
                }
              }
            }
          })
      }
    }
  }

  private def shutdownUnderlying(): Unit = {
    if (cascadeShutdown && (underlying ne null) && underlyingShutdownNowRequested.get) {
      if (underlyingShutdownState.getAndSet(UnderlyingShutdownNow) != UnderlyingShutdownNow) {
        underlying.shutdownNow()
      }
    } else if (cascadeShutdown && (underlying ne null) &&
      underlyingShutdownState.compareAndSet(UnderlyingRunning, UnderlyingShutdown)) {
      underlying.shutdown()
    }
  }

  override def submit[T](task: Callable[T]): Future[T] = {
    executor.submit(task)
  }

  override def submit[T](task: Runnable, result: T): Future[T] = {
    executor.submit(task, result)
  }

  override def submit(task: Runnable): Future[_] = {
    executor.submit(task)
  }

  override def invokeAll[T](tasks: util.Collection[_ <: Callable[T]]): util.List[Future[T]] = {
    executor.invokeAll(tasks)
  }

  override def invokeAll[T](
      tasks: util.Collection[_ <: Callable[T]], timeout: Long, unit: TimeUnit): util.List[Future[T]] = {
    executor.invokeAll(tasks, timeout, unit)
  }

  override def invokeAny[T](tasks: util.Collection[_ <: Callable[T]]): T = {
    executor.invokeAny(tasks)
  }

  override def invokeAny[T](tasks: util.Collection[_ <: Callable[T]], timeout: Long, unit: TimeUnit): T = {
    executor.invokeAny(tasks, timeout, unit)
  }

  override def execute(command: Runnable): Unit = {
    executor.execute(command)
  }
}
