/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed 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 com.google.common.util.concurrent;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.J2ktIncompatible;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicBoolean;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Utilities necessary for working with libraries that supply plain {@link Future} instances. Note
 * that, whenever possible, it is strongly preferred to modify those libraries to return {@code
 * ListenableFuture} directly.
 *
 * <p>For interoperability between {@code ListenableFuture} and <b>{@code CompletableFuture}</b>,
 * consider <a href="https://github.com/lukas-krecan/future-converter">Future Converter</a>.
 *
 * @author Sven Mawson
 * @since 10.0 (replacing {@code Futures.makeListenable}, which existed in 1.0)
 */
@J2ktIncompatible
@GwtIncompatible
@ElementTypesAreNonnullByDefault
public final class JdkFutureAdapters {
  /**
   * Assigns a thread to the given {@link Future} to provide {@link ListenableFuture} functionality.
   *
   * <p><b>Warning:</b> If the input future does not already implement {@code ListenableFuture}, the
   * returned future will emulate {@link ListenableFuture#addListener} by taking a thread from an
   * internal, unbounded pool at the first call to {@code addListener} and holding it until the
   * future is {@linkplain Future#isDone() done}.
   *
   * <p>Prefer to create {@code ListenableFuture} instances with {@link SettableFuture}, {@link
   * MoreExecutors#listeningDecorator( java.util.concurrent.ExecutorService)}, {@link
   * ListenableFutureTask}, {@link AbstractFuture}, and other utilities over creating plain {@code
   * Future} instances to be upgraded to {@code ListenableFuture} after the fact.
   */
  public static <V extends @Nullable Object> ListenableFuture<V> listenInPoolThread(
      Future<V> future) {
    if (future instanceof ListenableFuture) {
      return (ListenableFuture<V>) future;
    }
    return new ListenableFutureAdapter<>(future);
  }

  /**
   * Submits a blocking task for the given {@link Future} to provide {@link ListenableFuture}
   * functionality.
   *
   * <p><b>Warning:</b> If the input future does not already implement {@code ListenableFuture}, the
   * returned future will emulate {@link ListenableFuture#addListener} by submitting a task to the
   * given executor at the first call to {@code addListener}. The task must be started by the
   * executor promptly, or else the returned {@code ListenableFuture} may fail to work. The task's
   * execution consists of blocking until the input future is {@linkplain Future#isDone() done}, so
   * each call to this method may claim and hold a thread for an arbitrary length of time. Use of
   * bounded executors or other executors that may fail to execute a task promptly may result in
   * deadlocks.
   *
   * <p>Prefer to create {@code ListenableFuture} instances with {@link SettableFuture}, {@link
   * MoreExecutors#listeningDecorator( java.util.concurrent.ExecutorService)}, {@link
   * ListenableFutureTask}, {@link AbstractFuture}, and other utilities over creating plain {@code
   * Future} instances to be upgraded to {@code ListenableFuture} after the fact.
   *
   * @since 12.0
   */
  public static <V extends @Nullable Object> ListenableFuture<V> listenInPoolThread(
      Future<V> future, Executor executor) {
    checkNotNull(executor);
    if (future instanceof ListenableFuture) {
      return (ListenableFuture<V>) future;
    }
    return new ListenableFutureAdapter<>(future, executor);
  }

  /**
   * An adapter to turn a {@link Future} into a {@link ListenableFuture}. This will wait on the
   * future to finish, and when it completes, run the listeners. This implementation will wait on
   * the source future indefinitely, so if the source future never completes, the adapter will never
   * complete either.
   *
   * <p>If the delegate future is interrupted or throws an unexpected unchecked exception, the
   * listeners will not be invoked.
   */
  private static class ListenableFutureAdapter<V extends @Nullable Object>
      extends ForwardingFuture<V> implements ListenableFuture<V> {

    private static final ThreadFactory threadFactory =
        new ThreadFactoryBuilder()
            .setDaemon(true)
            .setNameFormat("ListenableFutureAdapter-thread-%d")
            .build();
    private static final Executor defaultAdapterExecutor =
        Executors.newCachedThreadPool(threadFactory);

    private final Executor adapterExecutor;

    // The execution list to hold our listeners.
    private final ExecutionList executionList = new ExecutionList();

    // This allows us to only start up a thread waiting on the delegate future when the first
    // listener is added.
    private final AtomicBoolean hasListeners = new AtomicBoolean(false);

    // The delegate future.
    private final Future<V> delegate;

    ListenableFutureAdapter(Future<V> delegate) {
      this(delegate, defaultAdapterExecutor);
    }

    ListenableFutureAdapter(Future<V> delegate, Executor adapterExecutor) {
      this.delegate = checkNotNull(delegate);
      this.adapterExecutor = checkNotNull(adapterExecutor);
    }

    @Override
    protected Future<V> delegate() {
      return delegate;
    }

    @Override
    public void addListener(Runnable listener, Executor exec) {
      executionList.add(listener, exec);

      // When a listener is first added, we run a task that will wait for the delegate to finish,
      // and when it is done will run the listeners.
      if (hasListeners.compareAndSet(false, true)) {
        if (delegate.isDone()) {
          // If the delegate is already done, run the execution list immediately on the current
          // thread.
          executionList.execute();
          return;
        }

        // TODO(lukes): handle RejectedExecutionException
        adapterExecutor.execute(
            () -> {
              try {
                /*
                 * Threads from our private pool are never interrupted. Threads from a
                 * user-supplied executor might be, but... what can we do? This is another reason
                 * to return a proper ListenableFuture instead of using listenInPoolThread.
                 */
                getUninterruptibly(delegate);
              } catch (ExecutionException | RuntimeException | Error e) {
                // (including CancellationException)
                // The task is presumably done, run the listeners.
                // TODO(cpovirk): Do *something* in case of Error (and maybe RuntimeException)?
              }
              executionList.execute();
            });
      }
    }
  }

  private JdkFutureAdapters() {}
}
