/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * 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.android.dialer.common.concurrent;

import static com.google.common.util.concurrent.Futures.immediateCancelledFuture;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;

import com.google.common.util.concurrent.AsyncCallable;
import com.google.common.util.concurrent.Futures;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.common.util.concurrent.SettableFuture;
import java.util.concurrent.Callable;
import java.util.concurrent.Executor;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicReference;

/**
 * Serializes execution of a set of operations. This class guarantees that a submitted callable will
 * not be called before previously submitted callables have completed.
 */
public final class DialerFutureSerializer {
  /** This reference acts as a pointer tracking the head of a linked list of ListenableFutures. */
  private final AtomicReference<ListenableFuture<?>> ref =
      new AtomicReference<>(immediateFuture(null));

  /** Enqueues a task to run when the previous task (if any) completes. */
  public <T> ListenableFuture<T> submit(final Callable<T> callable, Executor executor) {
    return submitAsync(() -> immediateFuture(callable.call()), executor);
  }

  /**
   * Enqueues a task to run when the previous task (if any) completes.
   *
   * <p>Cancellation does not propagate from the output future to the future returned from {@code
   * callable}, but if the output future is cancelled before {@link AsyncCallable#call()} is
   * invoked, {@link AsyncCallable#call()} will not be invoked.
   */
  public <T> ListenableFuture<T> submitAsync(final AsyncCallable<T> callable, Executor executor) {
    AtomicBoolean wasCancelled = new AtomicBoolean(false);
    final AsyncCallable<T> task =
        () -> {
          if (wasCancelled.get()) {
            return immediateCancelledFuture();
          }
          return callable.call();
        };
    /*
     * Three futures are at play here:
     * taskFuture is the future that comes from the callable.
     * newFuture is the future we use to track the serialization of our task.
     * oldFuture is the previous task's newFuture.
     *
     * newFuture is guaranteed to only complete once all tasks previously submitted to this instance
     * once the futures returned from those submissions have completed.
     */
    final SettableFuture<Object> newFuture = SettableFuture.create();

    final ListenableFuture<?> oldFuture = ref.getAndSet(newFuture);

    // Invoke our task once the previous future completes.
    final ListenableFuture<T> taskFuture =
        Futures.nonCancellationPropagating(
            Futures.submitAsync(task, runnable -> oldFuture.addListener(runnable, executor)));
    // newFuture's lifetime is determined by taskFuture, unless taskFuture is cancelled, in which
    // case it falls back to oldFuture's. This is to ensure that if the future we return is
    // cancelled, we don't begin execution of the next task until after oldFuture completes.
    taskFuture.addListener(
        () -> {
          if (taskFuture.isCancelled()) {
            // Since the value of oldFuture can only ever be immediateFuture(null) or setFuture of a
            // future that eventually came from immediateFuture(null), this doesn't leak throwables
            // or completion values.
            wasCancelled.set(true);
            newFuture.setFuture(oldFuture);
          } else {
            newFuture.set(null);
          }
        },
        directExecutor());

    return taskFuture;
  }
}