code: /*
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * 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 io.reactivex.rxjava3.internal.observers;
  
  import static io.reactivex.rxjava3.internal.util.ExceptionHelper.timeoutMessage;
  
  import java.util.NoSuchElementException;
  import java.util.concurrent.*;
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  import io.reactivex.rxjava3.core.Observer;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.internal.util.BlockingHelper;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * An Observer + Future that expects exactly one upstream value and provides it
   * via the (blocking) Future API.
   *
   * @param <T> the value type
   */
  public final class FutureObserver<T> extends CountDownLatch
  implements Observer<T>, Future<T>, Disposable {
  
      T value;
      Throwable error;
  
      final AtomicReference<Disposable> upstream;
  
      public FutureObserver() {
          super(1);
          this.upstream = new AtomicReference<>();
      }
  
      @Override
      public boolean cancel(boolean mayInterruptIfRunning) {
          for (;;) {
              Disposable a = upstream.get();
              if (a == this || a == DisposableHelper.DISPOSED) {
                  return false;
              }
  
              if (upstream.compareAndSet(a, DisposableHelper.DISPOSED)) {
                  if (a != null) {
                      a.dispose();
                  }
                  countDown();
                  return true;
              }
          }
      }
  
      @Override
      public boolean isCancelled() {
          return DisposableHelper.isDisposed(upstream.get());
      }
  
      @Override
      public boolean isDone() {
          return getCount() == 0;
      }
  
      @Override
      public T get() throws InterruptedException, ExecutionException {
          if (getCount() != 0) {
              BlockingHelper.verifyNonBlocking();
              await();
          }
  
          if (isCancelled()) {
              throw new CancellationException();
          }
          Throwable ex = error;
          if (ex != null) {
              throw new ExecutionException(ex);
          }
          return value;
      }
  
      @Override
      public T get(long timeout, @NonNull TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
          if (getCount() != 0) {
              BlockingHelper.verifyNonBlocking();
              if (!await(timeout, unit)) {
                  throw new TimeoutException(timeoutMessage(timeout, unit));
              }
          }
  
          if (isCancelled()) {
              throw new CancellationException();
          }
  
          Throwable ex = error;
          if (ex != null) {
              throw new ExecutionException(ex);
          }
          return value;
      }
  
      @Override
      public void onSubscribe(Disposable d) {
          DisposableHelper.setOnce(this.upstream, d);
      }
  
      @Override
      public void onNext(T t) {
          if (value != null) {
              upstream.get().dispose();
              onError(new IndexOutOfBoundsException("More than one element received"));
              return;
          }
          value = t;
      }
  
      @Override
      public void onError(Throwable t) {
          if (error == null) {
              Disposable a = upstream.get();
              if (a != this && a != DisposableHelper.DISPOSED
                      && upstream.compareAndSet(a, this)) {
                  error = t;
                  countDown();
                  return;
              }
          }
          RxJavaPlugins.onError(t);
      }
  
      @Override
      public void onComplete() {
          if (value == null) {
              onError(new NoSuchElementException("The source is empty"));
              return;
          }
          Disposable a = upstream.get();
          if (a == this || a == DisposableHelper.DISPOSED) {
              return;
          }
          if (upstream.compareAndSet(a, this)) {
              countDown();
          }
      }
  
      @Override
      public void dispose() {
          // ignoring as `this` means a finished Disposable only
      }
  
      @Override
      public boolean isDisposed() {
          return isDone();
      }
  }
  