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.operators.flowable;
  
  import org.reactivestreams.Subscription;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.fuseable.FuseToFlowable;
  import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  public final class FlowableSingleMaybe<T> extends Maybe<T> implements FuseToFlowable<T> {
  
      final Flowable<T> source;
  
      public FlowableSingleMaybe(Flowable<T> source) {
          this.source = source;
      }
  
      @Override
      protected void subscribeActual(MaybeObserver<? super T> observer) {
          source.subscribe(new SingleElementSubscriber<>(observer));
      }
  
      @Override
      public Flowable<T> fuseToFlowable() {
          return RxJavaPlugins.onAssembly(new FlowableSingle<>(source, null, false));
      }
  
      static final class SingleElementSubscriber<T>
      implements FlowableSubscriber<T>, Disposable {
  
          final MaybeObserver<? super T> downstream;
  
          Subscription upstream;
  
          boolean done;
  
          T value;
  
          SingleElementSubscriber(MaybeObserver<? super T> downstream) {
              this.downstream = downstream;
          }
  
          @Override
          public void onSubscribe(Subscription s) {
              if (SubscriptionHelper.validate(this.upstream, s)) {
                  this.upstream = s;
                  downstream.onSubscribe(this);
                  s.request(Long.MAX_VALUE);
              }
          }
  
          @Override
          public void onNext(T t) {
              if (done) {
                  return;
              }
              if (value != null) {
                  done = true;
                  upstream.cancel();
                  upstream = SubscriptionHelper.CANCELLED;
                  downstream.onError(new IllegalArgumentException("Sequence contains more than one element!"));
                  return;
              }
              value = t;
          }
  
          @Override
          public void onError(Throwable t) {
              if (done) {
                  RxJavaPlugins.onError(t);
                  return;
              }
              done = true;
              upstream = SubscriptionHelper.CANCELLED;
              downstream.onError(t);
          }
  
          @Override
          public void onComplete() {
              if (done) {
                  return;
              }
              done = true;
              upstream = SubscriptionHelper.CANCELLED;
              T v = value;
              value = null;
              if (v == null) {
                  downstream.onComplete();
              } else {
                  downstream.onSuccess(v);
              }
          }
  
          @Override
          public void dispose() {
              upstream.cancel();
              upstream = SubscriptionHelper.CANCELLED;
          }
  
          @Override
          public boolean isDisposed() {
              return upstream == SubscriptionHelper.CANCELLED;
          }
      }
  }
  