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.single;
  
  import java.util.Iterator;
  import java.util.Objects;
  import java.util.concurrent.atomic.AtomicLong;
  
  import org.reactivestreams.Subscriber;
  
  import io.reactivex.rxjava3.annotations.Nullable;
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.exceptions.Exceptions;
  import io.reactivex.rxjava3.functions.Function;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.internal.subscriptions.*;
  import io.reactivex.rxjava3.internal.util.BackpressureHelper;
  
  /**
   * Maps a success value into an Iterable and streams it back as a Flowable.
   *
   * @param <T> the source value type
   * @param <R> the element type of the Iterable
   */
  public final class SingleFlatMapIterableFlowable<T, R> extends Flowable<R> {
  
      final SingleSource<T> source;
  
      final Function<? super T, ? extends Iterable<? extends R>> mapper;
  
      public SingleFlatMapIterableFlowable(SingleSource<T> source,
              Function<? super T, ? extends Iterable<? extends R>> mapper) {
          this.source = source;
          this.mapper = mapper;
      }
  
      @Override
      protected void subscribeActual(Subscriber<? super R> s) {
          source.subscribe(new FlatMapIterableObserver<>(s, mapper));
      }
  
      static final class FlatMapIterableObserver<T, R>
      extends BasicIntQueueSubscription<R>
      implements SingleObserver<T> {
  
          private static final long serialVersionUID = -8938804753851907758L;
  
          final Subscriber<? super R> downstream;
  
          final Function<? super T, ? extends Iterable<? extends R>> mapper;
  
          final AtomicLong requested;
  
          Disposable upstream;
  
          volatile Iterator<? extends R> it;
  
          volatile boolean cancelled;
  
          boolean outputFused;
  
          FlatMapIterableObserver(Subscriber<? super R> actual,
                  Function<? super T, ? extends Iterable<? extends R>> mapper) {
              this.downstream = actual;
              this.mapper = mapper;
              this.requested = new AtomicLong();
          }
  
          @Override
          public void onSubscribe(Disposable d) {
              if (DisposableHelper.validate(this.upstream, d)) {
                  this.upstream = d;
  
                  downstream.onSubscribe(this);
              }
          }
  
          @Override
          public void onSuccess(T value) {
              Iterator<? extends R> iterator;
              boolean has;
              try {
                  iterator = mapper.apply(value).iterator();
  
                  has = iterator.hasNext();
              } catch (Throwable ex) {
                  Exceptions.throwIfFatal(ex);
                  downstream.onError(ex);
                  return;
              }
  
              if (!has) {
                  downstream.onComplete();
                  return;
              }
  
              this.it = iterator;
              drain();
          }
  
          @Override
          public void onError(Throwable e) {
              upstream = DisposableHelper.DISPOSED;
              downstream.onError(e);
          }
  
          @Override
          public void request(long n) {
              if (SubscriptionHelper.validate(n)) {
                  BackpressureHelper.add(requested, n);
                  drain();
              }
          }
  
          @Override
          public void cancel() {
              cancelled = true;
              upstream.dispose();
              upstream = DisposableHelper.DISPOSED;
          }
  
          void drain() {
              if (getAndIncrement() != 0) {
                  return;
              }
  
              Subscriber<? super R> a = downstream;
              Iterator<? extends R> iterator = this.it;
  
              if (outputFused && iterator != null) {
                  a.onNext(null);
                  a.onComplete();
                  return;
              }
  
              int missed = 1;
  
              for (;;) {
  
                  if (iterator != null) {
                      long r = requested.get();
                      long e = 0L;
  
                      if (r == Long.MAX_VALUE) {
                          fastPath(a, iterator);
                          return;
                      }
  
                      while (e != r) {
                          if (cancelled) {
                              return;
                          }
  
                          R v;
  
                          try {
                              v = Objects.requireNonNull(iterator.next(), "The iterator returned a null value");
                          } catch (Throwable ex) {
                              Exceptions.throwIfFatal(ex);
                              a.onError(ex);
                              return;
                          }
  
                          a.onNext(v);
  
                          if (cancelled) {
                              return;
                          }
  
                          e++;
  
                          boolean b;
  
                          try {
                              b = iterator.hasNext();
                          } catch (Throwable ex) {
                              Exceptions.throwIfFatal(ex);
                              a.onError(ex);
                              return;
                          }
  
                          if (!b) {
                              a.onComplete();
                              return;
                          }
                      }
  
                      if (e != 0L) {
                          BackpressureHelper.produced(requested, e);
                      }
                  }
  
                  missed = addAndGet(-missed);
                  if (missed == 0) {
                      break;
                  }
  
                  if (iterator == null) {
                      iterator = it;
                  }
              }
          }
  
          void fastPath(Subscriber<? super R> a, Iterator<? extends R> iterator) {
              for (;;) {
                  if (cancelled) {
                      return;
                  }
  
                  R v;
  
                  try {
                      v = iterator.next();
                  } catch (Throwable ex) {
                      Exceptions.throwIfFatal(ex);
                      a.onError(ex);
                      return;
                  }
  
                  a.onNext(v);
  
                  if (cancelled) {
                      return;
                  }
  
                  boolean b;
  
                  try {
                      b = iterator.hasNext();
                  } catch (Throwable ex) {
                      Exceptions.throwIfFatal(ex);
                      a.onError(ex);
                      return;
                  }
  
                  if (!b) {
                      a.onComplete();
                      return;
                  }
              }
          }
  
          @Override
          public int requestFusion(int mode) {
              if ((mode & ASYNC) != 0) {
                  outputFused = true;
                  return ASYNC;
              }
              return NONE;
          }
  
          @Override
          public void clear() {
              it = null;
          }
  
          @Override
          public boolean isEmpty() {
              return it == null;
          }
  
          @Nullable
          @Override
          public R poll() {
              Iterator<? extends R> iterator = it;
  
              if (iterator != null) {
                  R v = Objects.requireNonNull(iterator.next(), "The iterator returned a null value");
                  if (!iterator.hasNext()) {
                      it = null;
                  }
                  return v;
              }
              return null;
          }
  
      }
  }
  