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.completable;
  
  import java.util.concurrent.atomic.*;
  
  import org.reactivestreams.*;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.exceptions.*;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
  import io.reactivex.rxjava3.operators.*;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  public final class CompletableConcat extends Completable {
      final Publisher<? extends CompletableSource> sources;
      final int prefetch;
  
      public CompletableConcat(Publisher<? extends CompletableSource> sources, int prefetch) {
          this.sources = sources;
          this.prefetch = prefetch;
      }
  
      @Override
      public void subscribeActual(CompletableObserver observer) {
          sources.subscribe(new CompletableConcatSubscriber(observer, prefetch));
      }
  
      static final class CompletableConcatSubscriber
      extends AtomicInteger
      implements FlowableSubscriber<CompletableSource>, Disposable {
          private static final long serialVersionUID = 9032184911934499404L;
  
          final CompletableObserver downstream;
  
          final int prefetch;
  
          final int limit;
  
          final ConcatInnerObserver inner;
  
          final AtomicBoolean once;
  
          int sourceFused;
  
          int consumed;
  
          SimpleQueue<CompletableSource> queue;
  
          Subscription upstream;
  
          volatile boolean done;
  
          volatile boolean active;
  
          CompletableConcatSubscriber(CompletableObserver actual, int prefetch) {
              this.downstream = actual;
              this.prefetch = prefetch;
              this.inner = new ConcatInnerObserver(this);
              this.once = new AtomicBoolean();
              this.limit = prefetch - (prefetch >> 2);
          }
  
          @Override
          public void onSubscribe(Subscription s) {
              if (SubscriptionHelper.validate(this.upstream, s)) {
                  this.upstream = s;
  
                  long r = prefetch == Integer.MAX_VALUE ? Long.MAX_VALUE : prefetch;
  
                  if (s instanceof QueueSubscription) {
                      @SuppressWarnings("unchecked")
                      QueueSubscription<CompletableSource> qs = (QueueSubscription<CompletableSource>) s;
  
                      int m = qs.requestFusion(QueueSubscription.ANY);
  
                      if (m == QueueSubscription.SYNC) {
                          sourceFused = m;
                          queue = qs;
                          done = true;
                          downstream.onSubscribe(this);
                          drain();
                          return;
                      }
                      if (m == QueueSubscription.ASYNC) {
                          sourceFused = m;
                          queue = qs;
                          downstream.onSubscribe(this);
                          s.request(r);
                          return;
                      }
                  }
  
                  if (prefetch == Integer.MAX_VALUE) {
                      queue = new SpscLinkedArrayQueue<>(Flowable.bufferSize());
                  } else {
                      queue = new SpscArrayQueue<>(prefetch);
                  }
  
                  downstream.onSubscribe(this);
  
                  s.request(r);
              }
          }
  
          @Override
          public void onNext(CompletableSource t) {
              if (sourceFused == QueueSubscription.NONE) {
                  if (!queue.offer(t)) {
                      onError(new QueueOverflowException());
                      return;
                  }
              }
              drain();
          }
  
          @Override
          public void onError(Throwable t) {
              if (once.compareAndSet(false, true)) {
                  DisposableHelper.dispose(inner);
                  downstream.onError(t);
              } else {
                  RxJavaPlugins.onError(t);
              }
          }
  
          @Override
          public void onComplete() {
              done = true;
              drain();
          }
  
          @Override
          public void dispose() {
              upstream.cancel();
              DisposableHelper.dispose(inner);
          }
  
          @Override
          public boolean isDisposed() {
              return DisposableHelper.isDisposed(inner.get());
          }
  
          void drain() {
              if (getAndIncrement() != 0) {
                  return;
              }
  
              for (;;) {
                  if (isDisposed()) {
                      return;
                  }
  
                  if (!active) {
  
                      boolean d = done;
  
                      CompletableSource cs;
  
                      try {
                          cs = queue.poll();
                      } catch (Throwable ex) {
                          Exceptions.throwIfFatal(ex);
                          innerError(ex);
                          return;
                      }
  
                      boolean empty = cs == null;
  
                      if (d && empty) {
                          // errors never set done or call drain.
                          downstream.onComplete();
                          return;
                      }
  
                      if (!empty) {
                          active = true;
                          cs.subscribe(inner);
                          request();
                      }
                  }
  
                  if (decrementAndGet() == 0) {
                      break;
                  }
              }
          }
  
          void request() {
              if (sourceFused != QueueSubscription.SYNC) {
                  int p = consumed + 1;
                  if (p == limit) {
                      consumed = 0;
                      upstream.request(p);
                  } else {
                      consumed = p;
                  }
              }
          }
  
          void innerError(Throwable e) {
              if (once.compareAndSet(false, true)) {
                  upstream.cancel();
                  downstream.onError(e);
              } else {
                  RxJavaPlugins.onError(e);
              }
          }
  
          void innerComplete() {
              active = false;
              drain();
          }
  
          static final class ConcatInnerObserver extends AtomicReference<Disposable> implements CompletableObserver {
              private static final long serialVersionUID = -5454794857847146511L;
  
              final CompletableConcatSubscriber parent;
  
              ConcatInnerObserver(CompletableConcatSubscriber parent) {
                  this.parent = parent;
              }
  
              @Override
              public void onSubscribe(Disposable d) {
                  DisposableHelper.replace(this, d);
              }
  
              @Override
              public void onError(Throwable e) {
                  parent.innerError(e);
              }
  
              @Override
              public void onComplete() {
                  parent.innerComplete();
              }
          }
      }
  }
  