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.subjects;
  
  import java.util.concurrent.atomic.*;
  
  import io.reactivex.rxjava3.annotations.*;
  import io.reactivex.rxjava3.core.Observer;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.util.ExceptionHelper;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * A Subject that emits (multicasts) items to currently subscribed {@link Observer}s and terminal events to current
   * or late {@code Observer}s.
   * <p>
   * <img width="640" height="281" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/PublishSubject.png" alt="">
   * <p>
   * This subject does not have a public constructor by design; a new empty instance of this
   * {@code PublishSubject} can be created via the {@link #create()} method.
   * <p>
   * Since a {@code Subject} is conceptionally derived from the {@code Processor} type in the Reactive Streams specification,
   * {@code null}s are not allowed (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.13">Rule 2.13</a>) as
   * parameters to {@link #onNext(Object)} and {@link #onError(Throwable)}. Such calls will result in a
   * {@link NullPointerException} being thrown and the subject's state is not changed.
   * <p>
   * Since a {@code PublishSubject} is an {@link io.reactivex.rxjava3.core.Observable}, it does not support backpressure.
   * <p>
   * When this {@code PublishSubject} is terminated via {@link #onError(Throwable)} or {@link #onComplete()},
   * late {@link io.reactivex.rxjava3.core.Observer}s only receive the respective terminal event.
   * <p>
   * Unlike a {@link BehaviorSubject}, a {@code PublishSubject} doesn't retain/cache items, therefore, a new
   * {@code Observer} won't receive any past items.
   * <p>
   * Even though {@code PublishSubject} implements the {@code Observer} interface, calling
   * {@code onSubscribe} is not required (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.12">Rule 2.12</a>)
   * if the subject is used as a standalone source. However, calling {@code onSubscribe}
   * after the {@code PublishSubject} reached its terminal state will result in the
   * given {@code Disposable} being disposed immediately.
   * <p>
   * Calling {@link #onNext(Object)}, {@link #onError(Throwable)} and {@link #onComplete()}
   * is required to be serialized (called from the same thread or called non-overlappingly from different threads
   * through external means of serialization). The {@link #toSerialized()} method available to all {@code Subject}s
   * provides such serialization and also protects against reentrance (i.e., when a downstream {@code Observer}
   * consuming this subject also wants to call {@link #onNext(Object)} on this subject recursively).
   * <p>
   * This {@code PublishSubject} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
   * {@link #getThrowable()} and {@link #hasObservers()}.
   * <dl>
   *  <dt><b>Scheduler:</b></dt>
   *  <dd>{@code PublishSubject} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and
   *  the {@code Observer}s get notified on the thread the respective {@code onXXX} methods were invoked.</dd>
   *  <dt><b>Error handling:</b></dt>
   *  <dd>When the {@link #onError(Throwable)} is called, the {@code PublishSubject} enters into a terminal state
   *  and emits the same {@code Throwable} instance to the last set of {@code Observer}s. During this emission,
   *  if one or more {@code Observer}s dispose their respective {@code Disposable}s, the
   *  {@code Throwable} is delivered to the global error handler via
   *  {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)} (multiple times if multiple {@code Observer}s
   *  cancel at once).
   *  If there were no {@code Observer}s subscribed to this {@code PublishSubject} when the {@code onError()}
   *  was called, the global error handler is not invoked.
   *  </dd>
   * </dl>
   * <p>
   * Example usage:
   * <pre> {@code
  
    PublishSubject<Object> subject = PublishSubject.create();
    // observer1 will receive all onNext and onComplete events
    subject.subscribe(observer1);
    subject.onNext("one");
    subject.onNext("two");
    // observer2 will only receive "three" and onComplete
    subject.subscribe(observer2);
    subject.onNext("three");
    subject.onComplete();
  
    // late Observers only receive the terminal event
    subject.test().assertEmpty();
    } </pre>
   *
   * @param <T>
   *          the type of items observed and emitted by the Subject
   */
  public final class PublishSubject<T> extends Subject<T> {
      /** The terminated indicator for the subscribers array. */
      @SuppressWarnings("rawtypes")
      static final PublishDisposable[] TERMINATED = new PublishDisposable[0];
      /** An empty subscribers array to avoid allocating it all the time. */
      @SuppressWarnings("rawtypes")
      static final PublishDisposable[] EMPTY = new PublishDisposable[0];
  
      /** The array of currently subscribed subscribers. */
      final AtomicReference<PublishDisposable<T>[]> subscribers;
  
      /** The error, write before terminating and read after checking subscribers. */
      Throwable error;
  
      /**
       * Constructs a PublishSubject.
       * @param <T> the value type
       * @return the new PublishSubject
       */
      @CheckReturnValue
      @NonNull
      public static <T> PublishSubject<T> create() {
          return new PublishSubject<>();
      }
  
      /**
       * Constructs a PublishSubject.
       * @since 2.0
       */
      @SuppressWarnings("unchecked")
      PublishSubject() {
          subscribers = new AtomicReference<>(EMPTY);
      }
  
      @Override
      protected void subscribeActual(Observer<? super T> t) {
          PublishDisposable<T> ps = new PublishDisposable<>(t, this);
          t.onSubscribe(ps);
          if (add(ps)) {
              // if cancellation happened while a successful add, the remove() didn't work
              // so we need to do it again
              if (ps.isDisposed()) {
                  remove(ps);
              }
          } else {
              Throwable ex = error;
              if (ex != null) {
                  t.onError(ex);
              } else {
                  t.onComplete();
              }
          }
      }
  
      /**
       * Tries to add the given subscriber to the subscribers array atomically
       * or returns false if the subject has terminated.
       * @param ps the subscriber to add
       * @return true if successful, false if the subject has terminated
       */
      boolean add(PublishDisposable<T> ps) {
          for (;;) {
              PublishDisposable<T>[] a = subscribers.get();
              if (a == TERMINATED) {
                  return false;
              }
  
              int n = a.length;
              @SuppressWarnings("unchecked")
              PublishDisposable<T>[] b = new PublishDisposable[n + 1];
              System.arraycopy(a, 0, b, 0, n);
              b[n] = ps;
  
              if (subscribers.compareAndSet(a, b)) {
                  return true;
              }
          }
      }
  
      /**
       * Atomically removes the given subscriber if it is subscribed to the subject.
       * @param ps the subject to remove
       */
      @SuppressWarnings("unchecked")
      void remove(PublishDisposable<T> ps) {
          for (;;) {
              PublishDisposable<T>[] a = subscribers.get();
              if (a == TERMINATED || a == EMPTY) {
                  return;
              }
  
              int n = a.length;
              int j = -1;
              for (int i = 0; i < n; i++) {
                  if (a[i] == ps) {
                      j = i;
                      break;
                  }
              }
  
              if (j < 0) {
                  return;
              }
  
              PublishDisposable<T>[] b;
  
              if (n == 1) {
                  b = EMPTY;
              } else {
                  b = new PublishDisposable[n - 1];
                  System.arraycopy(a, 0, b, 0, j);
                  System.arraycopy(a, j + 1, b, j, n - j - 1);
              }
              if (subscribers.compareAndSet(a, b)) {
                  return;
              }
          }
      }
  
      @Override
      public void onSubscribe(Disposable d) {
          if (subscribers.get() == TERMINATED) {
              d.dispose();
          }
      }
  
      @Override
      public void onNext(T t) {
          ExceptionHelper.nullCheck(t, "onNext called with a null value.");
          for (PublishDisposable<T> pd : subscribers.get()) {
              pd.onNext(t);
          }
      }
  
      @SuppressWarnings("unchecked")
      @Override
      public void onError(Throwable t) {
          ExceptionHelper.nullCheck(t, "onError called with a null Throwable.");
          if (subscribers.get() == TERMINATED) {
              RxJavaPlugins.onError(t);
              return;
          }
          error = t;
  
          for (PublishDisposable<T> pd : subscribers.getAndSet(TERMINATED)) {
              pd.onError(t);
          }
      }
  
      @SuppressWarnings("unchecked")
      @Override
      public void onComplete() {
          if (subscribers.get() == TERMINATED) {
              return;
          }
          for (PublishDisposable<T> pd : subscribers.getAndSet(TERMINATED)) {
              pd.onComplete();
          }
      }
  
      @Override
      @CheckReturnValue
      public boolean hasObservers() {
          return subscribers.get().length != 0;
      }
  
      @Override
      @Nullable
      @CheckReturnValue
      public Throwable getThrowable() {
          if (subscribers.get() == TERMINATED) {
              return error;
          }
          return null;
      }
  
      @Override
      @CheckReturnValue
      public boolean hasThrowable() {
          return subscribers.get() == TERMINATED && error != null;
      }
  
      @Override
      @CheckReturnValue
      public boolean hasComplete() {
          return subscribers.get() == TERMINATED && error == null;
      }
  
      /**
       * Wraps the actual subscriber, tracks its requests and makes cancellation
       * to remove itself from the current subscribers array.
       *
       * @param <T> the value type
       */
      static final class PublishDisposable<T> extends AtomicBoolean implements Disposable {
  
          private static final long serialVersionUID = 3562861878281475070L;
          /** The actual subscriber. */
          final Observer<? super T> downstream;
          /** The subject state. */
          final PublishSubject<T> parent;
  
          /**
           * Constructs a PublishSubscriber, wraps the actual subscriber and the state.
           * @param actual the actual subscriber
           * @param parent the parent PublishProcessor
           */
          PublishDisposable(Observer<? super T> actual, PublishSubject<T> parent) {
              this.downstream = actual;
              this.parent = parent;
          }
  
          public void onNext(T t) {
              if (!get()) {
                  downstream.onNext(t);
              }
          }
  
          public void onError(Throwable t) {
              if (get()) {
                  RxJavaPlugins.onError(t);
              } else {
                  downstream.onError(t);
              }
          }
  
          public void onComplete() {
              if (!get()) {
                  downstream.onComplete();
              }
          }
  
          @Override
          public void dispose() {
              if (compareAndSet(false, true)) {
                  parent.remove(this);
              }
          }
  
          @Override
          public boolean isDisposed() {
              return get();
          }
      }
  }
  