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.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.util.ExceptionHelper;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Represents a hot Single-like source and consumer of events similar to Subjects.
   * <p>
   * <img width="640" height="236" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/SingleSubject.png" alt="">
   * <p>
   * This subject does not have a public constructor by design; a new non-terminated instance of this
   * {@code SingleSubject} can be created via the {@link #create()} method.
   * <p>
   * Since the {@code SingleSubject} 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 #onSuccess(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 SingleSubject} is a {@link io.reactivex.rxjava3.core.Single}, calling {@code onSuccess} or {@code onError}
   * will move this {@code SingleSubject} into its terminal state atomically.
   * <p>
   * All methods are thread safe. Calling {@link #onSuccess(Object)} multiple
   * times has no effect. Calling {@link #onError(Throwable)} multiple times relays the {@code Throwable} to
   * the {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)} global error handler.
   * <p>
   * Even though {@code SingleSubject} implements the {@code SingleObserver} 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 SingleSubject} reached its terminal state will result in the
   * given {@code Disposable} being disposed immediately.
   * <p>
   * This {@code SingleSubject} supports the standard state-peeking methods {@link #hasThrowable()},
   * {@link #getThrowable()} and {@link #hasObservers()} as well as means to read any success item in a non-blocking
   * and thread-safe manner via {@link #hasValue()} and {@link #getValue()}.
   * <p>
   * The {@code SingleSubject} does not support clearing its cached {@code onSuccess} value.
   * <dl>
   *  <dt><b>Scheduler:</b></dt>
   *  <dd>{@code SingleSubject} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and
   *  the {@code SingleObserver}s get notified on the thread where the terminating {@code onSuccess} or {@code onError}
   *  methods were invoked.</dd>
   *  <dt><b>Error handling:</b></dt>
   *  <dd>When the {@link #onError(Throwable)} is called, the {@code SingleSubject} enters into a terminal state
   *  and emits the same {@code Throwable} instance to the last set of {@code SingleObserver}s. During this emission,
   *  if one or more {@code SingleObserver}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 SingleObserver}s
   *  cancel at once).
   *  If there were no {@code SingleObserver}s subscribed to this {@code SingleSubject} when the {@code onError()}
   *  was called, the global error handler is not invoked.
   *  </dd>
   * </dl>
   * <p>
   * Example usage:
   * <pre><code>
   * SingleSubject&lt;Integer&gt; subject1 = SingleSubject.create();
   * 
   * TestObserver&lt;Integer&gt; to1 = subject1.test();
   * 
   * // SingleSubjects are empty by default
   * to1.assertEmpty();
   * 
   * subject1.onSuccess(1);
   * 
   * // onSuccess is a terminal event with SingleSubjects
   * // TestObserver converts onSuccess into onNext + onComplete
   * to1.assertResult(1);
   *
   * TestObserver&lt;Integer&gt; to2 = subject1.test();
   * 
   * // late Observers receive the terminal signal (onSuccess) too
   * to2.assertResult(1);
   * </code></pre>
   * <p>History: 2.0.5 - experimental
   * @param <T> the value type received and emitted
   * @since 2.1
   */
  public final class SingleSubject<T> extends Single<T> implements SingleObserver<T> {
  
      final AtomicReference<SingleDisposable<T>[]> observers;
  
      @SuppressWarnings("rawtypes")
      static final SingleDisposable[] EMPTY = new SingleDisposable[0];
  
      @SuppressWarnings("rawtypes")
      static final SingleDisposable[] TERMINATED = new SingleDisposable[0];
  
      final AtomicBoolean once;
      T value;
      Throwable error;
  
      /**
       * Creates a fresh SingleSubject.
       * @param <T> the value type received and emitted
       * @return the new SingleSubject instance
       */
      @CheckReturnValue
      @NonNull
      public static <T> SingleSubject<T> create() {
          return new SingleSubject<>();
      }
  
      @SuppressWarnings("unchecked")
      SingleSubject() {
          once = new AtomicBoolean();
          observers = new AtomicReference<>(EMPTY);
      }
  
      @Override
      public void onSubscribe(@NonNull Disposable d) {
          if (observers.get() == TERMINATED) {
              d.dispose();
          }
      }
  
      @SuppressWarnings("unchecked")
      @Override
      public void onSuccess(@NonNull T value) {
          ExceptionHelper.nullCheck(value, "onSuccess called with a null value.");
          if (once.compareAndSet(false, true)) {
              this.value = value;
              for (SingleDisposable<T> md : observers.getAndSet(TERMINATED)) {
                  md.downstream.onSuccess(value);
              }
          }
      }
  
      @SuppressWarnings("unchecked")
      @Override
      public void onError(@NonNull Throwable e) {
          ExceptionHelper.nullCheck(e, "onError called with a null Throwable.");
          if (once.compareAndSet(false, true)) {
              this.error = e;
              for (SingleDisposable<T> md : observers.getAndSet(TERMINATED)) {
                  md.downstream.onError(e);
              }
          } else {
              RxJavaPlugins.onError(e);
          }
      }
  
      @Override
      protected void subscribeActual(@NonNull SingleObserver<? super T> observer) {
          SingleDisposable<T> md = new SingleDisposable<>(observer, this);
          observer.onSubscribe(md);
          if (add(md)) {
              if (md.isDisposed()) {
                  remove(md);
              }
          } else {
              Throwable ex = error;
              if (ex != null) {
                  observer.onError(ex);
              } else {
                  observer.onSuccess(value);
              }
          }
      }
  
      boolean add(@NonNull SingleDisposable<T> inner) {
          for (;;) {
              SingleDisposable<T>[] a = observers.get();
              if (a == TERMINATED) {
                  return false;
              }
  
              int n = a.length;
              @SuppressWarnings("unchecked")
              SingleDisposable<T>[] b = new SingleDisposable[n + 1];
              System.arraycopy(a, 0, b, 0, n);
              b[n] = inner;
              if (observers.compareAndSet(a, b)) {
                  return true;
              }
          }
      }
  
      @SuppressWarnings("unchecked")
      void remove(@NonNull SingleDisposable<T> inner) {
          for (;;) {
              SingleDisposable<T>[] a = observers.get();
              int n = a.length;
              if (n == 0) {
                  return;
              }
  
              int j = -1;
  
              for (int i = 0; i < n; i++) {
                  if (a[i] == inner) {
                      j = i;
                      break;
                  }
              }
  
              if (j < 0) {
                  return;
              }
              SingleDisposable<T>[] b;
              if (n == 1) {
                  b = EMPTY;
              } else {
                  b = new SingleDisposable[n - 1];
                  System.arraycopy(a, 0, b, 0, j);
                  System.arraycopy(a, j + 1, b, j, n - j - 1);
              }
  
              if (observers.compareAndSet(a, b)) {
                  return;
              }
          }
      }
  
      /**
       * Returns the success value if this SingleSubject was terminated with a success value.
       * @return the success value or null
       */
      @Nullable
      public T getValue() {
          if (observers.get() == TERMINATED) {
              return value;
          }
          return null;
      }
  
      /**
       * Returns true if this SingleSubject was terminated with a success value.
       * @return true if this SingleSubject was terminated with a success value
       */
      public boolean hasValue() {
          return observers.get() == TERMINATED && value != null;
      }
  
      /**
       * Returns the terminal error if this SingleSubject has been terminated with an error, null otherwise.
       * @return the terminal error or null if not terminated or not with an error
       */
      @Nullable
      public Throwable getThrowable() {
          if (observers.get() == TERMINATED) {
              return error;
          }
          return null;
      }
  
      /**
       * Returns true if this SingleSubject has been terminated with an error.
       * @return true if this SingleSubject has been terminated with an error
       */
      public boolean hasThrowable() {
          return observers.get() == TERMINATED && error != null;
      }
  
      /**
       * Returns true if this SingleSubject has observers.
       * @return true if this SingleSubject has observers
       */
      public boolean hasObservers() {
          return observers.get().length != 0;
      }
  
      /**
       * Returns the number of current observers.
       * @return the number of current observers
       */
      /* test */ int observerCount() {
          return observers.get().length;
      }
  
      static final class SingleDisposable<T>
      extends AtomicReference<SingleSubject<T>> implements Disposable {
          private static final long serialVersionUID = -7650903191002190468L;
  
          final SingleObserver<? super T> downstream;
  
          SingleDisposable(SingleObserver<? super T> actual, SingleSubject<T> parent) {
              this.downstream = actual;
              lazySet(parent);
          }
  
          @Override
          public void dispose() {
              SingleSubject<T> parent = getAndSet(null);
              if (parent != null) {
                  parent.remove(this);
              }
          }
  
          @Override
          public boolean isDisposed() {
              return get() == null;
          }
      }
  }
  