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.observers;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  import io.reactivex.rxjava3.core.Observer;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.internal.util.*;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Serializes access to the {@link Observer#onNext(Object)}, {@link Observer#onError(Throwable)} and
   * {@link Observer#onComplete()} methods of another {@link Observer}.
   *
   * <p>Note that {@link #onSubscribe(Disposable)} is not serialized in respect of the other methods so
   * make sure the {@code onSubscribe()} is called with a non-null {@link Disposable}
   * before any of the other methods are called.
   *
   * <p>The implementation assumes that the actual {@code Observer}'s methods don't throw.
   *
   * @param <T> the value type
   */
  public final class SerializedObserver<T> implements Observer<T>, Disposable {
      final Observer<? super T> downstream;
      final boolean delayError;
  
      static final int QUEUE_LINK_SIZE = 4;
  
      Disposable upstream;
  
      boolean emitting;
      AppendOnlyLinkedArrayList<Object> queue;
  
      volatile boolean done;
  
      /**
       * Construct a {@code SerializedObserver} by wrapping the given actual {@link Observer}.
       * @param downstream the actual {@code Observer}, not {@code null} (not verified)
       */
      public SerializedObserver(@NonNull Observer<? super T> downstream) {
          this(downstream, false);
      }
  
      /**
       * Construct a SerializedObserver by wrapping the given actual {@link Observer} and
       * optionally delaying the errors till all regular values have been emitted
       * from the internal buffer.
       * @param actual the actual {@code Observer}, not {@code null} (not verified)
       * @param delayError if {@code true}, errors are emitted after regular values have been emitted
       */
      public SerializedObserver(@NonNull Observer<? super T> actual, boolean delayError) {
          this.downstream = actual;
          this.delayError = delayError;
      }
  
      @Override
      public void onSubscribe(@NonNull Disposable d) {
          if (DisposableHelper.validate(this.upstream, d)) {
              this.upstream = d;
  
              downstream.onSubscribe(this);
          }
      }
  
      @Override
      public void dispose() {
          done = true;
          upstream.dispose();
      }
  
      @Override
      public boolean isDisposed() {
          return upstream.isDisposed();
      }
  
      @Override
      public void onNext(@NonNull T t) {
          if (done) {
              return;
          }
          if (t == null) {
              upstream.dispose();
              onError(ExceptionHelper.createNullPointerException("onNext called with a null value."));
              return;
          }
          synchronized (this) {
              if (done) {
                  return;
              }
              if (emitting) {
                  AppendOnlyLinkedArrayList<Object> q = queue;
                  if (q == null) {
                      q = new AppendOnlyLinkedArrayList<>(QUEUE_LINK_SIZE);
                      queue = q;
                  }
                  q.add(NotificationLite.next(t));
                  return;
              }
              emitting = true;
          }
  
          downstream.onNext(t);
  
          emitLoop();
      }
  
      @Override
      public void onError(@NonNull Throwable t) {
          if (done) {
              RxJavaPlugins.onError(t);
              return;
          }
          boolean reportError;
          synchronized (this) {
              if (done) {
                  reportError = true;
              } else
              if (emitting) {
                  done = true;
                  AppendOnlyLinkedArrayList<Object> q = queue;
                  if (q == null) {
                      q = new AppendOnlyLinkedArrayList<>(QUEUE_LINK_SIZE);
                      queue = q;
                  }
                  Object err = NotificationLite.error(t);
                  if (delayError) {
                      q.add(err);
                  } else {
                      q.setFirst(err);
                  }
                  return;
              } else {
                  done = true;
                  emitting = true;
                  reportError = false;
              }
          }
  
          if (reportError) {
              RxJavaPlugins.onError(t);
              return;
          }
  
          downstream.onError(t);
          // no need to loop because this onError is the last event
      }
  
      @Override
      public void onComplete() {
          if (done) {
              return;
          }
          synchronized (this) {
              if (done) {
                  return;
              }
              if (emitting) {
                  AppendOnlyLinkedArrayList<Object> q = queue;
                  if (q == null) {
                      q = new AppendOnlyLinkedArrayList<>(QUEUE_LINK_SIZE);
                      queue = q;
                  }
                  q.add(NotificationLite.complete());
                  return;
              }
              done = true;
              emitting = true;
          }
  
          downstream.onComplete();
          // no need to loop because this onComplete is the last event
      }
  
      void emitLoop() {
          for (;;) {
              AppendOnlyLinkedArrayList<Object> q;
              synchronized (this) {
                  q = queue;
                  if (q == null) {
                      emitting = false;
                      return;
                  }
                  queue = null;
              }
  
              if (q.accept(downstream)) {
                  return;
              }
          }
      }
  }
  