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.subscribers;
  
  import org.reactivestreams.Subscription;
  
  import io.reactivex.rxjava3.exceptions.Exceptions;
  import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
  import io.reactivex.rxjava3.operators.ConditionalSubscriber;
  import io.reactivex.rxjava3.operators.QueueSubscription;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Base class for a fuseable intermediate subscriber.
   * @param <T> the upstream value type
   * @param <R> the downstream value type
   */
  public abstract class BasicFuseableConditionalSubscriber<T, R> implements ConditionalSubscriber<T>, QueueSubscription<R> {
  
      /** The downstream subscriber. */
      protected final ConditionalSubscriber<? super R> downstream;
  
      /** The upstream subscription. */
      protected Subscription upstream;
  
      /** The upstream's QueueSubscription if not null. */
      protected QueueSubscription<T> qs;
  
      /** Flag indicating no further onXXX event should be accepted. */
      protected boolean done;
  
      /** Holds the established fusion mode of the upstream. */
      protected int sourceMode;
  
      /**
       * Construct a BasicFuseableSubscriber by wrapping the given subscriber.
       * @param downstream the subscriber, not null (not verified)
       */
      public BasicFuseableConditionalSubscriber(ConditionalSubscriber<? super R> downstream) {
          this.downstream = downstream;
      }
  
      // final: fixed protocol steps to support fuseable and non-fuseable upstream
      @SuppressWarnings("unchecked")
      @Override
      public final void onSubscribe(Subscription s) {
          if (SubscriptionHelper.validate(this.upstream, s)) {
  
              this.upstream = s;
              if (s instanceof QueueSubscription) {
                  this.qs = (QueueSubscription<T>)s;
              }
  
              if (beforeDownstream()) {
  
                  downstream.onSubscribe(this);
  
                  afterDownstream();
              }
  
          }
      }
  
      /**
       * Override this to perform actions before the call {@code actual.onSubscribe(this)} happens.
       * @return true if onSubscribe should continue with the call
       */
      protected boolean beforeDownstream() {
          return true;
      }
  
      /**
       * Override this to perform actions after the call to {@code actual.onSubscribe(this)} happened.
       */
      protected void afterDownstream() {
          // default no-op
      }
  
      // -----------------------------------
      // Convenience and state-aware methods
      // -----------------------------------
  
      @Override
      public void onError(Throwable t) {
          if (done) {
              RxJavaPlugins.onError(t);
              return;
          }
          done = true;
          downstream.onError(t);
      }
  
      /**
       * Rethrows the throwable if it is a fatal exception or calls {@link #onError(Throwable)}.
       * @param t the throwable to rethrow or signal to the actual subscriber
       */
      protected final void fail(Throwable t) {
          Exceptions.throwIfFatal(t);
          upstream.cancel();
          onError(t);
      }
  
      @Override
      public void onComplete() {
          if (done) {
              return;
          }
          done = true;
          downstream.onComplete();
      }
  
      /**
       * Calls the upstream's QueueSubscription.requestFusion with the mode and
       * saves the established mode in {@link #sourceMode} if that mode doesn't
       * have the {@link QueueSubscription#BOUNDARY} flag set.
       * <p>
       * If the upstream doesn't support fusion ({@link #qs} is null), the method
       * returns {@link QueueSubscription#NONE}.
       * @param mode the fusion mode requested
       * @return the established fusion mode
       */
      protected final int transitiveBoundaryFusion(int mode) {
          QueueSubscription<T> qs = this.qs;
          if (qs != null) {
              if ((mode & BOUNDARY) == 0) {
                  int m = qs.requestFusion(mode);
                  if (m != NONE) {
                      sourceMode = m;
                  }
                  return m;
              }
          }
          return NONE;
      }
  
      // --------------------------------------------------------------
      // Default implementation of the RS and QS protocol (can be overridden)
      // --------------------------------------------------------------
  
      @Override
      public void request(long n) {
          upstream.request(n);
      }
  
      @Override
      public void cancel() {
          upstream.cancel();
      }
  
      @Override
      public boolean isEmpty() {
          return qs.isEmpty();
      }
  
      @Override
      public void clear() {
          qs.clear();
      }
  
      // -----------------------------------------------------------
      // The rest of the Queue interface methods shouldn't be called
      // -----------------------------------------------------------
  
      @Override
      public final boolean offer(R e) {
          throw new UnsupportedOperationException("Should not be called!");
      }
  
      @Override
      public final boolean offer(R v1, R v2) {
          throw new UnsupportedOperationException("Should not be called!");
      }
  }
  