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.operators;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  
  /**
   * Represents a {@link SimpleQueue} plus the means and constants for requesting a fusion mode.
   * @param <T> the value type returned by the SimpleQueue.poll()
   * @since 3.1.1
   */
  public interface QueueFuseable<@NonNull T> extends SimpleQueue<T> {
      /**
       * Returned by the {@link #requestFusion(int)} if the upstream doesn't support
       * the requested mode.
       */
      int NONE = 0;
  
      /**
       * Request a synchronous fusion mode and can be returned by {@link #requestFusion(int)}
       * for an accepted mode.
       * <p>
       * In synchronous fusion, all upstream values are either already available or is generated
       * when {@link #poll()} is called synchronously. When the {@link #poll()} returns null,
       * that is the indication if a terminated stream.
       * In this mode, the upstream won't call the onXXX methods and callers of
       * {@link #poll()} should be prepared to catch exceptions. Note that {@link #poll()} has
       * to be called sequentially (from within a serializing drain-loop).
       */
      int SYNC = 1;
  
      /**
       * Request an asynchronous fusion mode and can be returned by {@link #requestFusion(int)}
       * for an accepted mode.
       * <p>
       * In asynchronous fusion, upstream values may become available to {@link #poll()} eventually.
       * Upstream signals onError() and onComplete() as usual but onNext may not actually contain
       * the upstream value but have {@code null} instead. Downstream should treat such onNext as indication
       * that {@link #poll()} can be called. Note that {@link #poll()} has to be called sequentially
       * (from within a serializing drain-loop). In addition, callers of {@link #poll()} should be
       * prepared to catch exceptions.
       */
      int ASYNC = 2;
  
      /**
       * Request any of the {@link #SYNC} or {@link #ASYNC} modes.
       */
      int ANY = SYNC | ASYNC;
  
      /**
       * Used in binary or combination with the other constants as an input to {@link #requestFusion(int)}
       * indicating that the {@link #poll()} will be called behind an asynchronous boundary and thus
       * may change the non-trivial computation locations attached to the {@link #poll()} chain of
       * fused operators.
       * <p>
       * For example, fusing map() and observeOn() may move the computation of the map's function over to
       * the thread run after the observeOn(), which is generally unexpected.
       */
      int BOUNDARY = 4;
  
      /**
       * Request a fusion mode from the upstream.
       * <p>
       * This should be called before {@code onSubscribe} returns.
       * <p>
       * Calling this method multiple times or after {@code onSubscribe} finished is not allowed
       * and may result in undefined behavior.
       * <p>
       * @param mode the requested fusion mode, allowed values are {@link #SYNC}, {@link #ASYNC},
       * {@link #ANY} combined with {@link #BOUNDARY} (e.g., {@code requestFusion(SYNC | BOUNDARY)}).
       * @return the established fusion mode: {@link #NONE}, {@link #SYNC}, {@link #ASYNC}.
       */
      int requestFusion(int mode);
  
  }
  