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.fuseable;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  import io.reactivex.rxjava3.core.Observable;
  
  /**
   * Interface indicating a operator implementation can be macro-fused back to Observable in case
   * the operator goes from Observable to some other reactive type and then the sequence calls
   * for toObservable again:
   * <pre>
   * {@code
   * Single<Integer> single = Observable.range(1, 10).reduce((a, b) -> a + b);
   * Observable<Integer> observable = single.toObservable();
   * }
   * </pre>
   *
   * The {@code Single.toObservable()} will check for this interface and call the {@link #fuseToObservable()}
   * to return an Observable which could be the Observable-specific implementation of reduce(BiFunction).
   * <p>
   * This causes a slight overhead in assembly time (1 instanceof check, 1 operator allocation and 1 dropped
   * operator) but does not incur the conversion overhead at runtime.
   *
   * @param <T> the value type
   */
  public interface FuseToObservable<@NonNull T> {
  
      /**
       * Returns a (direct) Observable for the operator.
       * <p>The implementation should handle the necessary RxJavaPlugins wrapping.
       * @return the Observable instance
       */
      @NonNull
      Observable<T> fuseToObservable();
  }
  