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.operators.observable;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.exceptions.Exceptions;
  import io.reactivex.rxjava3.functions.BiFunction;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  import java.util.Objects;
  
  /**
   * Reduce a sequence of values into a single value via an aggregator function and emit the final value or complete
   * if the source is empty.
   *
   * @param <T> the source and result value type
   */
  public final class ObservableReduceMaybe<T> extends Maybe<T> {
  
      final ObservableSource<T> source;
  
      final BiFunction<T, T, T> reducer;
  
      public ObservableReduceMaybe(ObservableSource<T> source, BiFunction<T, T, T> reducer) {
          this.source = source;
          this.reducer = reducer;
      }
  
      @Override
      protected void subscribeActual(MaybeObserver<? super T> observer) {
          source.subscribe(new ReduceObserver<>(observer, reducer));
      }
  
      static final class ReduceObserver<T> implements Observer<T>, Disposable {
  
          final MaybeObserver<? super T> downstream;
  
          final BiFunction<T, T, T> reducer;
  
          boolean done;
  
          T value;
  
          Disposable upstream;
  
          ReduceObserver(MaybeObserver<? super T> observer, BiFunction<T, T, T> reducer) {
              this.downstream = observer;
              this.reducer = reducer;
          }
  
          @Override
          public void onSubscribe(Disposable d) {
              if (DisposableHelper.validate(this.upstream, d)) {
                  this.upstream = d;
  
                  downstream.onSubscribe(this);
              }
          }
  
          @Override
          public void onNext(T value) {
              if (!done) {
                  T v = this.value;
  
                  if (v == null) {
                      this.value = value;
                  } else {
                      try {
                          this.value = Objects.requireNonNull(reducer.apply(v, value), "The reducer returned a null value");
                      } catch (Throwable ex) {
                          Exceptions.throwIfFatal(ex);
                          upstream.dispose();
                          onError(ex);
                      }
                  }
              }
          }
  
          @Override
          public void onError(Throwable e) {
              if (done) {
                  RxJavaPlugins.onError(e);
                  return;
              }
              done = true;
              value = null;
              downstream.onError(e);
          }
  
          @Override
          public void onComplete() {
              if (done) {
                  return;
              }
              done = true;
              T v = value;
              value = null;
              if (v != null) {
                  downstream.onSuccess(v);
              } else {
                  downstream.onComplete();
              }
          }
  
          @Override
          public void dispose() {
              upstream.dispose();
          }
  
          @Override
          public boolean isDisposed() {
              return upstream.isDisposed();
          }
      }
  }
  