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.flowable;
  
  import org.reactivestreams.*;
  
  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.subscriptions.SubscriptionHelper;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  import java.util.Objects;
  
  /**
   * Reduce a sequence of values, starting from a seed value and by using
   * an accumulator function and return the last accumulated value.
   *
   * @param <T> the source value type
   * @param <R> the accumulated result type
   */
  public final class FlowableReduceSeedSingle<T, R> extends Single<R> {
  
      final Publisher<T> source;
  
      final R seed;
  
      final BiFunction<R, ? super T, R> reducer;
  
      public FlowableReduceSeedSingle(Publisher<T> source, R seed, BiFunction<R, ? super T, R> reducer) {
          this.source = source;
          this.seed = seed;
          this.reducer = reducer;
      }
  
      @Override
      protected void subscribeActual(SingleObserver<? super R> observer) {
          source.subscribe(new ReduceSeedObserver<>(observer, reducer, seed));
      }
  
      static final class ReduceSeedObserver<T, R> implements FlowableSubscriber<T>, Disposable {
  
          final SingleObserver<? super R> downstream;
  
          final BiFunction<R, ? super T, R> reducer;
  
          R value;
  
          Subscription upstream;
  
          ReduceSeedObserver(SingleObserver<? super R> actual, BiFunction<R, ? super T, R> reducer, R value) {
              this.downstream = actual;
              this.value = value;
              this.reducer = reducer;
          }
  
          @Override
          public void onSubscribe(Subscription s) {
              if (SubscriptionHelper.validate(this.upstream, s)) {
                  this.upstream = s;
  
                  downstream.onSubscribe(this);
  
                  s.request(Long.MAX_VALUE);
              }
          }
  
          @Override
          public void onNext(T value) {
              R v = this.value;
              if (v != null) {
                  try {
                      this.value = Objects.requireNonNull(reducer.apply(v, value), "The reducer returned a null value");
                  } catch (Throwable ex) {
                      Exceptions.throwIfFatal(ex);
                      upstream.cancel();
                      onError(ex);
                  }
              }
          }
  
          @Override
          public void onError(Throwable e) {
              if (value != null) {
                  value = null;
                  upstream = SubscriptionHelper.CANCELLED;
                  downstream.onError(e);
              } else {
                  RxJavaPlugins.onError(e);
              }
          }
  
          @Override
          public void onComplete() {
              R v = value;
              if (v != null) {
                  value = null;
                  upstream = SubscriptionHelper.CANCELLED;
                  downstream.onSuccess(v);
              }
          }
  
          @Override
          public void dispose() {
              upstream.cancel();
              upstream = SubscriptionHelper.CANCELLED;
          }
  
          @Override
          public boolean isDisposed() {
              return upstream == SubscriptionHelper.CANCELLED;
          }
      }
  }
  