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.Publisher;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.exceptions.Exceptions;
  import io.reactivex.rxjava3.functions.*;
  import io.reactivex.rxjava3.internal.disposables.EmptyDisposable;
  import io.reactivex.rxjava3.internal.operators.flowable.FlowableReduceSeedSingle.ReduceSeedObserver;
  
  import java.util.Objects;
  
  /**
   * Reduce a sequence of values, starting from a generated 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 FlowableReduceWithSingle<T, R> extends Single<R> {
  
      final Publisher<T> source;
  
      final Supplier<R> seedSupplier;
  
      final BiFunction<R, ? super T, R> reducer;
  
      public FlowableReduceWithSingle(Publisher<T> source, Supplier<R> seedSupplier, BiFunction<R, ? super T, R> reducer) {
          this.source = source;
          this.seedSupplier = seedSupplier;
          this.reducer = reducer;
      }
  
      @Override
      protected void subscribeActual(SingleObserver<? super R> observer) {
          R seed;
  
          try {
              seed = Objects.requireNonNull(seedSupplier.get(), "The seedSupplier returned a null value");
          } catch (Throwable ex) {
              Exceptions.throwIfFatal(ex);
              EmptyDisposable.error(ex, observer);
              return;
          }
          source.subscribe(new ReduceSeedObserver<>(observer, reducer, seed));
      }
  }
  