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 io.reactivex.rxjava3.annotations.NonNull;
  import io.reactivex.rxjava3.core.Flowable;
  import io.reactivex.rxjava3.exceptions.Exceptions;
  import io.reactivex.rxjava3.functions.BiFunction;
  import io.reactivex.rxjava3.functions.Supplier;
  import org.reactivestreams.Subscriber;
  
  import java.util.Objects;
  
  public final class FlowableOnBackpressureReduceWith<T, R> extends AbstractFlowableWithUpstream<T, R> {
  
      final BiFunction<R, ? super T, R> reducer;
      final Supplier<R> supplier;
  
      public FlowableOnBackpressureReduceWith(@NonNull Flowable<T> source,
                                              @NonNull Supplier<R> supplier,
                                              @NonNull BiFunction<R, ? super T, R> reducer) {
          super(source);
          this.reducer = reducer;
          this.supplier = supplier;
      }
  
      @Override
      protected void subscribeActual(@NonNull Subscriber<? super R> s) {
          source.subscribe(new BackpressureReduceWithSubscriber<>(s, supplier, reducer));
      }
  
      static final class BackpressureReduceWithSubscriber<T, R> extends AbstractBackpressureThrottlingSubscriber<T, R> {
  
          private static final long serialVersionUID = 8255923705960622424L;
  
          final BiFunction<R, ? super T, R> reducer;
          final Supplier<R> supplier;
  
          BackpressureReduceWithSubscriber(@NonNull Subscriber<? super R> downstream,
                                           @NonNull Supplier<R> supplier,
                                           @NonNull BiFunction<R, ? super T, R> reducer) {
              super(downstream);
              this.reducer = reducer;
              this.supplier = supplier;
          }
  
          @Override
          public void onNext(T t) {
              R v = current.get();
              if (v != null) {
                  v = current.getAndSet(null);
              }
              try {
                  if (v == null) {
                      current.lazySet(Objects.requireNonNull(
                              reducer.apply(Objects.requireNonNull(supplier.get(), "The supplier returned a null value"), t),
                              "The reducer returned a null value"
                      ));
                  } else {
                      current.lazySet(Objects.requireNonNull(reducer.apply(v, t), "The reducer returned a null value"));
                  }
              } catch (Throwable ex) {
                  Exceptions.throwIfFatal(ex);
                  upstream.cancel();
                  onError(ex);
                  return;
              }
              drain();
          }
      }
  }
  