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.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;
  
  public final class FlowableScan<T> extends AbstractFlowableWithUpstream<T, T> {
      final BiFunction<T, T, T> accumulator;
      public FlowableScan(Flowable<T> source, BiFunction<T, T, T> accumulator) {
          super(source);
          this.accumulator = accumulator;
      }
  
      @Override
      protected void subscribeActual(Subscriber<? super T> s) {
          source.subscribe(new ScanSubscriber<>(s, accumulator));
      }
  
      static final class ScanSubscriber<T> implements FlowableSubscriber<T>, Subscription {
          final Subscriber<? super T> downstream;
          final BiFunction<T, T, T> accumulator;
  
          Subscription upstream;
  
          T value;
  
          boolean done;
  
          ScanSubscriber(Subscriber<? super T> actual, BiFunction<T, T, T> accumulator) {
              this.downstream = actual;
              this.accumulator = accumulator;
          }
  
          @Override
          public void onSubscribe(Subscription s) {
              if (SubscriptionHelper.validate(this.upstream, s)) {
                  this.upstream = s;
                  downstream.onSubscribe(this);
              }
          }
  
          @Override
          public void onNext(T t) {
              if (done) {
                  return;
              }
              final Subscriber<? super T> a = downstream;
              T v = value;
              if (v == null) {
                  value = t;
                  a.onNext(t);
              } else {
                  T u;
  
                  try {
                      u = Objects.requireNonNull(accumulator.apply(v, t), "The value returned by the accumulator is null");
                  } catch (Throwable e) {
                      Exceptions.throwIfFatal(e);
                      upstream.cancel();
                      onError(e);
                      return;
                  }
  
                  value = u;
                  a.onNext(u);
              }
          }
  
          @Override
          public void onError(Throwable t) {
              if (done) {
                  RxJavaPlugins.onError(t);
                  return;
              }
              done = true;
              downstream.onError(t);
          }
  
          @Override
          public void onComplete() {
              if (done) {
                  return;
              }
              done = true;
              downstream.onComplete();
          }
  
          @Override
          public void request(long n) {
              upstream.request(n);
          }
  
          @Override
          public void cancel() {
              upstream.cancel();
          }
      }
  }
  