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.*;
  import io.reactivex.rxjava3.internal.disposables.*;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  import java.util.Objects;
  
  public final class ObservableScanSeed<T, R> extends AbstractObservableWithUpstream<T, R> {
      final BiFunction<R, ? super T, R> accumulator;
      final Supplier<R> seedSupplier;
  
      public ObservableScanSeed(ObservableSource<T> source, Supplier<R> seedSupplier, BiFunction<R, ? super T, R> accumulator) {
          super(source);
          this.accumulator = accumulator;
          this.seedSupplier = seedSupplier;
      }
  
      @Override
      public void subscribeActual(Observer<? super R> t) {
          R r;
  
          try {
              r = Objects.requireNonNull(seedSupplier.get(), "The seed supplied is null");
          } catch (Throwable e) {
              Exceptions.throwIfFatal(e);
              EmptyDisposable.error(e, t);
              return;
          }
  
          source.subscribe(new ScanSeedObserver<>(t, accumulator, r));
      }
  
      static final class ScanSeedObserver<T, R> implements Observer<T>, Disposable {
          final Observer<? super R> downstream;
          final BiFunction<R, ? super T, R> accumulator;
  
          R value;
  
          Disposable upstream;
  
          boolean done;
  
          ScanSeedObserver(Observer<? super R> actual, BiFunction<R, ? super T, R> accumulator, R value) {
              this.downstream = actual;
              this.accumulator = accumulator;
              this.value = value;
          }
  
          @Override
          public void onSubscribe(Disposable d) {
              if (DisposableHelper.validate(this.upstream, d)) {
                  this.upstream = d;
  
                  downstream.onSubscribe(this);
  
                  downstream.onNext(value);
              }
          }
  
          @Override
          public void dispose() {
              upstream.dispose();
          }
  
          @Override
          public boolean isDisposed() {
              return upstream.isDisposed();
          }
  
          @Override
          public void onNext(T t) {
              if (done) {
                  return;
              }
  
              R v = value;
  
              R u;
  
              try {
                  u = Objects.requireNonNull(accumulator.apply(v, t), "The accumulator returned a null value");
              } catch (Throwable e) {
                  Exceptions.throwIfFatal(e);
                  upstream.dispose();
                  onError(e);
                  return;
              }
  
              value = u;
  
              downstream.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();
          }
      }
  }
  