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.single;
  
  import java.util.Objects;
  import java.util.concurrent.atomic.AtomicReference;
  
  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.DisposableHelper;
  
  /**
   * Maps a source item to another SingleSource then calls a BiFunction with the
   * original item and the secondary item to generate the final result.
   *
   * @param <T> the main value type
   * @param <U> the second value type
   * @param <R> the result value type
   * @since 3.0.0
   */
  public final class SingleFlatMapBiSelector<T, U, R> extends Single<R> {
  
      final SingleSource<T> source;
  
      final Function<? super T, ? extends SingleSource<? extends U>> mapper;
  
      final BiFunction<? super T, ? super U, ? extends R> resultSelector;
  
      public SingleFlatMapBiSelector(SingleSource<T> source,
              Function<? super T, ? extends SingleSource<? extends U>> mapper,
              BiFunction<? super T, ? super U, ? extends R> resultSelector) {
          this.source = source;
          this.mapper = mapper;
          this.resultSelector = resultSelector;
      }
  
      @Override
      protected void subscribeActual(SingleObserver<? super R> observer) {
          source.subscribe(new FlatMapBiMainObserver<T, U, R>(observer, mapper, resultSelector));
      }
  
      static final class FlatMapBiMainObserver<T, U, R>
      implements SingleObserver<T>, Disposable {
  
          final Function<? super T, ? extends SingleSource<? extends U>> mapper;
  
          final InnerObserver<T, U, R> inner;
  
          FlatMapBiMainObserver(SingleObserver<? super R> actual,
                  Function<? super T, ? extends SingleSource<? extends U>> mapper,
                  BiFunction<? super T, ? super U, ? extends R> resultSelector) {
              this.inner = new InnerObserver<>(actual, resultSelector);
              this.mapper = mapper;
          }
  
          @Override
          public void dispose() {
              DisposableHelper.dispose(inner);
          }
  
          @Override
          public boolean isDisposed() {
              return DisposableHelper.isDisposed(inner.get());
          }
  
          @Override
          public void onSubscribe(Disposable d) {
              if (DisposableHelper.setOnce(inner, d)) {
                  inner.downstream.onSubscribe(this);
              }
          }
  
          @Override
          public void onSuccess(T value) {
              SingleSource<? extends U> next;
  
              try {
                  next = Objects.requireNonNull(mapper.apply(value), "The mapper returned a null MaybeSource");
              } catch (Throwable ex) {
                  Exceptions.throwIfFatal(ex);
                  inner.downstream.onError(ex);
                  return;
              }
  
              if (DisposableHelper.replace(inner, null)) {
                  inner.value = value;
                  next.subscribe(inner);
              }
          }
  
          @Override
          public void onError(Throwable e) {
              inner.downstream.onError(e);
          }
  
          static final class InnerObserver<T, U, R>
          extends AtomicReference<Disposable>
          implements SingleObserver<U> {
  
              private static final long serialVersionUID = -2897979525538174559L;
  
              final SingleObserver<? super R> downstream;
  
              final BiFunction<? super T, ? super U, ? extends R> resultSelector;
  
              T value;
  
              InnerObserver(SingleObserver<? super R> actual,
                      BiFunction<? super T, ? super U, ? extends R> resultSelector) {
                  this.downstream = actual;
                  this.resultSelector = resultSelector;
              }
  
              @Override
              public void onSubscribe(Disposable d) {
                  DisposableHelper.setOnce(this, d);
              }
  
              @Override
              public void onSuccess(U value) {
                  T t = this.value;
                  this.value = null;
  
                  R r;
  
                  try {
                      r = Objects.requireNonNull(resultSelector.apply(t, value), "The resultSelector returned a null value");
                  } catch (Throwable ex) {
                      Exceptions.throwIfFatal(ex);
                      downstream.onError(ex);
                      return;
                  }
  
                  downstream.onSuccess(r);
              }
  
              @Override
              public void onError(Throwable e) {
                  downstream.onError(e);
              }
          }
      }
  }
  