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.mixed;
  
  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.Function;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.internal.util.*;
  import io.reactivex.rxjava3.operators.SimpleQueue;
  
  /**
   * Maps each upstream item into a {@link SingleSource}, subscribes to them one after the other terminates
   * and relays their success values, optionally delaying any errors till the main and inner sources
   * terminate.
   * <p>History: 2.1.11 - experimental
   * @param <T> the upstream element type
   * @param <R> the output element type
   * @since 2.2
   */
  public final class ObservableConcatMapSingle<T, R> extends Observable<R> {
  
      final ObservableSource<T> source;
  
      final Function<? super T, ? extends SingleSource<? extends R>> mapper;
  
      final ErrorMode errorMode;
  
      final int prefetch;
  
      public ObservableConcatMapSingle(ObservableSource<T> source,
              Function<? super T, ? extends SingleSource<? extends R>> mapper,
                      ErrorMode errorMode, int prefetch) {
          this.source = source;
          this.mapper = mapper;
          this.errorMode = errorMode;
          this.prefetch = prefetch;
      }
  
      @Override
      protected void subscribeActual(Observer<? super R> observer) {
          if (!ScalarXMapZHelper.tryAsSingle(source, mapper, observer)) {
              source.subscribe(new ConcatMapSingleMainObserver<>(observer, mapper, prefetch, errorMode));
          }
      }
  
      static final class ConcatMapSingleMainObserver<T, R>
      extends ConcatMapXMainObserver<T> {
  
          private static final long serialVersionUID = -9140123220065488293L;
  
          final Observer<? super R> downstream;
  
          final Function<? super T, ? extends SingleSource<? extends R>> mapper;
  
          final ConcatMapSingleObserver<R> inner;
  
          R item;
  
          volatile int state;
  
          /** No inner SingleSource is running. */
          static final int STATE_INACTIVE = 0;
          /** An inner SingleSource is running but there are no results yet. */
          static final int STATE_ACTIVE = 1;
          /** The inner SingleSource succeeded with a value in {@link #item}. */
          static final int STATE_RESULT_VALUE = 2;
  
          ConcatMapSingleMainObserver(Observer<? super R> downstream,
                  Function<? super T, ? extends SingleSource<? extends R>> mapper,
                          int prefetch, ErrorMode errorMode) {
              super(prefetch, errorMode);
              this.downstream = downstream;
              this.mapper = mapper;
              this.inner = new ConcatMapSingleObserver<>(this);
          }
  
          void innerSuccess(R item) {
              this.item = item;
              this.state = STATE_RESULT_VALUE;
              drain();
          }
  
          void innerError(Throwable ex) {
              if (errors.tryAddThrowableOrReport(ex)) {
                  if (errorMode != ErrorMode.END) {
                      upstream.dispose();
                  }
                  this.state = STATE_INACTIVE;
                  drain();
              }
          }
  
          @Override
          void disposeInner() {
              inner.dispose();
          }
  
          @Override
          void onSubscribeDownstream() {
              downstream.onSubscribe(this);
          }
  
          @Override
          void clearValue() {
              item = null;
          }
  
          @Override
          void drain() {
              if (getAndIncrement() != 0) {
                  return;
              }
  
              int missed = 1;
              Observer<? super R> downstream = this.downstream;
              ErrorMode errorMode = this.errorMode;
              SimpleQueue<T> queue = this.queue;
              AtomicThrowable errors = this.errors;
  
              for (;;) {
  
                  for (;;) {
                      if (disposed) {
                          queue.clear();
                          item = null;
                          break;
                      }
  
                      int s = state;
  
                      if (errors.get() != null) {
                          if (errorMode == ErrorMode.IMMEDIATE
                                  || (errorMode == ErrorMode.BOUNDARY && s == STATE_INACTIVE)) {
                              queue.clear();
                              item = null;
                              errors.tryTerminateConsumer(downstream);
                              return;
                          }
                      }
  
                      if (s == STATE_INACTIVE) {
                          boolean d = done;
                          T v;
  
                          try {
                              v = queue.poll();
                          } catch (Throwable ex) {
                              Exceptions.throwIfFatal(ex);
                              disposed = true;
                              upstream.dispose();
                              errors.tryAddThrowableOrReport(ex);
                              errors.tryTerminateConsumer(downstream);
                              return;
                          }
                          boolean empty = v == null;
  
                          if (d && empty) {
                              errors.tryTerminateConsumer(downstream);
                              return;
                          }
  
                          if (empty) {
                              break;
                          }
  
                          SingleSource<? extends R> ss;
  
                          try {
                              ss = Objects.requireNonNull(mapper.apply(v), "The mapper returned a null SingleSource");
                          } catch (Throwable ex) {
                              Exceptions.throwIfFatal(ex);
                              upstream.dispose();
                              queue.clear();
                              errors.tryAddThrowableOrReport(ex);
                              errors.tryTerminateConsumer(downstream);
                              return;
                          }
  
                          state = STATE_ACTIVE;
                          ss.subscribe(inner);
                          break;
                      } else if (s == STATE_RESULT_VALUE) {
                          R w = item;
                          item = null;
                          downstream.onNext(w);
  
                          state = STATE_INACTIVE;
                      } else {
                          break;
                      }
                  }
  
                  missed = addAndGet(-missed);
                  if (missed == 0) {
                      break;
                  }
              }
          }
  
          static final class ConcatMapSingleObserver<R>
          extends AtomicReference<Disposable>
          implements SingleObserver<R> {
  
              private static final long serialVersionUID = -3051469169682093892L;
  
              final ConcatMapSingleMainObserver<?, R> parent;
  
              ConcatMapSingleObserver(ConcatMapSingleMainObserver<?, R> parent) {
                  this.parent = parent;
              }
  
              @Override
              public void onSubscribe(Disposable d) {
                  DisposableHelper.replace(this, d);
              }
  
              @Override
              public void onSuccess(R t) {
                  parent.innerSuccess(t);
              }
  
              @Override
              public void onError(Throwable e) {
                  parent.innerError(e);
              }
  
              void dispose() {
                  DisposableHelper.dispose(this);
              }
          }
      }
  }
  