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 java.util.Objects;
  import java.util.concurrent.atomic.AtomicInteger;
  
  import io.reactivex.rxjava3.annotations.Nullable;
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.exceptions.Exceptions;
  import io.reactivex.rxjava3.functions.*;
  import io.reactivex.rxjava3.internal.disposables.EmptyDisposable;
  import io.reactivex.rxjava3.operators.QueueDisposable;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Utility classes to work with scalar-sourced XMap operators (where X == { flat, concat, switch }).
   */
  public final class ObservableScalarXMap {
  
      /** Utility class. */
      private ObservableScalarXMap() {
          throw new IllegalStateException("No instances!");
      }
  
      /**
       * Tries to subscribe to a possibly Supplier source's mapped ObservableSource.
       * @param <T> the input value type
       * @param <R> the output value type
       * @param source the source ObservableSource
       * @param observer the subscriber
       * @param mapper the function mapping a scalar value into an ObservableSource
       * @return true if successful, false if the caller should continue with the regular path.
       */
      @SuppressWarnings("unchecked")
      public static <T, R> boolean tryScalarXMapSubscribe(ObservableSource<T> source,
              Observer<? super R> observer,
              Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
          if (source instanceof Supplier) {
              T t;
  
              try {
                  t = ((Supplier<T>)source).get();
              } catch (Throwable ex) {
                  Exceptions.throwIfFatal(ex);
                  EmptyDisposable.error(ex, observer);
                  return true;
              }
  
              if (t == null) {
                  EmptyDisposable.complete(observer);
                  return true;
              }
  
              ObservableSource<? extends R> r;
  
              try {
                  r = Objects.requireNonNull(mapper.apply(t), "The mapper returned a null ObservableSource");
              } catch (Throwable ex) {
                  Exceptions.throwIfFatal(ex);
                  EmptyDisposable.error(ex, observer);
                  return true;
              }
  
              if (r instanceof Supplier) {
                  R u;
  
                  try {
                      u = ((Supplier<R>)r).get();
                  } catch (Throwable ex) {
                      Exceptions.throwIfFatal(ex);
                      EmptyDisposable.error(ex, observer);
                      return true;
                  }
  
                  if (u == null) {
                      EmptyDisposable.complete(observer);
                      return true;
                  }
                  ScalarDisposable<R> sd = new ScalarDisposable<>(observer, u);
                  observer.onSubscribe(sd);
                  sd.run();
              } else {
                  r.subscribe(observer);
              }
  
              return true;
          }
          return false;
      }
  
      /**
       * Maps a scalar value into an Observable and emits its values.
       *
       * @param <T> the scalar value type
       * @param <U> the output value type
       * @param value the scalar value to map
       * @param mapper the function that gets the scalar value and should return
       * an ObservableSource that gets streamed
       * @return the new Observable instance
       */
      public static <T, U> Observable<U> scalarXMap(T value,
              Function<? super T, ? extends ObservableSource<? extends U>> mapper) {
          return RxJavaPlugins.onAssembly(new ScalarXMapObservable<>(value, mapper));
      }
  
      /**
       * Maps a scalar value to an ObservableSource and subscribes to it.
       *
       * @param <T> the scalar value type
       * @param <R> the mapped ObservableSource's element type.
       */
      static final class ScalarXMapObservable<T, R> extends Observable<R> {
  
          final T value;
  
          final Function<? super T, ? extends ObservableSource<? extends R>> mapper;
  
          ScalarXMapObservable(T value,
                  Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
              this.value = value;
              this.mapper = mapper;
          }
  
          @SuppressWarnings("unchecked")
          @Override
          public void subscribeActual(Observer<? super R> observer) {
              ObservableSource<? extends R> other;
              try {
                  other = Objects.requireNonNull(mapper.apply(value), "The mapper returned a null ObservableSource");
              } catch (Throwable e) {
                  Exceptions.throwIfFatal(e);
                  EmptyDisposable.error(e, observer);
                  return;
              }
              if (other instanceof Supplier) {
                  R u;
  
                  try {
                      u = ((Supplier<R>)other).get();
                  } catch (Throwable ex) {
                      Exceptions.throwIfFatal(ex);
                      EmptyDisposable.error(ex, observer);
                      return;
                  }
  
                  if (u == null) {
                      EmptyDisposable.complete(observer);
                      return;
                  }
                  ScalarDisposable<R> sd = new ScalarDisposable<>(observer, u);
                  observer.onSubscribe(sd);
                  sd.run();
              } else {
                  other.subscribe(observer);
              }
          }
      }
  
      /**
       * Represents a Disposable that signals one onNext followed by an onComplete.
       *
       * @param <T> the value type
       */
      public static final class ScalarDisposable<T>
      extends AtomicInteger
      implements QueueDisposable<T>, Runnable {
  
          private static final long serialVersionUID = 3880992722410194083L;
  
          final Observer<? super T> observer;
  
          final T value;
  
          static final int START = 0;
          static final int FUSED = 1;
          static final int ON_NEXT = 2;
          static final int ON_COMPLETE = 3;
  
          public ScalarDisposable(Observer<? super T> observer, T value) {
              this.observer = observer;
              this.value = value;
          }
  
          @Override
          public boolean offer(T value) {
              throw new UnsupportedOperationException("Should not be called!");
          }
  
          @Override
          public boolean offer(T v1, T v2) {
              throw new UnsupportedOperationException("Should not be called!");
          }
  
          @Nullable
          @Override
          public T poll() {
              if (get() == FUSED) {
                  lazySet(ON_COMPLETE);
                  return value;
              }
              return null;
          }
  
          @Override
          public boolean isEmpty() {
              return get() != FUSED;
          }
  
          @Override
          public void clear() {
              lazySet(ON_COMPLETE);
          }
  
          @Override
          public void dispose() {
              set(ON_COMPLETE);
          }
  
          @Override
          public boolean isDisposed() {
              return get() == ON_COMPLETE;
          }
  
          @Override
          public int requestFusion(int mode) {
              if ((mode & SYNC) != 0) {
                  lazySet(FUSED);
                  return SYNC;
              }
              return NONE;
          }
  
          @Override
          public void run() {
              if (get() == START && compareAndSet(START, ON_NEXT)) {
                  observer.onNext(value);
                  if (get() == ON_NEXT) {
                      lazySet(ON_COMPLETE);
                      observer.onComplete();
                  }
              }
          }
      }
  }
  