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 java.util.NoSuchElementException;
  
  import org.reactivestreams.*;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.internal.subscriptions.*;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  public final class FlowableElementAt<T> extends AbstractFlowableWithUpstream<T, T> {
      final long index;
      final T defaultValue;
      final boolean errorOnFewer;
  
      public FlowableElementAt(Flowable<T> source, long index, T defaultValue, boolean errorOnFewer) {
          super(source);
          this.index = index;
          this.defaultValue = defaultValue;
          this.errorOnFewer = errorOnFewer;
      }
  
      @Override
      protected void subscribeActual(Subscriber<? super T> s) {
          source.subscribe(new ElementAtSubscriber<>(s, index, defaultValue, errorOnFewer));
      }
  
      static final class ElementAtSubscriber<T> extends DeferredScalarSubscription<T> implements FlowableSubscriber<T> {
  
          private static final long serialVersionUID = 4066607327284737757L;
  
          final long index;
          final T defaultValue;
          final boolean errorOnFewer;
  
          Subscription upstream;
  
          long count;
  
          boolean done;
  
          ElementAtSubscriber(Subscriber<? super T> actual, long index, T defaultValue, boolean errorOnFewer) {
              super(actual);
              this.index = index;
              this.defaultValue = defaultValue;
              this.errorOnFewer = errorOnFewer;
          }
  
          @Override
          public void onSubscribe(Subscription s) {
              if (SubscriptionHelper.validate(this.upstream, s)) {
                  this.upstream = s;
                  downstream.onSubscribe(this);
                  s.request(Long.MAX_VALUE);
              }
          }
  
          @Override
          public void onNext(T t) {
              if (done) {
                  return;
              }
              long c = count;
              if (c == index) {
                  done = true;
                  upstream.cancel();
                  complete(t);
                  return;
              }
              count = c + 1;
          }
  
          @Override
          public void onError(Throwable t) {
              if (done) {
                  RxJavaPlugins.onError(t);
                  return;
              }
              done = true;
              downstream.onError(t);
          }
  
          @Override
          public void onComplete() {
              if (!done) {
                  done = true;
                  T v = defaultValue;
                  if (v == null) {
                      if (errorOnFewer) {
                          downstream.onError(new NoSuchElementException());
                      } else {
                          downstream.onComplete();
                      }
                  } else {
                      complete(v);
                  }
              }
          }
  
          @Override
          public void cancel() {
              super.cancel();
              upstream.cancel();
          }
      }
  }
  