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.Objects;
  import java.util.concurrent.atomic.*;
  
  import org.reactivestreams.*;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.exceptions.Exceptions;
  import io.reactivex.rxjava3.functions.*;
  import io.reactivex.rxjava3.internal.subscriptions.*;
  import io.reactivex.rxjava3.internal.util.BackpressureHelper;
  import io.reactivex.rxjava3.operators.SimplePlainQueue;
  import io.reactivex.rxjava3.operators.SpscArrayQueue;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  public final class FlowableScanSeed<T, R> extends AbstractFlowableWithUpstream<T, R> {
      final BiFunction<R, ? super T, R> accumulator;
      final Supplier<R> seedSupplier;
  
      public FlowableScanSeed(Flowable<T> source, Supplier<R> seedSupplier, BiFunction<R, ? super T, R> accumulator) {
          super(source);
          this.accumulator = accumulator;
          this.seedSupplier = seedSupplier;
      }
  
      @Override
      protected void subscribeActual(Subscriber<? super R> s) {
          R r;
  
          try {
              r = Objects.requireNonNull(seedSupplier.get(), "The seed supplied is null");
          } catch (Throwable e) {
              Exceptions.throwIfFatal(e);
              EmptySubscription.error(e, s);
              return;
          }
  
          source.subscribe(new ScanSeedSubscriber<>(s, accumulator, r, bufferSize()));
      }
  
      static final class ScanSeedSubscriber<T, R>
      extends AtomicInteger
      implements FlowableSubscriber<T>, Subscription {
          private static final long serialVersionUID = -1776795561228106469L;
  
          final Subscriber<? super R> downstream;
  
          final BiFunction<R, ? super T, R> accumulator;
  
          final SimplePlainQueue<R> queue;
  
          final AtomicLong requested;
  
          final int prefetch;
  
          final int limit;
  
          volatile boolean cancelled;
  
          volatile boolean done;
          Throwable error;
  
          Subscription upstream;
  
          R value;
  
          int consumed;
  
          ScanSeedSubscriber(Subscriber<? super R> actual, BiFunction<R, ? super T, R> accumulator, R value, int prefetch) {
              this.downstream = actual;
              this.accumulator = accumulator;
              this.value = value;
              this.prefetch = prefetch;
              this.limit = prefetch - (prefetch >> 2);
              this.queue = new SpscArrayQueue<>(prefetch);
              this.queue.offer(value);
              this.requested = new AtomicLong();
          }
  
          @Override
          public void onSubscribe(Subscription s) {
              if (SubscriptionHelper.validate(this.upstream, s)) {
                  this.upstream = s;
  
                  downstream.onSubscribe(this);
  
                  s.request(prefetch - 1);
              }
          }
  
          @Override
          public void onNext(T t) {
              if (done) {
                  return;
              }
  
              R v = value;
              try {
                  v = Objects.requireNonNull(accumulator.apply(v, t), "The accumulator returned a null value");
              } catch (Throwable ex) {
                  Exceptions.throwIfFatal(ex);
                  upstream.cancel();
                  onError(ex);
                  return;
              }
  
              value = v;
              queue.offer(v);
              drain();
          }
  
          @Override
          public void onError(Throwable t) {
              if (done) {
                  RxJavaPlugins.onError(t);
                  return;
              }
              error = t;
              done = true;
              drain();
          }
  
          @Override
          public void onComplete() {
              if (done) {
                  return;
              }
              done = true;
              drain();
          }
  
          @Override
          public void cancel() {
              cancelled = true;
              upstream.cancel();
              if (getAndIncrement() == 0) {
                  queue.clear();
              }
          }
  
          @Override
          public void request(long n) {
              if (SubscriptionHelper.validate(n)) {
                  BackpressureHelper.add(requested, n);
                  drain();
              }
          }
  
          void drain() {
              if (getAndIncrement() != 0) {
                  return;
              }
  
              int missed = 1;
              Subscriber<? super R> a = downstream;
              SimplePlainQueue<R> q = queue;
              int lim = limit;
              int c = consumed;
  
              for (;;) {
  
                  long r = requested.get();
                  long e = 0L;
  
                  while (e != r) {
                      if (cancelled) {
                          q.clear();
                          return;
                      }
                      boolean d = done;
  
                      if (d) {
                          Throwable ex = error;
                          if (ex != null) {
                              q.clear();
                              a.onError(ex);
                              return;
                          }
                      }
  
                      R v = q.poll();
                      boolean empty = v == null;
  
                      if (d && empty) {
                          a.onComplete();
                          return;
                      }
  
                      if (empty) {
                          break;
                      }
  
                      a.onNext(v);
  
                      e++;
                      if (++c == lim) {
                          c = 0;
                          upstream.request(lim);
                      }
                  }
  
                  if (e == r) {
                      if (done) {
                          Throwable ex = error;
                          if (ex != null) {
                              q.clear();
                              a.onError(ex);
                              return;
                          }
                          if (q.isEmpty()) {
                              a.onComplete();
                              return;
                          }
                      }
                  }
  
                  if (e != 0L) {
                      BackpressureHelper.produced(requested, e);
                  }
  
                  consumed = c;
                  missed = addAndGet(-missed);
                  if (missed == 0) {
                      break;
                  }
              }
          }
      }
  }
  