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.parallel;
  
  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.BiFunction;
  import io.reactivex.rxjava3.internal.subscriptions.*;
  import io.reactivex.rxjava3.internal.util.AtomicThrowable;
  import io.reactivex.rxjava3.parallel.ParallelFlowable;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Reduces all 'rails' into a single value which then gets reduced into a single
   * Publisher sequence.
   *
   * @param <T> the value type
   */
  public final class ParallelReduceFull<T> extends Flowable<T> {
  
      final ParallelFlowable<? extends T> source;
  
      final BiFunction<T, T, T> reducer;
  
      public ParallelReduceFull(ParallelFlowable<? extends T> source, BiFunction<T, T, T> reducer) {
          this.source = source;
          this.reducer = reducer;
      }
  
      @Override
      protected void subscribeActual(Subscriber<? super T> s) {
          ParallelReduceFullMainSubscriber<T> parent = new ParallelReduceFullMainSubscriber<>(s, source.parallelism(), reducer);
          s.onSubscribe(parent);
  
          source.subscribe(parent.subscribers);
      }
  
      static final class ParallelReduceFullMainSubscriber<T> extends DeferredScalarSubscription<T> {
  
          private static final long serialVersionUID = -5370107872170712765L;
  
          final ParallelReduceFullInnerSubscriber<T>[] subscribers;
  
          final BiFunction<T, T, T> reducer;
  
          final AtomicReference<SlotPair<T>> current = new AtomicReference<>();
  
          final AtomicInteger remaining = new AtomicInteger();
  
          final AtomicThrowable error = new AtomicThrowable();
  
          ParallelReduceFullMainSubscriber(Subscriber<? super T> subscriber, int n, BiFunction<T, T, T> reducer) {
              super(subscriber);
              @SuppressWarnings("unchecked")
              ParallelReduceFullInnerSubscriber<T>[] a = new ParallelReduceFullInnerSubscriber[n];
              for (int i = 0; i < n; i++) {
                  a[i] = new ParallelReduceFullInnerSubscriber<>(this, reducer);
              }
              this.subscribers = a;
              this.reducer = reducer;
              remaining.lazySet(n);
          }
  
          SlotPair<T> addValue(T value) {
              for (;;) {
                  SlotPair<T> curr = current.get();
  
                  if (curr == null) {
                      curr = new SlotPair<>();
                      if (!current.compareAndSet(null, curr)) {
                          continue;
                      }
                  }
  
                  int c = curr.tryAcquireSlot();
                  if (c < 0) {
                      current.compareAndSet(curr, null);
                      continue;
                  }
                  if (c == 0) {
                      curr.first = value;
                  } else {
                      curr.second = value;
                  }
  
                  if (curr.releaseSlot()) {
                      current.compareAndSet(curr, null);
                      return curr;
                  }
                  return null;
              }
          }
  
          @Override
          public void cancel() {
              for (ParallelReduceFullInnerSubscriber<T> inner : subscribers) {
                  inner.cancel();
              }
          }
  
          void innerError(Throwable ex) {
              if (error.compareAndSet(null, ex)) {
                  cancel();
                  downstream.onError(ex);
              } else {
                  if (ex != error.get()) {
                      RxJavaPlugins.onError(ex);
                  }
              }
          }
  
          void innerComplete(T value) {
              if (value != null) {
                  for (;;) {
                      SlotPair<T> sp = addValue(value);
  
                      if (sp != null) {
  
                          try {
                              value = Objects.requireNonNull(reducer.apply(sp.first, sp.second), "The reducer returned a null value");
                          } catch (Throwable ex) {
                              Exceptions.throwIfFatal(ex);
                              innerError(ex);
                              return;
                          }
  
                      } else {
                          break;
                      }
                  }
              }
  
              if (remaining.decrementAndGet() == 0) {
                  SlotPair<T> sp = current.get();
                  current.lazySet(null);
  
                  if (sp != null) {
                      complete(sp.first);
                  } else {
                      downstream.onComplete();
                  }
              }
          }
      }
  
      static final class ParallelReduceFullInnerSubscriber<T>
      extends AtomicReference<Subscription>
      implements FlowableSubscriber<T> {
  
          private static final long serialVersionUID = -7954444275102466525L;
  
          final ParallelReduceFullMainSubscriber<T> parent;
  
          final BiFunction<T, T, T> reducer;
  
          T value;
  
          boolean done;
  
          ParallelReduceFullInnerSubscriber(ParallelReduceFullMainSubscriber<T> parent, BiFunction<T, T, T> reducer) {
              this.parent = parent;
              this.reducer = reducer;
          }
  
          @Override
          public void onSubscribe(Subscription s) {
              SubscriptionHelper.setOnce(this, s, Long.MAX_VALUE);
          }
  
          @Override
          public void onNext(T t) {
              if (!done) {
                  T v = value;
  
                  if (v == null) {
                      value = t;
                  } else {
  
                      try {
                          v = Objects.requireNonNull(reducer.apply(v, t), "The reducer returned a null value");
                      } catch (Throwable ex) {
                          Exceptions.throwIfFatal(ex);
                          get().cancel();
                          onError(ex);
                          return;
                      }
  
                      value = v;
                  }
              }
          }
  
          @Override
          public void onError(Throwable t) {
              if (done) {
                  RxJavaPlugins.onError(t);
                  return;
              }
              done = true;
              parent.innerError(t);
          }
  
          @Override
          public void onComplete() {
              if (!done) {
                  done = true;
                  parent.innerComplete(value);
              }
          }
  
          void cancel() {
              SubscriptionHelper.cancel(this);
          }
      }
  
      static final class SlotPair<T> extends AtomicInteger {
  
          private static final long serialVersionUID = 473971317683868662L;
  
          T first;
  
          T second;
  
          final AtomicInteger releaseIndex = new AtomicInteger();
  
          int tryAcquireSlot() {
              for (;;) {
                  int acquired = get();
                  if (acquired >= 2) {
                      return -1;
                  }
  
                  if (compareAndSet(acquired, acquired + 1)) {
                      return acquired;
                  }
              }
          }
  
          boolean releaseSlot() {
              return releaseIndex.incrementAndGet() == 2;
          }
      }
  }
  