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.
/**
 * 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.internal.operators.parallel;

import java.util.concurrent.atomic.*;

import org.reactivestreams.*;

import io.reactivex.*;
import io.reactivex.exceptions.Exceptions;
import io.reactivex.functions.BiFunction;
import io.reactivex.internal.functions.ObjectHelper;
import io.reactivex.internal.subscriptions.*;
import io.reactivex.parallel.ParallelFlowable;
import io.reactivex.plugins.RxJavaPlugins;

Reduces all 'rails' into a single value which then gets reduced into a single
Publisher sequence.
Type parameters:
<T> – the value type/**
 * 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<T>(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<SlotPair<T>>();

        final AtomicInteger remaining = new AtomicInteger();

        final AtomicReference<Throwable> error = new AtomicReference<Throwable>();

        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<T>(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<T>();
                    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 = ObjectHelper.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 = ObjectHelper.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;
        }
    }
}