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.flowable;

import org.reactivestreams.*;

import io.reactivex.*;
import io.reactivex.disposables.Disposable;
import io.reactivex.exceptions.Exceptions;
import io.reactivex.functions.BiFunction;
import io.reactivex.internal.functions.ObjectHelper;
import io.reactivex.internal.fuseable.*;
import io.reactivex.internal.subscriptions.SubscriptionHelper;
import io.reactivex.plugins.RxJavaPlugins;

Reduce a Flowable into a single value exposed as Single or signal NoSuchElementException.
Type parameters:
<T> – the value type/**
 * Reduce a Flowable into a single value exposed as Single or signal NoSuchElementException.
 *
 * @param <T> the value type
 */
public final class FlowableReduceMaybe<T>
extends Maybe<T>
implements HasUpstreamPublisher<T>, FuseToFlowable<T> {

    final Flowable<T> source;

    final BiFunction<T, T, T> reducer;

    public FlowableReduceMaybe(Flowable<T> source, BiFunction<T, T, T> reducer) {
        this.source = source;
        this.reducer = reducer;
    }

    @Override
    public Publisher<T> source() {
        return source;
    }

    @Override
    public Flowable<T> fuseToFlowable() {
        return RxJavaPlugins.onAssembly(new FlowableReduce<T>(source, reducer));
    }

    @Override
    protected void subscribeActual(MaybeObserver<? super T> observer) {
        source.subscribe(new ReduceSubscriber<T>(observer, reducer));
    }

    static final class ReduceSubscriber<T> implements FlowableSubscriber<T>, Disposable {
        final MaybeObserver<? super T> downstream;

        final BiFunction<T, T, T> reducer;

        T value;

        Subscription upstream;

        boolean done;

        ReduceSubscriber(MaybeObserver<? super T> actual, BiFunction<T, T, T> reducer) {
            this.downstream = actual;
            this.reducer = reducer;
        }

        @Override
        public void dispose() {
            upstream.cancel();
            done = true;
        }

        @Override
        public boolean isDisposed() {
            return done;
        }

        @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;
            }
            T v = value;
            if (v == null) {
                value = t;
            } else {
                try {
                    value = ObjectHelper.requireNonNull(reducer.apply(v, t), "The reducer returned a null value");
                } catch (Throwable ex) {
                    Exceptions.throwIfFatal(ex);
                    upstream.cancel();
                    onError(ex);
                }
            }
        }

        @Override
        public void onError(Throwable t) {
            if (done) {
                RxJavaPlugins.onError(t);
                return;
            }
            done = true;
            downstream.onError(t);
         }

        @Override
        public void onComplete() {
            if (done) {
                return;
            }
            done = true;
            T v = value;
            if (v != null) {
//                value = null;
                downstream.onSuccess(v);
            } else {
                downstream.onComplete();
            }
        }
    }
}