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

Reduces a sequence via a function into a single value or signals NoSuchElementException for
an empty source.
Type parameters:
<T> – the value type/**
 * Reduces a sequence via a function into a single value or signals NoSuchElementException for
 * an empty source.
 *
 * @param <T> the value type
 */
public final class FlowableReduce<T> extends AbstractFlowableWithUpstream<T, T> {

    final BiFunction<T, T, T> reducer;

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

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

    static final class ReduceSubscriber<T> extends DeferredScalarSubscription<T> implements FlowableSubscriber<T> {

        private static final long serialVersionUID = -4663883003264602070L;

        final BiFunction<T, T, T> reducer;

        Subscription upstream;

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

        @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 (upstream == SubscriptionHelper.CANCELLED) {
                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 (upstream == SubscriptionHelper.CANCELLED) {
                RxJavaPlugins.onError(t);
                return;
            }
            upstream = SubscriptionHelper.CANCELLED;
            downstream.onError(t);
        }

        @Override
        public void onComplete() {
            if (upstream == SubscriptionHelper.CANCELLED) {
                return;
            }
            upstream = SubscriptionHelper.CANCELLED;

            T v = value;
            if (v != null) {
                complete(v);
            } else {
                downstream.onComplete();
            }
        }

        @Override
        public void cancel() {
            super.cancel();
            upstream.cancel();
            upstream = SubscriptionHelper.CANCELLED;
        }

    }
}