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.subscriptions.SubscriptionHelper;
import io.reactivex.plugins.RxJavaPlugins;

Reduce a sequence of values, starting from a seed value and by using
an accumulator function and return the last accumulated value.
Type parameters:
<T> – the source value type
<R> – the accumulated result type/**
 * Reduce a sequence of values, starting from a seed value and by using
 * an accumulator function and return the last accumulated value.
 *
 * @param <T> the source value type
 * @param <R> the accumulated result type
 */
public final class FlowableReduceSeedSingle<T, R> extends Single<R> {

    final Publisher<T> source;

    final R seed;

    final BiFunction<R, ? super T, R> reducer;

    public FlowableReduceSeedSingle(Publisher<T> source, R seed, BiFunction<R, ? super T, R> reducer) {
        this.source = source;
        this.seed = seed;
        this.reducer = reducer;
    }

    @Override
    protected void subscribeActual(SingleObserver<? super R> observer) {
        source.subscribe(new ReduceSeedObserver<T, R>(observer, reducer, seed));
    }

    static final class ReduceSeedObserver<T, R> implements FlowableSubscriber<T>, Disposable {

        final SingleObserver<? super R> downstream;

        final BiFunction<R, ? super T, R> reducer;

        R value;

        Subscription upstream;

        ReduceSeedObserver(SingleObserver<? super R> actual, BiFunction<R, ? super T, R> reducer, R value) {
            this.downstream = actual;
            this.value = value;
            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 value) {
            R v = this.value;
            if (v != null) {
                try {
                    this.value = ObjectHelper.requireNonNull(reducer.apply(v, value), "The reducer returned a null value");
                } catch (Throwable ex) {
                    Exceptions.throwIfFatal(ex);
                    upstream.cancel();
                    onError(ex);
                }
            }
        }

        @Override
        public void onError(Throwable e) {
            if (value != null) {
                value = null;
                upstream = SubscriptionHelper.CANCELLED;
                downstream.onError(e);
            } else {
                RxJavaPlugins.onError(e);
            }
        }

        @Override
        public void onComplete() {
            R v = value;
            if (v != null) {
                value = null;
                upstream = SubscriptionHelper.CANCELLED;
                downstream.onSuccess(v);
            }
        }

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

        @Override
        public boolean isDisposed() {
            return upstream == SubscriptionHelper.CANCELLED;
        }
    }
}