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.observable;

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

Reduce a sequence of values into a single value via an aggregator function and emit the final value or complete
if the source is empty.
Type parameters:
<T> – the source and result value type/**
 * Reduce a sequence of values into a single value via an aggregator function and emit the final value or complete
 * if the source is empty.
 *
 * @param <T> the source and result value type
 */
public final class ObservableReduceMaybe<T> extends Maybe<T> {

    final ObservableSource<T> source;

    final BiFunction<T, T, T> reducer;

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

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

    static final class ReduceObserver<T> implements Observer<T>, Disposable {

        final MaybeObserver<? super T> downstream;

        final BiFunction<T, T, T> reducer;

        boolean done;

        T value;

        Disposable upstream;

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

        @Override
        public void onSubscribe(Disposable d) {
            if (DisposableHelper.validate(this.upstream, d)) {
                this.upstream = d;

                downstream.onSubscribe(this);
            }
        }

        @Override
        public void onNext(T value) {
            if (!done) {
                T v = this.value;

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

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

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

        @Override
        public void dispose() {
            upstream.dispose();
        }

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