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/**
 * 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.observable.internal.operators;

import java.util.concurrent.Callable;

import io.reactivex.common.exceptions.Exceptions;
import io.reactivex.common.functions.BiFunction;
import io.reactivex.common.internal.functions.ObjectHelper;
import io.reactivex.observable.*;
import io.reactivex.observable.internal.disposables.EmptyDisposable;
import io.reactivex.observable.internal.operators.ObservableReduceSeedSingle.ReduceSeedObserver;

/**
 * Reduce a sequence of values, starting from a generated seed value and by using
 * an accumulator function and return the last accumulated value.
 *
 * @param  the source value type
 * @param  the accumulated result type
 */
public final class ObservableReduceWithSingle extends Single {

    final ObservableSource source;

    final Callable seedSupplier;

    final BiFunction reducer;

    public ObservableReduceWithSingle(ObservableSource source, Callable seedSupplier, BiFunction reducer) {
        this.source = source;
        this.seedSupplier = seedSupplier;
        this.reducer = reducer;
    }

    @Override
    protected void subscribeActual(SingleObserver observer) {
        R seed;

        try {
            seed = ObjectHelper.requireNonNull(seedSupplier.call(), "The seedSupplier returned a null value");
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            EmptyDisposable.error(ex, observer);
            return;
        }
        source.subscribe(new ReduceSeedObserver(observer, reducer, seed));
    }
}