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Reactive Extensions for Java
There is a newer version: 3.1.9
/*
 * 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.rxjava3.internal.operators.observable;

import io.reactivex.rxjava3.core.*;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.exceptions.Exceptions;
import io.reactivex.rxjava3.functions.*;
import io.reactivex.rxjava3.internal.disposables.*;
import io.reactivex.rxjava3.plugins.RxJavaPlugins;

import java.util.Objects;

public final class ObservableScanSeed extends AbstractObservableWithUpstream {
    final BiFunction accumulator;
    final Supplier seedSupplier;

    public ObservableScanSeed(ObservableSource source, Supplier seedSupplier, BiFunction accumulator) {
        super(source);
        this.accumulator = accumulator;
        this.seedSupplier = seedSupplier;
    }

    @Override
    public void subscribeActual(Observer t) {
        R r;

        try {
            r = Objects.requireNonNull(seedSupplier.get(), "The seed supplied is null");
        } catch (Throwable e) {
            Exceptions.throwIfFatal(e);
            EmptyDisposable.error(e, t);
            return;
        }

        source.subscribe(new ScanSeedObserver<>(t, accumulator, r));
    }

    static final class ScanSeedObserver implements Observer, Disposable {
        final Observer downstream;
        final BiFunction accumulator;

        R value;

        Disposable upstream;

        boolean done;

        ScanSeedObserver(Observer actual, BiFunction accumulator, R value) {
            this.downstream = actual;
            this.accumulator = accumulator;
            this.value = value;
        }

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

                downstream.onSubscribe(this);

                downstream.onNext(value);
            }
        }

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

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

        @Override
        public void onNext(T t) {
            if (done) {
                return;
            }

            R v = value;

            R u;

            try {
                u = Objects.requireNonNull(accumulator.apply(v, t), "The accumulator returned a null value");
            } catch (Throwable e) {
                Exceptions.throwIfFatal(e);
                upstream.dispose();
                onError(e);
                return;
            }

            value = u;

            downstream.onNext(u);
        }

        @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;
            downstream.onComplete();
        }
    }
}