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.maybe;

import io.reactivex.*;
import io.reactivex.disposables.Disposable;
import io.reactivex.internal.disposables.DisposableHelper;
import io.reactivex.internal.fuseable.HasUpstreamMaybeSource;

import java.util.concurrent.atomic.AtomicReference;

Subscribes to the other source if the main source is empty.
Type parameters:
<T> – the value type/**
 * Subscribes to the other source if the main source is empty.
 *
 * @param <T> the value type
 */
public final class MaybeSwitchIfEmptySingle<T> extends Single<T> implements HasUpstreamMaybeSource<T> {

    final MaybeSource<T> source;
    final SingleSource<? extends T> other;

    public MaybeSwitchIfEmptySingle(MaybeSource<T> source, SingleSource<? extends T> other) {
        this.source = source;
        this.other = other;
    }

    @Override
    public MaybeSource<T> source() {
        return source;
    }

    @Override
    protected void subscribeActual(SingleObserver<? super T> observer) {
        source.subscribe(new SwitchIfEmptyMaybeObserver<T>(observer, other));
    }

    static final class SwitchIfEmptyMaybeObserver<T>
    extends AtomicReference<Disposable>
    implements MaybeObserver<T>, Disposable {

        private static final long serialVersionUID = 4603919676453758899L;

        final SingleObserver<? super T> downstream;

        final SingleSource<? extends T> other;

        SwitchIfEmptyMaybeObserver(SingleObserver<? super T> actual, SingleSource<? extends T> other) {
            this.downstream = actual;
            this.other = other;
        }

        @Override
        public void dispose() {
            DisposableHelper.dispose(this);
        }

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

        @Override
        public void onSubscribe(Disposable d) {
            if (DisposableHelper.setOnce(this, d)) {
                downstream.onSubscribe(this);
            }
        }

        @Override
        public void onSuccess(T value) {
            downstream.onSuccess(value);
        }

        @Override
        public void onError(Throwable e) {
            downstream.onError(e);
        }

        @Override
        public void onComplete() {
            Disposable d = get();
            if (d != DisposableHelper.DISPOSED) {
                if (compareAndSet(d, null)) {
                    other.subscribe(new OtherSingleObserver<T>(downstream, this));
                }
            }
        }

        static final class OtherSingleObserver<T> implements SingleObserver<T> {

            final SingleObserver<? super T> downstream;

            final AtomicReference<Disposable> parent;
            OtherSingleObserver(SingleObserver<? super T> actual, AtomicReference<Disposable> parent) {
                this.downstream = actual;
                this.parent = parent;
            }

            @Override
            public void onSubscribe(Disposable d) {
                DisposableHelper.setOnce(parent, d);
            }

            @Override
            public void onSuccess(T value) {
                downstream.onSuccess(value);
            }

            @Override
            public void onError(Throwable e) {
                downstream.onError(e);
            }
        }

    }
}