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.internal.disposables.DisposableHelper;
import io.reactivex.plugins.RxJavaPlugins;
import java.util.NoSuchElementException;

public final class ObservableSingleSingle<T> extends Single<T> {

    final ObservableSource<? extends T> source;

    final T defaultValue;

    public ObservableSingleSingle(ObservableSource<? extends T> source, T defaultValue) {
        this.source = source;
        this.defaultValue = defaultValue;
    }

    @Override
    public void subscribeActual(SingleObserver<? super T> t) {
        source.subscribe(new SingleElementObserver<T>(t, defaultValue));
    }

    static final class SingleElementObserver<T> implements Observer<T>, Disposable {
        final SingleObserver<? super T> downstream;

        final T defaultValue;

        Disposable upstream;

        T value;

        boolean done;

        SingleElementObserver(SingleObserver<? super T> actual, T defaultValue) {
            this.downstream = actual;
            this.defaultValue = defaultValue;
        }

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

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

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

        @Override
        public void onNext(T t) {
            if (done) {
                return;
            }
            if (value != null) {
                done = true;
                upstream.dispose();
                downstream.onError(new IllegalArgumentException("Sequence contains more than one element!"));
                return;
            }
            value = t;
        }

        @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;
            T v = value;
            value = null;
            if (v == null) {
                v = defaultValue;
            }

            if (v != null) {
                downstream.onSuccess(v);
            } else {
                downstream.onError(new NoSuchElementException());
            }
        }
    }
}