/*
 * 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.flowable;

import java.util.concurrent.atomic.*;

import org.reactivestreams.*;

import io.reactivex.rxjava3.core.*;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
import io.reactivex.rxjava3.internal.util.*;
import io.reactivex.rxjava3.operators.SimplePlainQueue;
import io.reactivex.rxjava3.operators.SpscArrayQueue;

/**
 * Merges an Observable and a Maybe by emitting the items of the Observable and the success
 * value of the Maybe and waiting until both the Observable and Maybe terminate normally.
 * <p>History: 2.1.10 - experimental
 * @param <T> the element type of the Observable
 * @since 2.2
 */
public final class FlowableMergeWithSingle<T> extends AbstractFlowableWithUpstream<T, T> {

    final SingleSource<? extends T> other;

    public FlowableMergeWithSingle(Flowable<T> source, SingleSource<? extends T> other) {
        super(source);
        this.other = other;
    }

    @Override
    protected void subscribeActual(Subscriber<? super T> subscriber) {
        MergeWithObserver<T> parent = new MergeWithObserver<>(subscriber);
        subscriber.onSubscribe(parent);
        source.subscribe(parent);
        other.subscribe(parent.otherObserver);
    }

    static final class MergeWithObserver<T> extends AtomicInteger
    implements FlowableSubscriber<T>, Subscription {

        private static final long serialVersionUID = -4592979584110982903L;

        final Subscriber<? super T> downstream;

        final AtomicReference<Subscription> mainSubscription;

        final OtherObserver<T> otherObserver;

        final AtomicThrowable errors;

        final AtomicLong requested;

        final int prefetch;

        final int limit;

        volatile SimplePlainQueue<T> queue;

        T singleItem;

        volatile boolean cancelled;

        volatile boolean mainDone;

        volatile int otherState;

        long emitted;

        int consumed;

        static final int OTHER_STATE_HAS_VALUE = 1;

        static final int OTHER_STATE_CONSUMED_OR_EMPTY = 2;

        MergeWithObserver(Subscriber<? super T> downstream) {
            this.downstream = downstream;
            this.mainSubscription = new AtomicReference<>();
            this.otherObserver = new OtherObserver<>(this);
            this.errors = new AtomicThrowable();
            this.requested = new AtomicLong();
            this.prefetch = bufferSize();
            this.limit = prefetch - (prefetch >> 2);
        }

        @Override
        public void onSubscribe(Subscription s) {
            SubscriptionHelper.setOnce(mainSubscription, s, prefetch);
        }

        @Override
        public void onNext(T t) {
            if (compareAndSet(0, 1)) {
                long e = emitted;
                if (requested.get() != e) {
                    SimplePlainQueue<T> q = queue;
                    if (q == null || q.isEmpty()) {

                        emitted = e + 1;
                        downstream.onNext(t);

                        int c = consumed + 1;
                        if (c == limit) {
                            consumed = 0;
                            mainSubscription.get().request(c);
                        } else {
                            consumed = c;
                        }
                    } else {
                        q.offer(t);
                    }
                } else {
                    SimplePlainQueue<T> q = getOrCreateQueue();
                    q.offer(t);
                }
                if (decrementAndGet() == 0) {
                    return;
                }
            } else {
                SimplePlainQueue<T> q = getOrCreateQueue();
                q.offer(t);
                if (getAndIncrement() != 0) {
                    return;
                }
            }
            drainLoop();
        }

        @Override
        public void onError(Throwable ex) {
            if (errors.tryAddThrowableOrReport(ex)) {
                DisposableHelper.dispose(otherObserver);
                drain();
            }
        }

        @Override
        public void onComplete() {
            mainDone = true;
            drain();
        }

        @Override
        public void request(long n) {
            BackpressureHelper.add(requested, n);
            drain();
        }

        @Override
        public void cancel() {
            cancelled = true;
            SubscriptionHelper.cancel(mainSubscription);
            DisposableHelper.dispose(otherObserver);
            errors.tryTerminateAndReport();
            if (getAndIncrement() == 0) {
                queue = null;
                singleItem = null;
            }
        }

        void otherSuccess(T value) {
            if (compareAndSet(0, 1)) {
                long e = emitted;
                if (requested.get() != e) {

                    emitted = e + 1;
                    downstream.onNext(value);
                    otherState = OTHER_STATE_CONSUMED_OR_EMPTY;
                } else {
                    singleItem = value;
                    otherState = OTHER_STATE_HAS_VALUE;
                    if (decrementAndGet() == 0) {
                        return;
                    }
                }
            } else {
                singleItem = value;
                otherState = OTHER_STATE_HAS_VALUE;
                if (getAndIncrement() != 0) {
                    return;
                }
            }
            drainLoop();
        }

        void otherError(Throwable ex) {
            if (errors.tryAddThrowableOrReport(ex)) {
                SubscriptionHelper.cancel(mainSubscription);
                drain();
            }
        }

        SimplePlainQueue<T> getOrCreateQueue() {
            SimplePlainQueue<T> q = queue;
            if (q == null) {
                q = new SpscArrayQueue<>(bufferSize());
                queue = q;
            }
            return q;
        }

        void drain() {
            if (getAndIncrement() == 0) {
                drainLoop();
            }
        }

        void drainLoop() {
            Subscriber<? super T> actual = this.downstream;
            int missed = 1;
            long e = emitted;
            int c = consumed;
            int lim = limit;
            for (;;) {

                long r = requested.get();

                while (e != r) {
                    if (cancelled) {
                        singleItem = null;
                        queue = null;
                        return;
                    }

                    if (errors.get() != null) {
                        singleItem = null;
                        queue = null;
                        errors.tryTerminateConsumer(downstream);
                        return;
                    }

                    int os = otherState;
                    if (os == OTHER_STATE_HAS_VALUE) {
                        T v = singleItem;
                        singleItem = null;
                        otherState = OTHER_STATE_CONSUMED_OR_EMPTY;
                        os = OTHER_STATE_CONSUMED_OR_EMPTY;
                        actual.onNext(v);

                        e++;
                        continue;
                    }

                    boolean d = mainDone;
                    SimplePlainQueue<T> q = queue;
                    T v = q != null ? q.poll() : null;
                    boolean empty = v == null;

                    if (d && empty && os == OTHER_STATE_CONSUMED_OR_EMPTY) {
                        queue = null;
                        actual.onComplete();
                        return;
                    }

                    if (empty) {
                        break;
                    }

                    actual.onNext(v);

                    e++;

                    if (++c == lim) {
                        c = 0;
                        mainSubscription.get().request(lim);
                    }
                }

                if (e == r) {
                    if (cancelled) {
                        singleItem = null;
                        queue = null;
                        return;
                    }

                    if (errors.get() != null) {
                        singleItem = null;
                        queue = null;
                        errors.tryTerminateConsumer(downstream);
                        return;
                    }

                    boolean d = mainDone;
                    SimplePlainQueue<T> q = queue;
                    boolean empty = q == null || q.isEmpty();

                    if (d && empty && otherState == 2) {
                        queue = null;
                        actual.onComplete();
                        return;
                    }
                }

                emitted = e;
                consumed = c;
                missed = addAndGet(-missed);
                if (missed == 0) {
                    break;
                }
            }
        }

        static final class OtherObserver<T> extends AtomicReference<Disposable>
        implements SingleObserver<T> {

            private static final long serialVersionUID = -2935427570954647017L;

            final MergeWithObserver<T> parent;

            OtherObserver(MergeWithObserver<T> parent) {
                this.parent = parent;
            }

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

            @Override
            public void onSuccess(T t) {
                parent.otherSuccess(t);
            }

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