package org.schabi.newpipe.local.subscription;

import android.content.Context;
import androidx.annotation.NonNull;
import android.util.Log;

import org.schabi.newpipe.MainActivity;
import org.schabi.newpipe.NewPipeDatabase;
import org.schabi.newpipe.database.AppDatabase;
import org.schabi.newpipe.database.subscription.SubscriptionDAO;
import org.schabi.newpipe.database.subscription.SubscriptionEntity;
import org.schabi.newpipe.extractor.channel.ChannelInfo;
import org.schabi.newpipe.util.ExtractorHelper;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

import io.reactivex.Completable;
import io.reactivex.CompletableSource;
import io.reactivex.Flowable;
import io.reactivex.Maybe;
import io.reactivex.Scheduler;
import io.reactivex.functions.Function;
import io.reactivex.schedulers.Schedulers;

/**
 * Subscription Service singleton:
 * Provides a basis for channel Subscriptions.
 * Provides access to subscription table in database as well as
 * up-to-date observations on the subscribed channels
 */
public class SubscriptionService {

    private static volatile SubscriptionService instance;

    public static SubscriptionService getInstance(@NonNull Context context) {
        SubscriptionService result = instance;
        if (result == null) {
            synchronized (SubscriptionService.class) {
                result = instance;
                if (result == null) {
                    instance = (result = new SubscriptionService(context));
                }
            }
        }

        return result;
    }

    protected final String TAG = "SubscriptionService@" + Integer.toHexString(hashCode());
    protected static final boolean DEBUG = MainActivity.DEBUG;
    private static final int SUBSCRIPTION_DEBOUNCE_INTERVAL = 500;
    private static final int SUBSCRIPTION_THREAD_POOL_SIZE = 4;

    private final AppDatabase db;
    private final Flowable<List<SubscriptionEntity>> subscription;

    private final Scheduler subscriptionScheduler;

    private SubscriptionService(Context context) {
        db = NewPipeDatabase.getInstance(context.getApplicationContext());
        subscription = getSubscriptionInfos();

        final Executor subscriptionExecutor = Executors.newFixedThreadPool(SUBSCRIPTION_THREAD_POOL_SIZE);
        subscriptionScheduler = Schedulers.from(subscriptionExecutor);
    }

    /**
     * Part of subscription observation pipeline
     *
     * @see SubscriptionService#getSubscription()
     */
    private Flowable<List<SubscriptionEntity>> getSubscriptionInfos() {
        return subscriptionTable().getAll()
                // Wait for a period of infrequent updates and return the latest update
                .debounce(SUBSCRIPTION_DEBOUNCE_INTERVAL, TimeUnit.MILLISECONDS)
                .share()            // Share allows multiple subscribers on the same observable
                .replay(1)          // Replay synchronizes subscribers to the last emitted result
                .autoConnect();
    }

    /**
     * Provides an observer to the latest update to the subscription table.
     * <p>
     * This observer may be subscribed multiple times, where each subscriber obtains
     * the latest synchronized changes available, effectively share the same data
     * across all subscribers.
     * <p>
     * This observer has a debounce cooldown, meaning if multiple updates are observed
     * in the cooldown interval, only the latest changes are emitted to the subscribers.
     * This reduces the amount of observations caused by frequent updates to the database.
     */
    @androidx.annotation.NonNull
    public Flowable<List<SubscriptionEntity>> getSubscription() {
        return subscription;
    }

    public Maybe<ChannelInfo> getChannelInfo(final SubscriptionEntity subscriptionEntity) {
        if (DEBUG) Log.d(TAG, "getChannelInfo() called with: subscriptionEntity = [" + subscriptionEntity + "]");

        return Maybe.fromSingle(ExtractorHelper
                .getChannelInfo(subscriptionEntity.getServiceId(), subscriptionEntity.getUrl(), false))
                .subscribeOn(subscriptionScheduler);
    }

    /**
     * Returns the database access interface for subscription table.
     */
    public SubscriptionDAO subscriptionTable() {
        return db.subscriptionDAO();
    }

    public Completable updateChannelInfo(final ChannelInfo info) {
        final Function<List<SubscriptionEntity>, CompletableSource> update = new Function<List<SubscriptionEntity>, CompletableSource>() {
            @Override
            public CompletableSource apply(@NonNull List<SubscriptionEntity> subscriptionEntities) {
                if (DEBUG) Log.d(TAG, "updateChannelInfo() called with: subscriptionEntities = [" + subscriptionEntities + "]");
                if (subscriptionEntities.size() == 1) {
                    SubscriptionEntity subscription = subscriptionEntities.get(0);

                    // Subscriber count changes very often, making this check almost unnecessary.
                    // Consider removing it later.
                    if (!isSubscriptionUpToDate(info, subscription)) {
                        subscription.setData(info.getName(), info.getAvatarUrl(), info.getDescription(), info.getSubscriberCount());

                        return Completable.fromRunnable(() -> subscriptionTable().update(subscription));
                    }
                }

                return Completable.complete();
            }
        };

        return subscriptionTable().getSubscription(info.getServiceId(), info.getUrl())
                .firstOrError()
                .flatMapCompletable(update);
    }

    public List<SubscriptionEntity> upsertAll(final List<ChannelInfo> infoList) {
        final List<SubscriptionEntity> entityList = new ArrayList<>();
        for (ChannelInfo info : infoList) entityList.add(SubscriptionEntity.from(info));

        return subscriptionTable().upsertAll(entityList);
    }

    private boolean isSubscriptionUpToDate(final ChannelInfo info, final SubscriptionEntity entity) {
        return equalsAndNotNull(info.getUrl(), entity.getUrl()) &&
                info.getServiceId() == entity.getServiceId() &&
                info.getName().equals(entity.getName()) &&
                equalsAndNotNull(info.getAvatarUrl(), entity.getAvatarUrl()) &&
                equalsAndNotNull(info.getDescription(), entity.getDescription()) &&
                info.getSubscriberCount() == entity.getSubscriberCount();
    }

    private boolean equalsAndNotNull(final Object o1, final Object o2) {
        return (o1 != null && o2 != null)
                && o1.equals(o2);
    }
}