/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

package mozilla.components.service.fxa.sync

import mozilla.components.concept.sync.SyncableStore
import mozilla.components.service.fxa.SyncConfig
import mozilla.components.service.fxa.SyncEngine
import mozilla.components.service.fxa.manager.SyncEnginesStorage
import mozilla.components.support.base.log.logger.Logger
import mozilla.components.support.base.observer.Observable
import mozilla.components.support.base.observer.ObserverRegistry
import java.io.Closeable
import java.lang.Exception
import java.util.concurrent.TimeUnit

/**
 * A collection of objects describing a reason for running a sync.
 */
sealed class SyncReason {
    /**
     * Application is starting up, and wants to sync data.
     */
    object Startup : SyncReason()

    /**
     * User is requesting a sync (e.g. pressed a "sync now" button).
     */
    object User : SyncReason()

    /**
     * User changed enabled/disabled state of one or more [SyncEngine]s.
     */
    object EngineChange : SyncReason()

    /**
     * Internal use only: first time running a sync.
     */
    internal object FirstSync : SyncReason()

    /**
     * Internal use only: running a periodic sync.
     */
    internal object Scheduled : SyncReason()
}

/**
 * An interface for consumers that wish to observer "sync lifecycle" events.
 */
interface SyncStatusObserver {
    /**
     * Gets called at the start of a sync, before any configured syncable is synchronized.
     */
    fun onStarted()

    /**
     * Gets called at the end of a sync, after every configured syncable has been synchronized.
     * A set of enabled [SyncEngine]s could have changed, so observers are expected to query
     * [SyncEnginesStorage.getStatus].
     */
    fun onIdle()

    /**
     * Gets called if sync encounters an error that's worthy of processing by status observers.
     * @param error Optional relevant exception.
     */
    fun onError(error: Exception?)
}

/**
 * A singleton registry of [SyncableStore] objects. [WorkManagerSyncDispatcher] will use this to
 * access configured [SyncableStore] instances.
 *
 * This pattern provides a safe way for library-defined background workers to access globally
 * available instances of stores within an application.
 */
object GlobalSyncableStoreProvider {
    private val stores: MutableMap<String, Lazy<SyncableStore>> = mutableMapOf()

    /**
     * Configure an instance of [SyncableStore] for a [SyncEngine] enum.
     * @param storePair A pair associating [SyncableStore] with a [SyncEngine].
     */
    fun configureStore(storePair: Pair<SyncEngine, Lazy<SyncableStore>>) {
        stores[storePair.first.nativeName] = storePair.second
    }

    internal fun getStore(name: String): SyncableStore? {
        return stores[name]?.value
    }
}

/**
 * Internal interface to enable testing SyncManager implementations independently from SyncDispatcher.
 */
internal interface SyncDispatcher : Closeable, Observable<SyncStatusObserver> {
    fun isSyncActive(): Boolean
    fun syncNow(reason: SyncReason, debounce: Boolean = false)
    fun startPeriodicSync(unit: TimeUnit, period: Long, initialDelay: Long)
    fun stopPeriodicSync()
    fun workersStateChanged(isRunning: Boolean)
}

/**
 * A base sync manager implementation.
 * @param syncConfig A [SyncConfig] object describing how sync should behave.
 */
@SuppressWarnings("TooManyFunctions")
internal abstract class SyncManager(
    private val syncConfig: SyncConfig
) {
    open val logger = Logger("SyncManager")

    // A SyncDispatcher instance bound to an account and a set of syncable stores.
    private var syncDispatcher: SyncDispatcher? = null

    private val syncStatusObserverRegistry = ObserverRegistry<SyncStatusObserver>()

    // Manager encapsulates events emitted by the wrapped observers, and passes them on to the outside world.
    // This split allows us to define a nice public observer API (manager -> consumers), along with
    // a more robust internal observer API (dispatcher -> manager).
    // Currently the interfaces are the same, hence the name "pass-through".
    private val dispatcherStatusObserver = PassThroughSyncStatusObserver(syncStatusObserverRegistry)

    /**
     * Indicates if sync is currently running.
     */
    internal fun isSyncActive() = syncDispatcher?.isSyncActive() ?: false

    internal fun registerSyncStatusObserver(observer: SyncStatusObserver) {
        syncStatusObserverRegistry.register(observer)
    }

    /**
     * Request an immediate synchronization of all configured stores.
     *
     * @param reason A [SyncReason] indicating why this sync is being requested.
     * @param debounce Whether or not this sync should debounced.
     */
    internal fun now(reason: SyncReason, debounce: Boolean = false) = synchronized(this) {
        if (syncDispatcher == null) {
            logger.info("Sync is not enabled. Ignoring 'sync now' request.")
        }
        syncDispatcher?.syncNow(reason, debounce)
    }

    /**
     * Enables synchronization, with behaviour described in [syncConfig].
     */
    internal fun start() = synchronized(this) {
        logger.debug("Enabling...")
        syncDispatcher = initDispatcher(newDispatcher(syncDispatcher, syncConfig.supportedEngines))
        logger.debug("set and initialized new dispatcher: $syncDispatcher")
    }

    /**
     * Disables synchronization.
     */
    internal fun stop() = synchronized(this) {
        logger.debug("Disabling...")
        syncDispatcher?.close()
        syncDispatcher = null
    }

    internal abstract fun createDispatcher(supportedEngines: Set<SyncEngine>): SyncDispatcher

    internal abstract fun dispatcherUpdated(dispatcher: SyncDispatcher)

    private fun newDispatcher(
        currentDispatcher: SyncDispatcher?,
        supportedEngines: Set<SyncEngine>
    ): SyncDispatcher {
        // Let the existing dispatcher, if present, cleanup.
        currentDispatcher?.close()
        // TODO will events from old and new dispatchers overlap..? How do we ensure correct sequence
        // for outside observers?
        currentDispatcher?.unregister(dispatcherStatusObserver)

        // Create a new dispatcher bound to current stores and account.
        return createDispatcher(supportedEngines)
    }

    private fun initDispatcher(dispatcher: SyncDispatcher): SyncDispatcher {
        dispatcher.register(dispatcherStatusObserver)
        syncConfig.periodicSyncConfig?.let {
            dispatcher.startPeriodicSync(
                TimeUnit.MINUTES,
                period = it.periodMinutes.toLong(),
                initialDelay = it.initialDelayMinutes.toLong()
            )
        }
        dispatcherUpdated(dispatcher)
        return dispatcher
    }

    private class PassThroughSyncStatusObserver(
        private val passThroughRegistry: ObserverRegistry<SyncStatusObserver>
    ) : SyncStatusObserver {
        override fun onStarted() {
            passThroughRegistry.notifyObservers { onStarted() }
        }

        override fun onIdle() {
            passThroughRegistry.notifyObservers { onIdle() }
        }

        override fun onError(error: Exception?) {
            passThroughRegistry.notifyObservers { onError(error) }
        }
    }
}
