//
//  FervorController.swift
//  Reader
//
//  Created by Shadowfacts on 11/25/21.
//

@preconcurrency import Foundation
import Fervor
@preconcurrency import OSLog
import Combine

actor FervorController {
    
    static let oauthRedirectURI = URL(string: "frenzy://oauth-callback")!
    
    let instanceURL: URL
    
    private let logger = Logger(subsystem: Bundle.main.bundleIdentifier!, category: "FervorController")
    
    let client: FervorClient
    nonisolated let account: LocalData.Account?
    private(set) var clientID: String?
    private(set) var clientSecret: String?
    private(set) var accessToken: String?
    
    nonisolated let persistentContainer: PersistentContainer!
    
    nonisolated let syncState = PassthroughSubject<SyncState, Never>()
    private var lastSyncState = SyncState.done
    private var cancellables = Set<AnyCancellable>()
    
    init(instanceURL: URL, account: LocalData.Account?) async {
        self.instanceURL = instanceURL
        self.client = FervorClient(instanceURL: instanceURL, accessToken: account?.accessToken)
        self.account = account
        self.clientID = account?.clientID
        self.clientSecret = account?.clientSecret
        
        if let account = account {
            self.persistentContainer = PersistentContainer(account: account)
        } else {
            self.persistentContainer = nil
        }
        
        persistentContainer?.fervorController = self
    }
    
    convenience init(account: LocalData.Account) async {
        await self.init(instanceURL: account.instanceURL, account: account)
    }
    
    private func setSyncState(_ state: SyncState) {
        lastSyncState = state
        syncState.send(state)
    }
    
    func register() async throws -> ClientRegistration {
        let registration = try await client.register(clientName: "Frenzy iOS", website: nil, redirectURI: FervorController.oauthRedirectURI)
        clientID = registration.clientID
        clientSecret = registration.clientSecret
        return registration
    }
    
    func getToken(authCode: String) async throws {
        let token = try await client.token(authCode: authCode, redirectURI: FervorController.oauthRedirectURI, clientID: clientID!, clientSecret: clientSecret!)
        accessToken = token.accessToken
    }
    
    func syncAll() async throws {
        guard lastSyncState == .done else {
            return
        }
        // always return to .done, even if we throw and stop syncing early
        defer { setSyncState(.done) }
        
        setSyncState(.groupsAndFeeds)
        
        logger.info("Syncing groups and feeds")
        async let groups = try client.groups()
        async let feeds = try client.feeds()
        try await persistentContainer.sync(serverGroups: groups, serverFeeds: feeds)
        
        setSyncState(.items)
        
        let lastSync = try await persistentContainer.lastSyncDate()
        logger.info("Syncing items with last sync date: \(String(describing: lastSync), privacy: .public)")
        let update = try await client.syncItems(lastSync: lastSync)
        try await persistentContainer.syncItems(update, setSyncState: setSyncState(_:))
        try await persistentContainer.updateLastSyncDate(update.syncTimestamp)
        
        setSyncState(.excerpts)
        await ExcerptGenerator.generateAll(self)
    }
    
    func syncReadToServer() async {
        var count = 0
        
        // try to sync items which failed last time
        let req = Item.fetchRequest()
        req.predicate = NSPredicate(format: "needsReadStateSync = YES")
        if var needsSync = try? persistentContainer.viewContext.fetch(req) {
            let firstReadIndex = needsSync.partition(by: \.read)
            let unreadIDs = needsSync[..<firstReadIndex].map(\.id.unsafelyUnwrapped)
            let readIDs = needsSync[firstReadIndex...].map(\.id.unsafelyUnwrapped)
            var updatedIDs = Set<FervorID>()
            if !unreadIDs.isEmpty {
                do {
                    let ids = try await client.unread(ids: unreadIDs)
                    updatedIDs.formUnion(ids)
                } catch {
                    logger.error("Failed to sync unread state: \(String(describing: error), privacy: .public)")
                }
            }
            if !readIDs.isEmpty {
                do {
                    let ids = try await client.read(ids: readIDs)
                    updatedIDs.formUnion(ids)
                } catch {
                    logger.error("Failed to sync read state: \(String(describing: error), privacy: .public)")
                }
            }
            
            count += updatedIDs.count
            for item in needsSync where updatedIDs.contains(item.id!) {
                item.needsReadStateSync = false
            }
        }
        
        logger.info("Synced \(count, privacy: .public) read/unread to server")
        
        do {
            try persistentContainer.viewContext.save()
        } catch {
            logger.error("Failed to save view context: \(String(describing: error), privacy: .public)")
        }
    }
    
    @MainActor
    func markItem(_ item: Item, read: Bool) async {
        item.read = read
        do {
            let f = read ? client.read(item:) : client.unread(item:)
            _ = try await f(item.id!)
            item.needsReadStateSync = false
        } catch {
            logger.error("Failed to mark item (un)read: \(String(describing: error), privacy: .public)")
            item.needsReadStateSync = true
        }
        
        do {
            try self.persistentContainer.saveViewContext()
        } catch {
            logger.error("Failed to save view context: \(String(describing: error), privacy: .public)")
        }
    }
    
}

extension FervorController {
    enum SyncState: Equatable {
        case groupsAndFeeds
        case items
        case updateItems(current: Int, total: Int)
        case excerpts
        case done
    }
}