// App-wide state: whether the watch is paired, the navigation path, and the latest feed. It also
// keeps the complication current: it shares the mode and latest counts through the shared Keychain
// group and reloads the complication's timeline when they change.

import Foundation
import Observation
import UserNotifications
import WatchKit
import WidgetKit

@MainActor @Observable
final class AppModel {
    enum Mode: Equatable {
        case unpaired
        case paired(Credentials)
        /// Serves fixtures from `DemoGateway`. Nothing leaves the watch.
        case demo
    }

    /// The one instance the app delegate and notification handler share with the views.
    static let shared = AppModel(launchArguments: ProcessInfo.processInfo.arguments)

    var mode: Mode {
        didSet { if mode != oldValue { shareMode() } }
    }
    var path: [Route] = []
    private(set) var feed: Feed?
    private(set) var feedError: String?
    /// Ids of feed items whose settle call is still out. The feed hides them meanwhile.
    private(set) var settling: Set<String> = []
    /// Why settling an item failed, by item id. Its row shows the message.
    private(set) var settleErrors: [String: String] = [:]
    private var apnsToken: String?
    private var refreshing = false
    /// The counts the complication was last told about.
    private var sharedSummary: Summary?

    init(mode: Mode, feed: Feed? = nil, path: [Route] = []) {
        self.mode = mode
        self.feed = feed
        self.path = path
    }

    /// `-demo` starts in demo mode. `-screen new|status|notifications`, `-host <hostId>` or
    /// `-thread <hostId>/<threadId>` opens that screen, which is how the screenshots in
    /// watch/screenshots are taken.
    convenience init(launchArguments args: [String]) {
        DeviceStore.migrateToSharedGroup()
        let demo = args.contains("-demo")
        let mode: Mode = demo ? .demo : DeviceStore.load().map(Mode.paired) ?? .unpaired
        func argument(_ flag: String) -> String? {
            guard demo, let i = args.firstIndex(of: flag), i + 1 < args.count else { return nil }
            return args[i + 1]
        }
        var path: [Route] = []
        switch argument("-screen") {
        case "new": path = [.newThread]
        case "status": path = [.status]
        case "notifications": path = [.status, .notifications]
        default: break
        }
        if let hostId = argument("-host") {
            path = [.status, .host(id: hostId)]
        }
        if let thread = argument("-thread") {
            let parts = thread.split(separator: "/", maxSplits: 1).map(String.init)
            if parts.count == 2 { path = [.thread(hostId: parts[0], threadId: parts[1])] }
        }
        self.init(mode: mode, path: path)
        shareMode()
    }

    var isPaired: Bool {
        if case .paired = mode { return true }
        return false
    }

    var api: any GatewayAPI {
        switch mode {
        case let .paired(credentials):
            GatewayClient(baseURL: credentials.gatewayURL, token: credentials.token)
        case .demo:
            DemoGateway.shared
        case .unpaired:
            GatewayClient(baseURL: URL(string: DeviceStore.defaultGatewayURL)!, token: nil)
        }
    }

    // MARK: - Pairing

    func pair(gatewayURL: URL, code: String) async throws {
        let deviceName = WKInterfaceDevice.current().name
        let response = try await GatewayClient.pair(baseURL: gatewayURL, code: code, deviceName: deviceName)
        let credentials = Credentials(gatewayURL: gatewayURL, token: response.deviceToken)
        DeviceStore.save(credentials)
        mode = .paired(credentials)
        feed = nil
        feedError = nil
        path = []
        await enableNotifications()
    }

    func startDemo() {
        mode = .demo
        feed = nil
        feedError = nil
        path = []
    }

    /// Back to the pairing screen. A real token is forgotten; the gateway URL is kept.
    func signOut() {
        if isPaired { DeviceStore.clearToken() }
        mode = .unpaired
        feed = nil
        feedError = nil
        path = []
    }

    /// Revokes the device token on the gateway, then signs out. Signs out even when the gateway
    /// cannot be reached, since the token is forgotten either way.
    func unpair() async {
        if isPaired { try? await api.unpair() }
        signOut()
    }

    /// The gateway's host name, such as `wristcode.gbrown.org`. Nil in demo mode.
    var gatewayHost: String? {
        if case let .paired(credentials) = mode { return credentials.gatewayURL.host() }
        return nil
    }

    /// The message to show for a failed call. A 401 sends the app back to pairing and returns nil.
    func message(for error: any Error) -> String? {
        if error is CancellationError { return nil }
        if let error = error as? GatewayError, error == .unauthorized {
            signOut()
            return nil
        }
        return error.localizedDescription
    }

    // MARK: - Feed

    func refreshFeed() async {
        guard mode != .unpaired, !refreshing else { return }
        refreshing = true
        defer { refreshing = false }
        do {
            let feed = try await api.feed()
            self.feed = feed
            feedError = nil
            share(Summary(feed))
        } catch {
            feedError = message(for: error)
        }
    }

    /// Settles items concurrently. Each one leaves the feed at once and comes back with its error
    /// if its call fails. Refreshes once every call has answered.
    func settle(_ items: [FeedItem]) async {
        let ids = Set(items.map(\.id))
        settling.formUnion(ids)
        for id in ids { settleErrors[id] = nil }
        let api = api
        let failures = await withTaskGroup(of: (String, (any Error)?).self) { group in
            for item in items {
                group.addTask {
                    do {
                        try await api.settle(hostId: item.hostId, threadId: item.threadId)
                        return (item.id, nil)
                    } catch {
                        return (item.id, error)
                    }
                }
            }
            var failures: [String: any Error] = [:]
            for await (id, error) in group {
                if let error { failures[id] = error }
            }
            return failures
        }
        for (id, error) in failures {
            settleErrors[id] = message(for: error)
        }
        feed?.recent.removeAll { ids.contains($0.id) && failures[$0.id] == nil }
        settling.subtract(ids)
        await refreshFeed()
    }

    func open(_ route: Route) {
        path = [route]
    }

    /// Opens a `wristcode://` link from the complication.
    func open(url: URL) {
        guard mode != .unpaired, let route = Route(url: url) else { return }
        open(route)
    }

    // MARK: - Complication

    /// Tells the complication about the mode. Counts from another mode, or from before signing
    /// out, are forgotten.
    private func shareMode() {
        let demo = mode == .demo
        if SharedState.demo != demo || mode == .unpaired { SharedState.lastSummary = nil }
        SharedState.demo = demo
        sharedSummary = nil
        reloadComplication()
    }

    /// Hands the complication fresh counts. The feed refreshes every 10 s while on screen, so the
    /// timeline only reloads when the counts or the next thread change.
    private func share(_ summary: Summary) {
        guard summary != sharedSummary else { return }
        sharedSummary = summary
        SharedState.lastSummary = summary
        reloadComplication()
    }

    /// Asks the complication to fetch the summary again, as after a push or a notification action.
    func reloadComplication() {
        WidgetCenter.shared.reloadAllTimelines()
    }

    // MARK: - Notifications

    func enableNotifications() async {
        guard isPaired else { return }
        let center = UNUserNotificationCenter.current()
        let granted = (try? await center.requestAuthorization(options: [.alert, .sound, .badge])) ?? false
        if granted {
            WKApplication.shared().registerForRemoteNotifications()
        }
    }

    func didRegister(apnsToken data: Data) {
        apnsToken = data.map { String(format: "%02x", $0) }.joined()
        Task { await sendPushToken() }
    }

    private func sendPushToken() async {
        guard isPaired, let apnsToken else { return }
        #if DEBUG
        let environment = PushRegistration.Environment.sandbox
        #else
        let environment = PushRegistration.Environment.production
        #endif
        do {
            try await api.registerPush(PushRegistration(apnsToken: apnsToken, apnsEnvironment: environment))
        } catch {
            _ = message(for: error)
        }
    }

    func notificationArrived() {
        reloadComplication()
        Task { await refreshFeed() }
    }
}

extension AppModel {
    /// A demo model with the fixture feed already loaded, for previews.
    static func preview(path: [Route] = []) -> AppModel {
        AppModel(mode: .demo, feed: DemoFixtures.previewFeed(), path: path)
    }
}