import Foundation
import WatchConnectivity
import SwiftData

/// iPhone → Watch push of the week snapshot.
///
/// Two delivery paths, both needed:
/// - `updateApplicationContext`: latest value delivered when the watch app
///   next runs, even if unreachable now. Activation is asynchronous, so a
///   payload pushed before the session activates is kept and flushed from
///   `activationDidCompleteWith`.
/// - `didReceiveMessage` reply: the watch asks for data on launch (wakes this
///   app in background); we answer with the snapshot the iPhone keeps in its
///   own app group for the iOS week widget.
final class WatchSyncService: NSObject, WCSessionDelegate, @unchecked Sendable {
    static let shared = WatchSyncService()

    private var pendingData: Data?

    private override init() {
        super.init()
        guard WCSession.isSupported() else { return }
        WCSession.default.delegate = self
        WCSession.default.activate()
    }

    /// Touch from app launch so activation happens before the first push.
    func activate() { /* init side effect */ }

    func push(plan: WeekPlan?, dishes: [Dish], settings: AppSettings) {
        guard WCSession.isSupported() else { return }
        guard let payload = Self.makePayload(plan: plan, dishes: dishes, settings: settings),
              let data = payload.encoded() else { return }
        send(data)
    }

    private func send(_ data: Data) {
        guard WCSession.default.activationState == .activated else {
            pendingData = data
            return
        }
        try? WCSession.default.updateApplicationContext([WatchWeekPayload.storageKey: data])
    }

    static func makePayload(plan: WeekPlan?, dishes: [Dish], settings: AppSettings) -> WatchWeekPayload? {
        guard let plan else { return nil }
        let locale = Locale(identifier: settings.languageEnum.resolved().localeIdentifier)
        let dishById = Dictionary(dishes.map { ($0.id, $0) }, uniquingKeysWith: { first, _ in first })
        let dayFormatter = DateFormatter()
        dayFormatter.locale = locale
        dayFormatter.setLocalizedDateFormatFromTemplate("EEE d")

        let todayOffset = (Calendar.current.component(.weekday, from: Date()) + 5) % 7
        let isCurrentWeek = plan.weekStartDate == Date().startOfWeek()
        let schedule = settings.schedule(for: plan)

        let days: [WatchWeekPayload.Day] = schedule.dayRange.map { day in
            let meals: [WatchWeekPayload.Meal] = schedule.mealTypes.map { meal in
                let slot = plan.slotList.first { $0.dayOfWeek == day && $0.mealType == meal.rawValue }
                let label = String(localized: String.LocalizationValue(meal.localizedKey))
                var name: String?
                if let slot {
                    if slot.isEatingOut {
                        name = String(localized: "slot_eating_out")
                    } else if slot.isSkipped {
                        name = "—"
                    } else if let dishId = slot.dishId, let dish = dishById[dishId] {
                        name = dish.name
                        if let secondaryId = slot.secondaryDishId, let secondary = dishById[secondaryId] {
                            name! += " + \(secondary.name)"
                        }
                    }
                }
                return WatchWeekPayload.Meal(type: meal.rawValue, label: label, name: name)
            }
            let dayDate = plan.weekStartDate.addingDays(day)
            return WatchWeekPayload.Day(
                dayOfWeek: day,
                title: dayFormatter.string(from: dayDate).capitalized(with: locale),
                isToday: isCurrentWeek && day == todayOffset,
                meals: meals
            )
        }

        return WatchWeekPayload(
            weekTitle: plan.weekStartDate.formattedWeekRange(),
            days: days,
            updatedAt: Date()
        )
    }

    // MARK: - WCSessionDelegate

    func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {
        guard activationState == .activated else { return }
        if let data = pendingData {
            pendingData = nil
            try? session.updateApplicationContext([WatchWeekPayload.storageKey: data])
        } else if let stored = WatchWeekPayload.stored()?.encoded() {
            // Nothing pending this run, but ship the last known snapshot so a
            // freshly-paired watch has data without waiting for an edit.
            try? session.updateApplicationContext([WatchWeekPayload.storageKey: stored])
        }
    }

    /// The watch asks on launch; reply with the snapshot from the app group.
    func session(_ session: WCSession, didReceiveMessage message: [String: Any], replyHandler: @escaping ([String: Any]) -> Void) {
        if message["request"] as? String == "week", let data = WatchWeekPayload.stored()?.encoded() {
            replyHandler([WatchWeekPayload.storageKey: data])
        } else {
            replyHandler([:])
        }
    }

    func sessionDidBecomeInactive(_ session: WCSession) {}
    func sessionDidDeactivate(_ session: WCSession) { session.activate() }
}