//
//  CalendarManager.swift
//  DynamicIsland
//
//  Created by Harsh Vardhan  Goswami  on 08/09/24.
//

import Defaults
import EventKit
import SwiftUI

// MARK: - CalendarManager

@MainActor
class CalendarManager: ObservableObject {
    static let shared = CalendarManager()

    @Published var currentWeekStartDate: Date
    @Published var events: [EventModel] = []
    @Published var allCalendars: [CalendarModel] = []
    @Published var eventCalendars: [CalendarModel] = []
    @Published var reminderLists: [CalendarModel] = []
    @Published var selectedCalendarIDs: Set<String> = []
    @Published var calendarAuthorizationStatus: EKAuthorizationStatus = .notDetermined
    @Published var reminderAuthorizationStatus: EKAuthorizationStatus = .notDetermined

    private var selectedCalendars: [CalendarModel] = []
    private let calendarService = CalendarService()
    private var lastEventsFetchDate: Date?
    private let reloadRefreshInterval: TimeInterval = 15
    private var eventStoreChangedObserver: NSObjectProtocol?
    private var pendingEventStoreRefreshTask: Task<Void, Never>?
    private var nextAllowedEventStoreRefresh: Date = .distantPast
    private var ignoreEventStoreChangesUntil: Date = .distantPast
    private let eventStoreChangeThrottle: TimeInterval = 20
    private let selfInducedChangeSuppression: TimeInterval = 6
    private let eventFetchLimiter = EventFetchLimiter()

    var hasCalendarAccess: Bool { isAuthorized(calendarAuthorizationStatus) }
    var hasReminderAccess: Bool { isAuthorized(reminderAuthorizationStatus) }

    private init() {
        currentWeekStartDate = CalendarManager.startOfDay(Date())
        setupEventStoreChangedObserver()
        Task {
            await reloadCalendarAndReminderLists()
        }
    }

    deinit {
        if let observer = eventStoreChangedObserver {
            NotificationCenter.default.removeObserver(observer)
        }
        pendingEventStoreRefreshTask?.cancel()
    }

    private func setupEventStoreChangedObserver() {
        eventStoreChangedObserver = NotificationCenter.default.addObserver(
            forName: .EKEventStoreChanged,
            object: nil,
            queue: .main
        ) { [weak self] _ in
            self?.handleEventStoreChanged()
        }
    }

    private func handleEventStoreChanged() {
        Logger.log("CalendarManager: Event store changed notification received", category: .lifecycle)
        let now = Date()
        guard now >= ignoreEventStoreChangesUntil else { return }

        if now < nextAllowedEventStoreRefresh {
            let delay = max(nextAllowedEventStoreRefresh.timeIntervalSince(now), 0.05)
            scheduleEventStoreRefresh(after: delay)
            return
        }

        nextAllowedEventStoreRefresh = now.addingTimeInterval(eventStoreChangeThrottle)
        scheduleEventStoreRefresh(after: 0)
    }

    private func scheduleEventStoreRefresh(after delay: TimeInterval) {
        pendingEventStoreRefreshTask?.cancel()
        pendingEventStoreRefreshTask = Task { [weak self] in
            guard let self else { return }
            if delay > 0 {
                let nanoseconds = UInt64(delay * 1_000_000_000)
                try? await Task.sleep(nanoseconds: nanoseconds)
            }
            await self.performEventStoreRefresh()
        }
    }

    @MainActor
    private func performEventStoreRefresh() async {
        pendingEventStoreRefreshTask = nil
        await reloadCalendarAndReminderLists()
        await maybeRefreshEventsAfterReload()
        nextAllowedEventStoreRefresh = Date().addingTimeInterval(eventStoreChangeThrottle)
        ignoreEventStoreChangesUntil = Date().addingTimeInterval(selfInducedChangeSuppression)
    }

    @MainActor
    func reloadCalendarAndReminderLists() async {
        let allCalendars = await calendarService.calendars()
        eventCalendars = allCalendars.filter { !$0.isReminder }
        reminderLists = allCalendars.filter { $0.isReminder }
        self.allCalendars = allCalendars
        updateSelectedCalendars()
    }

    @MainActor
    private func maybeRefreshEventsAfterReload() async {
        guard hasCalendarAccess else { return }
        let now = Date()
        if let lastFetch = lastEventsFetchDate, now.timeIntervalSince(lastFetch) < reloadRefreshInterval {
            return
        }
        await updateEvents()
    }

    private func isAuthorized(_ status: EKAuthorizationStatus) -> Bool {
        switch status {
        case .authorized, .fullAccess:
            return true
        default:
            return false
        }
    }

    func checkCalendarAuthorization() async {
        let status = EKEventStore.authorizationStatus(for: .event)
        calendarAuthorizationStatus = status

        switch status {
        case .notDetermined:
            let granted = await calendarService.requestAccess(to: .event)
            calendarAuthorizationStatus = granted ? .fullAccess : .denied
            if granted {
                await reloadCalendarAndReminderLists()
                await updateEvents(force: true)
            }
        case .restricted, .denied:
            NSLog("Calendar access denied or restricted")
        case .authorized, .fullAccess:
            await reloadCalendarAndReminderLists()
            await updateEvents(force: true)
        case .writeOnly:
            NSLog("Calendar write only")
        @unknown default:
            NSLog("Unknown calendar authorization status")
        }
    }

    func checkReminderAuthorization() async {
        let status = EKEventStore.authorizationStatus(for: .reminder)
        reminderAuthorizationStatus = status

        switch status {
        case .notDetermined:
            let granted = await calendarService.requestAccess(to: .reminder)
            reminderAuthorizationStatus = granted ? .fullAccess : .denied
            if granted {
                await reloadCalendarAndReminderLists()
            }
        case .restricted, .denied:
            NSLog("Reminder access denied or restricted")
        case .authorized, .fullAccess:
            await reloadCalendarAndReminderLists()
        case .writeOnly:
            NSLog("Reminder write only")
        @unknown default:
            NSLog("Unknown reminder authorization status")
        }
    }

    func updateSelectedCalendars() {
        switch Defaults[.calendarSelectionState] {
        case .all:
            selectedCalendarIDs = Set(allCalendars.map { $0.id })
        case .selected(let identifiers):
            selectedCalendarIDs = identifiers
        }

        selectedCalendars = allCalendars.filter { selectedCalendarIDs.contains($0.id) }
    }

    func getCalendarSelected(_ calendar: CalendarModel) -> Bool {
        selectedCalendarIDs.contains(calendar.id)
    }

    func setCalendarSelected(_ calendar: CalendarModel, isSelected: Bool) async {
        var selectionState = Defaults[.calendarSelectionState]

        switch selectionState {
        case .all:
            if !isSelected {
                let identifiers = Set(allCalendars.map { $0.id }).subtracting([calendar.id])
                selectionState = .selected(identifiers)
            }
        case .selected(var identifiers):
            if isSelected {
                identifiers.insert(calendar.id)
            } else {
                identifiers.remove(calendar.id)
            }

            if identifiers.isEmpty || identifiers.count == allCalendars.count {
                selectionState = .all
            } else {
                selectionState = .selected(identifiers)
            }
        }

        Defaults[.calendarSelectionState] = selectionState
        updateSelectedCalendars()
        await updateEvents(force: true)
    }

    static func startOfDay(_ date: Date) -> Date {
        Calendar.current.startOfDay(for: date)
    }

    func updateCurrentDate(_ date: Date) async {
        currentWeekStartDate = Calendar.current.startOfDay(for: date)
        await updateEvents(force: true)
    }

    private func updateEvents(force: Bool = false) async {
        let now = Date()
        if !force, let lastFetch = lastEventsFetchDate, now.timeIntervalSince(lastFetch) < reloadRefreshInterval {
            return
        }
        
        Logger.log("CalendarManager: Updating events (force: \(force))", category: .lifecycle)

        let calendarIDs = selectedCalendars.map { $0.id }
        let startDate = currentWeekStartDate
        guard let endDate = Calendar.current.date(byAdding: .day, value: 1, to: currentWeekStartDate) else { return }
        let service = calendarService

        let events = await eventFetchLimiter.run {
            await service.events(
                from: startDate,
                to: endDate,
                calendars: calendarIDs
            )
        }

        self.events = events
        lastEventsFetchDate = Date()
    }

    func setReminderCompleted(reminderID: String, completed: Bool) async {
        await calendarService.setReminderCompleted(reminderID: reminderID, completed: completed)
        await updateEvents(force: true)
    }
}

// MARK: - Event Fetch Limiter

private actor EventFetchLimiter {
    private var waiters: [CheckedContinuation<Void, Never>] = []
    private var isRunning = false

    func run<T>(_ operation: @escaping @Sendable () async -> T) async -> T {
        await waitTurn()
        defer { resumeNext() }
        return await operation()
    }

    private func waitTurn() async {
        if !isRunning {
            isRunning = true
            return
        }

        await withCheckedContinuation { continuation in
            waiters.append(continuation)
        }
    }

    private func resumeNext() {
        if waiters.isEmpty {
            isRunning = false
            return
        }

        let continuation = waiters.removeFirst()
        continuation.resume()
    }
}