import Foundation
import UserNotifications

/**
 * Local reminders before each lesson of the coming week, from the saved timetable: they work
 * offline, no push server is involved. iOS keeps at most 64 pending requests per app.
 */
enum Reminders {
    private static let prefix = "lesson-"
    private static let limit = 48

    /** Asks for permission; false when the student declined. */
    static func authorize() async -> Bool {
        (try? await UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge])) ?? false
    }

    /** Replaces every scheduled reminder with the ones the timetable needs now. */
    static func reschedule(timetable: Timetable?, settings: UserSettings) {
        let center = UNUserNotificationCenter.current()
        Task {
            let pending = await center.pendingNotificationRequests().map(\.identifier).filter { $0.hasPrefix(prefix) }
            center.removePendingNotificationRequests(withIdentifiers: pending)
            guard settings.reminders, let t = timetable else { return }
            let now = Date()
            // A week ahead, so a Friday evening already holds Monday's reminders.
            let horizon = now.addingTimeInterval(7 * 24 * 3600)
            var count = 0
            for day in t.days {
                guard let date = CivilDate(day.date), count < limit else { continue }
                // One reminder per start time: parallel subgroups share the notification.
                for slot in t.slots(on: date, subgroup: settings.subgroup, hidden: settings.hidden) {
                    let begins = date.at(slot.start)
                    let at = begins.addingTimeInterval(-Double(settings.reminderMinutes) * 60)
                    guard at > now, begins < horizon, count < limit else { continue }
                    let content = UNMutableNotificationContent()
                    content.title = slot.lessons.map(\.subject).joined(separator: " / ")
                    let first = slot.lessons[0]
                    let rooms = Array(Set(slot.lessons.flatMap(\.rooms))).sorted().joined(separator: ", ")
                    let place = withPlace(rooms.isEmpty ? nil : "ауд. " + rooms, slot.lessons.compactMap(\.place).first)
                    var text = slot.start.description
                    if let pair = first.pair { text += " · \(pair) пара" }
                    if let place { text += " · \(place)" }
                    if slot.lessons.contains(where: \.uncertain) { text += " · сверьте недели" }
                    content.body = text
                    content.sound = .default
                    content.interruptionLevel = .timeSensitive
                    let parts = minskCalendar.dateComponents(in: minsk, from: at)
                    let trigger = UNCalendarNotificationTrigger(
                        dateMatching: DateComponents(timeZone: minsk, year: parts.year, month: parts.month, day: parts.day, hour: parts.hour, minute: parts.minute),
                        repeats: false
                    )
                    try? await center.add(UNNotificationRequest(identifier: prefix + slot.id, content: content, trigger: trigger))
                    count += 1
                }
            }
        }
    }
}