import BackgroundTasks
import UIKit
import UserNotifications

/// The number on the icon: All Subscriptions' unread count, as the page's badge shows it (#421).
///
/// Set when the app comes forward, after the player marks an item read, and by a background
/// refresh while it is closed. iOS decides when that runs (a few times a day for an app that is
/// opened often), so the number can be hours behind; push would make it exact and needs a sender
/// in ipx.
enum Badge {
    static let taskId = "net.sdf1.ipx.refresh"

    private struct Row: Decodable {
        let id: String
        let group: String?
        let unread: Int?
        let hide_from_all: Bool?
    }

    /// web/src/feeds.ts `unreadInAll`: a feed counts unless it, or the OPML it came from, is
    /// hidden from All Subscriptions. Nil when the body is not a feed list, so an error page
    /// does not turn the badge to zero.
    static func unread(inAll body: Data) -> Int? {
        guard let rows = try? JSONDecoder().decode([Row].self, from: body) else { return nil }
        let hidden = Set(rows.filter { $0.hide_from_all == true }.map(\.id))
        return rows.reduce(0) { n, f in
            if f.hide_from_all == true || f.group.map(hidden.contains) == true { return n }
            return n + max(0, f.unread ?? 0)
        }
    }

    /// Asks once, for the badge alone: no alerts, no sounds.
    static func requestPermission() {
        // `-ipx.noBadgePrompt YES` on the launch arguments: a simulator run that drives the page
        // has nobody to tap Allow, and the dialog covers it.
        if UserDefaults.standard.bool(forKey: "ipx.noBadgePrompt") { return }
        UNUserNotificationCenter.current().requestAuthorization(options: [.badge]) { granted, _ in
            // Each launch, not once: Apple can change the token, and says to ask every time.
            guard granted else { return }
            Task { @MainActor in UIApplication.shared.registerForRemoteNotifications() }
        }
    }

    /// The page's own count, sent as its list changes (#421).
    @MainActor static func set(_ n: Int) {
        lastSet = Date()
        Task { try? await UNUserNotificationCenter.current().setBadgeCount(max(0, n)) }
    }

    /// When the icon was last set from a count, by either way.
    @MainActor private static var lastSet = Date.distantPast

    /// While an episode plays with the screen locked the app is awake, unlike the page: so new
    /// items reach the icon every quarter of an hour, which background refresh does not promise.
    @MainActor static func refreshIfStale() {
        guard Date().timeIntervalSince(lastSet) > 15 * 60 else { return }
        lastSet = Date()
        Task { await refresh() }
    }

    /// Asks ipx and sets the icon. Left as it is when ipx cannot be reached or does not answer
    /// with a feed list: an old number is better than a wrong one.
    @discardableResult
    static func refresh() async -> Bool {
        guard let url = Server.resolveForPlayer("/api/feeds") else { return false }
        var req = URLRequest(url: url)
        req.timeoutInterval = 20
        req.setValue("application/json", forHTTPHeaderField: "Accept")
        Credentials.headers.forEach { req.setValue($1, forHTTPHeaderField: $0) }
        let cfg = URLSessionConfiguration.default
        cfg.httpCookieStorage = .shared
        guard let (data, resp) = try? await URLSession(configuration: cfg).data(for: req),
              (resp as? HTTPURLResponse)?.statusCode == 200,
              let n = unread(inAll: data) else { return false }
        await MainActor.run { set(n) }
        return true
    }

    // MARK: - while the app is closed

    /// Before the app finishes launching, which is when iOS wants the handler registered.
    static func registerBackgroundRefresh() {
        BGTaskScheduler.shared.register(forTaskWithIdentifier: taskId, using: nil) { task in
            // The next one first, so a run that fails or is cut short does not end the chain.
            scheduleBackgroundRefresh()
            let work = Task {
                task.setTaskCompleted(success: await refresh())
            }
            task.expirationHandler = { work.cancel() }
        }
    }

    static func scheduleBackgroundRefresh() {
        let req = BGAppRefreshTaskRequest(identifier: taskId)
        req.earliestBeginDate = Date(timeIntervalSinceNow: 30 * 60)
        try? BGTaskScheduler.shared.submit(req)
    }
}