import CoreLocation
import UserNotifications
import SwiftUI

/// Geofences favourite + closest stations and fires a local notification when
/// the user enters a station whose price is the cheapest within the radius.
///
/// Region budget: iOS allows 20 monitored regions per app — favourites are
/// always registered first, then the closest remaining stations fill the rest.
@MainActor
final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLocationManagerDelegate {
    @Published var monitoredStationIDs: [String] = []
    @Published var lastAlert: String?

    private let manager = CLLocationManager()
    private var stations: [FuelStation] = []
    private var favourites: [FuelStation] = []
    private var fuel: FuelType = .e10
    private var radiusKM: Double = 3.0
    private var lastNotified: [String: Date] = [:]   // dedup per station
    private var enabled = false

    override init() {
        super.init()
        manager.delegate = self
        manager.desiredAccuracy = kCLLocationAccuracyHundredMeters
        manager.pausesLocationUpdatesAutomatically = true
        // Restore persisted state on background relaunch so region events work
        // even before the view fully appears.
        enabled = FuelStore.loadAlertsEnabled()
        fuel = FuelStore.loadAlertsFuel()
        radiusKM = FuelStore.loadAlertsRadius()
        stations = FuelStore.loadStations()
        favourites = FuelStore.loadFavourites().filter { $0.fuel == fuel }.map(\.station)
    }

    /// Re-registers geofences. Call whenever stations/favourites/settings change.
    /// Favourites are fuel-scoped entries; only entries for the monitored fuel
    /// get priority slots (a Diesel favourite must not grab a slot while
    /// monitoring Unleaded).
    func update(stations: [FuelStation], favourites: [FavouriteEntry], fuel: FuelType, radiusKM: Double) {
        // Fall back to the shared cache when called before the first fetch
        // completes (launch, background region-event wake).
        self.stations = stations.isEmpty ? FuelStore.loadStations() : stations
        self.favourites = favourites.isEmpty
            ? FuelStore.loadFavourites().filter { $0.fuel == fuel }.map(\.station)
            : favourites.filter { $0.fuel == fuel }.map(\.station)
        self.fuel = fuel
        self.radiusKM = radiusKM

        for region in manager.monitoredRegions {
            manager.stopMonitoring(for: region)
        }
        monitoredStationIDs = []

        guard enabled else { return }

        // Favourites first (guaranteed slots), then closest stations, max 18.
        // `self.favourites` is already filtered to the monitored fuel.
        var candidates: [FuelStation] = self.favourites
        let favIDs = Set(self.favourites.map(\.id))
        let location = FuelStore.loadLocation()
        let others = stations
            .filter { !favIDs.contains($0.id) }
            .sorted { lhs, rhs in
                guard let location else { return false }
                return lhs.distanceKM(to: location.lat, lng2: location.lng) <
                       rhs.distanceKM(to: location.lat, lng2: location.lng)
            }
        candidates.append(contentsOf: others)

        var registered: [String] = []
        for station in candidates.prefix(18) where station.prices[fuel] != nil {
            let region = CLCircularRegion(
                center: CLLocationCoordinate2D(latitude: station.lat, longitude: station.lng),
                radius: 300,
                identifier: station.id
            )
            region.notifyOnEntry = true
            region.notifyOnExit = false
            manager.startMonitoring(for: region)
            registered.append(station.id)
        }
        monitoredStationIDs = registered
    }

    func setEnabled(_ enabled: Bool) {
        self.enabled = enabled
        if !enabled {
            for region in manager.monitoredRegions {
                manager.stopMonitoring(for: region)
            }
            monitoredStationIDs = []
        } else {
            requestPermissions()
            // Re-register geofences from restored cache immediately.
            update(stations: stations, favourites: FuelStore.loadFavourites(), fuel: fuel, radiusKM: radiusKM)
        }
    }

    var isEnabled: Bool { enabled }

    private func requestPermissions() {
        // Region monitoring needs Always location for background delivery.
        switch manager.authorizationStatus {
        case .notDetermined:
            manager.requestWhenInUseAuthorization()
            manager.requestAlwaysAuthorization()
        case .authorizedWhenInUse:
            manager.requestAlwaysAuthorization()
        default:
            break
        }
        UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge]) { _, _ in }
    }

    // MARK: - Region events

    nonisolated func locationManager(_ manager: CLLocationManager, didEnterRegion region: CLRegion) {
        Task { @MainActor in
            self.handleEntry(region)
        }
    }

    nonisolated func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) {
        // Ignore — alerts still work while the app is open.
    }

    private func handleEntry(_ region: CLRegion) {
        let stationID = region.identifier
        // Fall back to cached data on background wake (stations may not be
        // fetched yet when the app is relaunched by a region event).
        if stations.isEmpty { stations = FuelStore.loadStations() }
        guard let station = stations.first(where: { $0.id == stationID }) else { return }

        // Dedup: one alert per station per hour.
        if let last = lastNotified[stationID], Date().timeIntervalSince(last) < 3600 { return }

        Task {
            do {
                // Fresh prices around the entered station (alert radius in km).
                let fresh = try await FuelPriceProvider.active.fetchStations(
                    near: station.lat, lng: station.lng, fuel: fuel,
                    radiusKM: max(radiusKM, 10)  // fetch wider than trigger radius
                )
                let withinRadius = fresh.filter {
                    $0.prices[fuel] != nil &&
                    $0.distanceKM(to: station.lat, lng2: station.lng) <= radiusKM
                }
                guard let cheapest = withinRadius.min(by: { $0.prices[fuel]! < $1.prices[fuel]! }),
                      cheapest.id == station.id,
                      let price = cheapest.prices[fuel] else { return }

                fireAlert(for: cheapest, price: price)
                lastNotified[stationID] = Date()
            } catch {
                // Silent — geofence state stays valid for next entry.
            }
        }
    }

    private func fireAlert(for station: FuelStation, price: Double) {
        let brand = station.brand.isEmpty ? station.name : station.brand
        let unit = FuelStore.loadDistanceUnit()
        let content = UNMutableNotificationContent()
        content.title = "Cheapest \(fuel.displayName) nearby: \(brand)"
        content.body = "\(station.name) is the cheapest within \(unit.format(radiusKM)) at \(String(format: "%.1fp", price)). Tap to open."
        content.sound = .default

        let request = UNNotificationRequest(identifier: UUID().uuidString, content: content, trigger: nil)
        UNUserNotificationCenter.current().add(request)

        lastAlert = "\(brand) · \(String(format: "%.1fp", price)) · \(unit.format(radiusKM)) radius"
    }
}