// RoadDistanceService.swift — computes Apple-Maps-matched ROAD distances for
// nearby stations and caches them (keychain) so widgets + Live Activity can
// show real driving distance instead of straight-line haversine.
//
// Runs only in the APP: MKDirections is network-bound and the widget extension
// has a tiny execution budget + a ~40-70/day refresh budget, so routing belongs
// here, not in the widget. The widget/Live Activity just read the cache.
//
// Throttling: recompute at most every `throttleMinutes`, or when the user has
// moved `moveThresholdMeters` from where the cache was built. In
// "Accurate distances" mode a pass routes the WHOLE search area (up to
// `fillCapacity` stations) so the browse layer can serve routed distances for
// every row at once; the mode must also be ON for any pass to run at all.

import Foundation
import CoreLocation
import MapKit
import UIKit
import WidgetKit

struct RoadDistanceFillDebugStatus: Codable, Equatable {
    var state: State = .idle
    var reason: String = "Waiting"
    var updatedAt: Date = Date()
    var originLat: Double?
    var originLng: Double?
    var stationPoolCount: Int = 0
    var priorityCount: Int = 0
    var candidateCount: Int = 0
    var routedCount: Int = 0
    var cachedCount: Int = 0
    var failedCount: Int = 0
    var complete: Bool = false
    var capped: Bool = false
    var elapsedSeconds: Double?
    var movedMeters: Double?
    var throttleSecondsRemaining: Double?

    enum State: String, Codable {
        case idle
        case gated
        case running
        case completed
    }

    var summary: String {
        switch state {
        case .idle: return "Idle — \(reason)"
        case .gated: return "Gated — \(reason)"
        case .running: return "Running — \(reason)"
        case .completed: return complete ? "Complete — routed set ready" : "Incomplete — still Approx."
        }
    }

    static let idle = RoadDistanceFillDebugStatus()
}

enum RoadDistanceService {
    /// Upper bound on stations routed per pass. Accurate-distances mode routes the
    /// whole search area in ONE pass, because a 12-per-pass budget could never
    /// make a list fully routed — that is exactly why two devices (and two
    /// radii) disagreed about the same station. Capped so a pathological dataset
    /// can't turn one pass into thousands of route requests; a capped pass stays
    /// incomplete, and the browse layer keeps showing straight-line + "Approx."
    /// until a later pass finishes the job. Measured on device: ~3.2 route calls
    /// per second, so 600 stations (1,200 calls) is ~6 min at the very worst.
    static let fillCapacity = 600
    /// How many Worth-the-Drive candidates may claim slots in a pass. Those
    /// rows are ranked by NET SAVING and are typically FURTHER out than the
    /// nearest stations — routing only the nearest stations (the old behaviour)
    /// meant they never got a measured route and stayed on straight-line
    /// estimates forever.
    static let priorityCandidates = 6
    /// Only route stations within this straight-line radius (km). Covers the
    /// largest search radius the UI exposes (15 mi ≈ 24.1 km) plus margin, so
    /// every station a widget/Live Activity/list can actually show gets routed.
    static let maxRadiusKM: Double = 25
    /// Don't route again more often than this (minutes) once the set is whole.
    static let throttleMinutes: Double = 10
    /// Retry window for a set that is still INCOMPLETE — a pass cut short by
    /// backgrounding the app must be continued promptly, not in ten minutes.
    static let incompleteRetrySeconds: Double = 30
    /// Recompute when the user moves more than this (metres) from the last
    /// source location.
    static let moveThresholdMeters: Double = 400
    static private(set) var debugStatus = RoadDistanceFillDebugStatus.idle

    @MainActor
    static func currentDebugStatus() -> RoadDistanceFillDebugStatus {
        debugStatus
    }

    @MainActor
    private static func publishDebug(_ status: RoadDistanceFillDebugStatus) {
        debugStatus = status
        NotificationCenter.default.post(name: .fuelBoardRoadDistanceDebugUpdated, object: nil)
    }

    private static func updateDebug(_ status: RoadDistanceFillDebugStatus) async {
        await MainActor.run { publishDebug(status) }
    }

    private static func baseDebugStatus(state: RoadDistanceFillDebugStatus.State,
                                        reason: String,
                                        stations: [FuelStation],
                                        lat: Double,
                                        lng: Double,
                                        priorityCount: Int) -> RoadDistanceFillDebugStatus {
        RoadDistanceFillDebugStatus(
            state: state,
            reason: reason,
            updatedAt: Date(),
            originLat: lat,
            originLng: lng,
            stationPoolCount: stations.count,
            priorityCount: priorityCount
        )
    }

    /// Refreshes the cached road distances for the in-radius stations around
    /// `lat`/`lng`. Throttled by time + distance; safe to call on every fix.
    ///
    /// - Parameter priorityStations: stations to route BEFORE the nearest ones
    ///   (the Worth-the-Drive candidates and the comparison baseline), so the
    ///   numbers the user actually reads are road-routed rather than estimated.
    static func refreshIfNeeded(stations: [FuelStation],
                                lat: Double,
                                lng: Double,
                                prioritising priorityStations: [FuelStation] = []) async {
        // "Accurate distances" OFF (the default) means the browse layer never
        // serves a routed value, so any routing would be pure cost with nothing
        // visible to show for it: no pass, no requests, no widget reloads.
        guard FuelStore.accurateDistancesEnabled else {
            await updateDebug(baseDebugStatus(state: .gated,
                                              reason: "Driving distances off",
                                              stations: stations,
                                              lat: lat,
                                              lng: lng,
                                              priorityCount: priorityStations.count))
            return
        }
        // Only fill while the app is on screen. The browse layer is the only
        // consumer, so a background fill would spend radio and battery routing a
        // position nothing is displaying — worst case, a long drive.
        guard await appIsActive() else {
            await updateDebug(baseDebugStatus(state: .gated,
                                              reason: "App not active",
                                              stations: stations,
                                              lat: lat,
                                              lng: lng,
                                              priorityCount: priorityStations.count))
            return
        }
        guard await refreshGate.begin() else {
            await updateDebug(baseDebugStatus(state: .gated,
                                              reason: "Fill already running",
                                              stations: stations,
                                              lat: lat,
                                              lng: lng,
                                              priorityCount: priorityStations.count))
            return
        }
        defer { Task { await refreshGate.end() } }

        guard !stations.isEmpty else {
            await updateDebug(baseDebugStatus(state: .gated,
                                              reason: "No stations loaded",
                                              stations: stations,
                                              lat: lat,
                                              lng: lng,
                                              priorityCount: priorityStations.count))
            return
        }

        let existing = FuelStore.loadRoadDistances()
        // Entries can only be merged when they were routed from (near enough)
        // this same origin — the read path rejects anything else anyway.
        let originCompatible = existing.map {
            haversineMeters($0.sourceLat, $0.sourceLng, lat, lng) < moveThresholdMeters
        } ?? false

        // Throttle: keep cached values when fresh and the user hasn't moved far.
        // `FB_ROUTE_REFRESH=1` forces a pass (DEBUG verification hook).
#if DEBUG
        let forcePass = ProcessInfo.processInfo.environment["FB_ROUTE_REFRESH"] == "1"
#else
        let forcePass = false
#endif
        if !forcePass, let cache = existing {
            let elapsed = Date().timeIntervalSince1970 - cache.updatedAt
            let movedMeters = haversineMeters(cache.sourceLat, cache.sourceLng, lat, lng)
            // An INCOMPLETE set still has work to do, so it is not throttled for
            // the full ten minutes. The browse layer refuses to serve a partial
            // set (every row reads straight-line + "Approx." until the set is
            // whole), and a pass is routinely cut short by backgrounding the app
            // — so a ten-minute throttle on an incomplete set is what made
            // "Approx." look permanent: reopen the app inside the window and the
            // pass that would finish the job never even started, and refreshing
            // the prices could not help. Retry soon instead.
            let incomplete = cache.complete != true
            let throttleWindow = incomplete ? incompleteRetrySeconds : throttleMinutes * 60
            if elapsed < throttleWindow && movedMeters < moveThresholdMeters {
                var status = baseDebugStatus(state: .gated,
                                             reason: incomplete ? "Throttled — incomplete set, retrying"
                                                                : "Throttled",
                                             stations: stations,
                                             lat: lat,
                                             lng: lng,
                                             priorityCount: priorityStations.count)
                status.cachedCount = cache.entries.count
                status.complete = !incomplete
                status.movedMeters = movedMeters
                status.throttleSecondsRemaining = throttleWindow - elapsed
                if incomplete,
                   await repairCompletenessIfNothingLeftToRoute(cache, stations: stations,
                                                                lat: lat, lng: lng) {
                    status.complete = true
                    status.reason = "Throttled — stale incomplete flag repaired"
                }
                await updateDebug(status)
                if status.complete {
                    await notifyDistanceSurfaces()
                }
                return
            }
        }

        // Candidates: Worth-the-Drive priorities first, then the nearest
        // stations. Stations that already have a route from this origin are
        // skipped so the per-pass budget goes on the ones still missing one.
        var candidates: [FuelStation] = []
        var seen = Set<String>()
        func consider(_ station: FuelStation) {
            guard candidates.count < fillCapacity else { return }
            guard station.distanceKM(to: lat, lng2: lng) <= maxRadiusKM else { return }
            guard seen.insert(station.id).inserted else { return }
            guard !FuelStore.hasRoadDistance(for: station, userLat: lat, userLng: lng) else { return }
            candidates.append(station)
        }
        for station in priorityStations.prefix(priorityCandidates) { consider(station) }
        let nearest = stations.sorted { $0.distanceKM(to: lat, lng2: lng) < $1.distanceKM(to: lat, lng2: lng) }
        for station in nearest {
            if candidates.count >= fillCapacity { break }
            consider(station)
        }

        guard !candidates.isEmpty else {
            var status = baseDebugStatus(state: .gated,
                                         reason: "No unrouted candidates",
                                         stations: stations,
                                         lat: lat,
                                         lng: lng,
                                         priorityCount: priorityStations.count)
            status.cachedCount = existing?.entries.count ?? 0
            var complete = existing?.complete == true && FuelStore.accurateDistancesReady(userLat: lat, userLng: lng)
            if !complete, let existing,
               await repairCompletenessIfNothingLeftToRoute(existing, stations: stations, lat: lat, lng: lng) {
                complete = true
                status.reason = "No unrouted candidates — stale incomplete flag repaired"
            }
            status.complete = complete
            await updateDebug(status)
            if status.complete {
                await notifyDistanceSurfaces()
            }
            return
        }

        let origin = CLLocationCoordinate2D(latitude: lat, longitude: lng)
        var entries: [String: CachedRoadDistance] = originCompatible ? (existing?.entries ?? [:]) : [:]

        var running = baseDebugStatus(state: .running,
                                      reason: "Routing \(candidates.count) stations",
                                      stations: stations,
                                      lat: lat,
                                      lng: lng,
                                      priorityCount: priorityStations.count)
        running.candidateCount = candidates.count
        running.cachedCount = entries.count
        running.capped = candidates.count >= fillCapacity
        await updateDebug(running)

        let passStart = Date()

        var routedCount = 0
        for station in candidates {
            let dest = CLLocationCoordinate2D(latitude: station.lat, longitude: station.lng)
            async let outward = roadMeters(from: origin, to: dest)
            async let back = roadMeters(from: dest, to: origin)
            if let meters = await outward {
                routedCount += 1
                // Store the exact pin that was routed so the display layer can
                // refuse to serve this value if the station later appears with
                // a different coordinate (corrected pin / other data source).
                let returnMeters = await back
                entries[station.id] = CachedRoadDistance(meters: meters,
                                                         lat: station.lat,
                                                         lng: station.lng,
                                                         returnMeters: returnMeters)
            }
        }
        guard entries.count > (originCompatible ? (existing?.entries.count ?? 0) : 0) || !originCompatible else {
            var status = baseDebugStatus(state: .gated,
                                         reason: "Route pass added no distances",
                                         stations: stations,
                                         lat: lat,
                                         lng: lng,
                                         priorityCount: priorityStations.count)
            status.candidateCount = candidates.count
            status.routedCount = routedCount
            status.failedCount = candidates.count - routedCount
            status.cachedCount = entries.count
            status.elapsedSeconds = Date().timeIntervalSince(passStart)
            await updateDebug(status)
            return
        }

        // A pass is complete when nothing in the search area is left UNRESOLVED:
        // every station either has a route from this anchor, or was attempted in
        // THIS pass and Apple Maps returned none. A few pins genuinely have no
        // route (offshore, deleted forecourt, bad snapshot) and one of those must
        // never leave the area permanently incomplete — that would pin the whole
        // list on straight-line estimates forever. Those stations show their
        // straight line, marked Approx., and the radius filter uses that same
        // distance, so nothing is ever out of step with what a row displays.
        let attempted = Set(candidates.map(\.id))
        let capped = candidates.count >= fillCapacity
        let unresolved = capped || stations.contains { station in
            station.distanceKM(to: lat, lng2: lng) <= maxRadiusKM
                && !FuelStore.hasRoadDistance(for: station, userLat: lat, userLng: lng)
                && !attempted.contains(station.id)
        }
        FuelStore.saveRoadDistances(sourceLat: lat, sourceLng: lng, entries: entries,
                                    complete: !unresolved)
        var completed = baseDebugStatus(state: .completed,
                                        reason: !unresolved ? "Fill complete" : (capped ? "Capacity cap hit" : "Unresolved stations remain"),
                                        stations: stations,
                                        lat: lat,
                                        lng: lng,
                                        priorityCount: priorityStations.count)
        completed.candidateCount = candidates.count
        completed.routedCount = routedCount
        completed.failedCount = candidates.count - routedCount
        completed.cachedCount = entries.count
        completed.complete = !unresolved
        completed.capped = capped
        completed.elapsedSeconds = Date().timeIntervalSince(passStart)
        await updateDebug(completed)
#if DEBUG
        // `FB-ROUTE` prints what a pass actually cost (stations, route calls,
        // wall-clock, unroutable pins, whether the set is now complete) — the
        // evidence for any "how taxing is routing?" answer, and how a bigger pass
        // gets scaled from measurement instead of guesswork.
        print("FB-ROUTE pass stations=\(candidates.count) calls=\(candidates.count * 2) "
            + "elapsed=\(String(format: "%.2f", Date().timeIntervalSince(passStart)))s "
            + "cached=\(entries.count) failed=\(candidates.count - routedCount) "
            + "complete=\(!unresolved)")
#endif
        await notifyDistanceSurfaces()
    }

    /// A cache can hold a route for EVERY station in the search area and still be
    /// flagged incomplete — a previous pass hit the capacity cap, or was
    /// interrupted between routing and writing. Nothing would ever clear that
    /// flag on its own (the next pass finds nothing left to route, and the
    /// throttle path returns without writing), and every row would stay on
    /// straight-line + "Approx." while its routed value sat in the cache. When
    /// nothing in the area is left unrouted, the set IS complete: correct the
    /// flag so the routed values can be served.
    private static func repairCompletenessIfNothingLeftToRoute(_ cache: RoadDistanceCache,
                                                              stations: [FuelStation],
                                                              lat: Double,
                                                              lng: Double) async -> Bool {
        guard cache.complete != true, !stations.isEmpty else { return false }
        let nothingLeftToRoute = !stations.contains { station in
            station.distanceKM(to: lat, lng2: lng) <= maxRadiusKM
                && !FuelStore.hasRoadDistance(for: station, userLat: lat, userLng: lng)
        }
        guard nothingLeftToRoute else { return false }
        FuelStore.saveRoadDistances(sourceLat: cache.sourceLat, sourceLng: cache.sourceLng,
                                    entries: cache.entries, complete: true)
#if DEBUG
        print("FB-COMPLETE stale incomplete flag repaired cached=\(cache.entries.count) "
            + "(every in-radius station already routed — no re-routing needed)")
#endif
        return true
    }

    @MainActor
    private static func notifyDistanceSurfacesOnMain() {
        WidgetCenter.shared.reloadAllTimelines()
        NotificationCenter.default.post(name: .fuelBoardRoadDistancesUpdated, object: nil)
    }

    private static func notifyDistanceSurfaces() async {
        await MainActor.run { notifyDistanceSurfacesOnMain() }
    }

    private static let refreshGate = RoadDistanceRefreshGate()

    /// Accurate mode fills only while the app is on screen: the browse layer is
    /// the only consumer of routed distances, so a background fill would spend
    /// radio and battery routing a position nothing is looking at.
    private static func appIsActive() async -> Bool {
        await MainActor.run { UIApplication.shared.applicationState == .active }
    }

    /// Driving distance (metres) between two coordinates via Apple Maps routing.
    private static func roadMeters(from: CLLocationCoordinate2D, to: CLLocationCoordinate2D) async -> Double? {
        let request = MKDirections.Request()
        request.source = MKMapItem(
            location: CLLocation(latitude: from.latitude, longitude: from.longitude),
            address: nil
        )
        request.destination = MKMapItem(
            location: CLLocation(latitude: to.latitude, longitude: to.longitude),
            address: nil
        )
        request.transportType = .automobile
        request.requestsAlternateRoutes = false
        do {
            let response = try await MKDirections(request: request).calculate()
            return response.routes.first?.distance
        } catch {
            return nil
        }
    }

    /// Straight-line haversine distance between two coordinates, in metres.
    private static func haversineMeters(_ lat1: Double, _ lng1: Double, _ lat2: Double, _ lng2: Double) -> Double {
        let r = 6371000.0
        let dLat = (lat2 - lat1) * .pi / 180
        let dLng = (lng2 - lng1) * .pi / 180
        let a = sin(dLat / 2) * sin(dLat / 2) +
                cos(lat1 * .pi / 180) * cos(lat2 * .pi / 180) *
                sin(dLng / 2) * sin(dLng / 2)
        return r * 2 * atan2(sqrt(a), sqrt(1 - a))
    }
}

private actor RoadDistanceRefreshGate {
    private var isRefreshing = false

    func begin() -> Bool {
        guard !isRefreshing else { return false }
        isRefreshing = true
        return true
    }

    func end() {
        isRefreshing = false
    }
}