// MirrorFuelProvider.swift — GitHub price-mirror provider (P0 live chain).
//
// The mirror (aptonline/fuelboard-data, pushed daily by the LAN relay) is the
// PRODUCTION data source: its day files are the raw relay /api/v1/stations
// envelope, so the shared decode works verbatim. Fetching from
// raw.githubusercontent.com makes live prices work fully off-LAN — App
// Review cannot reach a LAN relay, and consumers never have one to reach —
// and the bundled dump (app target) is the no-network last resort.
//
// Chain (LiveChainProvider) — GitHub first, EVERYWHERE:
//   GitHub mirror → [dev-only LAN relay] → (app target: bundled dump in the
//   ContentView catch, widget: cache → placeholder)
// The relay is NOT part of the consumer chain at all: it is unreachable from
// any phone outside the developer's LAN, and even attempting it from a
// consumer phone would fire the iOS Local Network permission prompt for an
// address that can never be reached. It remains reachable only behind the
// hidden dev flag `fuelboard.relayFallback` (Settings → Debug) for home
// testing. The relay keeps its server-side job (GOV.UK ingest + daily mirror
// push) unchanged.
//
// Telemetry (FuelBeacon): raw GitHub fetches are invisible to repo analytics
// (page views/clones only), so every chain fetch opportunistically pings the
// relay's existing widget-diag route (zero relay changes) — but ONLY when the
// same dev flag is on, so consumers never make a local-network attempt.
// /stats app-hit spikes (dev flag on) = GitHub path failing → fallback active.

import Foundation

// MARK: - Mirror provider (GitHub raw)

/// Fetches CURRENT prices from the GitHub mirror. Same envelope as the
/// relay, so `FuelPriceProvider.decodeStations` applies unchanged.
struct MirrorFuelProvider: FuelPriceProviding {
    var baseURL = FuelHistoryStore.mirrorBase

    /// Envelope metadata from the most recent successful mirror fetch —
    /// surfaced via `LiveChainProvider.latestMeta` in Settings → About.
    static var latestMeta: RelayMeta?

    /// App-group cache of the last fetched FULL dump, keyed by snapshot day.
    /// The mirror pushes once/day, so between pushes the app reuses the
    /// cached dump instead of re-downloading ~2.8 MB at every 12 h gate.
    static let liveDumpCacheKey = "fuelboard.liveDumpCache"

    struct DumpCache: Codable {
        let day: String
        let data: Data
    }

    static func loadDumpCache() -> DumpCache? {
        guard let defaults = UserDefaults(suiteName: FuelStore.appGroupSuite),
              let raw = defaults.data(forKey: liveDumpCacheKey),
              let cache = try? JSONDecoder().decode(DumpCache.self, from: raw) else {
            return nil
        }
        return cache
    }

    static func saveDumpCache(day: String, data: Data) {
        guard let defaults = UserDefaults(suiteName: FuelStore.appGroupSuite),
              let raw = try? JSONEncoder().encode(DumpCache(day: day, data: data)) else { return }
        defaults.set(raw, forKey: liveDumpCacheKey)
    }

    /// True when the cached dump is already the freshest the mirror has —
    /// avoids the ~2.8 MB re-download when the mirror hasn't pushed a new day.
    static func canReuseCache(cachedDay: String?, latestDay: String?) -> Bool {
        guard let cachedDay, let latestDay else { return false }
        return cachedDay == latestDay
    }

    /// Focused-path projection, replacing what the relay's radius endpoint
    /// used to do server-side: stations WITH the fuel, within radius of the
    /// point, nearest first, bounded. The alert path re-checks radius and
    /// fuel itself afterwards, so this only narrows the payload.
    static func focused(_ stations: [FuelStation], near lat: Double?, lng: Double?,
                        fuel: FuelType, radiusKM: Double?, limit: Int = 500) -> [FuelStation] {
        guard let lat, let lng, let radiusKM else { return stations }
        return Array(
            stations
                .filter { $0.prices[fuel] != nil && $0.distanceKM(to: lat, lng2: lng) <= radiusKM }
                .sorted { $0.distanceKM(to: lat, lng2: lng) < $1.distanceKM(to: lat, lng2: lng) }
                .prefix(limit)
        )
    }

    func fetchStations(near lat: Double?, lng: Double?, fuel: FuelType, radiusKM: Double?) async throws -> [FuelStation] {
        // 1. Tiny pointer fetch — the freshest snapshot day.
        guard let latest = await FuelHistoryStore.fetchLatest(base: baseURL),
              let day = latest.availableTo ?? latest.date else {
            throw FuelProviderError.mirrorUnavailable
        }
        // 2. Reuse the cached full dump when the mirror hasn't pushed a new day.
        let cache = Self.loadDumpCache()
        let data: Data
        if Self.canReuseCache(cachedDay: cache?.day, latestDay: day), let cached = cache?.data {
            data = cached
        } else {
            let (fetched, response) = try await URLSession.shared.data(
                from: FuelHistoryStore.historyFileURL(day: day, base: baseURL)
            )
            guard (response as? HTTPURLResponse)?.statusCode == 200 else {
                throw FuelProviderError.mirrorUnavailable
            }
            Self.saveDumpCache(day: day, data: fetched)
            data = fetched
        }
        let stations = try FuelPriceProvider.decodeStations(from: data)
        Self.latestMeta = FuelPriceProvider.decodeRelayMeta(from: data)
        return Self.focused(stations, near: lat, lng: lng, fuel: fuel, radiusKM: radiusKM)
    }
}

// MARK: - Live chain provider

/// The live price chain — GitHub first, everywhere. The LAN relay is NOT part
/// of the consumer chain: consumers can never reach it (and must never even
/// attempt it — that would prompt for Local Network access on a stranger's
/// phone). It only joins behind the hidden dev flag `fuelboard.relayFallback`
/// (Settings → Debug) for the developer's own home testing. Whichever leg
/// serves is recorded for the About section + telemetry.
struct LiveChainProvider: FuelPriceProviding {
    var mirror: FuelPriceProviding
    var relay: FuelPriceProviding

    /// Which leg served the last fetch — Settings → About metadata + beacon.
    static var latestMeta: RelayMeta?
    static var lastSource: String?

    init(mirror: FuelPriceProviding = MirrorFuelProvider(),
         relay: FuelPriceProviding = RelayFuelProvider()) {
        self.mirror = mirror
        self.relay = relay
    }

    func fetchStations(near lat: Double?, lng: Double?, fuel: FuelType, radiusKM: Double?) async throws -> [FuelStation] {
        do {
            let stations = try await mirror.fetchStations(near: lat, lng: lng, fuel: fuel, radiusKM: radiusKM)
            Self.latestMeta = MirrorFuelProvider.latestMeta
            Self.lastSource = "github"
            FuelBeacon.fire(source: "github", n: stations.count)
            return stations
        } catch {
            // Dev-only fallback: consumers never reach this branch — the flag
            // is off by default and the flag itself lives in the Debug section.
            guard FuelStore.loadRelayFallbackEnabled() else { throw error }
            let stations = try await relay.fetchStations(near: lat, lng: lng, fuel: fuel, radiusKM: radiusKM)
            Self.latestMeta = RelayFuelProvider.latestMeta
            Self.lastSource = "relay"
            FuelBeacon.fire(source: "relay", n: stations.count)
            return stations
        }
    }
}

// MARK: - Telemetry beacon

/// Fire-and-forget attribution for the LIVE chain (see header note). Never
/// awaited, never user-visible; a failed ping is silent. Fires ONLY when the
/// dev relay flag is on — consumers must never make a local-network attempt,
/// and an off-LAN ping failing silently is itself the reachability datum.
/// Disabled in unit tests via `isEnabled`.
enum FuelBeacon {
    static var isEnabled = true

    static var shouldFire: Bool {
        isEnabled && FuelStore.loadRelayFallbackEnabled()
    }

    static func fire(source: String, n: Int, timeout: TimeInterval = 2) {
        guard shouldFire,
              let url = beaconURL(source: source, n: n) else { return }
        var request = RelayFuelProvider.relayRequest(url, client: "app", timeout: timeout)
        request.cachePolicy = .reloadIgnoringLocalCacheData
        Task { _ = try? await URLSession.shared.data(for: request) }
    }

    /// Testable URL construction — hits the relay's existing widget-diag
    /// beacon route with app attribution (zero relay changes).
    static func beaconURL(source: String, n: Int, base: URL = RelayFuelProvider().baseURL) -> URL? {
        guard var components = URLComponents(
            url: base.appendingPathComponent("api/v1/widget-diag"),
            resolvingAgainstBaseURL: false
        ) else { return nil }
        components.queryItems = [
            URLQueryItem(name: "intent", value: "app-live"),
            URLQueryItem(name: "source", value: source),
            URLQueryItem(name: "n", value: String(n)),
        ]
        return components.url
    }
}