//
//  AdminSession.swift
//  FTStore Admin
//
//  The authenticated half of the app. Owns the FTStore Admin session token
//  and is the only door every screen goes through, so there is exactly
//  one place that decides "are we allowed to talk to the server".
//
//  What the app ships: a public base URL. What it does not ship: any
//  credential, any API key, any token. FTStore Admin is dark until someone signs
//  in with a real FTStore Admin account, and the server re-checks that session
//  against its database on every single request — so pulling this IPA apart
//  yields a login form and nothing else.
//

import Foundation
import Combine
import LocalAuthentication

@MainActor
final class AdminSession: ObservableObject {
	static let shared = AdminSession()

	private static let _cachedUserKey = "admin.cachedUser"
	private static let _biometricsKey = "admin.requireBiometrics"

	/// Restoring a stored session at launch — distinct from "signed out" so the
	/// login form doesn't flash on every cold start.
	@Published private(set) var isRestoring = true
	@Published private(set) var user: AdminUser?
	/// Set when a stored session is present but the device owner hasn't passed
	/// the local biometric gate yet.
	@Published private(set) var isLocked = false
	/// Populated when the last `/api/auth/me` couldn't reach the server; the
	/// cached identity is still shown, flagged as unverified.
	@Published private(set) var connectionWarning: String?

	/// Optional second gate in front of FTStore Admin. It protects an unlocked,
	/// unattended device — the server credentials are what actually protect the
	/// data — so it is opt-in and never blocks the install flow.
	@Published var requireBiometrics: Bool {
		didSet {
			guard oldValue != requireBiometrics else { return }
			UserDefaults.standard.set(requireBiometrics, forKey: Self._biometricsKey)
		}
	}

	/// Last username, so the login form can prefill it. Not a secret.
	@Published var lastUsername: String {
		didSet { UserDefaults.standard.set(lastUsername, forKey: "admin.lastUsername") }
	}

	private let _api = AdminAPI.shared

	private init() {
		let defaults = UserDefaults.standard
		requireBiometrics = defaults.bool(forKey: Self._biometricsKey)
		lastUsername = defaults.string(forKey: "admin.lastUsername") ?? ""
	}

	// MARK: - State

	var isSignedIn: Bool { user != nil && !isLocked }
	var isAdmin: Bool { user?.isAdmin ?? false }

	/// The token, read fresh from the Keychain on every request rather than
	/// cached in memory for the lifetime of the process.
	private var _token: String? { AdminKeychain.get(AdminKeychain.sessionAccount) }

	var hasStoredSession: Bool { _token != nil }

	/// Host FTStore Admin talks to — shown on the login screen so it's obvious
	/// which server is being authenticated against.
	var serverDescription: String {
		URL(string: AdminConfig.baseURL)?.host ?? AdminConfig.baseURL
	}

	// MARK: - Lifecycle

	/// Validate a stored session at launch. A 401 means the operator revoked it
	/// (or it expired) — drop it. A network failure keeps it: being on a train
	/// is not a reason to make someone type their password again.
	func restore() async {
		defer { isRestoring = false }

		guard let token = _token else {
			user = nil
			isLocked = false
			return
		}

		user = _cachedUser()
		isLocked = requireBiometrics

		do {
			let fresh = try await _api.me(token: token)
			user = fresh
			_cacheUser(fresh)
			connectionWarning = nil
		} catch AdminAPIError.unauthorized {
			await _clearLocalSession()
		} catch let error as AdminAPIError {
			// Keep the cached identity, but say so.
			connectionWarning = error.errorDescription
		} catch {
			connectionWarning = error.localizedDescription
		}
	}

	func signIn(username: String, password: String) async throws {
		let result = try await _api.signIn(username: username, password: password)
		guard AdminKeychain.set(result.token, for: AdminKeychain.sessionAccount) else {
			throw AdminAPIError.network(.localized("Couldn't store the session securely."))
		}
		user = result.user
		lastUsername = result.user.username
		isLocked = false
		connectionWarning = nil
		_cacheUser(result.user)
	}

	/// Ends the session on the server too, so the token is dead even if it were
	/// somehow lifted off the device afterwards.
	func signOut() async {
		let token = _token
		await _clearLocalSession()
		if let token {
			await _api.signOut(token: token)
		}
	}

	private func _clearLocalSession() async {
		AdminKeychain.remove(AdminKeychain.sessionAccount)
		UserDefaults.standard.removeObject(forKey: Self._cachedUserKey)
		user = nil
		isLocked = false
		connectionWarning = nil
	}

	// MARK: - Local lock

	func lock() {
		guard user != nil else { return }
		isLocked = true
	}

	/// Face ID / Touch ID, falling back to the device passcode so a failed
	/// biometric can't strand someone out of their own FTStore Admin.
	func unlock() async -> String? {
		let context = LAContext()
		context.localizedFallbackTitle = .localized("Use Passcode")

		guard context.canEvaluatePolicy(.deviceOwnerAuthentication, error: nil) else {
			// No biometrics and no passcode set — the gate can't be enforced, so
			// don't let it become a lockout.
			isLocked = false
			return nil
		}

		do {
			let ok = try await context.evaluatePolicy(
				.deviceOwnerAuthentication,
				localizedReason: .localized("Unlock FTStore Admin")
			)
			if ok { isLocked = false }
			return nil
		} catch {
			return error.localizedDescription
		}
	}

	// MARK: - Cached identity

	private func _cacheUser(_ user: AdminUser) {
		guard let data = try? JSONEncoder().encode(user) else { return }
		UserDefaults.standard.set(data, forKey: Self._cachedUserKey)
	}

	private func _cachedUser() -> AdminUser? {
		guard let data = UserDefaults.standard.data(forKey: Self._cachedUserKey) else { return nil }
		return try? JSONDecoder().decode(AdminUser.self, from: data)
	}

	// MARK: - Authenticated requests
	//
	// Every FTStore Admin screen calls through here. A 401 anywhere means the session
	// is gone, so it tears the whole FTStore Admin down in one place instead of each
	// view inventing its own recovery.

	func get<Response: Decodable>(
		_ path: String,
		query: [String: String] = [:],
		as type: Response.Type
	) async throws -> Response {
		try await _guard { token in
			try await self._api.get(path, query: query, as: type, token: token)
		}
	}

	func send<Response: Decodable>(
		_ method: AdminAPI.Method,
		_ path: String,
		query: [String: String] = [:],
		as type: Response.Type
	) async throws -> Response {
		try await _guard { token in
			try await self._api.send(method, path, query: query, as: type, token: token)
		}
	}

	func send<Body: Encodable, Response: Decodable>(
		_ method: AdminAPI.Method,
		_ path: String,
		query: [String: String] = [:],
		body: Body,
		as type: Response.Type
	) async throws -> Response {
		try await _guard { token in
			try await self._api.send(method, path, query: query, body: body, as: type, token: token)
		}
	}

	/// Raw bytes in and out — the settings backup, which is a document rather
	/// than a model and must round-trip byte-for-byte.
	func rawData(
		_ method: AdminAPI.Method,
		_ path: String,
		query: [String: String] = [:],
		body: Data? = nil,
		contentType: String? = nil
	) async throws -> Data {
		try await _guard { token in
			try await self._api.rawData(
				method,
				path,
				query: query,
				body: body,
				contentType: contentType,
				token: token
			)
		}
	}

	func upload<Response: Decodable>(
		_ path: String,
		fields: [String: String],
		fileField: String,
		fileName: String,
		mimeType: String,
		fileData: Data,
		as type: Response.Type
	) async throws -> Response {
		try await _guard { token in
			try await self._api.upload(
				path,
				fields: fields,
				fileField: fileField,
				fileName: fileName,
				mimeType: mimeType,
				fileData: fileData,
				as: type,
				token: token
			)
		}
	}

	/// Convenience for the action endpoints, which all answer the same ack.
	@discardableResult
	func act(_ method: AdminAPI.Method, _ path: String, query: [String: String] = [:]) async throws -> AdminAck {
		try await send(method, path, query: query, as: AdminAck.self)
	}

	@discardableResult
	func act<Body: Encodable>(_ method: AdminAPI.Method, _ path: String, body: Body) async throws -> AdminAck {
		try await send(method, path, body: body, as: AdminAck.self)
	}

	private func _guard<Response>(
		_ operation: (String) async throws -> Response
	) async throws -> Response {
		guard let token = _token else {
			await _clearLocalSession()
			throw AdminAPIError.unauthorized
		}
		do {
			return try await operation(token)
		} catch AdminAPIError.unauthorized {
			await _clearLocalSession()
			throw AdminAPIError.unauthorized
		}
	}
}