import Combine
import Security
import Foundation
import CryptoKit

@MainActor
final class LicensingManager: ObservableObject {
    enum State: Equatable {
        case licensed(LicensePayload)
        case trial(daysRemaining: Int, expiresAt: Date)
        case expired
        case invalid(String)

        var canUseApp: Bool {
            switch self {
            case .licensed, .trial: true
            case .expired, .invalid: false
            }
        }
    }

    struct LicensePayload: Codable, Equatable {
        let licenseID: String
        let customerName: String?
        let customerEmail: String?
        let edition: String
        let installationID: String
        let issuedAt: Date
        let expiresAt: Date?
        let maxMajorVersion: Int?

        var displayOwner: String {
            if let customerName, !customerName.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty { return customerName }
            if let customerEmail, !customerEmail.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty { return customerEmail }
            return "Licensed User"
        }
    }

    struct ActivationResponse: Decodable {
        let licenseToken: String
    }

    private struct ActivationRequest: Encodable {
        let licenseKey: String
        let installationID: String
        let appVersion: String
        let platform: String
    }

    private struct DeactivationRequest: Encodable {
        let licenseToken: String
        let installationID: String
    }

    enum LicensingError: LocalizedError {
        case configuration(String)
        case invalidLicense(String)
        case server(String)

        var errorDescription: String? {
            switch self {
            case .configuration(let message), .invalidLicense(let message), .server(let message): message
            }
        }
    }

    static let trialLengthDays = 30

    @Published private(set) var state: State = .trial(daysRemaining: trialLengthDays, expiresAt: Date().addingTimeInterval(TimeInterval(trialLengthDays * 86_400)))
    @Published private(set) var installationID = ""
    @Published private(set) var isWorking = false
    @Published var lastMessage: String?

    private let calendar = Calendar(identifier: .gregorian)
    private let firstLaunchAccount = "licensing.trial.firstLaunch"
    private let lastSeenAccount = "licensing.trial.lastSeen"
    private let installationAccount = "licensing.installationID"
    private let tokenAccount = "licensing.token"

    init() {
        refresh()
    }

    var canUseApp: Bool { state.canUseApp }

    var statusTitle: String {
        switch state {
        case .licensed: "Licensed"
        case .trial: "Free Trial"
        case .expired: "Trial Expired"
        case .invalid: "Licence Problem"
        }
    }

    var statusDetail: String {
        switch state {
        case .licensed(let payload):
            let expiry = payload.expiresAt.map { " · expires \($0.formatted(date: .abbreviated, time: .omitted))" } ?? " · perpetual"
            return "\(payload.edition) · \(payload.displayOwner)\(expiry)"
        case .trial(let days, let expiresAt):
            return "\(days) day\(days == 1 ? "" : "s") remaining · ends \(expiresAt.formatted(date: .abbreviated, time: .omitted))"
        case .expired:
            return "The 30-day evaluation period has ended. Activate a licence to continue using sync jobs."
        case .invalid(let message):
            return message
        }
    }

    func refresh(now: Date = Date()) {
        installationID = loadOrCreateInstallationID()

        if let token = storedLicenseToken, !token.isEmpty {
            do {
                let payload = try verify(token: token, now: now)
                state = .licensed(payload)
                recordLastSeen(now)
                return
            } catch {
                state = .invalid(error.localizedDescription)
                recordLastSeen(now)
                return
            }
        }

        state = trialState(now: now)
    }

    func installSignedToken(_ token: String) throws {
        let clean = token.trimmingCharacters(in: .whitespacesAndNewlines)
        guard !clean.isEmpty else { throw LicensingError.invalidLicense("Paste a signed Rsync Studio licence token.") }
        let payload = try verify(token: clean, now: Date())
        guard KeychainStore.set(clean, for: tokenAccount) == errSecSuccess else {
            throw LicensingError.invalidLicense("Rsync Studio could not save the licence securely in Keychain.")
        }
        state = .licensed(payload)
        lastMessage = "Licence installed successfully."
    }

    func activate(licenseKey: String) async throws {
        let key = licenseKey.trimmingCharacters(in: .whitespacesAndNewlines)
        guard !key.isEmpty else { throw LicensingError.invalidLicense("Enter your licence key.") }
        guard let baseURL = configuredAPIBaseURL else {
            throw LicensingError.configuration("Online activation is not configured in this build. A signed licence token can still be installed manually.")
        }

        isWorking = true
        defer { isWorking = false }

        let requestBody = ActivationRequest(
            licenseKey: key,
            installationID: installationID,
            appVersion: Bundle.main.infoDictionary?["CFBundleShortVersionString"] as? String ?? "Unknown",
            platform: "macOS"
        )
        let response: ActivationResponse = try await post(path: "v1/activate", body: requestBody, baseURL: baseURL)
        try installSignedToken(response.licenseToken)
        lastMessage = "Rsync Studio has been activated on this Mac."
    }

    func deactivate() async throws {
        guard let token = storedLicenseToken, !token.isEmpty else {
            KeychainStore.delete(tokenAccount)
            refresh()
            return
        }

        isWorking = true
        defer { isWorking = false }

        if let baseURL = configuredAPIBaseURL {
            let body = DeactivationRequest(licenseToken: token, installationID: installationID)
            let _: EmptyResponse = try await post(path: "v1/deactivate", body: body, baseURL: baseURL)
        }

        guard KeychainStore.delete(tokenAccount) == errSecSuccess else {
            throw LicensingError.invalidLicense("Rsync Studio could not remove the local licence from Keychain.")
        }
        refresh()
        lastMessage = "Licence removed from this Mac."
    }

    func removeInvalidLocalLicence() {
        KeychainStore.delete(tokenAccount)
        refresh()
        lastMessage = "Invalid local licence removed."
    }

    private var storedLicenseToken: String? {
        let token = KeychainStore.string(for: tokenAccount)
        return token.isEmpty ? nil : token
    }

    private var configuredPublicKey: String? {
        guard let value = Bundle.main.infoDictionary?["RsyncStudioLicensePublicKey"] as? String else { return nil }
        let trimmed = value.trimmingCharacters(in: .whitespacesAndNewlines)
        return trimmed.isEmpty ? nil : trimmed
    }

    private var configuredAPIBaseURL: URL? {
        guard let value = Bundle.main.infoDictionary?["RsyncStudioLicenseAPIBaseURL"] as? String else { return nil }
        let trimmed = value.trimmingCharacters(in: .whitespacesAndNewlines)
        guard !trimmed.isEmpty, let url = URL(string: trimmed), url.scheme?.lowercased() == "https" else { return nil }
        return url
    }

    private func loadOrCreateInstallationID() -> String {
        let existing = KeychainStore.string(for: installationAccount)
        if UUID(uuidString: existing) != nil { return existing }
        let value = UUID().uuidString.lowercased()
        _ = KeychainStore.set(value, for: installationAccount)
        return value
    }

    private func trialState(now: Date) -> State {
        let firstLaunch: Date
        let storedFirst = KeychainStore.string(for: firstLaunchAccount)
        if let parsed = Self.parseDate(storedFirst) {
            firstLaunch = parsed
        } else {
            firstLaunch = now
            _ = KeychainStore.set(Self.formatDate(now), for: firstLaunchAccount)
        }

        if let lastSeen = Self.parseDate(KeychainStore.string(for: lastSeenAccount)), now < lastSeen.addingTimeInterval(-300) {
            recordLastSeen(lastSeen)
            return .invalid("The system clock appears to have moved backwards. Correct the date and time, then reopen Rsync Studio.")
        }

        recordLastSeen(now)
        guard let expiresAt = calendar.date(byAdding: .day, value: Self.trialLengthDays, to: firstLaunch) else { return .expired }
        if now >= expiresAt { return .expired }

        let startOfToday = calendar.startOfDay(for: now)
        let expiryDay = calendar.startOfDay(for: expiresAt)
        let days = max(1, calendar.dateComponents([.day], from: startOfToday, to: expiryDay).day ?? 1)
        return .trial(daysRemaining: days, expiresAt: expiresAt)
    }

    private func recordLastSeen(_ date: Date) {
        let previous = Self.parseDate(KeychainStore.string(for: lastSeenAccount))
        if previous == nil || date > previous! {
            _ = KeychainStore.set(Self.formatDate(date), for: lastSeenAccount)
        }
    }

    private func verify(token: String, now: Date) throws -> LicensePayload {
        guard let publicKeyText = configuredPublicKey else {
            throw LicensingError.configuration("Licence verification is not configured in this build. Add RsyncStudioLicensePublicKey to the signed app configuration.")
        }

        let parts = token.split(separator: ".", omittingEmptySubsequences: false)
        guard parts.count == 3, parts[0] == "RS3" else {
            throw LicensingError.invalidLicense("The licence token format is invalid.")
        }
        guard let payloadData = Self.base64URLDecode(String(parts[1])),
              let signature = Self.base64URLDecode(String(parts[2])),
              let keyData = Data(base64Encoded: publicKeyText) ?? Self.base64URLDecode(publicKeyText) else {
            throw LicensingError.invalidLicense("The licence token or embedded public key is not valid Base64.")
        }

        let publicKey: Curve25519.Signing.PublicKey
        do { publicKey = try Curve25519.Signing.PublicKey(rawRepresentation: keyData) }
        catch { throw LicensingError.configuration("The embedded Rsync Studio licence public key is invalid.") }

        guard publicKey.isValidSignature(signature, for: payloadData) else {
            throw LicensingError.invalidLicense("The licence signature is invalid.")
        }

        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = .iso8601
        let payload: LicensePayload
        do { payload = try decoder.decode(LicensePayload.self, from: payloadData) }
        catch { throw LicensingError.invalidLicense("The signed licence payload is invalid.") }

        guard payload.installationID.caseInsensitiveCompare(installationID) == .orderedSame else {
            throw LicensingError.invalidLicense("This licence was activated for a different Rsync Studio installation.")
        }
        if let expiresAt = payload.expiresAt, now >= expiresAt {
            throw LicensingError.invalidLicense("This licence expired on \(expiresAt.formatted(date: .abbreviated, time: .omitted)).")
        }
        if let maxMajor = payload.maxMajorVersion,
           let currentMajor = Self.currentMajorVersion,
           currentMajor > maxMajor {
            throw LicensingError.invalidLicense("This licence covers Rsync Studio through major version \(maxMajor).")
        }
        return payload
    }

    private func post<Request: Encodable, Response: Decodable>(path: String, body: Request, baseURL: URL) async throws -> Response {
        let url = baseURL.appending(path: path)
        guard url.scheme?.lowercased() == "https" else { throw LicensingError.configuration("Licence API must use HTTPS.") }
        var request = URLRequest(url: url)
        request.httpMethod = "POST"
        request.timeoutInterval = 20
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.setValue("application/json", forHTTPHeaderField: "Accept")
        request.httpBody = try JSONEncoder().encode(body)

        let (data, response) = try await URLSession.shared.data(for: request)
        guard let http = response as? HTTPURLResponse else { throw LicensingError.server("The licence server returned an invalid response.") }
        guard (200..<300).contains(http.statusCode) else {
            let serverMessage = (try? JSONDecoder().decode(ServerError.self, from: data).message) ?? "Activation server returned HTTP \(http.statusCode)."
            throw LicensingError.server(serverMessage)
        }
        if Response.self == EmptyResponse.self, data.isEmpty {
            return EmptyResponse() as! Response
        }
        do { return try JSONDecoder().decode(Response.self, from: data) }
        catch { throw LicensingError.server("The licence server returned malformed data.") }
    }

    private struct ServerError: Decodable { let message: String }
    private struct EmptyResponse: Codable { init() {} }

    private static var currentMajorVersion: Int? {
        guard let version = Bundle.main.infoDictionary?["CFBundleShortVersionString"] as? String else { return nil }
        return Int(version.split(separator: ".").first ?? "")
    }

    private static func formatDate(_ date: Date) -> String { ISO8601DateFormatter().string(from: date) }
    private static func parseDate(_ value: String) -> Date? { value.isEmpty ? nil : ISO8601DateFormatter().date(from: value) }

    private static func base64URLDecode(_ value: String) -> Data? {
        var text = value.replacingOccurrences(of: "-", with: "+").replacingOccurrences(of: "_", with: "/")
        let remainder = text.count % 4
        if remainder != 0 { text += String(repeating: "=", count: 4 - remainder) }
        return Data(base64Encoded: text)
    }
}