//
//  SDLDNSClient 2.swift
//  punchnet
//
//  Created by 安礼成 on 2026/4/9.
//
import Foundation
import Network

enum SDLIPV6AssistError: Error {
    case lostConnection
    case requestTimeout
}

actor SDLIPV6AssistClient {
    private struct PendingRequest {
        let continuation: CheckedContinuation<SDLV6AssistProbeReply, Error>
        let timeoutTask: Task<Void, Never>
    }
    
    private enum State {
        case idle
        case running
        case stopped
    }
    
    private var state: State = .idle
    private var connection: NWConnection?
    private var receiveTask: Task<Void, Never>?
    private let assistServerAddress: NWEndpoint
    
    private var packetId: UInt32 = 1
    private var pendingRequests: [UInt32: PendingRequest] = [:]

    init?(assistServerInfo: SDLV6Info) {
        guard assistServerInfo.port <= UInt32(UInt16.max), let host = SDLUtil.ipv6DataToString(assistServerInfo.v6) else {
            return nil
        }
        self.assistServerAddress = .hostPort(host: NWEndpoint.Host(host), port: NWEndpoint.Port(integerLiteral: UInt16(assistServerInfo.port)))
    }
    
    func start() {
        guard case .idle = self.state else {
            return
        }
        
        self.state = .running
        
        // 1. 配置参数：这是解决环路的关键
        let parameters = NWParameters.udp
        
        // 禁止此连接走 TUN 网卡（在 NE 中 TUN 通常被归类为 .other）
        parameters.prohibitedInterfaceTypes = [.other]
        // 2. 增强健壮性：启用多路径切换（替代 pathSelectionOptions 的意图）
        parameters.multipathServiceType = .handover
        
        // 只允许走 IPv6，避免在 assist 通道上退回到 IPv4 或双栈协商。
        if let ipOptions = parameters.defaultProtocolStack.internetProtocol as? NWProtocolIP.Options {
            ipOptions.version = .v6
        }
        
        // 2. 创建连接
        let connection = NWConnection(to: self.assistServerAddress, using: parameters)
        self.connection = connection
        
        connection.stateUpdateHandler = { [weak self] state in
            Task {
                await self?.handleConnectionStateUpdate(state, for: connection)
            }
        }
        
        // 启动连接队列
        connection.start(queue: .global())
    }
    
    /// 接收数据的递归循环
    private static func makeReceiveStream(for connection: NWConnection) -> AsyncStream<Data> {
        return AsyncStream(bufferingPolicy: .bufferingNewest(256)) { continuation in
            func receiveNext() {
                connection.receiveMessage { content, _, _, error in
                    if let data = content, !data.isEmpty {
                        // 将收到的 DNS 响应写回 AsyncStream
                        continuation.yield(data)
                    }
                    
                    if error == nil && connection.state == .ready {
                        receiveNext() // 继续监听下一个包
                    } else {
                        continuation.finish()
                    }
                }
            }
            
            receiveNext()
        }
    }
    
    func probe(requestTimeout: Duration = .seconds(5)) async throws -> SDLV6AssistProbeReply {
        guard case .running = self.state, let connection = self.connection, connection.state == .ready else {
            throw SDLIPV6AssistError.lostConnection
        }
        
        let pktId = self.nextPacketId()
        var assistProbe = SDLV6AssistProbe()
        assistProbe.pktID = pktId
        let data = try assistProbe.serializedData()
        
        return try await withCheckedThrowingContinuation { cont in
            let timeoutTask = Task { [weak self] in
                try? await Task.sleep(for: requestTimeout)
                await self?.handleRequestTimeout(packetId: pktId)
            }
            
            self.pendingRequests[pktId] = .init(continuation: cont, timeoutTask: timeoutTask)
            connection.send(content: data, completion: .contentProcessed { error in
                if let error {
                    Task {
                        await self.handleProcessError(packetId: pktId, error: error)
                    }
                }
            })
        }
        
    }
    
    private func handleProcessError(packetId: UInt32, error: NWError) {
        if let request = self.takePendingRequest(packetId: packetId) {
            request.continuation.resume(throwing: error)
        }
    }
    
    private func handleRequestTimeout(packetId: UInt32) {
        if let request = self.takePendingRequest(packetId: packetId) {
            request.continuation.resume(throwing: SDLIPV6AssistError.requestTimeout)
        }
    }
    
    func stop() {
        self.stop(pendingError: SDLIPV6AssistError.lostConnection)
    }
    
    private func stop(pendingError: any Error) {
        guard self.state != .stopped else {
            return
        }
        
        self.state = .stopped
        self.receiveTask?.cancel()
        self.receiveTask = nil
        self.connection?.cancel()
        self.connection = nil
        self.failAllPendingRequests(error: pendingError)
    }
    
    private func handleConnectionStateUpdate(_ state: NWConnection.State, for connection: NWConnection) {
        guard case .running = self.state else {
            return
        }
        
        switch state {
        case .ready:
            SDLLogger.log("[SDLIPV6AssistClient] Connection ready", for: .debug)
            self.startReceiveTask(for: connection)
        case .failed(let error):
            SDLLogger.log("[SDLIPV6AssistClient] Connection failed: \(error)", for: .debug)
            self.stop(pendingError: error)
        case .cancelled:
            self.stop()
        default:
            break
        }
    }
    
    private func startReceiveTask(for connection: NWConnection) {
        guard self.receiveTask == nil else {
            return
        }
        
        let stream = Self.makeReceiveStream(for: connection)
        self.receiveTask = Task { [weak self] in
            for await data in stream {
                guard let self else {
                    break
                }
                await self.handleReceivedPacket(data)
            }
            
            await self?.didFinishReceiving(for: connection)
        }
    }
    
    private func handleReceivedPacket(_ data: Data) {
        do {
            let packet = try SDLV6AssistProbeReply(serializedBytes: data)
            let pktId = packet.pktID
            if let request = self.takePendingRequest(packetId: pktId) {
                request.continuation.resume(returning: packet)
            }
        } catch {
            SDLLogger.log("[SDLIPV6AssistClient] Receive error: \(error)", for: .debug)
        }
    }
    
    private func didFinishReceiving(for connection: NWConnection) {
        guard case .running = self.state else {
            return
        }
        
        if self.connection === connection, connection.state != .ready {
            self.stop()
        } else {
            self.receiveTask = nil
        }
    }
    
    private func nextPacketId() -> UInt32 {
        let packetId = self.packetId
        self.packetId &+= 1
        
        return packetId
    }
    
    private func takePendingRequest(packetId: UInt32) -> PendingRequest? {
        guard let request = self.pendingRequests.removeValue(forKey: packetId) else {
            return nil
        }
        
        request.timeoutTask.cancel()
        return request
    }
    
    private func failAllPendingRequests(error: any Error) {
        let pendingRequests = self.pendingRequests
        self.pendingRequests.removeAll()
        
        pendingRequests.values.forEach { request in
            request.timeoutTask.cancel()
            request.continuation.resume(throwing: error)
        }
    }
    
    deinit {
        self.connection?.cancel()
    }
    
}