Music/Music/Remote/RemoteClient.swift

import Foundation
import Network
import os

@MainActor
@Observable
final class RemoteClient: RemoteCommandSender {

    // MARK: - Public State

    var connectionState = ConnectionState.disconnected
    var discoveredHosts: [(name: String, endpoint: NWEndpoint)] = []
    var onPlaybackState: ((PlaybackStatePayload) -> Void)?
    var onDBReady: (() -> Void)?

    // MARK: - Private State

    private var browser: NWBrowser?
    private var commandTransport: NDJSONTransport?
    private var hostEndpoint: NWEndpoint?
    private var pingTimer: Timer?
    private var missedPings = 0

    private let logger = RemoteLogger.client

    // MARK: - Remote DB Path

    static var remoteDBPath: String {
        let appSupport = FileManager.default.urls(
            for: .applicationSupportDirectory, in: .userDomainMask
        ).first!.appendingPathComponent("Music", isDirectory: true)
        return appSupport.appendingPathComponent("remote_db.sqlite").path
    }

    // MARK: - Discovery

    /// Start scanning for `_musicremote._tcp` services on the local network.
    func startDiscovery() {
        let descriptor = NWBrowser.Descriptor.bonjour(type: "_musicremote._tcp", domain: nil)
        let browser = NWBrowser(for: descriptor, using: .tcp)

        browser.stateUpdateHandler = { [weak self] state in
            Task { @MainActor [weak self] in
                guard let self else { return }
                switch state {
                case .ready:
                    self.logger.info("Browser ready")
                case .failed(let error):
                    self.logger.error("Browser failed: \(error.localizedDescription)")
                case .cancelled:
                    self.logger.info("Browser cancelled")
                default:
                    break
                }
            }
        }

        browser.browseResultsChangedHandler = { [weak self] results, _ in
            Task { @MainActor [weak self] in
                guard let self else { return }
                self.discoveredHosts = results.compactMap { result in
                    if case .service(let name, _, _, _) = result.endpoint {
                        return (name: name, endpoint: result.endpoint)
                    }
                    return nil
                }
                self.logger.info("Discovered \(self.discoveredHosts.count) host(s)")
            }
        }

        browser.start(queue: .main)
        self.browser = browser
        transition(to: .discovering)
    }

    /// Stop scanning and clear discovered hosts.
    func stopDiscovery() {
        browser?.cancel()
        browser = nil
        discoveredHosts = []
    }

    // MARK: - Connection

    /// Connect to a discovered host: download its DB, then establish the command channel.
    func connect(to host: (name: String, endpoint: NWEndpoint)) {
        stopDiscovery()
        hostEndpoint = host.endpoint
        transition(to: .foundHost(host.name))
        transition(to: .downloadingDB)
        downloadDatabase(on: host.endpoint, hostName: host.name)
    }

    /// Tear down everything: command channel, timers, and the local remote DB file.
    func disconnect() {
        pingTimer?.invalidate()
        pingTimer = nil
        missedPings = 0
        commandTransport?.close()
        commandTransport = nil
        hostEndpoint = nil
        stopDiscovery()
        deleteRemoteDB()
        transition(to: .disconnected)
    }

    // MARK: - RemoteCommandSender

    func sendCommand(_ command: RemoteCommand) {
        guard let transport = commandTransport else {
            logger.warning("Cannot send command — no command channel")
            return
        }
        transport.send(command)
    }

    // MARK: - DB Download

    /// Open a TCP connection to the host, send `GET /db`, and save the response body to disk.
    private func downloadDatabase(on endpoint: NWEndpoint, hostName: String) {
        let connection = NWConnection(to: endpoint, using: .tcp)

        connection.stateUpdateHandler = { [weak self] state in
            Task { @MainActor [weak self] in
                guard let self else { return }
                switch state {
                case .ready:
                    self.logger.info("DB connection ready")
                    self.sendDBRequest(on: connection, hostName: hostName)
                case .failed(let error):
                    self.logger.error("DB connection failed: \(error.localizedDescription)")
                    self.transition(to: .disconnected)
                default:
                    break
                }
            }
        }

        connection.start(queue: .main)
    }

    /// Send the HTTP GET request for the database.
    private func sendDBRequest(on connection: NWConnection, hostName: String) {
        let request = "GET /db HTTP/1.1\r\nHost: \(hostName)\r\nConnection: close\r\n\r\n"
        connection.send(content: Data(request.utf8), completion: .contentProcessed { [weak self] error in
            Task { @MainActor [weak self] in
                guard let self else { return }
                if let error {
                    self.logger.error("Failed to send DB request: \(error.localizedDescription)")
                    self.transition(to: .disconnected)
                    return
                }
                self.receiveDBResponse(on: connection, accumulated: Data(), hostName: hostName)
            }
        })
    }

    /// Accumulate the full HTTP response for the DB download, then strip headers and save.
    private func receiveDBResponse(on connection: NWConnection, accumulated: Data, hostName: String) {
        connection.receive(minimumIncompleteLength: 1, maximumLength: 1_048_576) { [weak self] data, _, isComplete, error in
            Task { @MainActor [weak self] in
                guard let self else { return }

                var buffer = accumulated
                if let data {
                    buffer.append(data)
                }

                if isComplete || error != nil {
                    // Done receiving — extract body after HTTP headers
                    connection.cancel()
                    self.handleDBData(buffer, hostName: hostName)
                } else {
                    // Keep reading
                    self.receiveDBResponse(on: connection, accumulated: buffer, hostName: hostName)
                }
            }
        }
    }

    /// Strip the HTTP headers from the response and write the SQLite body to disk.
    private func handleDBData(_ data: Data, hostName: String) {
        // Find the header/body separator: \r\n\r\n
        let separator: [UInt8] = [0x0D, 0x0A, 0x0D, 0x0A]
        guard let separatorRange = data.range(of: Data(separator)) else {
            logger.error("DB response missing HTTP header separator")
            transition(to: .disconnected)
            return
        }

        let body = data[separatorRange.upperBound...]
        guard !body.isEmpty else {
            logger.error("DB response body is empty")
            transition(to: .disconnected)
            return
        }

        // Ensure the directory exists
        let dirURL = URL(fileURLWithPath: Self.remoteDBPath).deletingLastPathComponent()
        try? FileManager.default.createDirectory(at: dirURL, withIntermediateDirectories: true)

        do {
            try body.write(to: URL(fileURLWithPath: Self.remoteDBPath))
            logger.info("Database saved (\(body.count) bytes) to \(Self.remoteDBPath)")
            connectCommandChannel(hostName: hostName)
        } catch {
            logger.error("Failed to write DB: \(error.localizedDescription)")
            transition(to: .disconnected)
        }
    }

    // MARK: - Command Channel

    /// Open a second TCP connection to the same endpoint and upgrade it to the command channel.
    private func connectCommandChannel(hostName: String) {
        guard let endpoint = hostEndpoint else {
            logger.error("No endpoint stored for command channel")
            transition(to: .disconnected)
            return
        }

        transition(to: .connectingCommandChannel)

        let connection = NWConnection(to: endpoint, using: .tcp)

        connection.stateUpdateHandler = { [weak self] state in
            Task { @MainActor [weak self] in
                guard let self else { return }
                switch state {
                case .ready:
                    self.logger.info("Command channel TCP ready")
                    self.sendCmdRequest(on: connection, hostName: hostName)
                case .failed(let error):
                    self.logger.error("Command channel connection failed: \(error.localizedDescription)")
                    self.transition(to: .disconnected)
                default:
                    break
                }
            }
        }

        connection.start(queue: .main)
    }

    /// Send the HTTP GET request to upgrade to the command channel.
    private func sendCmdRequest(on connection: NWConnection, hostName: String) {
        let request = "GET /cmd HTTP/1.1\r\nHost: \(hostName)\r\nConnection: keep-alive\r\n\r\n"
        connection.send(content: Data(request.utf8), completion: .contentProcessed { [weak self] error in
            Task { @MainActor [weak self] in
                guard let self else { return }
                if let error {
                    self.logger.error("Failed to send cmd request: \(error.localizedDescription)")
                    self.transition(to: .disconnected)
                    return
                }
                self.receiveCmdHeader(on: connection, accumulated: Data(), hostName: hostName)
            }
        })
    }

    /// Read the HTTP 200 response header before upgrading to NDJSON streaming.
    private func receiveCmdHeader(on connection: NWConnection, accumulated: Data, hostName: String) {
        connection.receive(minimumIncompleteLength: 1, maximumLength: 65_536) { [weak self] data, _, _, error in
            Task { @MainActor [weak self] in
                guard let self else { return }

                if let error {
                    self.logger.error("Failed to read cmd header: \(error.localizedDescription)")
                    self.transition(to: .disconnected)
                    return
                }

                var buffer = accumulated
                if let data {
                    buffer.append(data)
                }

                // Check if we've received the full HTTP header
                let separator: [UInt8] = [0x0D, 0x0A, 0x0D, 0x0A]
                if buffer.range(of: Data(separator)) != nil {
                    self.commandChannelReady(connection: connection, hostName: hostName)
                } else {
                    // Keep reading until we get the full header
                    self.receiveCmdHeader(on: connection, accumulated: buffer, hostName: hostName)
                }
            }
        }
    }

    /// The command channel HTTP handshake is complete — set up NDJSON transport.
    private func commandChannelReady(connection: NWConnection, hostName: String) {
        let transport = NDJSONTransport(connection: connection, logger: logger)
        self.commandTransport = transport

        transport.onLine = { [weak self] line in
            self?.handleEventLine(line)
        }

        transport.onClose = { [weak self] in
            Task { @MainActor [weak self] in
                guard let self else { return }
                self.logger.info("Command channel closed by host")
                self.pingTimer?.invalidate()
                self.pingTimer = nil
                self.commandTransport = nil
                if self.connectionState.isConnected {
                    self.transition(to: .connectionLost("Host closed connection"))
                }
            }
        }

        transport.startReceiving()

        // Send handshake
        let appVersion = Bundle.main.infoDictionary?["CFBundleShortVersionString"] as? String ?? "unknown"
        let handshake = HandshakeMessage(protocolVersion: RemoteProtocolVersion, appVersion: appVersion)
        transport.send(handshake)

        // Start keep-alive ping timer
        missedPings = 0
        pingTimer = Timer.scheduledTimer(withTimeInterval: 5.0, repeats: true) { [weak self] _ in
            Task { @MainActor [weak self] in
                guard let self else { return }
                self.missedPings += 1
                if self.missedPings >= 3 {
                    self.logger.warning("Keep-alive timeout — \(self.missedPings) consecutive pings unanswered")
                    self.pingTimer?.invalidate()
                    self.pingTimer = nil
                    self.commandTransport?.close()
                    self.commandTransport = nil
                    self.transition(to: .connectionLost("Host not responding"))
                }
            }
        }

        transition(to: .connected(hostName))
        logger.info("Command channel established with \(hostName)")
    }

    // MARK: - Event Handling

    /// Parse an incoming NDJSON line as a `HostEvent` and dispatch to the appropriate callback.
    private func handleEventLine(_ line: String) {
        guard let data = line.data(using: .utf8) else { return }
        let decoder = JSONDecoder()

        do {
            let event = try decoder.decode(HostEvent.self, from: data)
            switch event {
            case .playbackState(let payload):
                missedPings = 0
                onPlaybackState?(payload)
            case .dbReady:
                onDBReady?()
            case .error(let message):
                logger.error("Host error: \(message)")
            }
        } catch {
            logger.error("Failed to decode host event: \(error.localizedDescription)")
        }
    }

    // MARK: - State Machine

    /// Transition to a new connection state, validating via `canTransition(to:)`.
    private func transition(to newState: ConnectionState) {
        let oldState = connectionState
        guard oldState != newState else { return }

        guard oldState.canTransition(to: newState) else {
            logger.warning("Invalid transition: \(String(describing: oldState)) → \(String(describing: newState))")
            return
        }

        logger.info("State: \(String(describing: oldState)) → \(String(describing: newState))")
        connectionState = newState
    }

    // MARK: - Helpers

    /// Delete the local remote database file if it exists.
    private func deleteRemoteDB() {
        let path = Self.remoteDBPath
        if FileManager.default.fileExists(atPath: path) {
            do {
                try FileManager.default.removeItem(atPath: path)
                logger.info("Deleted remote DB at \(path)")
            } catch {
                logger.error("Failed to delete remote DB: \(error.localizedDescription)")
            }
        }
    }
}