import Foundation

/// Connects to the feishu-app change-notification service via Server-Sent
/// Events (SSE). When the assignee of a tracked Bitable record changes, the
/// server pushes a `change` event down this stream; Bugger responds by
/// triggering `PollerService.fetchNow()` so the bug list refreshes instantly.
///
/// The stream URL is built from `AppConfig.feishuAppBaseURL`:
/// `{feishuAppBaseURL}/api/v1/bitable/events?file_token=...&assignee_field=...&assignee_name=...`
///
/// When `feishuAppBaseURL` is empty the service stays disabled and no
/// network requests are made — polling remains the only refresh mechanism.
final class BitableEventService {
    static let shared = BitableEventService()

    private var session: URLSession?
    private var task: URLSessionDataTask?
    private var delegate: SSESessionDelegate?
    private var reconnectWorkItem: DispatchWorkItem?

    /// The user's intent to be connected. Distinguishes an unexpected stream
    /// drop (should auto-reconnect) from an explicit `disconnect()` (should not).
    private var isEnabled = false

    private let reconnectDelay: TimeInterval = 30

    private init() {}

    // MARK: - Public

    /// Open the SSE connection. Safe to call repeatedly — a no-op if already
    /// enabled, and a no-op if `feishuAppBaseURL` is not configured.
    func connect() {
        guard !isEnabled else { return }
        guard let url = buildURL() else {
            BuggerLog.info("BitableEventService: subscribe base URL not configured, skipping SSE")
            return
        }
        isEnabled = true
        openStream(at: url)
    }

    /// Close the SSE connection. No auto-reconnect will be attempted.
    func disconnect() {
        isEnabled = false
        closeStream()
    }

    /// Apply the current config: tear down any existing stream, then connect
    /// if the subscribe base URL is set. Call after the user changes the URL.
    func reconnect() {
        disconnect()
        connect()
    }

    // MARK: - Private

    private func openStream(at url: URL) {
        closeStream()

        let config = URLSessionConfiguration.ephemeral
        // The server sends a heartbeat every ~30s, so 5 min of silence is a
        // safe "something went wrong" threshold.
        config.timeoutIntervalForRequest = 300
        config.timeoutIntervalForResource = .infinity
        config.waitsForConnectivity = true

        let delegate = SSESessionDelegate(
            onEvent: { [weak self] event, data in
                self?.handleEvent(event, data: data)
            },
            onCompletion: { [weak self] error in
                self?.handleStreamEnd(error: error)
            }
        )
        self.delegate = delegate

        let session = URLSession(configuration: config, delegate: delegate, delegateQueue: nil)
        self.session = session

        var request = URLRequest(url: url)
        request.setValue("text/event-stream", forHTTPHeaderField: "Accept")
        request.timeoutInterval = 300

        let task = session.dataTask(with: request)
        self.task = task
        task.resume()

        BuggerLog.info("BitableEventService: connecting to \(url.absoluteString)")
    }

    private func closeStream() {
        reconnectWorkItem?.cancel()
        reconnectWorkItem = nil
        task?.cancel()
        task = nil
        session?.invalidateAndCancel()
        session = nil
        delegate = nil
    }

    private func handleStreamEnd(error: Error?) {
        task = nil
        session = nil
        delegate = nil

        if let error {
            BuggerLog.error("BitableEventService: stream ended (\(error.localizedDescription))")
        } else {
            BuggerLog.info("BitableEventService: stream ended")
        }

        // Only auto-reconnect if the user still wants to be connected.
        guard isEnabled else { return }

        let work = DispatchWorkItem { [weak self] in
            guard let self, self.isEnabled else { return }
            guard let url = self.buildURL() else { return }
            BuggerLog.info("BitableEventService: reconnecting...")
            self.openStream(at: url)
        }
        reconnectWorkItem = work
        DispatchQueue.main.asyncAfter(deadline: .now() + reconnectDelay, execute: work)
    }

    private func handleEvent(_ event: String, data: String) {
        switch event {
        case "change":
            BuggerLog.info("BitableEventService: change push received (\(data)), refreshing now")
            Task { await PollerService.shared.fetchNow() }
        case "error":
            BuggerLog.error("BitableEventService: server error \(data)")
        case "connected", "heartbeat":
            break
        default:
            BuggerLog.debug("BitableEventService: unknown event \(event)")
        }
    }

    private func buildURL() -> URL? {
        guard let config = AppStateService.shared.config,
              config.isConfigured,
              !config.feishuAppBaseURL.isEmpty else {
            return nil
        }

        let assigneeName = config.assigneeName
        let assigneeField = config.fieldMappings.assigneeField

        guard !assigneeName.isEmpty, !assigneeField.isEmpty else {
            BuggerLog.info("BitableEventService: assignee name or field not configured")
            return nil
        }

        let base = config.feishuAppBaseURL.trimmingCharacters(in: CharacterSet(charactersIn: "/"))
        guard var comps = URLComponents(string: "\(base)/api/v1/bitable/events") else {
            return nil
        }
        comps.queryItems = [
            URLQueryItem(name: "file_token", value: config.appToken),
            URLQueryItem(name: "assignee_field", value: assigneeField),
            URLQueryItem(name: "assignee_name", value: assigneeName),
        ]
        return comps.url
    }
}

// MARK: - SSE parsing

private final class SSESessionDelegate: NSObject, URLSessionDataDelegate {
    private let onEvent: (String, String) -> Void
    private let onCompletion: (Error?) -> Void
    private var buffer = ""

    init(onEvent: @escaping (String, String) -> Void, onCompletion: @escaping (Error?) -> Void) {
        self.onEvent = onEvent
        self.onCompletion = onCompletion
    }

    func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
        guard let chunk = String(data: data, encoding: .utf8) else { return }
        // Normalize line endings: SSE allows \n, \r, or \r\n.
        buffer.append(chunk.replacingOccurrences(of: "\r\n", with: "\n")
                           .replacingOccurrences(of: "\r", with: "\n"))

        // SSE frames are delimited by a blank line (\n\n).
        while let range = buffer.range(of: "\n\n") {
            let frame = String(buffer[..<range.lowerBound])
            buffer.removeSubrange(..<range.upperBound)
            processFrame(frame)
        }
    }

    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        // Flush any trailing frame that wasn't followed by a blank line.
        if !buffer.isEmpty {
            processFrame(buffer)
            buffer = ""
        }
        // Route back to the main queue so BitableEventService state is only
        // ever touched from one thread.
        DispatchQueue.main.async { [self] in
            self.onCompletion(error)
        }
    }

    private func processFrame(_ frame: String) {
        var eventType = ""
        var data = ""

        for line in frame.split(separator: "\n", omittingEmptySubsequences: false) {
            if line.hasPrefix("event:") {
                eventType = String(line.dropFirst("event:".count)).trimmingCharacters(in: .whitespaces)
            } else if line.hasPrefix("data:") {
                data = String(line.dropFirst("data:".count)).trimmingCharacters(in: .whitespaces)
            }
            // Lines starting with ":" are comments (heartbeats) — ignored.
        }

        // Frames without an event type (e.g. heartbeat-only frames) are skipped.
        guard !eventType.isEmpty else { return }

        DispatchQueue.main.async { [self] in
            self.onEvent(eventType, data)
        }
    }
}