using System.Net.Http.Headers;
using System.Net.WebSockets;
using System.Text;
using System.Text.Json;
using Microsoft.Extensions.Options;

namespace Nexus.Api.Services;

/// <summary>
/// BackgroundService that maintains a persistent WebSocket connection to the
/// OpenClaw Gateway for real-time event streaming and health monitoring.
///
/// Uses exponential backoff for reconnects, never logs the gateway token,
/// and reports connection state via <see cref="IGatewayConnector"/>.
/// </summary>
public sealed class GatewayConnector : BackgroundService, IGatewayConnector
{
    private readonly IHttpClientFactory _httpClientFactory;
    private readonly IConfiguration _configuration;
    private readonly ILogger<GatewayConnector> _logger;
    private readonly GatewayConnectorOptions _options;

    // ── Connection state (lock-protected) ──
    private readonly object _lock = new();
    private GatewayConnectionState _state = GatewayConnectionState.Initializing;
    private string? _gatewayVersion;
    private string? _requiredVersion;
    private DateTimeOffset? _lastConnectedAt;
    private int _reconnectAttempts;
    private string? _statusMessage;

    public GatewayConnector(
        IHttpClientFactory httpClientFactory,
        IConfiguration configuration,
        IOptions<GatewayConnectorOptions> options,
        ILogger<GatewayConnector> logger)
    {
        _httpClientFactory = httpClientFactory;
        _configuration = configuration;
        _logger = logger;
        _options = options.Value;
    }

    // ── IGatewayConnector ──

    public GatewayConnectionState ConnectionState
    {
        get { lock (_lock) return _state; }
    }

    public string? GatewayVersion
    {
        get { lock (_lock) return _gatewayVersion; }
    }

    public string? RequiredVersion
    {
        get { lock (_lock) return _requiredVersion; }
    }

    public DateTimeOffset? LastConnectedAt
    {
        get { lock (_lock) return _lastConnectedAt; }
    }

    public int ReconnectAttempts
    {
        get { lock (_lock) return _reconnectAttempts; }
    }

    public string? StatusMessage
    {
        get { lock (_lock) return _statusMessage; }
    }

    // ── BackgroundService ──

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        // Load version pin from configuration once at startup
        var pinnedVersion = _configuration["Integrations:OpenClaw:RequiredVersion"];
        lock (_lock)
        {
            _requiredVersion = string.IsNullOrWhiteSpace(pinnedVersion) ? null : pinnedVersion.Trim();
        }

        if (_requiredVersion is null)
        {
            _logger.LogWarning(
                "Gateway version is UNPINNED (Integrations:OpenClaw:RequiredVersion is empty). "
                + "Drift detection is disabled; set a required version to enable fail-fast on mismatch.");
        }

        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                await ConnectAndReceiveAsync(stoppingToken);
            }
            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
            {
                break;
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "GatewayConnector connection loop exception (will retry)");
            }

            // Exponential backoff before next reconnect attempt
            if (!stoppingToken.IsCancellationRequested)
            {
                var delay = ComputeBackoff();
                _logger.LogInformation(
                    "GatewayConnector reconnecting in {DelayMs}ms (attempt {Attempt})",
                    (int)delay.TotalMilliseconds, GetReconnectAttempts() + 1);

                SetState(GatewayConnectionState.Reconnecting);

                try { await Task.Delay(delay, stoppingToken); }
                catch (OperationCanceledException) { break; }
            }
        }

        _logger.LogInformation("GatewayConnector background service stopped");
    }

    private async Task ConnectAndReceiveAsync(CancellationToken stoppingToken)
    {
        using var ws = new ClientWebSocket();
        ConfigureWebSocketWithAuth(ws);

        var baseUrl = _configuration["Integrations:OpenClaw:BaseUrl"] ?? "http://127.0.0.1:18789";
        var wsBase = ConvertToWebSocketUrl(baseUrl);

        // ── Step 1: Pre-flight version check via HTTP ──
        string? detectedVersion = null;
        try
        {
            detectedVersion = await FetchGatewayVersionAsync(stoppingToken);
        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex, "Pre-flight version check failed (non-fatal)");
        }

        // Version check against the pin
        var (versionOk, versionWarning) = EvaluateVersion(detectedVersion);

        if (!versionOk)
        {
            if (_options.FailFastOnVersionMismatch)
            {
                _logger.LogError(
                    "Gateway version mismatch — fail-fast enabled. Detected: {Detected}, Required: {Required}",
                    detectedVersion ?? "none", _requiredVersion);

                IncrementReconnectAttempts();
                SetState(GatewayConnectionState.Failed,
                    $"Version mismatch — fail-fast: detected '{detectedVersion}', required '{_requiredVersion}'.");
                return;
            }

            _logger.LogWarning("Gateway version mismatch (non-fatal): {Warning}", versionWarning);
        }

        // ── Step 2: Establish WebSocket connection ──
        var wsUri = new Uri(new Uri(wsBase), _options.WebSocketPath);
        _logger.LogInformation("GatewayConnector connecting to {Uri}", wsUri);

        try
        {
            await ws.ConnectAsync(wsUri, stoppingToken);
        }
        catch (WebSocketException ex)
        {
            _logger.LogWarning(ex, "WebSocket connection failed to {Uri} — gateway may not expose a WS endpoint", wsUri);
            IncrementReconnectAttempts();
            SetState(GatewayConnectionState.Disconnected,
                $"Connection refused — WebSocket endpoint may not be available at {wsUri}. Error: {ex.Message}");
            return;
        }
        catch (HttpRequestException ex)
        {
            _logger.LogWarning(ex, "WebSocket HTTP upgrade failed to {Uri}", wsUri);
            IncrementReconnectAttempts();
            SetState(GatewayConnectionState.Disconnected,
                $"HTTP upgrade failed: {ex.Message}");
            return;
        }

        // ── Step 3: Submit authentication token (if configured) ──
        var token = ResolveToken();
        if (token is not null)
        {
            try
            {
                var authFrame = Encoding.UTF8.GetBytes(
                    JsonSerializer.Serialize(new { type = "auth", token }));
                await ws.SendAsync(
                    new ArraySegment<byte>(authFrame),
                    WebSocketMessageType.Text,
                    endOfMessage: true,
                    cancellationToken: stoppingToken);

                _logger.LogDebug("WebSocket auth token sent");
            }
            catch (Exception ex)
            {
                _logger.LogWarning(ex, "Failed to send WebSocket auth frame");
            }
        }

        // ── After successful connection ──
        lock (_lock)
        {
            _state = GatewayConnectionState.Connected;
            _gatewayVersion = detectedVersion;
            _lastConnectedAt = DateTimeOffset.UtcNow;
            _reconnectAttempts = 0;
            _statusMessage = versionWarning;
        }

        _logger.LogInformation(
            "GatewayConnector connected. Version: {Version}, Required: {Required}",
            detectedVersion ?? "unknown", _requiredVersion ?? "unpinned");

        // ── Step 4: Receive loop (heartbeat / event processing) ──
        await ReceiveLoopAsync(ws, stoppingToken);
    }

    private async Task ReceiveLoopAsync(ClientWebSocket ws, CancellationToken stoppingToken)
    {
        var buffer = new byte[4096];

        try
        {
            while (ws.State == WebSocketState.Open && !stoppingToken.IsCancellationRequested)
            {
                var result = await ws.ReceiveAsync(new ArraySegment<byte>(buffer), stoppingToken);

                if (result.MessageType == WebSocketMessageType.Close)
                {
                    _logger.LogInformation(
                        "GatewayConnector received close frame: {Description}",
                        result.CloseStatusDescription ?? "no description");
                    break;
                }

                // Process text frames (events, heartbeats, status updates)
                if (result.MessageType == WebSocketMessageType.Text)
                {
                    var message = Encoding.UTF8.GetString(buffer, 0, result.Count);
                    ProcessGatewayMessage(message);
                }
            }
        }
        catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
        {
            // Expected on shutdown
        }
        catch (WebSocketException ex)
        {
            _logger.LogWarning(ex, "WebSocket connection lost");
        }
        finally
        {
            if (ws.State == WebSocketState.Open || ws.State == WebSocketState.CloseReceived)
            {
                try
                {
                    await ws.CloseAsync(
                        WebSocketCloseStatus.NormalClosure, "Connector shutdown", CancellationToken.None);
                }
                catch { /* Best effort */ }
            }

            SetState(GatewayConnectionState.Disconnected, "Connection closed");
        }
    }

    private void ProcessGatewayMessage(string message)
    {
        try
        {
            using var doc = JsonDocument.Parse(message);
            var root = doc.RootElement;
            var type = root.TryGetProperty("type", out var typeEl) ? typeEl.GetString() : null;

            switch (type)
            {
                case "heartbeat":
                case "pong":
                    _logger.LogDebug("Gateway heartbeat received");
                    break;

                case "event":
                    var eventName = root.TryGetProperty("name", out var nameEl)
                        ? nameEl.GetString() : "unknown";
                    _logger.LogDebug("Gateway event: {EventName}", eventName);
                    // Future: fan-out to other services via a channel/event bus
                    break;

                case "error":
                    var errorMsg = root.TryGetProperty("message", out var msgEl)
                        ? msgEl.GetString() : "unknown error";
                    _logger.LogWarning("Gateway error frame received: {Error}", errorMsg);
                    break;

                default:
                    _logger.LogDebug("Gateway message (type={Type}): {Preview}",
                        type ?? "none", Truncate(message, 120));
                    break;
            }
        }
        catch (JsonException)
        {
            _logger.LogDebug("Non-JSON gateway message: {Preview}", Truncate(message, 80));
        }
    }

    // ── Helpers ──

    private (bool Ok, string? Warning) EvaluateVersion(string? detectedVersion)
    {
        if (_requiredVersion is null)
            return (true, null); // Unpinned — always ok

        if (detectedVersion is null)
            return (_options.FailFastOnMissingVersion ? false : true,
                "Gateway version not detected while version pin is set.");

        if (!string.Equals(detectedVersion, _requiredVersion, StringComparison.OrdinalIgnoreCase))
            return (false, $"Version drift: detected '{detectedVersion}', required '{_requiredVersion}'.");

        return (true, null);
    }

    private async Task<string?> FetchGatewayVersionAsync(CancellationToken ct)
    {
        var client = _httpClientFactory.CreateClient("gateway");
        try
        {
            using var request = new HttpRequestMessage(HttpMethod.Get, "/health");
            ApplyAuth(request);

            using var response = await client.SendAsync(request, ct);
            if (!response.IsSuccessStatusCode)
                return null;

            // Check X-OpenClaw-Version header first
            if (response.Headers.TryGetValues("X-OpenClaw-Version", out var headerValues))
            {
                var version = headerValues.FirstOrDefault();
                if (!string.IsNullOrWhiteSpace(version))
                    return version.Trim();
            }

            // Try JSON body
            var body = await response.Content.ReadAsStringAsync(ct);
            if (!string.IsNullOrWhiteSpace(body))
            {
                try
                {
                    using var doc = JsonDocument.Parse(body);
                    var root = doc.RootElement;
                    var v = TryGetString(root, "version")
                         ?? TryGetString(root, "gatewayVersion")
                         ?? TryGetString(root, "openclawVersion");
                    if (v is not null) return v;
                }
                catch { /* Not JSON */ }
            }

            return null;
        }
        catch
        {
            return null;
        }
    }

    private TimeSpan ComputeBackoff()
    {
        var attempts = GetReconnectAttempts();
        var backoffMs = (int)Math.Min(
            _options.ReconnectInitialDelayMs * Math.Pow(2, attempts),
            _options.ReconnectMaxDelayMs);

        // Add jitter (±25%)
        var jitter = (int)(backoffMs * 0.25 * (Random.Shared.NextDouble() * 2 - 1));
        return TimeSpan.FromMilliseconds(Math.Max(100, backoffMs + jitter));
    }

    private void IncrementReconnectAttempts()
    {
        lock (_lock) { _reconnectAttempts++; }
    }

    private int GetReconnectAttempts()
    {
        lock (_lock) { return _reconnectAttempts; }
    }

    private void SetState(GatewayConnectionState state, string? message = null)
    {
        lock (_lock)
        {
            _state = state;
            if (message is not null) _statusMessage = message;
        }
    }

    private string? ResolveToken()
    {
        // Token NEVER logged — only read from config here
        var token = _configuration["Integrations:OpenClaw:Password"]
            ?? _configuration["Integrations:OpenClaw:Token"];

        return string.IsNullOrWhiteSpace(token) ? null : token;
    }

    private void ApplyAuth(HttpRequestMessage request)
    {
        var token = ResolveToken();
        if (token is not null)
            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
    }

    private static void ConfigureWebSocket(ClientWebSocket ws)
    {
        // Add auth header to the initial HTTP upgrade request
        ws.Options.KeepAliveInterval = TimeSpan.FromSeconds(30);
        ws.Options.UseDefaultCredentials = false;
    }

    // Overload with auth header via cookie (WebSocket can't do Authorization header natively in all runtimes)
    private void ConfigureWebSocketWithAuth(ClientWebSocket ws)
    {
        var token = ResolveToken();
        if (token is not null)
        {
            ws.Options.SetRequestHeader("Authorization", "Bearer " + token);
        }
    }

    private static string ConvertToWebSocketUrl(string baseUrl)
    {
        var trimmed = baseUrl.TrimEnd('/');
        if (trimmed.StartsWith("https://", StringComparison.OrdinalIgnoreCase))
            return "wss://" + trimmed["https://".Length..];
        if (trimmed.StartsWith("http://", StringComparison.OrdinalIgnoreCase))
            return "ws://" + trimmed["http://".Length..];
        return "ws://" + trimmed;
    }

    private static string? TryGetString(JsonElement root, string property)
        => root.ValueKind == JsonValueKind.Object
        && root.TryGetProperty(property, out var value)
        && value.ValueKind == JsonValueKind.String
            ? value.GetString()
            : null;

    private static string Truncate(string value, int maxLength)
        => value.Length <= maxLength ? value : value[..maxLength] + "\u2026";
}

/// <summary>
/// Configuration options for the GatewayConnector background service.
/// </summary>
public sealed class GatewayConnectorOptions
{
    public const string SectionName = "GatewayConnector";

    /// <summary>
    /// WebSocket path relative to the gateway base URL.
    /// Default: "/ws"
    /// </summary>
    public string WebSocketPath { get; set; } = "/ws";

    /// <summary>
    /// Initial reconnect delay in milliseconds.
    /// Default: 1000 (1 second)
    /// </summary>
    public int ReconnectInitialDelayMs { get; set; } = 1000;

    /// <summary>
    /// Maximum reconnect delay in milliseconds.
    /// Default: 300_000 (5 minutes)
    /// </summary>
    public int ReconnectMaxDelayMs { get; set; } = 300_000;

    /// <summary>
    /// Maximum number of consecutive reconnect attempts before entering Failed state.
    /// Default: 0 (no limit — keep retrying forever)
    /// </summary>
    public int ReconnectMaxAttempts { get; set; }

    /// <summary>
    /// When true, the connector enters Failed state immediately if the gateway version
    /// doesn't match the pinned required version (instead of connecting with a warning).
    /// Default: true
    /// </summary>
    public bool FailFastOnVersionMismatch { get; set; } = true;

    /// <summary>
    /// When true, missing version (gateway doesn't report one) with a version pin set
    /// also triggers fail-fast. Default: false (allows pinned-version gateways that
    /// don't expose a version endpoint).
    /// </summary>
    public bool FailFastOnMissingVersion { get; set; }
}