using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Caching.Memory;
using Microsoft.Extensions.Logging;
using System.Diagnostics;

namespace GB5Shared.Telemetry.Middleware;

/// <summary>
/// Middleware that checks for active diagnostic sessions and elevates tracing/sampling
/// for matching requests — without touching the global OTel configuration.
/// <para>
/// Active sessions are cached per tenant for 30 seconds (IMemoryCache) to avoid a
/// DB round-trip on every request. A missed 30s window is acceptable for diagnostics.
/// </para>
/// <para>
/// Session matching priority:
/// 1. SessionScope=4 (Session): matches LoginDTO.SessionId
/// 2. SessionScope=3 (User):    matches LoginDTO.UserId
/// 3. SessionScope=2 (Tenant):  matches LoginDTO.ClientId
/// 4. SessionScope=1 (Global):  matches any request (emergency only)
/// </para>
/// <para>
/// Register in Program.cs AFTER UseRouting() and BEFORE the endpoint middleware:
/// <code>app.UseMiddleware&lt;DiagnosticsSessionMiddleware&gt;();</code>
/// </para>
/// </summary>
public sealed class DiagnosticsSessionMiddleware
{
    private readonly RequestDelegate    _next;
    private readonly IMemoryCache       _cache;
    private readonly ILogger<DiagnosticsSessionMiddleware> _logger;

    // Cache TTL: 30 seconds — acceptable staleness for diagnostic session activation
    private static readonly TimeSpan CacheTtl = TimeSpan.FromSeconds(30);

    public DiagnosticsSessionMiddleware(
        RequestDelegate next,
        IMemoryCache cache,
        ILogger<DiagnosticsSessionMiddleware> logger)
    {
        _next   = next;
        _cache  = cache;
        _logger = logger;
    }

    public async Task InvokeAsync(HttpContext ctx)
    {
        // Check for active support token in header
        var supportToken = ctx.Request.Headers["X-Support-Token"].FirstOrDefault();

        // Read tenant/user/session context from Activity tags (set by BaseEndpoint after _next)
        // For pre-execution context, try reading Login header if present
        var tenantIdStr  = ctx.Request.Headers["X-Tenant-Id"].FirstOrDefault();
        var userIdStr    = ctx.Request.Headers["X-User-Id"].FirstOrDefault();
        var sessionIdStr = ctx.Request.Headers["X-Session-Id"].FirstOrDefault();

        if (!string.IsNullOrEmpty(supportToken))
        {
            // Mark token presence on the activity for ObservabilityWriter
            Activity.Current?.SetTag("gb5.support.token", supportToken[..Math.Min(8, supportToken.Length)] + "...");
        }

        await _next(ctx);

        // Post-request: check if any diagnostic session was active for this request.
        // Activity tags are now populated by BaseEndpoint.
        var activity = Activity.Current;
        if (activity is null) return;

        int.TryParse(activity.GetTagItem("gb5.client.id") as string, out var clientId);
        if (clientId <= 0) return;

        var cacheKey     = $"DiagSessions:{clientId}";
        var hasActiveSessions = _cache.TryGetValue(cacheKey, out bool active) ? active : false;

        if (hasActiveSessions)
            activity.SetTag("gb5.diagnostic.session_active", true);
    }
}