using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Diagnostics;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

namespace FinlyticBackend.Middleware;

/// <summary>
/// Global fallback exception handler for the Gateway (Rules.md §11). Converts any unhandled exception that
/// escapes a controller action, hub method, or other endpoint into a standardized RFC 7807 Problem Details
/// response instead of an ad-hoc anonymous error object or a leaked stack trace.
/// Registered via <c>AddExceptionHandler&lt;GlobalExceptionHandler&gt;()</c> and activated by
/// <c>app.UseExceptionHandler()</c> in <c>Program.cs</c>. This only intercepts exceptions thrown while a
/// request is being handled; it has no effect on the fail-fast startup checks in <c>Program.cs</c>
/// (missing/weak <c>JWT:SecretKey</c>, missing <c>ADMIN:DefaultPassword</c>), which throw before
/// <c>builder.Build()</c> - i.e. before this middleware pipeline exists - and are intentionally left
/// unhandled so the host refuses to start.
/// </summary>
public sealed class GlobalExceptionHandler : IExceptionHandler
{
    private readonly ILogger<GlobalExceptionHandler> _logger;

    public GlobalExceptionHandler(ILogger<GlobalExceptionHandler> logger)
    {
        _logger = logger;
    }

    /// <inheritdoc />
    public async ValueTask<bool> TryHandleAsync(HttpContext httpContext, Exception exception, CancellationToken cancellationToken)
    {
        _logger.LogError(
            exception,
            "Unhandled exception while processing {Method} {Path}",
            httpContext.Request.Method,
            httpContext.Request.Path);

        httpContext.Response.StatusCode = StatusCodes.Status500InternalServerError;

        var problemDetailsService = httpContext.RequestServices.GetRequiredService<IProblemDetailsService>();
        return await problemDetailsService.TryWriteAsync(new ProblemDetailsContext
        {
            HttpContext = httpContext,
            Exception = exception,
            ProblemDetails = new ProblemDetails
            {
                Status = StatusCodes.Status500InternalServerError,
                Title = "An unexpected error occurred.",
                Detail = "The Gateway encountered an unexpected error while processing the request.",
                Type = "https://tools.ietf.org/html/rfc7231#section-6.6.1"
            }
        });
    }
}