using System.Text;
using System.Xml;
using System.Xml.Linq;
using System.Text.Encodings.Web;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Formatters;
using Microsoft.Net.Http.Headers;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddSingleton(DemoAlert.Load());
builder.Services.AddControllers(options =>
{
    options.RespectBrowserAcceptHeader = true;
    options.ReturnHttpNotAcceptable = true;
    // This standalone app serves only these two representations.
    options.OutputFormatters.Clear();
    options.OutputFormatters.Add(new AlertFormatter("application/cap+xml"));
    options.OutputFormatters.Add(new AlertFormatter("text/html"));
});
var app = builder.Build();
app.Use(async (context, next) =>
{
    context.Response.Headers.Append("Vary", "Accept");
    context.Response.Headers.Append("X-Content-Type-Options", "nosniff");
    context.Response.Headers.Append("Cache-Control", "no-store");
    // Normalize supported candidates using the framework parser. This prevents
    // MVC fallback from reviving q=0 types, including exclusions under */*.
    if (context.Request.Path.StartsWithSegments("/alerts") &&
        !context.Request.Path.Value!.EndsWith("/cap", StringComparison.Ordinal))
    {
        IList<MediaTypeHeaderValue> ranges;
        try { ranges = context.Request.GetTypedHeaders().Accept ?? []; }
        catch (FormatException) { context.Response.StatusCode = 400; return; }
        if (ranges.Count == 0) ranges = [MediaTypeHeaderValue.Parse("*/*")];
        var candidates = new[] { "application/cap+xml", "text/html" }
            .Select(type => (Type: type, Q: EffectiveQuality(type, ranges)))
            .Where(c => c.Q > 0).OrderByDescending(c => c.Q).ToArray();
        if (candidates.Length == 0) { context.Response.StatusCode = 406; return; }
        context.Request.Headers.Accept = string.Join(",", candidates.Select(c =>
            new MediaTypeHeaderValue(c.Type, c.Q).ToString()));
    }
    await next();
});
app.MapControllers();
app.Run();

static double EffectiveQuality(string type, IList<MediaTypeHeaderValue> ranges)
{
    var candidate = MediaTypeHeaderValue.Parse(type + "; charset=utf-8");
    var matched = ranges.Where(range =>
    {
        var withoutQuality = MediaTypeHeaderValue.Parse(range.ToString());
        withoutQuality.Quality = null;
        return candidate.IsSubsetOf(withoutQuality);
    }).OrderByDescending(r => r.MatchesAllTypes ? 0 : r.SubType.Value == "*" ? 1 : 2)
      .ThenByDescending(r => r.Parameters.Count(p =>
          !p.Name.Equals("q", StringComparison.OrdinalIgnoreCase))).FirstOrDefault();
    return matched?.Quality ?? (matched is null ? 0 : 1);
}

public sealed record DemoAlert(string Cap, string Headline, string Instruction)
{
    public static DemoAlert Load()
    {
        string cap = File.ReadAllText(Path.Combine(AppContext.BaseDirectory, "alert.xml"), Encoding.UTF8);
        using var reader = XmlReader.Create(new StringReader(cap), new XmlReaderSettings
        { DtdProcessing = DtdProcessing.Prohibit, XmlResolver = null });
        var xml = XDocument.Load(reader);
        XNamespace ns = "urn:oasis:names:tc:emergency:cap:1.2";
        var info = xml.Root!.Element(ns + "info")!;
        return new(cap, info.Element(ns + "headline")!.Value, info.Element(ns + "instruction")!.Value);
    }
    public string Html()
    {
        var e = HtmlEncoder.Default;
        return $"<!doctype html><html lang=\"en-GB\"><head><meta charset=\"utf-8\"><title>Fictional CAP example</title></head><body><main><h1>{e.Encode(Headline)}</h1><p>{e.Encode(Instruction)}</p><a href=\"/alerts/123/cap\">CAP XML</a></main></body></html>";
    }
}

public sealed class AlertFormatter : TextOutputFormatter
{
    private readonly bool html;
    public AlertFormatter(string mediaType)
    {
        html = mediaType == "text/html";
        SupportedMediaTypes.Add(MediaTypeHeaderValue.Parse(mediaType));
        SupportedEncodings.Add(new UTF8Encoding(false));
    }
    protected override bool CanWriteType(Type? type) => type == typeof(DemoAlert);
    public override Task WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding encoding)
    {
        var alert = (DemoAlert)context.Object!;
        return context.HttpContext.Response.WriteAsync(html ? alert.Html() : alert.Cap,
            encoding, context.HttpContext.RequestAborted);
    }
}

[ApiController]
public sealed class AlertsController(DemoAlert alert) : ControllerBase
{
    [HttpGet("/alerts/{id}")]
    [HttpHead("/alerts/{id}")]
    [Produces("application/cap+xml", "text/html")]
    public IActionResult Get(string id) => id == "123" ? Ok(alert) : NotFound();

    // Explicit link: a separate route, not header negotiation.
    [HttpGet("/alerts/123/cap")]
    [HttpHead("/alerts/123/cap")]
    public IActionResult Cap() => Content(alert.Cap, "application/cap+xml", Encoding.UTF8);
}