using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;

namespace WebDrop.Frontend;

/// <summary>在同源模式下,从本程序集资源提供 Vite 前端。</summary>
public static class EmbeddedFrontend
{
    private const string ResourcePrefix = "WebDrop.Frontend.Assets/";

    public static void Use(IApplicationBuilder app, string? extraConnectOrigin = null)
    {
        var assembly = typeof(EmbeddedFrontend).Assembly;
        var resources = assembly.GetManifestResourceNames()
            .Where(name => name.StartsWith(ResourcePrefix, StringComparison.Ordinal))
            .ToDictionary(
                name => name[ResourcePrefix.Length..].Replace('\\', '/'),
                StringComparer.Ordinal);

        if (!resources.ContainsKey("index.html")) return;

        var contentSecurityPolicy = BuildContentSecurityPolicy(extraConnectOrigin);
        app.Use((HttpContext context, RequestDelegate next) =>
            HandleAsync(context, next, assembly, resources, contentSecurityPolicy));
    }

    private static async Task HandleAsync(
        HttpContext context,
        RequestDelegate next,
        System.Reflection.Assembly assembly,
        IReadOnlyDictionary<string, string> resources,
        string contentSecurityPolicy)
    {
        if (!(HttpMethods.IsGet(context.Request.Method) || HttpMethods.IsHead(context.Request.Method)))
        {
            await next(context);
            return;
        }

        var normalized = NormalizePath(context.Request.Path.Value);
        if (normalized is null)
        {
            context.Response.StatusCode = StatusCodes.Status404NotFound;
            return;
        }
        if (IsApiPath(normalized))
        {
            await next(context);
            return;
        }

        if (resources.TryGetValue(normalized, out var resourceName))
        {
            await SendAsync(context, assembly, resources, normalized, contentSecurityPolicy, resourceName);
            return;
        }

        // A missing Vite asset must stay a 404. Other unknown paths are client-side routes.
        if (normalized.StartsWith("assets/", StringComparison.Ordinal) || normalized.Equals("favicon.ico", StringComparison.OrdinalIgnoreCase))
        {
            context.Response.StatusCode = StatusCodes.Status404NotFound;
            return;
        }

        await SendAsync(context, assembly, resources, "index.html", contentSecurityPolicy);
    }

    private static async Task SendAsync(
        HttpContext context,
        System.Reflection.Assembly assembly,
        IReadOnlyDictionary<string, string> resources,
        string path,
        string contentSecurityPolicy,
        string? resourceName = null)
    {
        resourceName ??= resources[path];
        await using var stream = assembly.GetManifestResourceStream(resourceName);
        if (stream is null)
        {
            context.Response.StatusCode = StatusCodes.Status404NotFound;
            return;
        }

        context.Response.StatusCode = StatusCodes.Status200OK;
        context.Response.ContentType = ContentType(path);
        context.Response.Headers["Cache-Control"] = path.StartsWith("assets/", StringComparison.Ordinal)
            ? "public, max-age=31536000, immutable"
            : "no-store";

        // Dedicated workers enforce the CSP returned with their own script response.
        // Give bundled JS the same app policy so the crypto worker can call the same-origin API.
        if (path.Equals("index.html", StringComparison.Ordinal) || path.EndsWith(".js", StringComparison.OrdinalIgnoreCase))
            context.Response.Headers["Content-Security-Policy"] = contentSecurityPolicy;

        if (!HttpMethods.IsHead(context.Request.Method))
            await stream.CopyToAsync(context.Response.Body, context.RequestAborted);
    }

    private static string? NormalizePath(string? path)
    {
        if (string.IsNullOrEmpty(path)) return "index.html";
        var normalized = path.Replace('\\', '/').Trim('/');
        if (normalized.Length == 0) return "index.html";
        if (normalized.Contains('\0') || normalized.Split('/').Any(segment => segment is ".." or "." or ""))
            return null;
        return normalized;
    }

    private static bool IsApiPath(string path) =>
        path.Equals("api", StringComparison.OrdinalIgnoreCase) ||
        path.StartsWith("api/", StringComparison.OrdinalIgnoreCase);

    private static string ContentType(string path) => Path.GetExtension(path).ToLowerInvariant() switch
    {
        ".html" => "text/html; charset=utf-8",
        ".js" => "text/javascript; charset=utf-8",
        ".css" => "text/css; charset=utf-8",
        ".json" => "application/json; charset=utf-8",
        ".svg" => "image/svg+xml",
        ".png" => "image/png",
        ".jpg" or ".jpeg" => "image/jpeg",
        ".webp" => "image/webp",
        ".ico" => "image/x-icon",
        ".woff2" => "font/woff2",
        ".wasm" => "application/wasm",
        _ => "application/octet-stream",
    };

    private static string BuildContentSecurityPolicy(string? extraConnectOrigin)
    {
        var connectSources = new List<string> { "'self'", "ws:", "wss:" };
        if (Uri.TryCreate(extraConnectOrigin, UriKind.Absolute, out var uri) && uri.Scheme is "http" or "https")
            connectSources.Add(uri.GetLeftPart(UriPartial.Authority));

        return "default-src 'none'; base-uri 'none'; object-src 'none'; frame-ancestors 'none'; " +
               "form-action 'self'; script-src 'self' 'wasm-unsafe-eval'; style-src 'self' 'unsafe-inline'; " +
               "img-src 'self' data: blob:; media-src 'self' blob:; font-src 'self'; worker-src 'self' blob:; " +
               $"connect-src {string.Join(' ', connectSources)}";
    }
}