using System.Net;
using System.Net.Http.Headers;
using Microsoft.Extensions.Options;

namespace Eis.Web.Legacy;

public sealed class LegacyApiProxy(
    HttpClient httpClient,
    IOptions<LegacyNodeOptions> options,
    ILogger<LegacyApiProxy> logger)
{
    private static readonly HashSet<string> HopByHopHeaders = new(StringComparer.OrdinalIgnoreCase)
    {
        "Connection",
        "Keep-Alive",
        "Proxy-Authenticate",
        "Proxy-Authorization",
        "TE",
        "Trailer",
        "Transfer-Encoding",
        "Upgrade"
    };

    private readonly LegacyNodeOptions _options = options.Value;

    public async Task ForwardAsync(HttpContext context)
    {
        if (!_options.Enabled)
        {
            context.Response.StatusCode = StatusCodes.Status501NotImplemented;
            await context.Response.WriteAsJsonAsync(new
            {
                ok = false,
                message = "该接口尚未迁移到 ASP.NET Core",
                migration = new { native = false, legacyProxyEnabled = false }
            }, context.RequestAborted);
            return;
        }

        var target = new Uri(_options.BaseUrl, $"{context.Request.PathBase}{context.Request.Path}{context.Request.QueryString}");
        using var outbound = CreateRequest(context, target);

        try
        {
            using var upstream = await httpClient.SendAsync(
                outbound,
                HttpCompletionOption.ResponseHeadersRead,
                context.RequestAborted);

            context.Response.StatusCode = (int)upstream.StatusCode;
            CopyResponseHeaders(upstream, context.Response);

            if (context.Request.Method != HttpMethods.Head && upstream.StatusCode != HttpStatusCode.NoContent)
            {
                await upstream.Content.CopyToAsync(context.Response.Body, context.RequestAborted);
            }
        }
        catch (HttpRequestException exception)
        {
            logger.LogWarning(exception, "Legacy Node API at {Target} is unavailable", target);
            if (context.Response.HasStarted)
            {
                context.Abort();
                return;
            }

            context.Response.StatusCode = StatusCodes.Status503ServiceUnavailable;
            await context.Response.WriteAsJsonAsync(new
            {
                ok = false,
                message = "迁移期间的旧版 API 服务暂时不可用",
                migration = new { native = false, legacyProxyEnabled = true }
            }, context.RequestAborted);
        }
    }

    public async Task<bool> IsAvailableAsync(CancellationToken cancellationToken)
    {
        if (!_options.Enabled)
        {
            return true;
        }

        try
        {
            using var response = await httpClient.GetAsync("api/public/home", cancellationToken);
            return response.IsSuccessStatusCode;
        }
        catch (HttpRequestException)
        {
            return false;
        }
    }

    private static HttpRequestMessage CreateRequest(HttpContext context, Uri target)
    {
        var request = new HttpRequestMessage(new HttpMethod(context.Request.Method), target);
        var hasBody = context.Request.ContentLength > 0 || context.Request.Headers.TransferEncoding.Count > 0;
        if (hasBody)
        {
            request.Content = new StreamContent(context.Request.Body);
        }

        foreach (var (name, values) in context.Request.Headers)
        {
            if (name.Equals("Host", StringComparison.OrdinalIgnoreCase) || HopByHopHeaders.Contains(name))
            {
                continue;
            }

            var valueArray = values.ToArray();
            if (!request.Headers.TryAddWithoutValidation(name, valueArray) && request.Content is not null)
            {
                request.Content.Headers.TryAddWithoutValidation(name, valueArray);
            }
        }

        request.Headers.TryAddWithoutValidation("X-Forwarded-Host", context.Request.Host.Value);
        request.Headers.TryAddWithoutValidation("X-Forwarded-Proto", context.Request.Scheme);
        return request;
    }

    private static void CopyResponseHeaders(HttpResponseMessage upstream, HttpResponse response)
    {
        foreach (var header in upstream.Headers.Concat(upstream.Content.Headers))
        {
            if (!HopByHopHeaders.Contains(header.Key))
            {
                response.Headers.Append(header.Key, header.Value.ToArray());
            }
        }

        response.Headers.Remove("transfer-encoding");
    }
}