using System.Linq;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;

namespace XWorld.Server.Gateway
{
    // 取本机内网主 IPv4，用于广播 advertise（仿 C++ XBroadcastServer.GetLocalIPAddress）。
    // 优先选 Up、非 loopback、非隧道、带网关的网卡上的 IPv4；都没有则回退 127.0.0.1。
    public static class LanIp
    {
        public static string Resolve()
        {
            // 1) 优先：有默认网关的活动网卡（最可能是真实内网口）
            string withGateway = PickFromNics(requireGateway: true);
            if (withGateway != null) return withGateway;

            // 2) 退一步：任意 Up 的非 loopback/隧道网卡上的私有 IPv4
            string anyUp = PickFromNics(requireGateway: false);
            if (anyUp != null) return anyUp;

            // 3) 兜底：DNS 解析本机名里的第一个 IPv4
            foreach (IPAddress addr in Dns.GetHostAddresses(Dns.GetHostName()))
            {
                if (addr.AddressFamily == AddressFamily.InterNetwork && !IPAddress.IsLoopback(addr))
                    return addr.ToString();
            }

            return "127.0.0.1";
        }

        private static string PickFromNics(bool requireGateway)
        {
            foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
            {
                if (nic.OperationalStatus != OperationalStatus.Up) continue;
                if (nic.NetworkInterfaceType == NetworkInterfaceType.Loopback) continue;
                if (nic.NetworkInterfaceType == NetworkInterfaceType.Tunnel) continue;

                IPInterfaceProperties props = nic.GetIPProperties();
                if (requireGateway && !props.GatewayAddresses.Any(g => g.Address.AddressFamily == AddressFamily.InterNetwork))
                    continue;

                foreach (UnicastIPAddressInformation ua in props.UnicastAddresses)
                {
                    IPAddress ip = ua.Address;
                    if (ip.AddressFamily != AddressFamily.InterNetwork) continue;
                    if (IPAddress.IsLoopback(ip)) continue;
                    return ip.ToString();
                }
            }
            return null;
        }
    }
}