using System.Net;
using System.Net.Sockets;
using MoShiro.Protocol;

namespace MoShiro.Backend.Application;

/// <summary>
/// 自动识别 Agent 的对外地址：Agent 上报本机的公网地址，注册请求的来源地址作为补充（来源是私网或隧道出口时不用）。
/// 管理员手填的（<c>*_manual</c>）永远优先；已经选中的自动地址还在候选里就不换，不会因为网卡顺序变化来回改。
/// </summary>
public static class AgentAddressPolicy
{
    /// <summary>返回要写入的 <c>(public_host, ipv6_host)</c>；没有候选的那一项是 null（保持不变）。</summary>
    public static (string? PublicHost, string? Ipv6Host) Choose(AgentRecord current, IReadOnlyList<string> reported, string? observed)
    {
        var v4 = new List<string>();
        var v6 = new List<string>();
        foreach (string text in reported.Append(observed ?? ""))
        {
            if (!IPAddress.TryParse(text, out IPAddress? address))
            {
                continue;
            }

            if (address.AddressFamily == AddressFamily.InterNetwork && PublicAddress.IsPublicIpv4(address) && !v4.Contains(address.ToString()))
            {
                v4.Add(address.ToString());
            }
            else if (address.AddressFamily == AddressFamily.InterNetworkV6 && PublicAddress.IsGlobalIpv6(address) && !v6.Contains(address.ToString()))
            {
                v6.Add(address.ToString());
            }
        }

        return (current.PublicHostManual ? null : Pick(v4, current.PublicHost), current.Ipv6HostManual ? null : Pick(v6, current.Ipv6Host));
    }

    private static string? Pick(List<string> candidates, string current) =>
        candidates.Count == 0 ? null : candidates.Contains(current) ? current : candidates[0];
}