using System.Globalization;
using System.Net;

namespace MoShiro.Protocol;

/// <summary>
/// 单个 Agent 任务的目标：<c>http</c> 是 <c>http(s)://主机[:端口][/路径]</c>，其余 Kind 是 <c>主机[:端口]</c>（IP 或域名，IPv6 带端口时用方括号）。
/// Backend 只检查格式，不做 DNS 解析；解析在 Agent 上进行，每个解析出的地址都要在 <c>allowed_target_cidrs</c> 里。
/// </summary>
public static class TargetHost
{
    public const string HttpKind = "http_request";

    /// <summary>解析目标；<paramref name="port"/> 为 0 表示没有写端口，<paramref name="url"/> 只在 http(s) URL 时有值。</summary>
    public static bool TryParse(string? target, out string host, out int port, out Uri? url)
    {
        host = "";
        port = 0;
        url = null;
        if (string.IsNullOrWhiteSpace(target) || target.Length > 2048 || target.Any(char.IsWhiteSpace))
        {
            return false;
        }

        if (target.Contains("://", StringComparison.Ordinal))
        {
            if (!Uri.TryCreate(target, UriKind.Absolute, out Uri? uri) || uri.Scheme is not ("http" or "https") || uri.UserInfo.Length > 0 || !ValidHost(uri.IdnHost))
            {
                return false;
            }

            url = uri;
            host = uri.IdnHost.Trim('[', ']');
            port = uri.IsDefaultPort ? 0 : uri.Port;
            return true;
        }

        // IPAddress.TryParse 会把「[v6]:端口」的端口丢掉，带方括号的先按端点解析
        if (!target.StartsWith('[') && IPAddress.TryParse(target, out IPAddress? ip))
        {
            host = ip.ToString();
            return true;
        }

        if (IPEndPoint.TryParse(target, out IPEndPoint? endpoint))
        {
            if (endpoint.Port is 0)
            {
                return false;
            }

            host = endpoint.Address.ToString();
            port = endpoint.Port;
            return true;
        }

        int colon = target.LastIndexOf(':');
        string name = colon < 0 ? target : target[..colon];
        if (colon >= 0 && (!int.TryParse(target.AsSpan(colon + 1), NumberStyles.None, CultureInfo.InvariantCulture, out port) || port is < 1 or > 65535))
        {
            return false;
        }

        if (!ValidHost(name))
        {
            return false;
        }

        host = name.ToLowerInvariant();
        return true;
    }

    /// <summary>按 Kind 检查并规范化：<c>http</c> 必须是 http(s) URL，其余 Kind 必须是主机[:端口]；不合格返回 null。</summary>
    public static string? Normalize(string kindId, string? target)
    {
        if (!TryParse(target, out string host, out int port, out Uri? url) || (url is not null) != (kindId == HttpKind))
        {
            return null;
        }

        if (url is not null)
        {
            return url.AbsoluteUri;
        }

        string shown = IPAddress.TryParse(host, out IPAddress? ip) && ip.AddressFamily == System.Net.Sockets.AddressFamily.InterNetworkV6 && port != 0 ? "[" + host + "]" : host;
        return port == 0 ? host : shown + ":" + port.ToString(CultureInfo.InvariantCulture);
    }

    private static bool ValidHost(string name)
    {
        if (IPAddress.TryParse(name.Trim('[', ']'), out _))
        {
            return true;
        }

        if (name.Length is 0 or > 253 || name[^1] == '-' || name[0] is '-' or '.')
        {
            return false;
        }

        return name.TrimEnd('.').Split('.').All(label => label.Length is > 0 and <= 63 && label.All(c => char.IsAsciiLetterOrDigit(c) || c == '-'));
    }
}