using System.Net.Security;
using System.Net.Sockets;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Text.RegularExpressions;
using ZA.CoreService.ESBCertificateManager.Models;

namespace ZA.CoreService.ESBCertificateManager.Services;

/// <summary>
/// Liest das präsentierte TLS-Zertifikat eines Endpoints
/// (entspricht grob <c>curl -v https://host:port</c>).
/// </summary>
public sealed class TlsEndpointProbeService
{
    private static readonly Regex HostPortRegex = new(
        @"^(?<host>[^:/]+)(:(?<port>\d{1,5}))?$",
        RegexOptions.Compiled | RegexOptions.CultureInvariant);

    public CertificateProbeResult Probe(
        string? checkUrl,
        int timeoutMilliseconds = 8000)
    {
        if (!TryParseEndpoint(
                checkUrl,
                out string host,
                out int port,
                out string serverName,
                out string? parseError))
        {
            return CertificateProbeResult.Failed(
                parseError ?? "Ungültige Prüf-URL.");
        }

        try
        {
            using CancellationTokenSource cts = new(timeoutMilliseconds);
            using TcpClient client = new();

            using (cts.Token.Register(
                       () =>
                       {
                           try
                           {
                               client.Close();
                           }
                           catch
                           {
                               // Timeout beendet die Verbindung.
                           }
                       }))
            {
                client.Connect(host, port);
            }

            using SslStream ssl = new(
                client.GetStream(),
                leaveInnerStreamOpen: false,
                userCertificateValidationCallback:
                static (_, _, _, _) => true);

            ssl.AuthenticateAsClient(serverName);

            if (ssl.RemoteCertificate is null)
            {
                return CertificateProbeResult.Failed(
                    $"Kein Zertifikat von {host}:{port} erhalten.");
            }

            using X509Certificate2 certificate = new(ssl.RemoteCertificate);
            CertificateInfo info = ToCertificateInfo(certificate);

            return CertificateProbeResult.Found(
                $"{host}:{port}",
                info);
        }
        catch (Exception ex)
        {
            return CertificateProbeResult.Failed(
                $"TLS-Prüfung {host}:{port} fehlgeschlagen: {ex.Message}");
        }
    }

    public static bool TryParseEndpoint(
        string? value,
        out string host,
        out int port,
        out string serverName,
        out string? error)
    {
        host = string.Empty;
        port = 443;
        serverName = string.Empty;
        error = null;

        if (string.IsNullOrWhiteSpace(value))
        {
            error = "Keine Prüf-URL hinterlegt.";
            return false;
        }

        string trimmed = value.Trim();

        if (Uri.TryCreate(trimmed, UriKind.Absolute, out Uri? uri)
            && (uri.Scheme.Equals("https", StringComparison.OrdinalIgnoreCase)
                || uri.Scheme.Equals("http", StringComparison.OrdinalIgnoreCase)
                || uri.Scheme.Equals("ssl", StringComparison.OrdinalIgnoreCase)
                || uri.Scheme.Equals("tcp", StringComparison.OrdinalIgnoreCase)))
        {
            if (string.IsNullOrWhiteSpace(uri.Host))
            {
                error = "Host in der Prüf-URL fehlt.";
                return false;
            }

            host = uri.Host;
            serverName = uri.Host;
            port = uri.IsDefaultPort
                ? (uri.Scheme.Equals("http", StringComparison.OrdinalIgnoreCase)
                    ? 80
                    : 443)
                : uri.Port;

            if (port is < 1 or > 65535)
            {
                error = "Port muss zwischen 1 und 65535 liegen.";
                return false;
            }

            return true;
        }

        Match match = HostPortRegex.Match(trimmed);

        if (!match.Success)
        {
            error =
                "Prüf-URL ungültig. Beispiele:\n"
                + "https://server:8443\n"
                + "server:8443\n"
                + "server";
            return false;
        }

        host = match.Groups["host"].Value;
        serverName = host;

        if (match.Groups["port"].Success)
        {
            if (!int.TryParse(match.Groups["port"].Value, out port)
                || port is < 1 or > 65535)
            {
                error = "Port muss zwischen 1 und 65535 liegen.";
                return false;
            }
        }

        return !string.IsNullOrWhiteSpace(host);
    }

    public static string? NormalizeCheckUrl(string? value)
    {
        if (string.IsNullOrWhiteSpace(value))
        {
            return null;
        }

        string trimmed = value.Trim();

        if (!TryParseEndpoint(
                trimmed,
                out _,
                out _,
                out _,
                out string? error))
        {
            throw new ArgumentException(
                error ?? "Ungültige Prüf-URL.",
                nameof(value));
        }

        if (trimmed.Length > 500)
        {
            throw new ArgumentException(
                "Die Prüf-URL darf maximal 500 Zeichen enthalten.",
                nameof(value));
        }

        return trimmed;
    }

    private static CertificateInfo ToCertificateInfo(
        X509Certificate2 certificate)
    {
        string subject = certificate.GetNameInfo(
            X509NameType.SimpleName,
            forIssuer: false);

        string issuer = certificate.GetNameInfo(
            X509NameType.SimpleName,
            forIssuer: true);

        if (string.IsNullOrWhiteSpace(subject))
        {
            subject = certificate.Subject;
        }

        if (string.IsNullOrWhiteSpace(issuer))
        {
            issuer = certificate.Issuer;
        }

        string fingerprint = certificate.GetCertHashString(
            HashAlgorithmName.SHA256);

        fingerprint = string.Join(
            ":",
            Enumerable.Range(0, fingerprint.Length / 2)
                .Select(index => fingerprint.Substring(index * 2, 2)));

        DateTimeOffset validFrom = new(certificate.NotBefore);
        DateTimeOffset validUntil = new(certificate.NotAfter);
        DateTimeOffset now = DateTimeOffset.Now;

        return new CertificateInfo
        {
            Subject = subject,
            Issuer = issuer,
            FingerprintSha256 = fingerprint,
            ValidFrom = validFrom,
            ValidUntil = validUntil,
            IsCurrentlyValid = now >= validFrom && now <= validUntil
        };
    }
}