discovery/src/Discovery.Core/Services/Scanner.cs

using Discovery.Models;
using System.Diagnostics;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;
using System.Runtime.InteropServices;
using System.Text.Json;
using System.Text.RegularExpressions;

namespace Discovery.Services;

public partial class Scanner<T> where T : Target
{
    public ScannerState State => _state;

    private CancellationTokenSource? _cts;

    private readonly ScannerState _state = new();
    private readonly static Dictionary<string, string>? _vendors = BuildVendorDictionary();

    public async Task<ScannerState> ScanAsync(
        SynchronizedCollection<T> targets,
        ScannerOptions options,
        ScannerEvents events,
        CancellationToken cancellationToken = default)
    {
        // cancel if already running
        if (_state.Operation == ScanOperation.Scanning && _cts != null)
        {
            await _cts.CancelAsync();
            return _state;
        }

        // renew state
        _state.Operation = ScanOperation.Scanning;
        _state.Progress = 0;

        // set cts
        _cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);

        var orderedTargets = targets.InternalList.OrderBy(p => p.Index);
        var start = Stopwatch.GetTimestamp();

        try
        {
            var tasks = new List<Task>(targets.InternalList.Count);
            var con = new SemaphoreSlim(options.Concurrency);

            var pingOptions = new PingOptions
            {
                DontFragment = options.DontFragment,
                Ttl = options.Ttl
            };

            foreach (var target in targets.InternalList)
            {
                tasks.Add(Task.Run(async () =>
                {
                    try
                    {
                        await con.WaitAsync(cancellationToken).ConfigureAwait(false);

                        target.Ping = await PingAsync(target.IPAddress, options.PingTimeout, null, pingOptions, cancellationToken).ConfigureAwait(false);
                        target.Mac = await ArpAsync(target.IPAddress, options.ArpTimeout, cancellationToken).ConfigureAwait(false);

                        if (target.Mac != null)
                        {
                            target.Arp = true;
                            target.Vendor = ResolveVendor(target.Mac!);
                        }

                        // local resolve fix
                        string? sAddress = null;
                        if (TryGetPrimaryInterface(out var pInterface, out var pAddress))
                            sAddress = pAddress?.ToString();

                        if (target.Address == sAddress)
                        {
                            target.Hostname = Dns.GetHostName();
                        }
                        else if (options.Nameservers == null)
                        {
                            target.Hostname = "N/A";
                        }
                        else
                        {
                            if (target.Online || (!target.Online && options.ResolveAll))
                            {
                                var result = await ResolveHostnameAsync(target.IPAddress, options.Nameservers, cancellationToken).ConfigureAwait(false);
                                if (result != null)
                                    target.Hostname = result;
                            }
                        }
                    }
                    catch (OperationCanceledException) { }
                    finally
                    {
                        _state.Progress += (100 / (double)targets.InternalList.Count);

                        if (events.OnStateUpdate != null)
                            await events.OnStateUpdate(_state);

                        con.Release();
                    }
                }, cancellationToken));
            }

            await Task.WhenAll(tasks);

            _state.Operation = ScanOperation.Idle;
        }
        catch (OperationCanceledException)
        {
            _state.Operation = ScanOperation.Aborted;
        }
        finally
        {
            _state.Progress = 100;
            _state.Elapsed = Stopwatch.GetElapsedTime(start);
            _cts = null;
        }

        return _state;
    }

    public async Task CancelAsync()
    {
        if (_cts == null)
            return;

        await _cts.CancelAsync().ConfigureAwait(false);

        _state.Operation = ScanOperation.Aborted;
    }

    public bool TryGetPrimaryInterface(out NetworkInterface? pInterface, out IPAddress? pAddress)
    {
        pInterface = null;
        pAddress = null;

        try
        {
            using var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, 0);
            socket.Connect("8.8.8.8", 65530);

            var ep = socket.LocalEndPoint as IPEndPoint;

            if (ep?.Address == null)
                return false;

            pAddress = ep.Address;

            pInterface = NetworkInterface
                .GetAllNetworkInterfaces()
                .FirstOrDefault(p => p
                .GetIPProperties().UnicastAddresses.Any(x => x.Address.ToString() == ep.Address.ToString()));

            if (pInterface == null)
                return false;

            return true;
        }
        catch
        {
            return false;
        }
    }

    public NetworkInfo? GetPrimaryNetwork()
    {
        if (!TryGetPrimaryInterface(out var pInterface, out var pAddress))
            return null;

        var pUnicast = pInterface!.GetIPProperties().UnicastAddresses.Single(p => p.Address.ToString() == pAddress!.ToString());

        if (!IPNetwork2.TryParse(pAddress!, pUnicast!.IPv4Mask, out var net))
            return null;

        var nameservers = pInterface!.GetIPProperties().DnsAddresses;

        return new NetworkInfo(pAddress!, net.Network, net.Cidr, [.. nameservers]);
    }

    private static Dictionary<string, string> BuildVendorDictionary()
    {
        var dictionary = new Dictionary<string, string>();

        using var fileStream = new FileStream("vendors.json", FileMode.Open, FileAccess.Read);

        var vendors = JsonSerializer.Deserialize(
            fileStream,
            VendorsJsonContext.Default.VendorArray
        ) ?? throw new InvalidOperationException("vendors.json");

        foreach (var item in vendors)
        {
            if (item is null)
                continue;

            if (item.MacPrefix == null || item.VendorName == null)
                continue;

            dictionary.Add(item.MacPrefix, item.VendorName);
        }

        return dictionary;
    }

    private static async Task<PingReply?> PingAsync(IPAddress address, TimeSpan timeout, byte[]? buffer, PingOptions? options, CancellationToken cancellationToken)
    {
        if (cancellationToken.IsCancellationRequested)
            return null;

        using var ping = new Ping();
        var reply = await ping.SendPingAsync(address, timeout, buffer, options, cancellationToken: cancellationToken).ConfigureAwait(false);
        ping.SendAsyncCancel();

        return reply;
    }

    private static async Task<string?> ArpAsync(IPAddress address, TimeSpan timeout, CancellationToken cancellationToken)
    {
        if (cancellationToken.IsCancellationRequested) return null;
        if (address.AddressFamily != AddressFamily.InterNetwork) return null; // IPv4 only

        // 1) Try ARP cache (GetIpNetTable)
        if (WinArpInterop.TryGetFromArpCache(address, out var mac))
            return mac;

        // 2) Active probe on a dedicated thread; still honor the timeout
        using var linked = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        var probeTask = Task.Factory.StartNew(
            () => WinArpInterop.ProbeWithSendArp(address),
            linked.Token,
            TaskCreationOptions.LongRunning | TaskCreationOptions.DenyChildAttach,
            TaskScheduler.Default);

        var completed = await Task.WhenAny(probeTask, Task.Delay(timeout, linked.Token)).ConfigureAwait(false);
        if (completed != probeTask) return null;

        linked.Cancel(); // cancel the delay if still pending
        return await probeTask.ConfigureAwait(false);
    }

    private static async Task<string?> ResolveHostnameAsync(IPAddress address, List<IPAddress> nameservers, CancellationToken cancellationToken)
    {
        if (cancellationToken.IsCancellationRequested)
            return null;

        string? hostname = null;

        for (int i = 0; i < nameservers.Count; i++)
        {
            using var process = new Process
            {
                StartInfo = new ProcessStartInfo
                {
                    FileName = "nslookup",
                    Arguments = $"{address} {nameservers[i]}",
                    RedirectStandardOutput = true,
                    UseShellExecute = false,
                    CreateNoWindow = true
                }
            };

            process.Start();

            var output = await process.StandardOutput
                .ReadToEndAsync(cancellationToken)
                .ConfigureAwait(false);

            await process.WaitForExitAsync(cancellationToken).ConfigureAwait(false);

            var match = NameRegex().Match(output);
            if (match.Success)
                hostname = match.Groups["Name"].Value;

            if (hostname != null)
                break;
        }

        return hostname;
    }

    private static string? ResolveVendor(string mac)
    {
        if (!_vendors!.TryGetValue(mac[..8].ToUpperInvariant(), out string? vendor))
            return null;

        return vendor;
    }

    [GeneratedRegex(@"Name:\s*(?<Name>[^\s]+)")]
    private static partial Regex NameRegex();
}