Nonemm/src/Nonemm.Network/StationNetwork.cs

using System.Net;
using System.Net.Sockets;
using System.Text;
using Nonemm.Core;

namespace Nonemm.Network;

/// Keeps the stations of a multi-operator entry in step. Each contact is sent
/// to every peer as it is logged, and contacts that arrive from a peer are
/// handed to whoever owns the log.
public sealed class StationNetwork : IDisposable
{
    private readonly int port;
    private readonly string stationName;
    private readonly IReadOnlyList<IPEndPoint> peers;
    private readonly CancellationTokenSource stopping = new();
    private readonly UdpClient listener;
    private readonly UdpClient sender = new();
    private Task? loop;

    public StationNetwork(int port, string stationName, IEnumerable<string> peerAddresses)
    {
        this.port = port;
        this.stationName = stationName;
        peers = peerAddresses.Select(a => Endpoint(a, port)).OfType<IPEndPoint>().ToList();
        listener = new UdpClient(new IPEndPoint(IPAddress.Any, port));
        sender.EnableBroadcast = true;
    }

    public string StationName => stationName;

    public IReadOnlyList<IPEndPoint> Peers => peers;

    /// A contact another station logged. It arrives already scored; the log
    /// works its points and multipliers out again from the rules.
    public event EventHandler<Qso>? ContactArrived;

    public event EventHandler<string>? Failed;

    public void Start() => loop ??= Task.Run(() => ListenAsync(stopping.Token));

    public async Task SendAsync(Qso qso, string myCallsign, CancellationToken cancellation = default)
    {
        byte[] message = Encoding.UTF8.GetBytes(ContactMessage.Write(qso, myCallsign, stationName));
        foreach (IPEndPoint peer in Destinations())
        {
            try
            {
                await sender.SendAsync(message, peer, cancellation).ConfigureAwait(false);
            }
            catch (SocketException e)
            {
                Failed?.Invoke(this, $"could not reach {peer}: {e.Message}");
            }
        }
    }

    public void Dispose()
    {
        stopping.Cancel();
        listener.Dispose();
        sender.Dispose();
        stopping.Dispose();
    }

    /// With no peers named, the contact goes out as a broadcast, which is how a
    /// station that has just joined is found without configuring anything.
    private IEnumerable<IPEndPoint> Destinations() =>
        peers.Count > 0 ? peers : [new IPEndPoint(IPAddress.Broadcast, port)];

    private async Task ListenAsync(CancellationToken cancellation)
    {
        while (!cancellation.IsCancellationRequested)
        {
            try
            {
                UdpReceiveResult received = await listener.ReceiveAsync(cancellation).ConfigureAwait(false);
                Qso? qso = ContactMessage.Read(Encoding.UTF8.GetString(received.Buffer));
                if (qso is not null && qso.StationName != stationName)
                {
                    ContactArrived?.Invoke(this, qso);
                }
            }
            catch (OperationCanceledException)
            {
                return;
            }
            catch (SocketException e)
            {
                Failed?.Invoke(this, e.Message);
            }
        }
    }

    private static IPEndPoint? Endpoint(string address, int defaultPort)
    {
        string[] parts = address.Split(':');
        if (!IPAddress.TryParse(parts[0].Trim(), out IPAddress? host))
        {
            return null;
        }
        int port = parts.Length > 1 && int.TryParse(parts[1], out int given) ? given : defaultPort;
        return new IPEndPoint(host, port);
    }
}