Nonemm/src/Nonemm.Keying/CwDaemonSender.cs

using System.Net;
using System.Net.Sockets;
using System.Text;

namespace Nonemm.Keying;

/// Sends CW through `cwdaemon`, which keys the radio from a serial or parallel
/// port and is what Linux stations usually run. Its protocol is one UDP
/// datagram per command; escape sequences start with a 0x1B byte.
///
/// `<ESC>h<text>` asks for a reply once the next message has been played, and
/// cwdaemon answers with `h` and that text. The request covers one message
/// only, so it goes out in front of every message.
///
/// Why the timing is cwdaemon's job and not ours: `docs/keying.md`.
public sealed class CwDaemonSender : MessageSender
{
    private const string ReplyToken = "nonemm";

    // bound before anything is sent, so the reply has somewhere to arrive
    private readonly UdpClient socket = new(new IPEndPoint(IPAddress.Any, 0));
    private readonly IPEndPoint daemon;
    private readonly CancellationTokenSource reading = new();

    public CwDaemonSender(string host = "127.0.0.1", int port = 6789)
    {
        daemon = new IPEndPoint(IPAddress.Parse(host), port);
        IsReady = true;
        _ = ReadRepliesAsync(reading.Token);
    }

    public bool IsReady { get; private set; }

    public bool ReportsCompletion => true;

    public event EventHandler? Finished;

    public async Task SendAsync(string text, CancellationToken cancellation = default)
    {
        await WriteAsync(Escape('h', ReplyToken), cancellation).ConfigureAwait(false);
        await WriteAsync(Encoding.ASCII.GetBytes(text.ToUpperInvariant()), cancellation)
            .ConfigureAwait(false);
    }

    public Task AbortAsync(CancellationToken cancellation = default) =>
        WriteAsync([0x1B, (byte)'4'], cancellation);

    public Task SetSpeedAsync(int wordsPerMinute, CancellationToken cancellation = default) =>
        WriteAsync(Escape('2', wordsPerMinute.ToString()), cancellation);

    public void Dispose()
    {
        reading.Cancel();
        reading.Dispose();
        socket.Dispose();
    }

    private static byte[] Escape(char command, string argument) =>
        [0x1B, (byte)command, .. Encoding.ASCII.GetBytes(argument)];

    /// cwdaemon's reply arrives on the same socket the commands went out on.
    /// A reply for anything other than our own token is another program's
    /// business and is passed over.
    private async Task ReadRepliesAsync(CancellationToken cancellation)
    {
        while (!cancellation.IsCancellationRequested)
        {
            UdpReceiveResult received;
            try
            {
                received = await socket.ReceiveAsync(cancellation).ConfigureAwait(false);
            }
            catch (Exception e) when (e is OperationCanceledException or ObjectDisposedException)
            {
                return;
            }
            catch (SocketException)
            {
                // a datagram that could not be read says nothing about the next
                continue;
            }
            if (Encoding.ASCII.GetString(received.Buffer).TrimEnd('\r', '\n') == $"h{ReplyToken}")
            {
                Finished?.Invoke(this, EventArgs.Empty);
            }
        }
    }

    private async Task WriteAsync(byte[] message, CancellationToken cancellation)
    {
        try
        {
            await socket.SendAsync(message, daemon, cancellation).ConfigureAwait(false);
        }
        catch (SocketException e)
        {
            IsReady = false;
            throw new InvalidOperationException($"could not reach cwdaemon at {daemon}: {e.Message}", e);
        }
    }
}