Nonemm/src/Nonemm.Session/AlternatingCq.cs

using Nonemm.Keying;

namespace Nonemm.Session;

/// Calls CQ on one radio, and when that message has gone out, moves to the
/// other radio and calls there. N1MM calls it dueling CQs and toggles it with
/// Ctrl+B.
///
/// Only the CQ message carries the alternation on. A station answers, the
/// operator sends their call and the exchange, and those messages leave the
/// alternation where it is until the next CQ goes out. N1MM works the same
/// way: it holds on any other message and on Escape, and carries on once the
/// CQ message is sent again, which is what happens after the contact is
/// logged.
///
/// It runs off the keyer's completion signal. Working out when a message ends
/// from the length of the text would be a guess, and a guess that runs short
/// keys the second radio while the first is still sending.
///
/// There is a keyer per mode — the CW keyer and the voice keyer — and a CQ
/// goes out through whichever the radio's mode calls for, so it listens to all
/// of them.
///
/// Two radios, because that is what an SO2R station has.
public sealed class AlternatingCq : IDisposable
{
    private readonly IReadOnlyList<MessageSender> keyers;
    private readonly Func<int, Task> callCqOn;
    private readonly Func<TimeSpan, CancellationToken, Task> wait;
    private readonly Lock gate = new();
    private CancellationTokenSource? running;
    private bool carriesOn = true;

    /// `callCqOn` moves the operator to that radio and sends the CQ message.
    /// `wait` is there so a test does not have to sleep.
    public AlternatingCq(
        IReadOnlyList<MessageSender> keyers,
        Func<int, Task> callCqOn,
        TimeSpan gap,
        Func<TimeSpan, CancellationToken, Task>? wait = null)
    {
        this.keyers = keyers;
        this.callCqOn = callCqOn;
        Gap = gap;
        this.wait = wait ?? Task.Delay;
        foreach (MessageSender keyer in keyers)
        {
            keyer.Finished += OnFinished;
        }
    }

    /// How long to leave between the message ending and the other radio
    /// starting. The relays of an SO2R box need a moment.
    public TimeSpan Gap { get; }

    public bool IsRunning
    {
        get
        {
            lock (gate)
            {
                return running is not null;
            }
        }
    }

    /// The radio it is calling on now.
    public int RadioNumber { get; private set; }

    /// True while a message finishing moves the CQ to the other radio, false
    /// while the operator is working a station.
    public bool CarriesOn
    {
        get
        {
            lock (gate)
            {
                return carriesOn;
            }
        }
    }

    /// Says the CQ message has gone out, so the next message that finishes
    /// moves the CQ to the other radio.
    public void CarryOn() => Set(true);

    /// Says something else has gone out, or that the operator has stopped the
    /// keyer, so the CQ stays where it is until the next one is sent.
    public void Hold() => Set(false);

    private void Set(bool value)
    {
        lock (gate)
        {
            carriesOn = value;
        }
    }

    /// Raised when it stops by itself, with the reason. Stopping by hand does
    /// not raise it.
    public event EventHandler<string>? Stopped;

    /// True when a keyer that can drive this at all is connected.
    public bool IsPossible => keyers.Any(keyer => keyer.ReportsCompletion);

    public void Start(int radioNumber)
    {
        if (!IsPossible)
        {
            throw new InvalidOperationException("this keyer does not report when a message has gone out");
        }
        CancellationToken token;
        lock (gate)
        {
            running?.Cancel();
            running?.Dispose();
            running = new CancellationTokenSource();
            token = running.Token;
        }
        RadioNumber = radioNumber;
        Set(true);
        _ = CallAsync(radioNumber, token);
    }

    public void Stop()
    {
        lock (gate)
        {
            running?.Cancel();
            running?.Dispose();
            running = null;
        }
    }

    public void Dispose()
    {
        foreach (MessageSender keyer in keyers)
        {
            keyer.Finished -= OnFinished;
        }
        Stop();
    }

    private void OnFinished(object? sender, EventArgs e)
    {
        CancellationToken token;
        lock (gate)
        {
            if (running is null || !carriesOn)
            {
                return;
            }
            token = running.Token;
        }
        _ = NextAsync(token);
    }

    private async Task NextAsync(CancellationToken token)
    {
        try
        {
            await wait(Gap, token).ConfigureAwait(false);
        }
        catch (OperationCanceledException)
        {
            return;
        }
        if (token.IsCancellationRequested)
        {
            return;
        }
        RadioNumber = RadioNumber == 1 ? 2 : 1;
        await CallAsync(RadioNumber, token).ConfigureAwait(false);
    }

    /// A keyer that has gone away stops the whole thing rather than leaving one
    /// radio calling into a dead port.
    private async Task CallAsync(int radioNumber, CancellationToken token)
    {
        try
        {
            await callCqOn(radioNumber).ConfigureAwait(false);
        }
        catch (Exception e) when (e is InvalidOperationException or IOException)
        {
            // read before Stop, which cancels this same token
            bool stoppedByHand = token.IsCancellationRequested;
            Stop();
            if (!stoppedByHand)
            {
                Stopped?.Invoke(this, e.Message);
            }
        }
    }
}