feat(discovery): add NdiDiscoveryService with diff-based event emission

src/TeamsISO.Engine/Discovery/NdiDiscoveryService.cs

@@ -0,0 +1,79 @@
+using System.Threading.Channels;
+using Microsoft.Extensions.Logging;
+using TeamsISO.Engine.Interop;
+
+namespace TeamsISO.Engine.Discovery;
+
+/// <summary>
+/// Polls <see cref="INdiInterop.GetCurrentSources"/> at a fixed cadence, diffs the
+/// resulting set against the previous poll, and emits <see cref="DiscoveryEvent"/>s
+/// on a channel for downstream consumers.
+/// </summary>
+public sealed class NdiDiscoveryService
+{
+    private readonly INdiInterop _interop;
+    private readonly ChannelWriter<DiscoveryEvent> _writer;
+    private readonly ILogger<NdiDiscoveryService> _logger;
+    private readonly NdiFindHandle _finder;
+    private readonly HashSet<string> _previous = new();
+
+    public NdiDiscoveryService(
+        INdiInterop interop,
+        ChannelWriter<DiscoveryEvent> writer,
+        ILogger<NdiDiscoveryService> logger)
+    {
+        _interop = interop;
+        _writer = writer;
+        _logger = logger;
+        _finder = interop.CreateFinder();
+    }
+
+    /// <summary>
+    /// Runs a single poll cycle. Public for unit testing; production uses <see cref="RunAsync"/>.
+    /// </summary>
+    public void PollOnce()
+    {
+        var current = _interop.GetCurrentSources(_finder);
+        var currentSet = new HashSet<string>(current);
+
+        foreach (var name in currentSet.Except(_previous))
+        {
+            var parsed = NdiSourceParser.Parse(name);
+            if (parsed is null)
+            {
+                _logger.LogTrace("Ignoring unrecognized source: {Name}", name);
+                continue;
+            }
+            _writer.TryWrite(new DiscoveryEvent.Added(parsed));
+        }
+
+        foreach (var name in _previous.Except(currentSet))
+        {
+            var parsed = NdiSourceParser.Parse(name);
+            if (parsed is null) continue;
+            _writer.TryWrite(new DiscoveryEvent.Removed(parsed));
+        }
+
+        _previous.Clear();
+        foreach (var name in currentSet) _previous.Add(name);
+    }
+
+    /// <summary>Long-running poll loop. Cancel the token to stop.</summary>
+    public async Task RunAsync(TimeSpan pollInterval, CancellationToken cancellationToken)
+    {
+        using var timer = new PeriodicTimer(pollInterval);
+        try
+        {
+            while (await timer.WaitForNextTickAsync(cancellationToken))
+            {
+                try { PollOnce(); }
+                catch (Exception ex) { _logger.LogWarning(ex, "Discovery poll failed; will retry on next tick."); }
+            }
+        }
+        catch (OperationCanceledException) { /* expected */ }
+        finally
+        {
+            _finder.Dispose();
+        }
+    }
+}

src/tests/TeamsISO.Engine.Tests/Discovery/NdiDiscoveryServiceTests.cs

@@ -0,0 +1,69 @@
+using System.Threading.Channels;
+using Microsoft.Extensions.Logging.Abstractions;
+using TeamsISO.Engine.Discovery;
+using TeamsISO.Engine.Tests.Fakes;
+
+namespace TeamsISO.Engine.Tests.Discovery;
+
+public class NdiDiscoveryServiceTests
+{
+    [Fact]
+    public void PollOnce_AddsNewParticipantSources_AndIgnoresMalformedStrings()
+    {
+        var interop = new FakeNdiInterop();
+        interop.Sources.Add("PC1 (Teams - Jane)");
+        interop.Sources.Add("PC1 (Teams)");
+        interop.Sources.Add("Just A Camera");
+        interop.Sources.Add("BAD (Teams - )");
+        var channel = Channel.CreateUnbounded<DiscoveryEvent>();
+
+        var svc = new NdiDiscoveryService(interop, channel.Writer, NullLogger<NdiDiscoveryService>.Instance);
+
+        svc.PollOnce();
+
+        var emitted = DrainChannel(channel.Reader);
+        emitted.OfType<DiscoveryEvent.Added>().Select(a => a.Source.FullName)
+            .Should().BeEquivalentTo(new[] { "PC1 (Teams - Jane)", "PC1 (Teams)" });
+    }
+
+    [Fact]
+    public void PollOnce_EmitsRemoved_WhenSourceDisappears()
+    {
+        var interop = new FakeNdiInterop();
+        interop.Sources.Add("PC1 (Teams - Jane)");
+        var channel = Channel.CreateUnbounded<DiscoveryEvent>();
+
+        var svc = new NdiDiscoveryService(interop, channel.Writer, NullLogger<NdiDiscoveryService>.Instance);
+        svc.PollOnce();
+        DrainChannel(channel.Reader);
+
+        interop.Sources.Clear();
+        svc.PollOnce();
+
+        var emitted = DrainChannel(channel.Reader);
+        emitted.OfType<DiscoveryEvent.Removed>().Select(r => r.Source.FullName)
+            .Should().BeEquivalentTo(new[] { "PC1 (Teams - Jane)" });
+    }
+
+    [Fact]
+    public void PollOnce_NoChange_EmitsNothing()
+    {
+        var interop = new FakeNdiInterop();
+        interop.Sources.Add("PC1 (Teams - Jane)");
+        var channel = Channel.CreateUnbounded<DiscoveryEvent>();
+
+        var svc = new NdiDiscoveryService(interop, channel.Writer, NullLogger<NdiDiscoveryService>.Instance);
+        svc.PollOnce(); DrainChannel(channel.Reader);
+
+        svc.PollOnce();
+
+        DrainChannel(channel.Reader).Should().BeEmpty();
+    }
+
+    private static List<DiscoveryEvent> DrainChannel(ChannelReader<DiscoveryEvent> reader)
+    {
+        var list = new List<DiscoveryEvent>();
+        while (reader.TryRead(out var ev)) list.Add(ev);
+        return list;
+    }
+}