chesscubing/ChessCubing.Server/Social/ConnectedUserTracker.cs

namespace ChessCubing.Server.Social;

public sealed class ConnectedUserTracker
{
    private readonly object _sync = new();
    private readonly Dictionary<string, string> _subjectByConnection = new(StringComparer.Ordinal);
    private readonly Dictionary<string, HashSet<string>> _connectionsBySubject = new(StringComparer.Ordinal);

    public bool TrackConnection(string connectionId, string subject)
    {
        lock (_sync)
        {
            _subjectByConnection[connectionId] = subject;

            if (!_connectionsBySubject.TryGetValue(subject, out var connections))
            {
                connections = new HashSet<string>(StringComparer.Ordinal);
                _connectionsBySubject[subject] = connections;
            }

            var wasOffline = connections.Count == 0;
            connections.Add(connectionId);
            return wasOffline;
        }
    }

    public (string? Subject, bool BecameOffline) RemoveConnection(string connectionId)
    {
        lock (_sync)
        {
            if (!_subjectByConnection.Remove(connectionId, out var subject))
            {
                return (null, false);
            }

            if (!_connectionsBySubject.TryGetValue(subject, out var connections))
            {
                return (subject, false);
            }

            connections.Remove(connectionId);
            if (connections.Count > 0)
            {
                return (subject, false);
            }

            _connectionsBySubject.Remove(subject);
            return (subject, true);
        }
    }

    public bool IsOnline(string subject)
    {
        lock (_sync)
        {
            return _connectionsBySubject.TryGetValue(subject, out var connections)
                && connections.Count > 0;
        }
    }
}