Relay/RelayServer/Services/Chat/ConnectedClientService.cs

using System.Collections.Concurrent;

namespace RelayServer.Services.Chat;

public static class ConnectedClientService
{
    private static readonly ConcurrentDictionary<string, string> SessionToUsername = new();
    private static readonly ConcurrentDictionary<string, HashSet<string>> UsernameToSessions =
        new(StringComparer.OrdinalIgnoreCase);

    public static void Register(string sessionId, string username)
    {
        if (SessionToUsername.TryGetValue(sessionId, out var oldUsername) &&
            !string.Equals(oldUsername, username, StringComparison.OrdinalIgnoreCase))
        {
            RemoveSessionFromUsername(sessionId, oldUsername);
        }

        SessionToUsername[sessionId] = username;

        var sessions = UsernameToSessions.GetOrAdd(username, _ => new HashSet<string>(StringComparer.Ordinal));
        lock (sessions)
            sessions.Add(sessionId);
    }

    public static void Unregister(string sessionId)
    {
        if (SessionToUsername.TryRemove(sessionId, out var username))
            RemoveSessionFromUsername(sessionId, username);
    }

    public static IReadOnlyCollection<string> GetSessionsForUser(string username)
    {
        if (UsernameToSessions.TryGetValue(username, out var sessions))
            lock (sessions)
                return sessions.ToList();

        return Array.Empty<string>();
    }

    public static string? GetUsernameForSession(string sessionId) =>
        SessionToUsername.TryGetValue(sessionId, out var u) ? u : null;

    private static void RemoveSessionFromUsername(string sessionId, string username)
    {
        if (!UsernameToSessions.TryGetValue(username, out var sessions))
            return;

        lock (sessions)
        {
            sessions.Remove(sessionId);
            if (sessions.Count == 0)
                UsernameToSessions.TryRemove(username, out _);
        }
    }
}