SocialPub/PrivaPub/Domain/Timelines/Streams.cs

using System.Collections.Concurrent;
using System.Threading.Channels;

namespace PrivaPub.Domain.Timelines
{
	// Mastodon's streaming, inside this one process: what happens (a post arriving in someone's home or a public feed, a
	// notification, an edit, a deletion) is told to the connections listening for it, by id only. Each connection maps a
	// post or a notification for its own persona when it sends it (StreamingController), so blocks, mutes and visibility
	// are decided there, never here. Nothing is kept: a connection that is gone misses what happened meanwhile, and a
	// client reads the timeline again when it comes back.
	public static class Streams
	{
		public const string User = "user";
		public const string UserNotification = "user:notification";
		public const string Public = "public";
		public const string PublicLocal = "public:local";
		public const string PublicRemote = "public:remote";
		public const string Hashtag = "hashtag";
		public const string HashtagLocal = "hashtag:local";
		public const string List = "list";
		public const string Direct = "direct";

		public static readonly string[] Names = { User, UserNotification, Public, PublicLocal, PublicRemote, Hashtag, HashtagLocal, List, Direct };

		// what happened: Mastodon's event name, the id of what it is about and, for some, what else the event says (an
		// announcement reaction's emoji)
		public sealed record Event(string Name, string Id, string Data = default);

		// one stream a connection listens to: its name and, for a hashtag or a list, which one
		public sealed record Key(string Name, string Parameter = default)
		{
			public string[] Wire => Parameter == default ? new[] { Name } : new[] { Name, Parameter };
		}

		public sealed class Listener
		{
			readonly ConcurrentDictionary<Key, byte> _keys = new();

			public Listener(string avatarId)
			{
				AvatarId = avatarId;
			}

			public string AvatarId { get; }

			// what reached it and was not sent yet; the oldest goes first when a slow connection falls behind
			public Channel<(Key Key, Event Event)> Queue { get; } =
				Channel.CreateBounded<(Key, Event)>(new BoundedChannelOptions(1000) { FullMode = BoundedChannelFullMode.DropOldest, SingleReader = true });

			public void Listen(Key key) => _keys[key] = 0;

			public void Stop(Key key) => _keys.TryRemove(key, out _);

			public bool Hears(Key key) => _keys.ContainsKey(key);

			public IEnumerable<Key> Keys => _keys.Keys;

			public void Tell(Key key, Event e) => Queue.Writer.TryWrite((key, e));
		}

		static readonly ConcurrentDictionary<Listener, byte> Listeners = new();

		public static Listener Open(string avatarId)
		{
			var listener = new Listener(avatarId);
			Listeners[listener] = 0;
			return listener;
		}

		public static void Close(Listener listener)
		{
			Listeners.TryRemove(listener, out _);
			listener.Queue.Writer.TryComplete();
		}

		public static int Count => Listeners.Count;

		// whether the persona has a connection open, so the work of finding its lists can be skipped when it has none
		public static bool Listening(string avatarId) => Listeners.Keys.Any(l => l.AvatarId == avatarId);

		// to the persona's own streams: its home (user) and, for a notification, user:notification too
		public static void ToPersona(string avatarId, Key key, Event e)
		{
			foreach (var listener in Listeners.Keys)
				if (listener.AvatarId == avatarId && listener.Hears(key))
					listener.Tell(key, e);
		}

		// to everyone listening to a shared stream (public, a hashtag)
		public static void ToAll(Key key, Event e)
		{
			foreach (var listener in Listeners.Keys)
				if (listener.Hears(key))
					listener.Tell(key, e);
		}

		// to every stream the persona listens to (a deletion: wherever its client may show the post)
		public static void ToPersonaEverywhere(string avatarId, Event e)
		{
			foreach (var listener in Listeners.Keys)
				if (listener.AvatarId == avatarId)
					foreach (var key in listener.Keys)
						listener.Tell(key, e);
		}

		// to every persona's home stream (user), as Mastodon tells its announcements: each connection maps it for its own
		// persona when it sends it
		public static void ToUsers(Event e)
		{
			var user = new Key(User);
			foreach (var listener in Listeners.Keys)
				if (listener.Hears(user))
					listener.Tell(user, e);
		}

		public static bool Anyone => !Listeners.IsEmpty;
	}
}