watchparty/src/WatchParty/Parties/PartyManager.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;

namespace WatchParty.Parties
{
    /// <summary>
    /// In-memory store of active parties. Nothing is persisted to disk - a server
    /// restart clears every party, which is fine for v1 (a party is meant to live
    /// for one watch session, not persist for weeks). See docs/architecture.md.
    /// </summary>
    public class PartyManager
    {
        private static readonly Lazy<PartyManager> LazyInstance =
            new Lazy<PartyManager>(() => new PartyManager());

        public static PartyManager Instance => LazyInstance.Value;

        private readonly ConcurrentDictionary<Guid, Party> _partiesById = new ConcurrentDictionary<Guid, Party>();
        private readonly ConcurrentDictionary<string, Guid> _partyIdByJoinCode =
            new ConcurrentDictionary<string, Guid>(StringComparer.OrdinalIgnoreCase);

        public Party Create(string hostUserId, string hostUserName, string hostSessionId, string itemId)
        {
            var party = new Party
            {
                ItemId = itemId,
                HostUserId = hostUserId,
                ReferenceUserId = hostUserId,
                JoinCode = GenerateJoinCode()
            };

            party.Members[hostUserId] = new PartyMember
            {
                UserId = hostUserId,
                UserName = hostUserName,
                SessionId = hostSessionId,
                JoinedUtc = DateTime.UtcNow,
                LastUpdatedUtc = DateTime.UtcNow
            };

            _partiesById[party.Id] = party;
            _partyIdByJoinCode[party.JoinCode] = party.Id;
            return party;
        }

        public Party GetById(Guid partyId)
        {
            _partiesById.TryGetValue(partyId, out var party);
            return party;
        }

        public Party GetByJoinCode(string joinCode)
        {
            if (joinCode != null && _partyIdByJoinCode.TryGetValue(joinCode, out var partyId))
            {
                return GetById(partyId);
            }

            return null;
        }

        public PartyMember Join(Party party, string userId, string userName, string sessionId)
        {
            var member = new PartyMember
            {
                UserId = userId,
                UserName = userName,
                SessionId = sessionId,
                JoinedUtc = DateTime.UtcNow,
                LastUpdatedUtc = DateTime.UtcNow
            };

            party.Members[userId] = member;
            party.LastActivityUtc = DateTime.UtcNow;
            return member;
        }

        public void Leave(Party party, string userId)
        {
            party.Members.TryRemove(userId, out _);
            party.LastActivityUtc = DateTime.UtcNow;

            if (party.ReferenceUserId == userId)
            {
                // Reference left: fall back to whoever's left (arbitrary but
                // deterministic-ish; good enough for v1 - see docs/architecture.md).
                var next = party.Members.Keys.FirstOrDefault();
                if (next != null)
                {
                    party.ReferenceUserId = next;
                }
            }

            if (party.Members.IsEmpty)
            {
                _partiesById.TryRemove(party.Id, out _);
                _partyIdByJoinCode.TryRemove(party.JoinCode, out _);
            }
        }

        public IEnumerable<Party> GetAllActive() => _partiesById.Values;

        public void RemoveIdleParties(int idleTimeoutMinutes)
        {
            var cutoff = DateTime.UtcNow.AddMinutes(-idleTimeoutMinutes);
            foreach (var party in _partiesById.Values.Where(p => p.LastActivityUtc < cutoff).ToList())
            {
                _partiesById.TryRemove(party.Id, out _);
                _partyIdByJoinCode.TryRemove(party.JoinCode, out _);
            }
        }

        private static string GenerateJoinCode()
        {
            const string alphabet = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789"; // no 0/O/1/I
            var bytes = new byte[6];
            using (var rng = System.Security.Cryptography.RandomNumberGenerator.Create())
            {
                rng.GetBytes(bytes);
            }

            var chars = new char[6];
            for (var i = 0; i < 6; i++)
            {
                chars[i] = alphabet[bytes[i] % alphabet.Length];
            }

            return new string(chars);
        }
    }
}