SVSimServer/SVSim.EmulatedEntrypoint/Matching/InProcessPairUp.cs

using System.Collections.Concurrent;
using SVSim.BattleNode.Bridge;
using SVSim.BattleNode.Sessions;

namespace SVSim.EmulatedEntrypoint.Matching;

/// <summary>
/// In-process FCFS pair-up: one waiting slot per mode, plus a per-viewer cache of
/// resolved matches for the first arriver's next poll (consume-on-read).
/// </summary>
public sealed class InProcessPairUp : IMatchingPairUpService
{
    private readonly IMatchingBridge _bridge;
    private readonly ConcurrentDictionary<string, ModeSlot> _slots = new();

    public InProcessPairUp(IMatchingBridge bridge)
    {
        _bridge = bridge;
    }

    public Task<PairUpResult?> TryPairAsync(string mode, BattlePlayer player, CancellationToken ct)
    {
        var slot = _slots.GetOrAdd(mode, _ => new ModeSlot());
        lock (slot.Lock)
        {
            // 1. Already-resolved match cached for this viewer? Consume + return.
            //    This caller is the FIRST arriver picking up their cached pair — owner role.
            if (slot.Resolved.TryGetValue(player.ViewerId, out var cached))
            {
                slot.Resolved.Remove(player.ViewerId);
                return Task.FromResult<PairUpResult?>(new PairUpResult(cached, IsOwner: true, IsAiFallback: false));
            }

            // 2. Someone already waiting in this slot? Pair with them.
            //    This caller is the SECOND arriver who triggered the pair — joiner role.
            if (slot.Waiting is not null)
            {
                if (slot.Waiting.ViewerId == player.ViewerId)
                {
                    // Same viewer polled twice while parked — keep them parked.
                    return Task.FromResult<PairUpResult?>(null);
                }
                var p1 = slot.Waiting;
                var p2 = player;
                slot.Waiting = null;
                var match = _bridge.RegisterBattle(p1, p2, BattleType.Pvp);
                // Cache the result for the FIRST arriver's next poll (consume-on-read).
                slot.Resolved[p1.ViewerId] = match;
                return Task.FromResult<PairUpResult?>(new PairUpResult(match, IsOwner: false, IsAiFallback: false));
            }

            // 3. Empty slot — park this caller.
            slot.Waiting = player;
            return Task.FromResult<PairUpResult?>(null);
        }
    }

    private sealed class ModeSlot
    {
        public BattlePlayer? Waiting { get; set; }
        public Dictionary<long, PendingMatch> Resolved { get; } = new();
        public object Lock { get; } = new();
    }
}