using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Text.Json.Nodes;
using CommonwealthOnline.Server;

namespace CommonwealthOnline.Server.Acceptance;

internal static class Program
{
    private static int _passed;
    private static int _failed;

    public static async Task<int> Main()
    {
        Console.WriteLine("Commonwealth Online - end-to-end acceptance harness (TCP)");

        if (!CanBindLoopback())
        {
            Console.WriteLine("SKIP: environment cannot bind a loopback TCP listener; end-to-end acceptance not executed here.");
            return 0;
        }

        await Run("baseline: two clients handshake welcome->hello->sessionReady", BaselineHandshake);
        await Run("baseline: server-owned ids are unique and not client-spoofable", ServerOwnedIdsNotSpoofable);
        await Run("baseline: malformed json rejected before gameplay mutation", MalformedJsonRejected);
        await Run("baseline: oversized payload rejected before gameplay mutation", OversizeRejected);
        await Run("baseline: sustained packet-rate abuse triggers rate limit", RateLimitTrips);
        await Run("tcp: two-client transform/playerState/worldState smoke", TwoClientSmoke);
        await Run("player-state: action events are not replay-cached to late joiners", ActionEventsNotReplayCached);
        await Run("tcp: disconnect leaves no stale session; reconnect works", DisconnectNoStale);
        await Run("movement: impossible movement is rejected, corrected, and not relayed", ImpossibleMovementRejected);
        await Run("combat: self-targeted combat hit is rejected", SelfTargetedCombatRejected);
        await Run("combat: replayed/out-of-order combat sequence is rejected", ReplayedCombatSequenceRejected);
        await Run("combat: combat hit at a disconnected target is rejected", DisconnectedTargetCombatRejected);
        await Run("combat: out-of-interest combat hit is rejected", OutOfInterestCombatRejected);
        await Run("interest: same-cell relays, distant is filtered", InterestSameCellVsDistant);
        await Run("authority: independent populated scopes get independent authorities", IndependentNpcAuthorities);
        await Run("authority: a stale npc authority epoch is rejected over transport", StaleNpcAuthorityEpochRejected);
        await Run("authority: disconnect deterministically hands off to a new owner", AuthorityHandoffOnDisconnect);
        await Run("authority: cell transition hands off and blocks the previous owner", AuthorityHandoffOnCellTransition);
        await Run("baseline: handshake timeout closes an unactivated session", HandshakeTimeoutCloses);
        await Run("baseline: idle timeout closes a stale active session", IdleTimeoutCloses);
        await Run("load: 16 clients across cells, no cross-cell transform spam", () => NoCrossCellSpam(16));
        await Run("load: 32 clients across cells, no cross-cell transform spam", () => NoCrossCellSpam(32));
        await Run("load: 64 clients across cells, no cross-cell transform spam", () => NoCrossCellSpam(64));
        await Run("load: burst transform traffic keeps the server responsive", BurstTransformTraffic);
        await Run("load: reconnect churn leaves no stale sessions", ReconnectChurn);
        await Run("load: authority handoff stays deterministic under churn", AuthorityHandoffUnderLoad);

        Console.WriteLine();
        Console.WriteLine($"acceptance: {_passed} passed, {_failed} failed");
        Console.WriteLine("deferred (follow-ups): packet loss/reorder on snapshot traffic and all GNS sections (both need the unreliable GNS path, blocked on #2).");
        return _failed == 0 ? 0 : 1;
    }

    // ---- baseline protocol --------------------------------------------------

    private static async Task BaselineHandshake()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        True(a.PlayerId != 0, "client a received a server-owned playerId");
        True(b.PlayerId != 0, "client b received a server-owned playerId");
        True(a.PlayerId != b.PlayerId, "player ids are unique");
    }

    private static async Task ServerOwnedIdsNotSpoofable()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await Drain(a, 300); await Drain(b, 300);

        var spoof = TransformPacket("0000AAAA", "0000BBBB", 0, 0, "spawn");
        spoof["playerId"] = 999999;
        await a.SendAsync(spoof);

        var relay = await ReceiveUntil(b, p => Type(p) == "transform", 3000);
        NotNull(relay, "b received a's transform");
        Equal(a.PlayerId, UInt(relay!["playerId"]), "relayed transform carries the server-owned id");
    }

    private static async Task MalformedJsonRejected()
    {
        await using var s = new TestServer();
        using var raw = await RawClient.Connect(s.Port);
        await raw.ExpectType("welcome", 2000);
        await raw.WriteLine(Encoding.UTF8.GetBytes("{\"type\":\"hello\",\"protocolVersion\":2}"));
        await raw.ExpectType("sessionReady", 2000);
        var before = Stat(s.Server, "packetsRejected");
        var transformsBefore = Stat(s.Server, "transformPacketsReceived");

        await raw.WriteLine(Encoding.UTF8.GetBytes("{ this is not valid json"));
        await SpinUntil(() => Stat(s.Server, "packetsRejected") > before, 2000);

        True(Stat(s.Server, "packetsRejected") > before, "malformed json incremented packetsRejected");
        Equal(transformsBefore, Stat(s.Server, "transformPacketsReceived"), "no gameplay mutation from malformed json");
    }

    private static async Task OversizeRejected()
    {
        await using var s = new TestServer();
        using var raw = await RawClient.Connect(s.Port);
        await raw.ExpectType("welcome", 2000);
        await raw.WriteLine(Encoding.UTF8.GetBytes("{\"type\":\"hello\",\"protocolVersion\":2}"));
        await raw.ExpectType("sessionReady", 2000);
        var transformsBefore = Stat(s.Server, "transformPacketsReceived");

        await raw.WriteRaw(new byte[ProtocolConstants.MaxMessageBytes + 1024]);

        True(await raw.WaitClosed(4000), "server closed the connection on oversize input");
        Equal(transformsBefore, Stat(s.Server, "transformPacketsReceived"), "oversize input caused no gameplay mutation");
    }

    private static async Task RateLimitTrips()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        for (var i = 0; i < 400; i++)
        {
            try { await a.SendAsync(new JsonObject { ["type"] = "keepAlive" }); }
            catch { break; }
        }
        await SpinUntil(() => Stat(s.Server, "rateLimitedPackets") > 0, 3000);
        True(Stat(s.Server, "rateLimitedPackets") > 0, "sustained burst tripped the rate limiter");
    }

    // ---- tcp compatibility --------------------------------------------------

    private static async Task TwoClientSmoke()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000CAFE", "0000F00D", 0, 0, "spawn"));
        await b.SendAsync(TransformPacket("0000CAFE", "0000F00D", 0, 0, "spawn"));
        await Drain(a, 400); await Drain(b, 400);

        await a.SendAsync(TransformPacket("0000CAFE", "0000F00D", 10, 0));
        NotNull(await ReceiveUntil(b, p => Type(p) == "transform" && UInt(p["playerId"]) == a.PlayerId, 3000),
            "b received a's transform");

        await a.SendAsync(new JsonObject
        {
            ["type"] = "playerState",
            ["characterName"] = "Nomad",
            ["actionEvents"] = new JsonArray(new JsonObject { ["sequence"] = 1, ["type"] = 3, ["eventName"] = "fireSingle" })
        });
        var ps = await ReceiveUntil(b, p => Type(p) == "playerState" && UInt(p["playerId"]) == a.PlayerId, 3000);
        NotNull(ps, "b received a's playerState");
        Equal("Nomad", JsonHelpers.String(ps!["characterName"]), "playerState carried the character name");

        await a.SendAsync(new JsonObject { ["type"] = "worldState", ["timeHours"] = 12.0 });
        NotNull(await ReceiveUntil(b, p => Type(p) == "worldState", 3000), "b received world state from the host");
    }

    private static async Task ActionEventsNotReplayCached()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000AC70", "0000AC70", 0, 0, "spawn"));
        await a.SendAsync(new JsonObject
        {
            ["type"] = "playerState",
            ["characterName"] = "Nomad",
            ["actionEvents"] = new JsonArray(new JsonObject { ["sequence"] = 1, ["type"] = 3, ["eventName"] = "fireSingle" })
        });
        await Drain(a, 300);

        await using var late = await Connect(s.Port);
        var replay = await ReceiveUntil(late, p => Type(p) == "playerState" && UInt(p["playerId"]) == a.PlayerId, 3000);
        NotNull(replay, "late joiner received a's durable player state");
        Equal("Nomad", JsonHelpers.String(replay!["characterName"]), "durable state replayed the character name");
        True(!replay.ContainsKey("actionEvents"), "discrete action events were not replay-cached to the late joiner");
    }

    private static async Task DisconnectNoStale()
    {
        await using var s = new TestServer();
        var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await SpinUntil(() => Stat(s.Server, "connectedClients") == 2, 3000);
        Equal(2u, Stat(s.Server, "connectedClients"), "two clients active");

        await a.DisposeAsync();
        await SpinUntil(() => Stat(s.Server, "connectedClients") == 1, 4000);
        Equal(1u, Stat(s.Server, "connectedClients"), "disconnect removed the session");

        await using var a2 = await Connect(s.Port);
        await SpinUntil(() => Stat(s.Server, "connectedClients") == 2, 3000);
        Equal(2u, Stat(s.Server, "connectedClients"), "reconnect restored two active sessions");
        True(a2.PlayerId != 0, "reconnected client received a fresh server-owned id");
    }

    // ---- interest management ------------------------------------------------

    private static async Task InterestSameCellVsDistant()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000BEEF", "0000BEEF", 0, 0, "spawn"));
        await b.SendAsync(TransformPacket("0000BEEF", "0000BEEF", 0, 0, "spawn"));
        await Drain(a, 400); await Drain(b, 400);

        await a.SendAsync(TransformPacket("0000BEEF", "0000BEEF", 5, 5));
        NotNull(await ReceiveUntil(b, p => Type(p) == "transform" && UInt(p["playerId"]) == a.PlayerId, 3000),
            "same-cell transform relayed to b");

        var filteredBefore = Stat(s.Server, "transformPacketsInterestFiltered");
        await a.SendAsync(TransformPacket("00000111", "00000222", 5_000_000, 5_000_000, "cell_change"));
        await Drain(a, 300);
        True(await ExpectNone(b, p => Type(p) == "transform" && UInt(p["playerId"]) == a.PlayerId, 1200),
            "distant transform was not relayed to b");
        await SpinUntil(() => Stat(s.Server, "transformPacketsInterestFiltered") > filteredBefore, 1500);
        True(Stat(s.Server, "transformPacketsInterestFiltered") > filteredBefore, "distant transform counted as interest-filtered");
    }

    private static async Task NoCrossCellSpam(int count)
    {
        await using var s = new TestServer(maxPlayers: Math.Max(64, count),
            tuning: new ServerTuning { MaxConnectAttempts = count * 4 });
        var clients = new List<SyntheticProtocolClient>();
        try
        {
            for (var i = 0; i < count; i++)
            {
                var c = await Connect(s.Port);
                clients.Add(c);
                await c.SendAsync(TransformPacket((0x2000 + i).ToString("X8"), (0x9000 + i).ToString("X8"), i * 1000.0, 0, "spawn"));
            }
            var ids = clients.Select(c => c.PlayerId).ToArray();
            Equal(count, ids.Distinct().Count(), "unique server-owned ids for every client");
            foreach (var c in clients) await Drain(c, 100);

            for (var i = 0; i < clients.Count; i++)
                await clients[i].SendAsync(TransformPacket((0x2000 + i).ToString("X8"), (0x9000 + i).ToString("X8"), i * 1000.0 + 10, 0));

            foreach (var c in clients)
                True(await ExpectNone(c, p => Type(p) == "transform", 400), "client received no cross-cell transform spam");
        }
        finally
        {
            foreach (var c in clients) await c.DisposeAsync();
        }
    }

    // ---- movement / combat validation --------------------------------------

    private static async Task ImpossibleMovementRejected()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000D00D", "0000D00D", 0, 0, "spawn"));
        await b.SendAsync(TransformPacket("0000D00D", "0000D00D", 0, 0, "spawn"));
        await Drain(a, 300); await Drain(b, 300);
        var rejectedBefore = Stat(s.Server, "movementPacketsRejected");

        await a.SendAsync(TransformPacket("0000D00D", "0000D00D", 1_000_000, 0, "normal"));

        NotNull(await ReceiveUntil(a, p => Type(p) == "positionCorrection", 3000),
            "sender received a position correction");
        True(await ExpectNone(b, p => Type(p) == "transform" && UInt(p["playerId"]) == a.PlayerId, 1000),
            "impossible movement was not relayed to b");
        await SpinUntil(() => Stat(s.Server, "movementPacketsRejected") > rejectedBefore, 1500);
        True(Stat(s.Server, "movementPacketsRejected") > rejectedBefore, "impossible movement counted as rejected");
    }

    private static async Task SelfTargetedCombatRejected()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000C0FE", "0000C0FE", 0, 0, "spawn"));
        await Drain(a, 300);
        var rejectedBefore = Stat(s.Server, "packetsRejected");
        var routedBefore = Stat(s.Server, "combatHitsRouted");

        await a.SendAsync(new JsonObject { ["type"] = "combatHit", ["targetPlayerId"] = a.PlayerId, ["sequence"] = 1, ["damage"] = 10.0 });

        await SpinUntil(() => Stat(s.Server, "packetsRejected") > rejectedBefore, 2000);
        True(Stat(s.Server, "packetsRejected") > rejectedBefore, "self-targeted combat hit was rejected");
        Equal(routedBefore, Stat(s.Server, "combatHitsRouted"), "self-targeted combat hit was not routed");
    }

    private static async Task ReplayedCombatSequenceRejected()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000CB01", "0000CB01", 0, 0, "spawn"));
        await b.SendAsync(TransformPacket("0000CB01", "0000CB01", 0, 0, "spawn"));
        await Drain(a, 300); await Drain(b, 300);

        await a.SendAsync(CombatHit(b.PlayerId, 5));
        NotNull(await ReceiveUntil(b, p => Type(p) == "combatHit", 3000), "in-interest combat hit routed to the target");
        var routedBefore = Stat(s.Server, "combatHitsRouted");
        var rejectedBefore = Stat(s.Server, "packetsRejected");

        await a.SendAsync(CombatHit(b.PlayerId, 3));
        await SpinUntil(() => Stat(s.Server, "packetsRejected") > rejectedBefore, 2000);
        True(Stat(s.Server, "packetsRejected") > rejectedBefore, "replayed combat sequence was rejected");
        Equal(routedBefore, Stat(s.Server, "combatHitsRouted"), "replayed combat sequence was not routed");
        True(await ExpectNone(b, p => Type(p) == "combatHit", 500), "target did not receive the replayed hit");
    }

    private static async Task DisconnectedTargetCombatRejected()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000CB02", "0000CB02", 0, 0, "spawn"));
        await Drain(a, 300);
        var rejectedBefore = Stat(s.Server, "packetsRejected");
        var routedBefore = Stat(s.Server, "combatHitsRouted");

        await a.SendAsync(CombatHit(999999, 1));
        await SpinUntil(() => Stat(s.Server, "packetsRejected") > rejectedBefore, 2000);
        True(Stat(s.Server, "packetsRejected") > rejectedBefore, "combat hit at a disconnected target was rejected");
        Equal(routedBefore, Stat(s.Server, "combatHitsRouted"), "disconnected-target combat was not routed");
    }

    private static async Task OutOfInterestCombatRejected()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000CB03", "0000CB03", 0, 0, "spawn"));
        await b.SendAsync(TransformPacket("0000CB04", "0000CB04", 1000, 0, "spawn"));
        await Drain(a, 300); await Drain(b, 300);
        var rejectedBefore = Stat(s.Server, "packetsRejected");
        var routedBefore = Stat(s.Server, "combatHitsRouted");

        await a.SendAsync(CombatHit(b.PlayerId, 1));
        await SpinUntil(() => Stat(s.Server, "packetsRejected") > rejectedBefore, 2000);
        True(Stat(s.Server, "packetsRejected") > rejectedBefore, "out-of-interest combat hit was rejected");
        Equal(routedBefore, Stat(s.Server, "combatHitsRouted"), "out-of-interest combat was not routed");
        True(await ExpectNone(b, p => Type(p) == "combatHit", 500), "distant target did not receive the hit");
    }

    private static JsonObject CombatHit(uint targetPlayerId, uint sequence) => new()
    {
        ["type"] = "combatHit", ["targetPlayerId"] = targetPlayerId, ["sequence"] = sequence, ["damage"] = 10.0
    };

    // ---- npc authority ------------------------------------------------------

    private static async Task IndependentNpcAuthorities()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000A111", "0000A111", 0, 0, "spawn"));
        await b.SendAsync(TransformPacket("0000B222", "0000B222", 5_000_000, 0, "spawn"));

        var grantA = await ReceiveUntil(a, p => Type(p) == "npcAuthority" && UInt(p["authorityPlayerId"]) == a.PlayerId, 3000);
        var grantB = await ReceiveUntil(b, p => Type(p) == "npcAuthority" && UInt(p["authorityPlayerId"]) == b.PlayerId, 3000);
        NotNull(grantA, "a received authority for its own scope");
        NotNull(grantB, "b received authority for its own scope");
        True(UInt(grantA!["authorityEpoch"]) >= 1, "a's authority carries an epoch");
        True(UInt(grantB!["authorityEpoch"]) >= 1, "b's authority carries an epoch");
        Equal(2u, Stat(s.Server, "npcAuthorityScopes"), "two independent authority scopes exist");
    }

    private static async Task StaleNpcAuthorityEpochRejected()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000E999", "0000E999", 0, 0, "spawn"));
        var grant = await ReceiveUntil(a, p => Type(p) == "npcAuthority" && UInt(p["authorityPlayerId"]) == a.PlayerId, 3000);
        NotNull(grant, "a became the authority for its scope");
        var epoch = UInt(grant!["authorityEpoch"]);
        await Drain(a, 200);
        var rejectsBefore = Stat(s.Server, "npcAuthorityRejects");

        await a.SendAsync(new JsonObject
        {
            ["type"] = "npcState",
            ["authorityEpoch"] = epoch + 1,
            ["authorityCellId"] = "0000E999",
            ["authorityWorldspaceId"] = "0000E999",
            ["npcs"] = new JsonArray()
        });

        await SpinUntil(() => Stat(s.Server, "npcAuthorityRejects") > rejectsBefore, 2000);
        True(Stat(s.Server, "npcAuthorityRejects") > rejectsBefore, "npcState with a stale/wrong epoch was rejected");
    }

    private static async Task AuthorityHandoffOnDisconnect()
    {
        await using var s = new TestServer();
        var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000A0FF", "0000A0FF", 0, 0, "spawn"));
        var grantA = await ReceiveUntil(a, p => Type(p) == "npcAuthority" && UInt(p["authorityPlayerId"]) == a.PlayerId, 3000);
        NotNull(grantA, "a is the initial authority for the shared scope");
        var epochA = UInt(grantA!["authorityEpoch"]);
        await b.SendAsync(TransformPacket("0000A0FF", "0000A0FF", 0, 0, "spawn"));
        await Drain(a, 300); await Drain(b, 300);

        await a.DisposeAsync();
        var grantB = await ReceiveUntil(b, p => Type(p) == "npcAuthority" && UInt(p["authorityPlayerId"]) == b.PlayerId, 4000);
        NotNull(grantB, "authority handed off to b after a disconnected");
        True(UInt(grantB!["authorityEpoch"]) > epochA, "handoff carries a newer epoch");
    }

    private static async Task AuthorityHandoffOnCellTransition()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000CE10", "0000CE10", 0, 0, "spawn"));
        var grantA = await ReceiveUntil(a, p => Type(p) == "npcAuthority" && UInt(p["authorityPlayerId"]) == a.PlayerId, 3000);
        NotNull(grantA, "a is authority for scope X");
        var epochX1 = UInt(grantA!["authorityEpoch"]);
        await b.SendAsync(TransformPacket("0000CE10", "0000CE10", 0, 0, "spawn"));
        await Drain(a, 300); await Drain(b, 300);

        await a.SendAsync(TransformPacket("0000CE20", "0000CE20", 0, 0, "cell_change"));
        var grantB = await ReceiveUntil(b, p => Type(p) == "npcAuthority" && UInt(p["authorityPlayerId"]) == b.PlayerId, 4000);
        NotNull(grantB, "scope X handed off to b after a transitioned away");
        True(UInt(grantB!["authorityEpoch"]) > epochX1, "handoff carries a newer epoch");

        await Drain(a, 200);
        var rejectsBefore = Stat(s.Server, "npcAuthorityRejects");
        await a.SendAsync(new JsonObject
        {
            ["type"] = "npcState",
            ["authorityEpoch"] = epochX1,
            ["authorityCellId"] = "0000CE10",
            ["authorityWorldspaceId"] = "0000CE10",
            ["npcs"] = new JsonArray()
        });
        await SpinUntil(() => Stat(s.Server, "npcAuthorityRejects") > rejectsBefore, 2000);
        True(Stat(s.Server, "npcAuthorityRejects") > rejectsBefore, "previous owner cannot submit for the reassigned scope");
    }

    // ---- load / fault -------------------------------------------------------

    private static async Task BurstTransformTraffic()
    {
        await using var s = new TestServer();
        await using var a = await Connect(s.Port);
        await using var b = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000B057", "0000B057", 5, 5, "spawn"));
        await b.SendAsync(TransformPacket("0000B057", "0000B057", 5, 5, "spawn"));
        await Drain(a, 300); await Drain(b, 300);
        var receivedBefore = Stat(s.Server, "transformPacketsReceived");

        for (var i = 0; i < 400; i++)
        {
            try { await a.SendAsync(TransformPacket("0000B057", "0000B057", 5, 5, "normal")); }
            catch { break; }
        }
        await SpinUntil(() => Stat(s.Server, "rateLimitedPackets") > 0, 3000);
        True(Stat(s.Server, "rateLimitedPackets") > 0, "the burst engaged the rate limiter");
        True(Stat(s.Server, "transformPacketsReceived") > receivedBefore, "the server processed transforms during the burst");
        Equal(2u, Stat(s.Server, "connectedClients"), "both sessions survived the burst");

        await Drain(b, 300);
        var relayed = false;
        for (var attempt = 0; attempt < 5 && !relayed; attempt++)
            relayed = await RelayWorks(a, b);
        True(relayed, "server relays a fresh transform after the burst");
    }

    private static async Task<bool> RelayWorks(SyntheticProtocolClient a, SyntheticProtocolClient b)
    {
        await a.SendAsync(TransformPacket("0000B057", "0000B057", 6, 6, "normal"));
        return await ReceiveUntil(b, p => Type(p) == "transform" && UInt(p["playerId"]) == a.PlayerId, 800) is not null;
    }

    private static async Task ReconnectChurn()
    {
        await using var s = new TestServer(tuning: new ServerTuning { MaxConnectAttempts = 128 });
        uint lastId = 0;
        for (var k = 0; k < 12; k++)
        {
            var c = await Connect(s.Port);
            True(c.PlayerId > lastId, "server-owned ids are monotonic and never reused across reconnects");
            lastId = c.PlayerId;
            await c.SendAsync(TransformPacket("0000C401", "0000C401", 0, 0, "spawn"));
            await SpinUntil(() => Stat(s.Server, "connectedClients") == 1, 3000);
            await c.DisposeAsync();
            await SpinUntil(() => Stat(s.Server, "connectedClients") == 0, 3000);
            Equal(0u, Stat(s.Server, "connectedClients"), "each disconnect leaves no stale active session");
        }
        await using var final = await Connect(s.Port);
        True(final.PlayerId > lastId, "server still assigns fresh ids after churn");
    }

    private static async Task AuthorityHandoffUnderLoad()
    {
        const int n = 6;
        await using var s = new TestServer(tuning: new ServerTuning { MaxConnectAttempts = 64 });
        var clients = new List<SyntheticProtocolClient>();
        try
        {
            for (var i = 0; i < n; i++)
            {
                var c = await Connect(s.Port);
                clients.Add(c);
                await c.SendAsync(TransformPacket("0000A11D", "0000A11D", 0, 0, "spawn"));
            }
            var watcher = clients[n - 1];
            await Drain(watcher, 300);
            uint lastEpoch = 0;

            for (var i = 0; i < 3; i++)
            {
                await clients[i].DisposeAsync();
                var expected = clients[i + 1].PlayerId;
                var grant = await ReceiveUntil(watcher, p => Type(p) == "npcAuthority" && UInt(p["authorityPlayerId"]) == expected, 4000);
                NotNull(grant, $"authority handed off to the next owner after retiring holder {i}");
                var epoch = UInt(grant!["authorityEpoch"]);
                True(epoch > lastEpoch, "each handoff carries a strictly newer epoch");
                lastEpoch = epoch;
            }
        }
        finally
        {
            foreach (var c in clients) await c.DisposeAsync();
        }
    }

    // ---- session teardown ---------------------------------------------------

    private static async Task HandshakeTimeoutCloses()
    {
        await using var s = new TestServer();
        using var raw = await RawClient.Connect(s.Port);
        await raw.ExpectType("welcome", 2000);
        Equal(0u, Stat(s.Server, "connectedClients"), "pending session is not counted as active");
        True(await raw.WaitClosed(14000), "server reaped the unactivated session after the handshake timeout");
        Equal(0u, Stat(s.Server, "connectedClients"), "still no active clients after reap");
    }

    private static async Task IdleTimeoutCloses()
    {
        await using var s = new TestServer(tuning: new ServerTuning { ClientIdleTimeoutSeconds = 2.0 });
        await using var a = await Connect(s.Port);
        await a.SendAsync(TransformPacket("0000171E", "0000171E", 0, 0, "spawn"));
        await SpinUntil(() => Stat(s.Server, "connectedClients") == 1, 3000);
        Equal(1u, Stat(s.Server, "connectedClients"), "client is active before going idle");

        await SpinUntil(() => Stat(s.Server, "connectedClients") == 0, 6000);
        Equal(0u, Stat(s.Server, "connectedClients"), "idle active session was reaped");
    }

    // ---- infrastructure -----------------------------------------------------

    private static async Task Run(string name, Func<Task> test)
    {
        try { await test(); _passed++; Console.WriteLine($"PASS {name}"); }
        catch (Exception ex) { _failed++; Console.Error.WriteLine($"FAIL {name}: {ex.Message}"); }
    }

    private static bool CanBindLoopback()
    {
        try { var p = FreePort(); return p > 0; }
        catch (SocketException) { return false; }
    }

    private static int FreePort()
    {
        var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        try { return ((IPEndPoint)listener.LocalEndpoint).Port; }
        finally { listener.Stop(); }
    }

    private static async Task<SyntheticProtocolClient> Connect(int port)
    {
        var client = new SyntheticProtocolClient();
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));
        await client.ConnectAsync("127.0.0.1", port, timeout.Token);
        return client;
    }

    private static JsonObject TransformPacket(string cell, string world, double x, double y, string movement = "normal") => new()
    {
        ["type"] = "transform", ["x"] = x, ["y"] = y, ["z"] = 0.0, ["angleZ"] = 0.0,
        ["cellId"] = cell, ["worldspaceId"] = world, ["movementType"] = movement
    };

    private static string? Type(JsonObject packet) => JsonHelpers.String(packet["type"]);
    private static uint UInt(JsonNode? node) => JsonHelpers.TryUInt32(node, 0, uint.MaxValue, out var v) ? v : 0;
    private static uint Stat(AuthoritativeServer server, string name) => JsonHelpers.TryUInt32(server.GetCoreStats()[name], 0, uint.MaxValue, out var v) ? v : 0;

    private static async Task<JsonObject?> TryReceive(SyntheticProtocolClient client, int timeoutMs)
    {
        using var cts = new CancellationTokenSource(timeoutMs);
        try { return await client.ReceiveAsync(cts.Token); }
        catch (OperationCanceledException) { return null; }
        catch (EndOfStreamException) { return null; }
        catch (IOException) { return null; }
        catch (InvalidDataException) { return null; }
    }

    private static async Task<JsonObject?> ReceiveUntil(SyntheticProtocolClient client, Func<JsonObject, bool> predicate, int timeoutMs)
    {
        var deadline = DateTime.UtcNow.AddMilliseconds(timeoutMs);
        while (DateTime.UtcNow < deadline)
        {
            var remaining = (int)Math.Max(1, (deadline - DateTime.UtcNow).TotalMilliseconds);
            var packet = await TryReceive(client, remaining);
            if (packet is null) break;
            if (predicate(packet)) return packet;
        }
        return null;
    }

    private static async Task<bool> ExpectNone(SyntheticProtocolClient client, Func<JsonObject, bool> predicate, int windowMs)
    {
        var deadline = DateTime.UtcNow.AddMilliseconds(windowMs);
        while (DateTime.UtcNow < deadline)
        {
            var remaining = (int)Math.Max(1, (deadline - DateTime.UtcNow).TotalMilliseconds);
            var packet = await TryReceive(client, remaining);
            if (packet is null) break;
            if (predicate(packet)) return false;
        }
        return true;
    }

    private static Task Drain(SyntheticProtocolClient client, int windowMs) => ExpectNone(client, _ => false, windowMs);

    private static async Task SpinUntil(Func<bool> condition, int timeoutMs)
    {
        var deadline = DateTime.UtcNow.AddMilliseconds(timeoutMs);
        while (DateTime.UtcNow < deadline)
        {
            if (condition()) return;
            await Task.Delay(50);
        }
    }

    private static void True(bool value, string what) { if (!value) throw new Exception($"expected: {what}"); }
    private static void NotNull(object? value, string what) { if (value is null) throw new Exception($"expected non-null: {what}"); }
    private static void Equal<T>(T expected, T actual, string what)
    {
        if (!EqualityComparer<T>.Default.Equals(expected, actual)) throw new Exception($"{what}: expected {expected}, got {actual}");
    }

    private sealed class TestServer : IAsyncDisposable
    {
        private readonly TcpServerTransport _tcp;
        private readonly string _dir;

        public TestServer(int maxPlayers = 32, ServerTuning? tuning = null)
        {
            Port = FreePort();
            _dir = Path.Combine(Path.GetTempPath(), "co-accept-" + Guid.NewGuid().ToString("N"));
            Directory.CreateDirectory(_dir);
            var options = new ServerOptions
            {
                ConfigPath = Path.Combine(_dir, "commonwealth-server.json"),
                Host = "127.0.0.1", Port = Port, AdminPort = FreePort(), MaxPlayers = maxPlayers
            };
            Server = new AuthoritativeServer(options, tuning);
            _tcp = new TcpServerTransport(options, Server);
            _tcp.Start();
        }

        public int Port { get; }
        public AuthoritativeServer Server { get; }

        public async ValueTask DisposeAsync()
        {
            await _tcp.DisposeAsync();
            await Server.DisposeAsync();
            try { Directory.Delete(_dir, true); } catch { }
        }
    }

    private sealed class RawClient : IDisposable
    {
        private readonly TcpClient _client;
        private readonly NetworkStream _stream;

        private RawClient(TcpClient client) { _client = client; _stream = client.GetStream(); }

        public static async Task<RawClient> Connect(int port)
        {
            var client = new TcpClient { NoDelay = true };
            await client.ConnectAsync(IPAddress.Loopback, port);
            return new RawClient(client);
        }

        public async Task WriteLine(byte[] payload)
        {
            await _stream.WriteAsync(payload);
            await _stream.WriteAsync(new byte[] { (byte)'\n' });
        }

        public Task WriteRaw(byte[] bytes) => _stream.WriteAsync(bytes).AsTask();

        public async Task ExpectType(string type, int timeoutMs)
        {
            var packet = await ReadLine(timeoutMs) ?? throw new Exception($"expected {type}, got connection close");
            var actual = JsonHelpers.String(packet["type"]);
            if (actual != type) throw new Exception($"expected {type}, got {actual}");
        }

        public async Task<JsonObject?> ReadLine(int timeoutMs)
        {
            using var cts = new CancellationTokenSource(timeoutMs);
            var buffer = new MemoryStream();
            var one = new byte[1];
            try
            {
                while (buffer.Length <= ProtocolConstants.MaxMessageBytes)
                {
                    var n = await _stream.ReadAsync(one, cts.Token);
                    if (n == 0) return null;
                    if (one[0] == (byte)'\n')
                    {
                        var data = buffer.ToArray();
                        if (data.Length > 0 && data[^1] == (byte)'\r') Array.Resize(ref data, data.Length - 1);
                        return JsonNode.Parse(data) as JsonObject;
                    }
                    buffer.WriteByte(one[0]);
                }
            }
            catch (OperationCanceledException) { return null; }
            catch (IOException) { return null; }
            return null;
        }

        public async Task<bool> WaitClosed(int timeoutMs)
        {
            using var cts = new CancellationTokenSource(timeoutMs);
            var one = new byte[1];
            try
            {
                while (true)
                {
                    var n = await _stream.ReadAsync(one, cts.Token);
                    if (n == 0) return true;
                }
            }
            catch (OperationCanceledException) { return false; }
            catch (IOException) { return true; }
            catch (SocketException) { return true; }
        }

        public void Dispose()
        {
            try { _stream.Dispose(); } catch { }
            try { _client.Dispose(); } catch { }
        }
    }
}