using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using AcDream.App.Rendering.Vfx;
using AcDream.App.Rendering.Wb;
using AcDream.App.World;
using AcDream.Core.World;

namespace AcDream.App.Streaming;

/// <summary>
/// Exact result of one committed landblock bucket mutation. Fallible mesh-pin
/// and static-script reconciliation consume this receipt separately, so a
/// callback failure cannot force the spatial mutation to replay.
/// </summary>
internal sealed record GpuLandblockSpatialPublication(
    uint LandblockId,
    LoadedLandblock Landblock,
    IReadOnlyList<ulong> AdditionalRenderIds,
    IReadOnlyList<WorldEntity> StaticEntities,
    bool RequiresActivation = true);

/// <summary>
/// Render-thread-owned registry of currently-loaded landblocks and their
/// entities. All mutation happens in <see cref="StreamingController.Tick"/>
/// on the render thread; the renderer reads <see cref="Entities"/> once per
/// frame.
///
/// <para>
/// Replaces the pre-streaming flat <c>_entities</c> list. This class is the
/// single point of truth for "what's in the world right now" and the only
/// thing that mutates spatial buckets. Logical live-object identity and
/// create-time resources are owned by <see cref="AcDream.App.World.LiveEntityRuntime"/>.
/// </para>
///
/// <para>
/// <b>Pending live entities.</b> Live <c>CreateObject</c> spawns can race
/// against streaming: the server may send a spawn for landblock X before
/// X is loaded into <see cref="_loaded"/> (frequently true on the first
/// frame after login, where the entire post-login spawn flood drains
/// before the streaming controller has finished loading the visible
/// window). To survive this race, <see cref="PlaceLiveEntityProjection"/> stores
/// orphaned spawns in a per-landblock pending bucket. When
/// <see cref="AddLandblock"/> later loads the landblock, the matching
/// pending entries are merged into the loaded record before the flat
/// view rebuild. <see cref="RemoveLandblock"/> retains non-persistent live
/// entries in that pending bucket so a later reload restores the same identity;
/// only dat-static entries are discarded with streaming residence.
/// </para>
///
/// <remarks>
/// Threading: not thread-safe. All calls must happen on the render thread.
/// </remarks>
/// </summary>
public sealed class GpuWorldState : ILiveEntitySpatialQuery
{
    private readonly LandblockSpawnAdapter? _wbSpawnAdapter;
    private readonly EntityScriptActivator? _entityScriptActivator;

    /// <summary>
    /// Phase Post-A.5 #53 (Task 12): optional callback fired before
    /// <see cref="RemoveEntitiesFromLandblock"/> zeroes a landblock's entity
    /// list. Wired by <c>GameWindow</c> to
    /// <c>EntityClassificationCache.InvalidateLandblock</c> so Tier 1 cache
    /// entries get swept on LB demote (Near to Far) and unload. Receives
    /// the canonicalized landblock id (low 16 bits forced to <c>0xFFFF</c>),
    /// matching the <c>LandblockHint</c> stored at <c>Populate</c> time.
    /// Null when the cache isn't relevant (tests).
    /// </summary>
    private readonly System.Action<uint>? _onLandblockUnloaded;

    public GpuWorldState(
        LandblockSpawnAdapter? wbSpawnAdapter = null,
        System.Action<uint>? onLandblockUnloaded = null,
        EntityScriptActivator? entityScriptActivator = null)
    {
        _wbSpawnAdapter = wbSpawnAdapter;
        _onLandblockUnloaded = onLandblockUnloaded;
        _entityScriptActivator = entityScriptActivator;
    }

    private readonly Dictionary<uint, LoadedLandblock> _loaded = new();
    private readonly Dictionary<uint, LandblockStreamTier> _tierByLandblock = new();
    private readonly Dictionary<uint, (Vector3 Min, Vector3 Max)> _aabbs = new();
    private readonly Dictionary<uint, AnimatedEntityIndex> _animatedIndexByLandblock = new();

    private sealed record AnimatedEntityIndex(
        LoadedLandblock Source,
        IReadOnlyDictionary<uint, WorldEntity> EntitiesById);

    /// <summary>
    /// Per-landblock buffer of live entities awaiting their landblock's
    /// arrival. Keyed by canonical landblock id (<c>0xAAAA0xFFFF</c>).
    /// Drained into <see cref="_loaded"/> in <see cref="AddLandblock"/>.
    /// </summary>
    private readonly Dictionary<uint, List<WorldEntity>> _pendingByLandblock = new();
    // Far-to-near promotion can complete before the base far landblock is
    // published. Preserve its independently-prepared EnvCell geometry ids
    // alongside the parked entity layer so the later AddLandblock transaction
    // cannot open the render gate before those shells upload.
    private readonly Dictionary<uint, HashSet<ulong>> _pendingRenderIdsByLandblock = new();
    private readonly HashSet<uint> _pendingNearTierLandblocks = new();

    /// <summary>
    /// Entities that must survive landblock unloads (e.g. the player character).
    /// On RemoveLandblock, these are rescued and re-parked as pending for their
    /// current canonical landblock.
    /// </summary>
    private readonly HashSet<uint> _persistentGuids = new();

    // Render-thread-owned flat view over all entities across loaded landblocks.
    // The update and render phases are serialized, so mutating this list during
    // the update phase is both safe and substantially cheaper than rebuilding a
    // full-world array for every CreateObject in an inbound spawn flood.
    private readonly List<WorldEntity> _flatEntities = new();
    private readonly Dictionary<WorldEntity, int> _flatEntityIndices =
        new(ReferenceEqualityComparer.Instance);
    private readonly Dictionary<WorldEntity, ProjectionLocation> _projectionLocations =
        new(ReferenceEqualityComparer.Instance);
    // Short-range consumers must never scan each landblock's DAT-static list.
    // This index mirrors only loaded live projections and changes at the same
    // transaction boundary as _projectionLocations.
    private readonly Dictionary<uint, HashSet<WorldEntity>> _loadedLiveByLandblock = new();
    // A server GUID normally has exactly one spatial projection. Keep that
    // allocation-free common case directly in the primary map; the secondary
    // list exists only for the rare overlap during GUID reuse/re-entrant
    // lifetime transitions.
    private readonly Dictionary<uint, WorldEntity> _primaryProjectionByGuid = new();
    private readonly Dictionary<uint, List<WorldEntity>> _additionalProjectionsByGuid = new();
    private readonly Dictionary<uint, int> _visibleLiveProjectionCounts = new();
    private readonly Dictionary<uint, bool> _visibilityBeforeMutation = new();
    private readonly Queue<(uint ServerGuid, bool Visible)> _visibilityTransitions = new();
    private bool _dispatchingVisibilityTransitions;
    private int _mutationDepth;
    private long _visibilityCommitCount;
    private ulong _flatViewGeneration;
    private bool _flatMembershipDirty;

    public IReadOnlyList<WorldEntity> Entities => _flatEntities;
    public ulong FlatViewGeneration => _flatViewGeneration;
    public IReadOnlyCollection<uint> LoadedLandblockIds => _loaded.Keys;
    public event Action<uint, bool>? LiveProjectionVisibilityChanged;

    public bool IsLoaded(uint landblockId) => _loaded.ContainsKey(landblockId);
    public bool IsNearTier(uint landblockId) =>
        _tierByLandblock.TryGetValue(landblockId, out var tier)
        && tier == LandblockStreamTier.Near;
    public bool IsNearTierOrPending(uint landblockId) =>
        IsNearTier(landblockId) || _pendingNearTierLandblocks.Contains(landblockId);
    public bool IsRenderReady(uint landblockId) =>
        _loaded.ContainsKey(landblockId)
        && (_wbSpawnAdapter?.IsLandblockRenderReady(landblockId) ?? true);
    public bool IsLiveEntityVisible(uint serverGuid) =>
        serverGuid != 0 && _visibleLiveProjectionCounts.ContainsKey(serverGuid);

    /// <summary>
    /// Try to grab the loaded record for a landblock — useful for callers
    /// that need to enumerate entities before the landblock is dropped
    /// (e.g. unregistering dynamic lights on a RemoveLandblock).
    /// </summary>
    public bool TryGetLandblock(uint landblockId, out LoadedLandblock? lb)
    {
        if (_loaded.TryGetValue(landblockId, out var found))
        {
            lb = found;
            return true;
        }
        lb = null;
        return false;
    }

    /// <summary>
    /// Store the axis-aligned bounding box for a loaded landblock. Called from
    /// the render thread after the terrain mesh is built and uploaded.
    /// </summary>
    public void SetLandblockAabb(uint landblockId, Vector3 min, Vector3 max)
    {
        _aabbs[landblockId] = (min, max);
    }

    /// <summary>
    /// Per-landblock iteration with AABB data for use by the frustum-culling
    /// draw path. Landblocks without a stored AABB yield <see cref="Vector3.Zero"/>
    /// for both corners, which the culler will conservatively treat as visible.
    ///
    /// <para>
    /// A.5 T17: also yields an <c>AnimatedById</c> dictionary derived from the
    /// landblock's entity list. This lets <see cref="WbDrawDispatcher"/>
    /// skip the full entity walk when the landblock is frustum-culled but animated
    /// entities inside it must still be processed (Change #1). The lookup is
    /// cached against the <see cref="LoadedLandblock"/> record identity, so a
    /// stable scene does not allocate one dictionary per landblock per frame.
    /// Live projection mutations update the mutable resident bucket in place and
    /// explicitly invalidate exactly the affected index generation.
    /// </para>
    /// </summary>
    public IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
                        IReadOnlyList<WorldEntity> Entities,
                        IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> LandblockEntries
        => EnumerateLandblockEntries(includeAnimatedIndex: true);

    /// <summary>
    /// Per-landblock render entries without the animated lookup dictionary.
    /// Static render passes use this to avoid rebuilding an index they cannot
    /// consume.
    /// </summary>
    public IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
                        IReadOnlyList<WorldEntity> Entities,
                        IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> LandblockEntriesWithoutAnimatedIndex
        => EnumerateLandblockEntries(includeAnimatedIndex: false);

    /// <summary>
    /// Lightweight bounds-only enumeration for overlays and diagnostics.
    /// Does not walk entity lists.
    /// </summary>
    public IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax)> LandblockBounds
    {
        get
        {
            foreach (var kvp in _loaded)
            {
                if (_aabbs.TryGetValue(kvp.Key, out var aabb))
                    yield return (kvp.Key, aabb.Min, aabb.Max);
                else
                    yield return (kvp.Key, Vector3.Zero, Vector3.Zero);
            }
        }
    }

    private IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
                         IReadOnlyList<WorldEntity> Entities,
                         IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> EnumerateLandblockEntries(
        bool includeAnimatedIndex)
    {
        foreach (var kvp in _loaded)
        {
            IReadOnlyDictionary<uint, WorldEntity>? byId = null;
            if (includeAnimatedIndex)
            {
                if (!_animatedIndexByLandblock.TryGetValue(kvp.Key, out var cached)
                    || !ReferenceEquals(cached.Source, kvp.Value))
                {
                    var rebuilt = new Dictionary<uint, WorldEntity>(kvp.Value.Entities.Count);
                    foreach (var entity in kvp.Value.Entities)
                        rebuilt[entity.Id] = entity;

                    cached = new AnimatedEntityIndex(kvp.Value, rebuilt);
                    _animatedIndexByLandblock[kvp.Key] = cached;
                }

                byId = cached.EntitiesById;
            }

            if (_aabbs.TryGetValue(kvp.Key, out var aabb))
                yield return (kvp.Key, aabb.Min, aabb.Max, kvp.Value.Entities, byId);
            else
                yield return (kvp.Key, Vector3.Zero, Vector3.Zero, kvp.Value.Entities, byId);
        }
    }

    /// <summary>
    /// Total live entities currently parked in the pending bucket waiting
    /// for their landblock to arrive. Useful diagnostic for verifying the
    /// pending path is doing its job.
    /// </summary>
    public int PendingLiveEntityCount => _pendingByLandblock.Values.Sum(list => list.Count);
    public int PendingBucketCount => _pendingByLandblock.Count;
    public int PendingRescueCount => _persistentRescued.Count;
    public int PersistentGuidCount => _persistentGuids.Count;
    public int PendingVisibilityTransitionCount => _visibilityTransitions.Count;
    internal long VisibilityCommitCount => _visibilityCommitCount;

    /// <summary>
    /// Groups spatial mutations into one visibility publication transaction.
    /// Bucket and flat-view contents become readable immediately on the render
    /// thread; observer edges are computed from the before/after state and drain
    /// only when the outermost scope ends. This is what keeps a loaded-to-loaded
    /// rebucket from exposing an implementation-only hidden/visible pulse and
    /// lets one inbound network drain commit thousands of spawns without global
    /// rebuilds between them.
    /// </summary>
    public MutationBatch BeginMutationBatch()
    {
        _mutationDepth++;
        return new MutationBatch(this);
    }

    /// <summary>
    /// Copy live server projections from the bounded landblock neighborhood
    /// around <paramref name="centerCellOrLandblockId"/>. This is the spatial
    /// candidate seam for short-range consumers such as retail radar; they
    /// still apply their own Hidden/classification/range rules.
    /// </summary>
    public void CopyLiveEntitiesNearLandblock(
        uint centerCellOrLandblockId,
        int landblockRadius,
        List<KeyValuePair<uint, WorldEntity>> destination)
    {
        ArgumentNullException.ThrowIfNull(destination);
        ArgumentOutOfRangeException.ThrowIfNegative(landblockRadius);
        destination.Clear();

        int centerX = (int)((centerCellOrLandblockId >> 24) & 0xFFu);
        int centerY = (int)((centerCellOrLandblockId >> 16) & 0xFFu);
        for (int dx = -landblockRadius; dx <= landblockRadius; dx++)
        for (int dy = -landblockRadius; dy <= landblockRadius; dy++)
        {
            int x = centerX + dx;
            int y = centerY + dy;
            if ((uint)x > 0xFFu || (uint)y > 0xFFu)
                continue;

            uint landblockId = ((uint)x << 24) | ((uint)y << 16) | 0xFFFFu;
            if (!_loadedLiveByLandblock.TryGetValue(
                    landblockId,
                    out HashSet<WorldEntity>? liveEntities))
                continue;

            foreach (WorldEntity entity in liveEntities)
                destination.Add(new KeyValuePair<uint, WorldEntity>(entity.ServerGuid, entity));
        }
    }

    public readonly ref struct MutationBatch
    {
        private readonly GpuWorldState _owner;

        public MutationBatch(GpuWorldState owner) => _owner = owner;

        public void Dispose() => _owner.EndMutationBatch();
    }

    private readonly record struct ProjectionLocation(
        uint LandblockId,
        bool IsLoaded,
        int BucketIndex);

    private void EndMutationBatch()
    {
        if (_mutationDepth <= 0)
            throw new InvalidOperationException("GpuWorldState mutation scope underflow.");
        if (--_mutationDepth != 0)
            return;

        foreach ((uint guid, bool wasVisible) in _visibilityBeforeMutation)
        {
            bool isVisible = _visibleLiveProjectionCounts.ContainsKey(guid);
            if (wasVisible != isVisible)
                _visibilityTransitions.Enqueue((guid, isVisible));
        }
        _visibilityBeforeMutation.Clear();
        _visibilityCommitCount++;
        if (_flatMembershipDirty)
        {
            _flatViewGeneration++;
            _flatMembershipDirty = false;
        }

        DrainVisibilityTransitions();
        if (EntityVanishProbe.Enabled)
            ProbeFlatViewTransitions();
    }

    private void AdjustVisibleLiveProjection(WorldEntity entity, int delta)
    {
        uint guid = entity.ServerGuid;
        if (guid == 0 || delta == 0)
            return;

        _visibleLiveProjectionCounts.TryGetValue(guid, out int priorCount);
        if (!_visibilityBeforeMutation.ContainsKey(guid))
            _visibilityBeforeMutation.Add(guid, priorCount > 0);

        int nextCount = checked(priorCount + delta);
        if (nextCount < 0)
            throw new InvalidOperationException(
                $"Live projection visibility count underflow for 0x{guid:X8}.");

        if (nextCount == 0)
        {
            _visibleLiveProjectionCounts.Remove(guid);
        }
        else
        {
            _visibleLiveProjectionCounts[guid] = nextCount;
        }
    }

    private static LoadedLandblock NormalizeLoadedLandblock(
        LoadedLandblock source,
        List<WorldEntity>? entities = null) =>
        new(
            source.LandblockId,
            source.Heightmap,
            entities ?? new List<WorldEntity>(source.Entities),
            source.PhysicsDats);

    private static List<WorldEntity> MutableEntities(LoadedLandblock landblock) =>
        (List<WorldEntity>)landblock.Entities;

    private void AddFlatEntity(WorldEntity entity)
    {
        if (!_flatEntityIndices.TryAdd(entity, _flatEntities.Count))
            throw new InvalidOperationException(
                $"Entity 0x{entity.Id:X8} already exists in the flat render view.");
        _flatEntities.Add(entity);
        _flatMembershipDirty = true;
    }

    private void RemoveFlatEntity(WorldEntity entity)
    {
        if (!_flatEntityIndices.Remove(entity, out int index))
            throw new InvalidOperationException(
                $"Loaded entity 0x{entity.Id:X8} was absent from the flat render view.");

        int lastIndex = _flatEntities.Count - 1;
        if (index != lastIndex)
        {
            WorldEntity moved = _flatEntities[lastIndex];
            _flatEntities[index] = moved;
            _flatEntityIndices[moved] = index;
        }
        _flatEntities.RemoveAt(lastIndex);
        _flatMembershipDirty = true;
    }

    private void SetProjectionLocation(
        WorldEntity entity,
        uint landblockId,
        bool isLoaded,
        int bucketIndex)
    {
        if (entity.ServerGuid == 0)
            return;

        bool isNew = !_projectionLocations.TryGetValue(
            entity,
            out ProjectionLocation priorLocation);
        if (!isNew
            && priorLocation.IsLoaded
            && (!isLoaded || priorLocation.LandblockId != landblockId))
        {
            RemoveLoadedLiveIndex(priorLocation.LandblockId, entity);
        }
        _projectionLocations[entity] = new ProjectionLocation(
            landblockId,
            isLoaded,
            bucketIndex);
        if (isLoaded
            && (isNew || !priorLocation.IsLoaded || priorLocation.LandblockId != landblockId))
        {
            AddLoadedLiveIndex(landblockId, entity);
        }
        if (!isNew)
            return;

        uint guid = entity.ServerGuid;
        if (_primaryProjectionByGuid.TryAdd(guid, entity))
            return;

        if (!_additionalProjectionsByGuid.TryGetValue(guid, out List<WorldEntity>? projections))
        {
            projections = new List<WorldEntity>(1);
            _additionalProjectionsByGuid.Add(guid, projections);
        }
        projections.Add(entity);
    }

    private void RemoveProjectionLocation(WorldEntity entity)
    {
        if (!_projectionLocations.Remove(entity, out ProjectionLocation location)
            || entity.ServerGuid == 0)
            return;
        if (location.IsLoaded)
            RemoveLoadedLiveIndex(location.LandblockId, entity);

        uint guid = entity.ServerGuid;
        if (!_primaryProjectionByGuid.TryGetValue(guid, out WorldEntity? primary))
            throw new InvalidOperationException(
                $"Live projection 0x{entity.Id:X8} had no server-guid index entry.");

        if (ReferenceEquals(primary, entity))
        {
            if (_additionalProjectionsByGuid.TryGetValue(guid, out List<WorldEntity>? projections)
                && projections.Count > 0)
            {
                int lastIndex = projections.Count - 1;
                _primaryProjectionByGuid[guid] = projections[lastIndex];
                projections.RemoveAt(lastIndex);
                if (projections.Count == 0)
                    _additionalProjectionsByGuid.Remove(guid);
            }
            else
            {
                _primaryProjectionByGuid.Remove(guid);
            }
            return;
        }

        if (!_additionalProjectionsByGuid.TryGetValue(guid, out List<WorldEntity>? additional))
            throw new InvalidOperationException(
                $"Live projection 0x{entity.Id:X8} was absent from its server-guid index.");

        int index = -1;
        for (int i = 0; i < additional.Count; i++)
        {
            if (!ReferenceEquals(additional[i], entity))
                continue;
            index = i;
            break;
        }
        if (index < 0)
            throw new InvalidOperationException(
                $"Live projection 0x{entity.Id:X8} was absent from its server-guid index.");

        int lastAdditionalIndex = additional.Count - 1;
        if (index != lastAdditionalIndex)
            additional[index] = additional[lastAdditionalIndex];
        additional.RemoveAt(lastAdditionalIndex);
        if (additional.Count == 0)
            _additionalProjectionsByGuid.Remove(guid);
    }

    private void AddLoadedLiveIndex(uint landblockId, WorldEntity entity)
    {
        if (!_loadedLiveByLandblock.TryGetValue(
                landblockId,
                out HashSet<WorldEntity>? entities))
        {
            entities = new HashSet<WorldEntity>(ReferenceEqualityComparer.Instance);
            _loadedLiveByLandblock.Add(landblockId, entities);
        }
        if (!entities.Add(entity))
        {
            throw new InvalidOperationException(
                $"Live entity 0x{entity.Id:X8} already exists in landblock index 0x{landblockId:X8}.");
        }
    }

    private void RemoveLoadedLiveIndex(uint landblockId, WorldEntity entity)
    {
        if (!_loadedLiveByLandblock.TryGetValue(
                landblockId,
                out HashSet<WorldEntity>? entities)
            || !entities.Remove(entity))
        {
            throw new InvalidOperationException(
                $"Live entity 0x{entity.Id:X8} is absent from landblock index 0x{landblockId:X8}.");
        }
        if (entities.Count == 0)
            _loadedLiveByLandblock.Remove(landblockId);
    }

    private void AddLoadedProjection(uint landblockId, WorldEntity entity)
    {
        LoadedLandblock landblock = _loaded[landblockId];
        List<WorldEntity> entities = MutableEntities(landblock);
        int bucketIndex = entities.Count;
        entities.Add(entity);
        _animatedIndexByLandblock.Remove(landblockId);
        AddFlatEntity(entity);
        SetProjectionLocation(entity, landblockId, isLoaded: true, bucketIndex);
        AdjustVisibleLiveProjection(entity, +1);
    }

    private void RemoveLoadedProjection(WorldEntity entity)
    {
        if (!_projectionLocations.TryGetValue(entity, out ProjectionLocation location)
            || !location.IsLoaded
            || !_loaded.TryGetValue(location.LandblockId, out LoadedLandblock? landblock))
        {
            throw new InvalidOperationException(
                $"Live entity 0x{entity.Id:X8} has no loaded projection location.");
        }

        List<WorldEntity> entities = MutableEntities(landblock);
        int lastIndex = entities.Count - 1;
        if ((uint)location.BucketIndex >= (uint)entities.Count
            || !ReferenceEquals(entities[location.BucketIndex], entity))
        {
            throw new InvalidOperationException(
                $"Loaded projection index for entity 0x{entity.Id:X8} is stale.");
        }
        if (location.BucketIndex != lastIndex)
        {
            WorldEntity moved = entities[lastIndex];
            entities[location.BucketIndex] = moved;
            if (moved.ServerGuid != 0)
            {
                SetProjectionLocation(
                    moved,
                    location.LandblockId,
                    isLoaded: true,
                    location.BucketIndex);
            }
        }
        entities.RemoveAt(lastIndex);

        _animatedIndexByLandblock.Remove(location.LandblockId);
        RemoveFlatEntity(entity);
        RemoveProjectionLocation(entity);
        AdjustVisibleLiveProjection(entity, -1);
    }

    private void RemoveLoadedLandblockFromFlatView(LoadedLandblock landblock)
    {
        foreach (WorldEntity entity in landblock.Entities)
        {
            RemoveFlatEntity(entity);
            if (entity.ServerGuid != 0)
                RemoveProjectionLocation(entity);
            AdjustVisibleLiveProjection(entity, -1);
        }
    }

    private void AddLoadedLandblockToFlatView(LoadedLandblock landblock)
    {
        for (int i = 0; i < landblock.Entities.Count; i++)
        {
            WorldEntity entity = landblock.Entities[i];
            AddFlatEntity(entity);
            if (entity.ServerGuid != 0)
                SetProjectionLocation(entity, landblock.LandblockId, isLoaded: true, i);
            AdjustVisibleLiveProjection(entity, +1);
        }
    }

    public void AddLandblock(
        LoadedLandblock landblock,
        IEnumerable<ulong>? additionalRenderIds = null,
        LandblockStreamTier tier = LandblockStreamTier.Near)
    {
        using MutationBatch mutation = BeginMutationBatch();
        GpuLandblockSpatialPublication publication =
            CommitLandblockSpatialCore(landblock, additionalRenderIds, tier);
        ActivateLandblockPresentation(publication);
    }

    /// <summary>
    /// Commits only canonical buckets, indexes, pending-live merge, tier, and
    /// readiness-id ownership. The caller holds one outer
    /// <see cref="MutationBatch"/> through the subsequent
    /// <see cref="ActivateLandblockPresentation"/> call so live visibility
    /// observers retain the shipped commit boundary.
    /// </summary>
    internal GpuLandblockSpatialPublication CommitLandblockSpatial(
        LoadedLandblock landblock,
        IEnumerable<ulong>? additionalRenderIds,
        LandblockStreamTier tier) =>
        CommitLandblockSpatialCore(landblock, additionalRenderIds, tier);

    private GpuLandblockSpatialPublication CommitLandblockSpatialCore(
        LoadedLandblock landblock,
        IEnumerable<ulong>? additionalRenderIds,
        LandblockStreamTier tier)
    {
        ArgumentNullException.ThrowIfNull(landblock);

        // A stale Far completion must never replace a newer Near entity layer.
        // StreamingController normally filters it before render-side apply;
        // keep the state boundary independently monotonic as well.
        if (tier == LandblockStreamTier.Far && IsNearTier(landblock.LandblockId))
        {
            LoadedLandblock retained = _loaded[landblock.LandblockId];
            return new GpuLandblockSpatialPublication(
                retained.LandblockId,
                retained,
                Array.Empty<ulong>(),
                Array.Empty<WorldEntity>(),
                RequiresActivation: false);
        }

        // Reconcile the only fallible logical-owner edge before consuming any
        // pending live/render/tier maps. A retry then sees the exact pending
        // suffix even when a resolver or owner callback fails partway through.
        landblock = NormalizeLoadedLandblock(landblock);
        if (_loaded.TryGetValue(landblock.LandblockId, out LoadedLandblock? displaced))
            ReconcileRetainedStaticEntityScripts(displaced, landblock);

        // If pending live entities have been waiting for this landblock,
        // merge them into the render-thread-owned mutable entity bucket before
        // storing. Subsequent live spawns append to that bucket in O(1).
        if (_pendingByLandblock.TryGetValue(landblock.LandblockId, out var pending) && pending.Count > 0)
        {
            var merged = new List<WorldEntity>(landblock.Entities.Count + pending.Count);
            merged.AddRange(landblock.Entities);
            merged.AddRange(pending);
            landblock = NormalizeLoadedLandblock(landblock, merged);
            _pendingByLandblock.Remove(landblock.LandblockId);
        }
        HashSet<ulong>? mergedRenderIds = additionalRenderIds is null
            ? null
            : new HashSet<ulong>(additionalRenderIds);
        if (_pendingRenderIdsByLandblock.Remove(landblock.LandblockId, out var pendingRenderIds))
        {
            mergedRenderIds ??= new HashSet<ulong>();
            mergedRenderIds.UnionWith(pendingRenderIds);
        }

        bool pendingNear = _pendingNearTierLandblocks.Remove(landblock.LandblockId);
        if (pendingNear
            || (_tierByLandblock.TryGetValue(landblock.LandblockId, out var currentTier)
                && currentTier == LandblockStreamTier.Near))
        {
            tier = LandblockStreamTier.Near;
        }

        if (_loaded.Remove(landblock.LandblockId, out displaced))
        {
            RemoveLoadedLandblockFromFlatView(displaced);
            _animatedIndexByLandblock.Remove(landblock.LandblockId);
        }

        _loaded[landblock.LandblockId] = landblock;
        AddLoadedLandblockToFlatView(landblock);
        _tierByLandblock[landblock.LandblockId] = tier;
        return CreateSpatialPublication(
            _loaded[landblock.LandblockId],
            (IEnumerable<ulong>?)mergedRenderIds ?? Array.Empty<ulong>());
    }

    /// <summary>
    /// A same-landblock rehydrate is an acdream streaming replacement, not a
    /// retail logical create/delete edge. Retained static IDs rebind their
    /// Setup-owned resources without replaying defaults; omitted IDs perform
    /// the same exact teardown used by ordinary retirement.
    /// </summary>
    private void ReconcileRetainedStaticEntityScripts(
        LoadedLandblock displaced,
        LoadedLandblock replacement)
    {
        if (_entityScriptActivator is null)
            return;

        var replacementsById = new Dictionary<uint, WorldEntity>();
        for (int i = 0; i < replacement.Entities.Count; i++)
        {
            WorldEntity entity = replacement.Entities[i];
            if (entity.ServerGuid == 0)
                replacementsById.Add(entity.Id, entity);
        }

        for (int i = 0; i < displaced.Entities.Count; i++)
        {
            WorldEntity prior = displaced.Entities[i];
            if (prior.ServerGuid != 0)
                continue;
            if (replacementsById.TryGetValue(prior.Id, out WorldEntity? retained))
                _entityScriptActivator.OnRebindStatic(prior, retained);
            else
                _entityScriptActivator.OnRemove(prior);
        }
    }

    internal void ActivateLandblockPresentation(
        GpuLandblockSpatialPublication publication)
    {
        ArgumentNullException.ThrowIfNull(publication);
        if (!publication.RequiresActivation)
            return;
        if (!_loaded.TryGetValue(publication.LandblockId, out LoadedLandblock? current)
            || !ReferenceEquals(current, publication.Landblock))
        {
            throw new InvalidOperationException(
                $"Landblock 0x{publication.LandblockId:X8} changed before presentation activation.");
        }

        _wbSpawnAdapter?.OnLandblockLoaded(
            publication.Landblock,
            publication.AdditionalRenderIds);

        // C.1.5b: fire DefaultScript for dat-hydrated entities only.
        // EntityScriptActivator owns a retryable per-static acquisition ledger,
        // so replaying this activation suffix after a later owner failure does
        // not replay already-started defaults.
        if (_entityScriptActivator is not null)
        {
            for (int i = 0; i < publication.StaticEntities.Count; i++)
                _entityScriptActivator.OnCreate(publication.StaticEntities[i]);
        }
    }

    private static GpuLandblockSpatialPublication CreateSpatialPublication(
        LoadedLandblock landblock,
        IEnumerable<ulong> additionalRenderIds)
    {
        ulong[] renderIds = additionalRenderIds as ulong[]
            ?? additionalRenderIds.ToArray();
        WorldEntity[] staticEntities = landblock.Entities
            .Where(static entity => entity.ServerGuid == 0)
            .ToArray();
        return new GpuLandblockSpatialPublication(
            landblock.LandblockId,
            landblock,
            renderIds,
            staticEntities);
    }

    /// <summary>
    /// Mark a server-GUID as persistent — this entity survives landblock unloads
    /// and gets re-parked as pending for its current canonical landblock.
    /// </summary>
    public void MarkPersistent(uint serverGuid)
    {
        _persistentGuids.Add(serverGuid);
    }

    /// <summary>
    /// Move a persistent entity from its current landblock slot to a new one.
    /// Called every frame for the player entity so it stays in the landblock
    /// matching its actual position (not its spawn landblock). Without this,
    /// the entity stays in the spawn landblock and gets frustum-culled when
    /// the player walks away.
    /// </summary>
    public void RebucketLiveEntity(WorldEntity entity, uint newCanonicalLb)
    {
        if (entity.ServerGuid == 0) return;

        using MutationBatch mutation = BeginMutationBatch();

        uint canonical = (newCanonicalLb & 0xFFFF0000u) | 0xFFFFu;

        // Fast path: already drawn in the correct loaded bucket → nothing to do
        // (avoids per-frame list churn for a settled, stationary entity).
        if (_projectionLocations.TryGetValue(entity, out ProjectionLocation current)
            && current.IsLoaded
            && current.LandblockId == canonical)
        {
            return;
        }

        // Remove the entity from wherever it currently lives — a loaded bucket
        // OR a pending bucket — then re-append to its current landblock.
        //
        // The projection-location index is the 2026-07-03 fix for the cold-spawn /
        // run-out "invisible player" bug: a persistent (server-spawned) entity
        // that spawned into a not-yet-loaded landblock sits in _pendingByLandblock,
        // and the old code scanned ONLY _loaded — so it silently no-op'd and left
        // the player stranded, hidden, even after its landblock finished loading
        // (the AddLandblock pending-drain had already run empty before the churn
        // re-parked the player, and the player is excluded from the server-object
        // re-hydrate — so RebucketLiveEntity was the ONLY path that could recover it,
        // and it couldn't reach a pending entity). The index now reaches either
        // residency class in O(1). Re-appending routes the entity
        // to _loaded (drawn) when its landblock is loaded, or back to pending to
        // await AddLandblock otherwise.
        // Remove + place is one spatial transaction. Rebuilding between the
        // two operations exposes a false/true implementation pulse for a
        // loaded-to-loaded move and lets reentrant visibility callbacks observe
        // a state that never exists at the LiveEntityRuntime boundary.
        RemoveEntityFromAllBuckets(entity);
        PlaceLiveEntityProjection(canonical, entity);
    }

    /// <summary>
    /// Remove <paramref name="entity"/> (by reference) from whichever
    /// <see cref="_loaded"/> or <see cref="_pendingByLandblock"/> bucket it
    /// currently occupies. At most one bucket holds a given entity, so this
    /// stops after the first hit. Called by <see cref="RebucketLiveEntity"/> before
    /// re-appending, so a stranded pending entity can be promoted.
    /// </summary>
    private void RemoveEntityFromAllBuckets(WorldEntity entity)
    {
        if (!_projectionLocations.TryGetValue(entity, out ProjectionLocation location))
            return;

        if (location.IsLoaded)
        {
            RemoveLoadedProjection(entity);
            return;
        }

        if (!_pendingByLandblock.TryGetValue(location.LandblockId, out List<WorldEntity>? pending)
            || (uint)location.BucketIndex >= (uint)pending.Count
            || !ReferenceEquals(pending[location.BucketIndex], entity))
        {
            throw new InvalidOperationException(
                $"Indexed live entity 0x{entity.Id:X8} was absent from pending landblock 0x{location.LandblockId:X8}.");
        }

        int lastIndex = pending.Count - 1;
        if (location.BucketIndex != lastIndex)
        {
            WorldEntity moved = pending[lastIndex];
            pending[location.BucketIndex] = moved;
            SetProjectionLocation(
                moved,
                location.LandblockId,
                isLoaded: false,
                location.BucketIndex);
        }
        pending.RemoveAt(lastIndex);

        RemoveProjectionLocation(entity);
        if (pending.Count == 0)
            _pendingByLandblock.Remove(location.LandblockId);
    }

    /// <summary>
    /// Commits the world-state half of a full landblock retirement and returns
    /// the exact entity context required by presentation owners. This method
    /// deliberately performs no renderer, script, or cache callbacks: those
    /// owners can fail independently and are advanced by
    /// <see cref="LandblockRetirementCoordinator"/> after the old state has
    /// become unreachable.
    /// </summary>
    public GpuLandblockRetirement? DetachLandblock(uint landblockId)
    {
        using MutationBatch mutation = BeginMutationBatch();

        uint canonical = (landblockId & 0xFFFF0000u) | 0xFFFFu;
        bool hadState = _loaded.ContainsKey(canonical)
            || _pendingByLandblock.ContainsKey(canonical)
            || _pendingRenderIdsByLandblock.ContainsKey(canonical)
            || _pendingNearTierLandblocks.Contains(canonical)
            || _tierByLandblock.ContainsKey(canonical)
            || _aabbs.ContainsKey(canonical);
        if (!hadState)
            return null;

        IReadOnlyList<WorldEntity> detachedEntities =
            _loaded.TryGetValue(canonical, out LoadedLandblock? residentLandblock)
                ? residentLandblock.Entities
                : Array.Empty<WorldEntity>();
        if (_pendingByLandblock.TryGetValue(canonical, out List<WorldEntity>? detachedPending)
            && detachedPending.Count > 0)
        {
            if (detachedEntities.Count == 0)
            {
                detachedEntities = detachedPending;
            }
            else
            {
                var combined = new List<WorldEntity>(
                    detachedEntities.Count + detachedPending.Count);
                var seen = new HashSet<WorldEntity>(ReferenceEqualityComparer.Instance);
                foreach (WorldEntity entity in detachedEntities)
                    if (seen.Add(entity))
                        combined.Add(entity);
                foreach (WorldEntity entity in detachedPending)
                    if (seen.Add(entity))
                        combined.Add(entity);
                detachedEntities = combined;
            }
        }

        // A logical live object survives streaming residence. Non-persistent
        // projections move to the pending bucket for this landblock and merge
        // back as the same WorldEntity when it reloads. Persistent projections
        // (the local player) are rescued because their current bucket may be a
        // different landblock by the time the caller reinjects them.
        var retainedLive = new List<WorldEntity>();
        var retainedLiveSet = new HashSet<WorldEntity>(ReferenceEqualityComparer.Instance);
        void RetainOrRescue(WorldEntity entity, string source)
        {
            if (entity.ServerGuid == 0)
                return;
            if (_persistentGuids.Contains(entity.ServerGuid))
            {
                _persistentRescued.Add(entity);
                EntityVanishProbe.Log(
                    $"[ent] RESCUE guid=0x{entity.ServerGuid:X8} from={source} lb=0x{canonical:X8}");
                return;
            }
            if (retainedLiveSet.Add(entity))
                retainedLive.Add(entity);
        }

        // Rescue persistent entities before removal. These get appended
        // to the _persistentRescued list; the caller is responsible for
        // re-injecting them through LiveEntityRuntime rebucketing into whatever landblock
        // the player is currently on.
        if (_loaded.TryGetValue(canonical, out var lb))
        {
            foreach (var entity in lb.Entities)
                RetainOrRescue(entity, "loaded");
        }

        // #138 (secondary): rescue persistent entities sitting in the PENDING
        // bucket too, not just the loaded list. The player is re-injected via
        // LiveEntityRuntime rebucketing into its current landblock every frame
        // (GameWindow's DrainRescued loop); right after a teleport that
        // landblock often hasn't streamed in yet, so the player lands in
        // _pendingByLandblock. If that same landblock is then unloaded (a
        // streaming churn / re-teleport before it finishes loading), the
        // pending entry was silently dropped here — violating the
        // "persistent ⇒ survives unload" contract and making the avatar
        // vanish after a couple round-trips. Rescue them so DrainRescued
        // re-parks them at the next valid landblock.
        if (_pendingByLandblock.TryGetValue(canonical, out var pendingForLb))
        {
            foreach (var entity in pendingForLb)
            {
                RetainOrRescue(entity, "pending");
                RemoveProjectionLocation(entity);
            }
        }

        _pendingByLandblock.Remove(canonical);
        _pendingRenderIdsByLandblock.Remove(canonical);
        _pendingNearTierLandblocks.Remove(canonical);
        if (retainedLive.Count > 0)
            _pendingByLandblock[canonical] = retainedLive;
        _aabbs.Remove(canonical);
        _animatedIndexByLandblock.Remove(canonical);

        _tierByLandblock.Remove(canonical);
        if (_loaded.Remove(canonical, out LoadedLandblock? removed))
            RemoveLoadedLandblockFromFlatView(removed);

        for (int i = 0; i < retainedLive.Count; i++)
            SetProjectionLocation(retainedLive[i], canonical, isLoaded: false, i);

        return new GpuLandblockRetirement(
            canonical,
            LandblockRetirementKind.Full,
            detachedEntities);
    }

    /// <summary>
    /// Compatibility edge for direct state tests. Production streaming uses
    /// <see cref="LandblockRetirementCoordinator"/> so each presentation owner
    /// has a retryable committed marker.
    /// </summary>
    public void RemoveLandblock(uint landblockId)
    {
        GpuLandblockRetirement? retirement = DetachLandblock(landblockId);
        if (retirement is null)
            return;

        ReleaseLandblockMeshReferences(retirement.LandblockId);
        InvalidateLandblockClassification(retirement.LandblockId);
        for (int i = 0; i < retirement.Entities.Count; i++)
        {
            WorldEntity entity = retirement.Entities[i];
            if (entity.ServerGuid == 0)
                StopStaticEntityScript(entity);
        }
    }

    internal void ReleaseLandblockMeshReferences(uint landblockId) =>
        _wbSpawnAdapter?.OnLandblockUnloaded(landblockId);

    internal void InvalidateLandblockClassification(uint landblockId) =>
        _onLandblockUnloaded?.Invoke(landblockId);

    internal void StopStaticEntityScript(WorldEntity entity) =>
        _entityScriptActivator?.OnRemove(entity);

    private readonly List<WorldEntity> _persistentRescued = new();

    /// <summary>
    /// Drain entities rescued from unloaded landblocks. The caller should
    /// re-inject each through <c>LiveEntityRuntime.RebucketLiveEntity</c> with its current position.
    /// </summary>
    public List<WorldEntity> DrainRescued()
    {
        if (_persistentRescued.Count == 0) return _persistentRescued;
        var result = new List<WorldEntity>(_persistentRescued);
        _persistentRescued.Clear();
        return result;
    }

    /// <summary>
    /// Remove every entity with the given <paramref name="serverGuid"/> from
    /// all loaded landblocks AND all pending buckets. Called by
    /// <c>LiveEntityRuntime</c> before rebucketing, temporary
    /// world withdrawal, or logical teardown. It owns no renderer/script
    /// lifecycle and is safe for a still-live incarnation.
    ///
    /// Safe to call with a server guid that's not currently present — no-op.
    /// </summary>
    public void RemoveLiveEntityProjection(uint serverGuid)
    {
        if (serverGuid == 0) return;

        using MutationBatch mutation = BeginMutationBatch();

        // Canonical live projections are indexed by server GUID and entity
        // identity, so delete/withdraw never scans every resident landblock.
        while (_primaryProjectionByGuid.TryGetValue(serverGuid, out WorldEntity? projection))
        {
            RemoveEntityFromAllBuckets(projection);
        }

        // A persistent projection may have been rescued by RemoveLandblock
        // and not yet drained by the next GameWindow frame. Rebucketing or
        // logical teardown before that drain must remove the stale rescue
        // reference or it can later re-inject a deleted/duplicated object.
        _persistentRescued.RemoveAll(e => e.ServerGuid == serverGuid);

    }

    /// <summary>
    /// Removes one exact live projection incarnation. Unlike the GUID overload,
    /// this cannot detach a replacement that reused the same server GUID from a
    /// re-entrant logical teardown callback.
    /// </summary>
    public void RemoveLiveEntityProjection(WorldEntity entity)
    {
        ArgumentNullException.ThrowIfNull(entity);
        if (entity.ServerGuid == 0) return;

        using MutationBatch mutation = BeginMutationBatch();

        RemoveEntityFromAllBuckets(entity);

        _persistentRescued.RemoveAll(candidate => ReferenceEquals(candidate, entity));

    }

    /// <summary>
    /// Ends all spatial lifetime retained for one server object. Temporary
    /// rebucketing uses <see cref="RemoveLiveEntityProjection"/> and keeps the
    /// persistence classification; logical delete also forgets that class so
    /// a later session/GUID reuse cannot be rescued as the old player.
    /// </summary>
    public void ForgetLiveEntity(uint serverGuid)
    {
        RemoveLiveEntityProjection(serverGuid);
        _persistentGuids.Remove(serverGuid);
        _persistentInFlatProbe.Remove(serverGuid);
    }

    /// <summary>Clears session-scoped persistence and undrained rescues.</summary>
    public void ClearLiveEntityLifetimeState()
    {
        _persistentGuids.Clear();
        _persistentRescued.Clear();
        _persistentInFlatProbe.Clear();
        _visibilityTransitions.Clear();
        _visibleLiveProjectionCounts.Clear();
        _visibilityBeforeMutation.Clear();
        _projectionLocations.Clear();
        _loadedLiveByLandblock.Clear();
        _primaryProjectionByGuid.Clear();
        _additionalProjectionsByGuid.Clear();
    }

    /// <summary>
    /// Place an already-registered live entity in a landblock slot whose
    /// terrain may or may not be loaded yet.
    ///
    /// <para>
    /// The server's <c>landblockId</c> is in cell-resolved form
    /// (<c>0xAAAA00CC</c>: high byte X, second byte Y, low 16 bits cell
    /// index within the landblock). The streaming system stores landblocks
    /// keyed by their canonical <c>0xAAAA0xFFFF</c> form. Canonicalize
    /// on the way in so callers don't have to think about it.
    /// </para>
    ///
    /// <para>
    /// Outcome:
    /// <list type="bullet">
    ///   <item>If the landblock is already loaded, the entity is appended
    ///         to its landblock and flat render-thread lists in O(1).</item>
    ///   <item>If the landblock is not yet loaded, the entity is parked
    ///         in <see cref="_pendingByLandblock"/> and will be merged
    ///         into the next <see cref="AddLandblock"/> for the same id.</item>
    /// </list>
    /// </para>
    /// </summary>
    public void PlaceLiveEntityProjection(uint landblockId, WorldEntity entity)
    {
        using MutationBatch mutation = BeginMutationBatch();

        if (_projectionLocations.ContainsKey(entity))
        {
            throw new InvalidOperationException(
                $"Live entity 0x{entity.Id:X8} is already spatially projected.");
        }

        // Spatial placement only. LiveEntityRuntime has already registered
        // this incarnation's renderer and script resources exactly once.
        uint canonicalLandblockId = (landblockId & 0xFFFF0000u) | 0xFFFFu;
        bool probePersistent = EntityVanishProbe.Enabled
            && entity.ServerGuid != 0 && _persistentGuids.Contains(entity.ServerGuid);

        if (_loaded.ContainsKey(canonicalLandblockId))
        {
            // Hot path — append directly to the render-thread-owned mutable
            // resident bucket and flat view.
            AddLoadedProjection(canonicalLandblockId, entity);
            if (probePersistent)
                EntityVanishProbe.Log($"[ent] APPEND guid=0x{entity.ServerGuid:X8} lb=0x{canonicalLandblockId:X8} -> LOADED(drawn)");
            return;
        }

        // Cold path — landblock not yet loaded. Park the entity in the
        // pending bucket; AddLandblock will pick it up when the streamer
        // delivers the matching landblock.
        if (!_pendingByLandblock.TryGetValue(canonicalLandblockId, out var bucket))
        {
            bucket = new List<WorldEntity>();
            _pendingByLandblock[canonicalLandblockId] = bucket;
        }
        int bucketIndex = bucket.Count;
        bucket.Add(entity);
        SetProjectionLocation(entity, canonicalLandblockId, isLoaded: false, bucketIndex);
        if (probePersistent)
            EntityVanishProbe.Log($"[ent] APPEND guid=0x{entity.ServerGuid:X8} lb=0x{canonicalLandblockId:X8} -> PENDING(hidden)");
    }

    /// <summary>
    /// Drop all entities from a landblock without removing the terrain. Used
    /// by two-tier streaming when a landblock crosses Near→Far hysteresis.
    /// Per Phase A.5 spec §4.4.
    ///
    /// <para>
    /// Only dat-static entity layers demote. Live server projections retain
    /// their exact WorldEntity and bucket while terrain remains resident; no
    /// logical or spatial lifetime callback is replayed.
    /// </para>
    /// </summary>
    public GpuLandblockRetirement? DetachNearLayer(uint landblockId)
    {
        using MutationBatch mutation = BeginMutationBatch();

        // A.5 T14 follow-up: canonicalize for symmetry with live projection placement.
        // Streaming callers always pass canonical (0xAAAA0xFFFF) ids; this
        // protects against future callers that mirror live projection placement's
        // cell-resolved-id pattern.
        uint canonical = (landblockId & 0xFFFF0000u) | 0xFFFFu;
        // A promotion may have parked its Near layer before the Far base load
        // exists. Demotion must retire that pending tier just as completely as
        // an installed tier, while preserving live server projections.
        bool hadNearLayer = _pendingNearTierLandblocks.Remove(canonical);
        hadNearLayer |= _pendingRenderIdsByLandblock.Remove(canonical);
        hadNearLayer |= IsNearTier(canonical);
        if (!hadNearLayer)
            return null;

        var retiredEntities = new List<WorldEntity>();
        if (_pendingByLandblock.TryGetValue(canonical, out var pending))
        {
            int pendingWriteIndex = 0;
            for (int readIndex = 0; readIndex < pending.Count; readIndex++)
            {
                WorldEntity entity = pending[readIndex];
                if (entity.ServerGuid == 0)
                {
                    retiredEntities.Add(entity);
                    continue;
                }
                if (pendingWriteIndex != readIndex)
                    pending[pendingWriteIndex] = entity;
                SetProjectionLocation(
                    entity,
                    canonical,
                    isLoaded: false,
                    pendingWriteIndex);
                pendingWriteIndex++;
            }
            if (pendingWriteIndex < pending.Count)
                pending.RemoveRange(pendingWriteIndex, pending.Count - pendingWriteIndex);
            if (pendingWriteIndex == 0)
                _pendingByLandblock.Remove(canonical);
        }

        if (_loaded.TryGetValue(canonical, out var lb))
        {

        // Phase Post-A.5 #53 (Task 12): invalidate the EntityClassificationCache
        // for this landblock BEFORE we drop the entity list. The cache stores
        // canonical landblock ids (the dispatcher's _walkScratch carries
        // entry.LandblockId from GpuWorldState.LandblockEntries, whose keys are
        // canonicalized). Null when the cache isn't wired (tests). Per spec §5.3 W3b.

        // C.1.5b: stop DefaultScript for each dat-hydrated entity about to
        // be dropped. Demote-tier entities are always atlas-tier (ServerGuid==0
        // per this method's class doc-comment); the filter is belt-and-suspenders.
        var retainedLive = new List<WorldEntity>();
        for (int readIndex = 0; readIndex < lb.Entities.Count; readIndex++)
        {
            WorldEntity entity = lb.Entities[readIndex];
            if (entity.ServerGuid != 0)
            {
                int liveWriteIndex = retainedLive.Count;
                retainedLive.Add(entity);
                SetProjectionLocation(
                    entity,
                    canonical,
                    isLoaded: true,
                    liveWriteIndex);
                continue;
            }

            retiredEntities.Add(entity);
            RemoveFlatEntity(entity);
        }
        _loaded[canonical] = NormalizeLoadedLandblock(lb, retainedLive);
        _animatedIndexByLandblock.Remove(canonical);
        _tierByLandblock[canonical] = LandblockStreamTier.Far;
        }

        return new GpuLandblockRetirement(
            canonical,
            LandblockRetirementKind.NearLayer,
            retiredEntities);
    }

    /// <summary>
    /// Compatibility edge for direct state tests. Production streaming uses
    /// the retryable retirement coordinator.
    /// </summary>
    public void RemoveEntitiesFromLandblock(uint landblockId)
    {
        GpuLandblockRetirement? retirement = DetachNearLayer(landblockId);
        if (retirement is null)
            return;

        ReleaseLandblockMeshReferences(retirement.LandblockId);
        InvalidateLandblockClassification(retirement.LandblockId);
        for (int i = 0; i < retirement.Entities.Count; i++)
            StopStaticEntityScript(retirement.Entities[i]);
    }

    /// <summary>
    /// Merge entities into an existing-loaded landblock. Used by two-tier
    /// streaming for the Far→Near promotion case (terrain already loaded;
    /// entity layer streaming in). Falls back to the pending bucket if the
    /// landblock isn't loaded yet (handles the rare "promote arrives before
    /// far load completes" race).
    /// Per Phase A.5 spec §4.4.
    ///
    /// <para>
    /// <b>Landblock id is canonicalized</b> (low 16 bits forced to 0xFFFF) —
    /// callers may pass cell-resolved ids and they will key correctly.
    /// </para>
    /// </summary>
    public bool AddEntitiesToExistingLandblock(
        uint landblockId,
        IReadOnlyList<WorldEntity> entities,
        IEnumerable<ulong>? additionalRenderIds = null)
    {
        using MutationBatch mutation = BeginMutationBatch();
        GpuLandblockSpatialPublication? publication =
            CommitEntitiesToExistingLandblockSpatialCore(
                landblockId,
                entities,
                additionalRenderIds,
                parkIfMissing: true);
        if (publication is null)
            return false;

        ActivateLandblockPresentation(publication);
        return true;
    }

    internal GpuLandblockSpatialPublication CommitEntitiesToExistingLandblockSpatial(
        uint landblockId,
        IReadOnlyList<WorldEntity> entities,
        IEnumerable<ulong>? additionalRenderIds)
    {
        GpuLandblockSpatialPublication? publication =
            CommitEntitiesToExistingLandblockSpatialCore(
                landblockId,
                entities,
                additionalRenderIds,
                parkIfMissing: false);
        return publication
            ?? throw new InvalidOperationException(
                $"Landblock 0x{landblockId:X8} is not resident for an accepted promotion.");
    }

    private GpuLandblockSpatialPublication? CommitEntitiesToExistingLandblockSpatialCore(
        uint landblockId,
        IReadOnlyList<WorldEntity> entities,
        IEnumerable<ulong>? additionalRenderIds,
        bool parkIfMissing)
    {
        ArgumentNullException.ThrowIfNull(entities);

        // A.5 T14 follow-up: canonicalize for symmetry with live projection placement.
        uint canonical = (landblockId & 0xFFFF0000u) | 0xFFFFu;
        if (!_loaded.TryGetValue(canonical, out var lb))
        {
            if (!parkIfMissing)
                return null;

            // Park as pending — same pattern as live projections for not-yet-loaded LBs.
            if (!_pendingByLandblock.TryGetValue(canonical, out var bucket))
            {
                bucket = new List<WorldEntity>();
                _pendingByLandblock[canonical] = bucket;
            }
            int firstIndex = bucket.Count;
            bucket.AddRange(entities);
            for (int i = 0; i < entities.Count; i++)
            {
                WorldEntity entity = entities[i];
                if (entity.ServerGuid != 0)
                {
                    SetProjectionLocation(
                        entity,
                        canonical,
                        isLoaded: false,
                        firstIndex + i);
                }
            }
            _pendingNearTierLandblocks.Add(canonical);
            if (additionalRenderIds is not null)
            {
                if (!_pendingRenderIdsByLandblock.TryGetValue(canonical, out var renderIds))
                {
                    renderIds = new HashSet<ulong>();
                    _pendingRenderIdsByLandblock[canonical] = renderIds;
                }
                renderIds.UnionWith(additionalRenderIds);
            }
            return null;
        }
        List<WorldEntity> resident = MutableEntities(lb);
        for (int i = 0; i < entities.Count; i++)
        {
            WorldEntity entity = entities[i];
            int bucketIndex = resident.Count;
            resident.Add(entity);
            AddFlatEntity(entity);
            if (entity.ServerGuid != 0)
                SetProjectionLocation(entity, canonical, isLoaded: true, bucketIndex);
            AdjustVisibleLiveProjection(entity, +1);
        }
        _animatedIndexByLandblock.Remove(canonical);
        _tierByLandblock[canonical] = LandblockStreamTier.Near;
        ulong[] effectiveRenderIds = additionalRenderIds?.ToArray() ?? Array.Empty<ulong>();
        WorldEntity[] staticEntities = entities
            .Where(static entity => entity.ServerGuid == 0)
            .ToArray();
        return new GpuLandblockSpatialPublication(
            canonical,
            _loaded[canonical],
            effectiveRenderIds,
            staticEntities);
    }

    private void DrainVisibilityTransitions()
    {
        if (_dispatchingVisibilityTransitions)
            return;

        List<Exception>? failures = null;
        _dispatchingVisibilityTransitions = true;
        try
        {
            while (_visibilityTransitions.TryDequeue(out var transition))
            {
                Delegate[] subscribers = LiveProjectionVisibilityChanged?
                    .GetInvocationList()
                    ?? Array.Empty<Delegate>();
                for (int i = 0; i < subscribers.Length; i++)
                {
                    try
                    {
                        ((Action<uint, bool>)subscribers[i])(
                            transition.ServerGuid,
                            transition.Visible);
                    }
                    catch (Exception error)
                    {
                        (failures ??= new List<Exception>()).Add(error);
                    }
                }
            }
        }
        finally
        {
            _dispatchingVisibilityTransitions = false;
        }

        if (failures is not null)
        {
            throw new AggregateException(
                "One or more live projection visibility observers failed.",
                failures);
        }
    }

    // TEMP (#138-B): persistent guids currently present in the drawn flat
    // view, for EntityVanishProbe transition logging. Strip with the probe.
    private readonly HashSet<uint> _persistentInFlatProbe = new();

    // TEMP (#138-B): log when a persistent (player) entity enters/leaves the
    // drawn flat view. Transition-gated → low volume (fires at teleport
    // boundaries, not every rebuild). Strip with EntityVanishProbe.
    private void ProbeFlatViewTransitions()
    {
        var now = new HashSet<uint>();
        foreach (var e in _flatEntities)
            if (e.ServerGuid != 0 && _persistentGuids.Contains(e.ServerGuid))
                now.Add(e.ServerGuid);
        foreach (var g in now)
            if (!_persistentInFlatProbe.Contains(g))
                EntityVanishProbe.Log($"[ent] DRAWSET guid=0x{g:X8} -> PRESENT (flatCount={_flatEntities.Count})");
        foreach (var g in _persistentInFlatProbe)
            if (!now.Contains(g))
                EntityVanishProbe.Log($"[ent] DRAWSET guid=0x{g:X8} -> ABSENT (flatCount={_flatEntities.Count})");
        _persistentInFlatProbe.Clear();
        foreach (var g in now) _persistentInFlatProbe.Add(g);
    }
}