acdream/tests/AcDream.Core.Tests/Rendering/Wb/WbDrawDispatcherBucketingTests.cs

using System.Collections.Generic;
using System.Numerics;
using AcDream.App.Rendering;
using AcDream.App.Rendering.Wb;
using AcDream.Core.Meshing;
using AcDream.Core.World;
using Xunit;

namespace AcDream.Core.Tests.Rendering.Wb;

/// <summary>
/// Tests for <see cref="WbDrawDispatcher.WalkEntities"/> — the pure-CPU
/// visibility filter extracted in A.5 T17. These tests exercise the two
/// key perf changes from Phase A.5 spec §4.6:
///
/// <list type="bullet">
///   <item>Change #1 (T17): invisible LB + animated set → iterate
///         <c>animatedEntityIds</c> directly, not the full entity list.</item>
///   <item>Change #2 (T18): per-entity AABB cull reads the cached AABB
///         (<see cref="WorldEntity.AabbMin"/>/<c>AabbMax</c>) rather than
///         recomputing Position±5 per frame.</item>
/// </list>
/// </summary>
public sealed class WbDrawDispatcherBucketingTests
{
    // ── helpers ──────────────────────────────────────────────────────────────

    private static WorldEntity MakeEntity(uint id, Vector3 position)
        => new WorldEntity
        {
            Id = id,
            SourceGfxObjOrSetupId = 0,
            Position = position,
            Rotation = Quaternion.Identity,
            MeshRefs = System.Array.Empty<MeshRef>(),
        };

    private static WorldEntity MakeEntityWithMesh(uint id, Vector3 position)
        => new WorldEntity
        {
            Id = id,
            SourceGfxObjOrSetupId = 0,
            Position = position,
            Rotation = Quaternion.Identity,
            // Single dummy MeshRef so it passes the MeshRefs.Count == 0 guard.
            MeshRefs = new[] { new MeshRef { GfxObjId = 0x01000001u } },
        };

    private static Dictionary<uint, WorldEntity> BuildById(IEnumerable<WorldEntity> entities)
    {
        var d = new Dictionary<uint, WorldEntity>();
        foreach (var e in entities) d[e.Id] = e;
        return d;
    }

    /// <summary>
    /// A frustum positioned at (1e6+1, 1e6+1, 1e6+1) looking toward (1e6, 1e6, 1e6)
    /// with a very narrow near/far. Any AABB near the origin (0..20000) is
    /// far behind the near plane and fails all six planes.
    /// </summary>
    private static FrustumPlanes MakeFarAwayFrustum()
    {
        var view = Matrix4x4.CreateLookAt(
            new Vector3(1e6f + 1f, 1e6f + 1f, 1e6f + 1f),
            new Vector3(1e6f, 1e6f, 1e6f),
            Vector3.UnitZ);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(
            MathF.PI / 4f, 1f, 0.1f, 1f);
        return FrustumPlanes.FromViewProjection(view * proj);
    }

    // ── T17 Change #1 tests ───────────────────────────────────────────────

    [Fact]
    public void WalkEntities_InvisibleLb_NoAnimated_SkipsEntireBlock()
    {
        // When LB is invisible AND animatedEntityIds is empty/null,
        // WalkEntities should not walk any entities at all.
        var entities = new List<WorldEntity>();
        for (int i = 0; i < 500; i++)
            entities.Add(MakeEntityWithMesh((uint)i, new Vector3(i, 0, 0)));

        var byId = BuildById(entities);
        var entries = new[]
        {
            new WbDrawDispatcher.LandblockEntry(
                0xAAAA_FFFFu,
                new Vector3(10000, 10000, 10000),
                new Vector3(20000, 20000, 20000),
                entities,
                byId),
        };

        var result = WbDrawDispatcher.WalkEntities(
            entries,
            frustum: MakeFarAwayFrustum(),
            neverCullLandblockId: null,
            visibleCellIds: null,
            animatedEntityIds: null);

        Assert.Equal(0, result.EntitiesWalked);
        Assert.Empty(result.ToDraw);
    }

    [Fact]
    public void WalkEntities_InvisibleLb_AnimatedSet_WalksOnlyAnimatedEntities()
    {
        // 1000 entities in an LB whose AABB is far outside the frustum.
        // Only entity Id=42 is in animatedEntityIds.
        // Pre-T17 behavior: walk all 1000 entities just to find #42.
        // Post-T17: walk only the 1 animated entity (EntitiesWalked == 1).
        const int Total = 1000;
        var entities = new List<WorldEntity>(Total);
        for (int i = 0; i < Total; i++)
            entities.Add(MakeEntityWithMesh((uint)i, new Vector3(i, 0, 0)));

        var byId = BuildById(entities);
        var animatedSet = new HashSet<uint> { 42 };

        var entries = new[]
        {
            new WbDrawDispatcher.LandblockEntry(
                0xAAAA_FFFFu,
                new Vector3(10000, 10000, 10000),
                new Vector3(20000, 20000, 20000),
                entities,
                byId),
        };

        var result = WbDrawDispatcher.WalkEntities(
            entries,
            frustum: MakeFarAwayFrustum(),
            neverCullLandblockId: null,
            visibleCellIds: null,
            animatedEntityIds: animatedSet);

        // Only the 1 animated entity should be walked — not 1000.
        Assert.Equal(1, result.EntitiesWalked);
        Assert.Single(result.ToDraw);
        Assert.Equal(42u, result.ToDraw[0].Entity.Id);
    }

    [Fact]
    public void WalkEntities_InvisibleLb_AnimatedIdAbsent_ZeroWalked()
    {
        // Animated entity ids 200 and 300 are NOT in this LB (which only
        // has ids 0..99). Should produce zero walks.
        var entities = new List<WorldEntity>();
        for (int i = 0; i < 100; i++)
            entities.Add(MakeEntityWithMesh((uint)i, Vector3.Zero));

        var byId = BuildById(entities);
        var animatedSet = new HashSet<uint> { 200, 300 }; // not in this LB

        var entries = new[]
        {
            new WbDrawDispatcher.LandblockEntry(
                0xBBBB_FFFFu,
                new Vector3(10000, 10000, 10000),
                new Vector3(20000, 20000, 20000),
                entities,
                byId),
        };

        var result = WbDrawDispatcher.WalkEntities(
            entries,
            frustum: MakeFarAwayFrustum(),
            neverCullLandblockId: null,
            visibleCellIds: null,
            animatedEntityIds: animatedSet);

        Assert.Equal(0, result.EntitiesWalked);
        Assert.Empty(result.ToDraw);
    }

    [Fact]
    public void WalkEntities_NeverCullLb_WalksAllEntitiesRegardlessOfFrustum()
    {
        // neverCullLandblockId bypasses the LB AABB check entirely.
        // All entities with at least one MeshRef should be walked.
        var entities = new List<WorldEntity>
        {
            MakeEntityWithMesh(1, Vector3.Zero),
            MakeEntityWithMesh(2, Vector3.Zero),
            MakeEntityWithMesh(3, Vector3.Zero),
        };

        var byId = BuildById(entities);
        const uint lbId = 0xCCCC_FFFFu;

        var entries = new[]
        {
            new WbDrawDispatcher.LandblockEntry(
                lbId,
                new Vector3(10000, 10000, 10000), // AABB would fail frustum
                new Vector3(20000, 20000, 20000),
                entities,
                byId),
        };

        var result = WbDrawDispatcher.WalkEntities(
            entries,
            frustum: MakeFarAwayFrustum(),
            neverCullLandblockId: lbId, // exempt from LB cull
            visibleCellIds: null,
            animatedEntityIds: null);

        Assert.Equal(3, result.EntitiesWalked);
    }

    [Fact]
    public void WalkEntities_NullFrustum_WalksEntitiesWithMeshRefs()
    {
        // Null frustum means no culling — all entities with MeshRefs pass.
        // Entities without MeshRefs are still filtered out.
        var entities = new List<WorldEntity>
        {
            MakeEntityWithMesh(1, Vector3.Zero),
            MakeEntity(2, Vector3.Zero),          // no MeshRefs — must be skipped
            MakeEntityWithMesh(3, Vector3.Zero),
        };

        var byId = BuildById(entities);
        var entries = new[]
        {
            new WbDrawDispatcher.LandblockEntry(
                0xDDDD_FFFFu, Vector3.Zero, Vector3.Zero,
                entities, byId),
        };

        var result = WbDrawDispatcher.WalkEntities(
            entries,
            frustum: null,
            neverCullLandblockId: null,
            visibleCellIds: null,
            animatedEntityIds: null);

        Assert.Equal(2, result.EntitiesWalked);
        Assert.Equal(2, result.ToDraw.Count);
    }

    // ── T18 Change #2 tests ───────────────────────────────────────────────

    [Fact]
    public void WalkEntities_VisibleLb_EntityFarAway_CulledViaCachedAabb()
    {
        // LB passes the LB-level cull; entity AABB is far from the frustum.
        // After RefreshAabb the entity should be culled by the per-entity check.
        var entity = MakeEntityWithMesh(1, new Vector3(50000, 50000, 50000));
        entity.RefreshAabb(); // populate cached AABB at (50000±5)

        var byId = BuildById(new[] { entity });
        var entries = new[]
        {
            // LB AABB near origin so it passes the LB cull; entity is far away.
            new WbDrawDispatcher.LandblockEntry(
                0xEEEE_FFFFu,
                new Vector3(-10, -10, -10),
                new Vector3(10, 10, 10),
                new List<WorldEntity> { entity },
                byId),
        };

        // Frustum centered at origin, range ±100.
        var view = Matrix4x4.CreateLookAt(new Vector3(-50, 0, 0), Vector3.Zero, Vector3.UnitZ);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 2f, 1f, 0.5f, 100f);
        var tightFrustum = FrustumPlanes.FromViewProjection(view * proj);

        var result = WbDrawDispatcher.WalkEntities(
            entries,
            frustum: tightFrustum,
            neverCullLandblockId: null,
            visibleCellIds: null,
            animatedEntityIds: null);

        // Entity at (50000,50000,50000) is outside the frustum — should be culled.
        Assert.Equal(0, result.EntitiesWalked);
    }

    [Fact]
    public void WalkEntities_AnimatedEntity_BypassesPerEntityAabbCull()
    {
        // Animated entities must always pass even if their AABB would be culled.
        var entity = MakeEntityWithMesh(7, new Vector3(50000, 50000, 50000));
        entity.RefreshAabb();

        var byId = BuildById(new[] { entity });
        var entries = new[]
        {
            new WbDrawDispatcher.LandblockEntry(
                0xEEEF_FFFFu,
                new Vector3(-10, -10, -10),
                new Vector3(10, 10, 10),
                new List<WorldEntity> { entity },
                byId),
        };

        var view = Matrix4x4.CreateLookAt(new Vector3(-50, 0, 0), Vector3.Zero, Vector3.UnitZ);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 2f, 1f, 0.5f, 100f);
        var tightFrustum = FrustumPlanes.FromViewProjection(view * proj);

        var animatedSet = new HashSet<uint> { 7 };

        var result = WbDrawDispatcher.WalkEntities(
            entries,
            frustum: tightFrustum,
            neverCullLandblockId: null,
            visibleCellIds: null,
            animatedEntityIds: animatedSet);

        // Animated entity bypasses per-entity cull.
        Assert.Equal(1, result.EntitiesWalked);
        Assert.Single(result.ToDraw);
        Assert.Equal(7u, result.ToDraw[0].Entity.Id);
    }

    [Fact]
    public void WalkEntities_AabbDirty_RefreshedLazilyBeforeCull()
    {
        // An entity with AabbDirty=true (initial state) should get its AABB
        // refreshed lazily by WalkEntities before the cull check.
        var entity = MakeEntityWithMesh(5, new Vector3(0, 0, 0));
        // AabbDirty starts true by default — do NOT call RefreshAabb manually.
        Assert.True(entity.AabbDirty);

        var byId = BuildById(new[] { entity });
        var entries = new[]
        {
            new WbDrawDispatcher.LandblockEntry(
                0xF0F0_FFFFu,
                new Vector3(-10, -10, -10),
                new Vector3(10, 10, 10),
                new List<WorldEntity> { entity },
                byId),
        };

        // A frustum that accepts things near origin.
        var view = Matrix4x4.CreateLookAt(new Vector3(-50, 0, 0), Vector3.Zero, Vector3.UnitZ);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 2f, 1f, 0.1f, 200f);
        var nearOriginFrustum = FrustumPlanes.FromViewProjection(view * proj);

        var result = WbDrawDispatcher.WalkEntities(
            entries,
            frustum: nearOriginFrustum,
            neverCullLandblockId: null,
            visibleCellIds: null,
            animatedEntityIds: null);

        // Entity at origin is inside the frustum after lazy RefreshAabb.
        Assert.Equal(1, result.EntitiesWalked);
        // AabbDirty should have been cleared by the lazy refresh.
        Assert.False(entity.AabbDirty);
    }

    // ── Tier 1 cache (#53) dispatcher integration tests ──────────────────────
    //
    // Tasks 9 & 10 wire the EntityClassificationCache into Draw's per-entity
    // loop. These tests exercise the populate + cache-hit fast-path algorithm
    // through the static helpers Draw uses (MaybeFlushOnEntityChange,
    // FinalFlushPopulate, ApplyCacheHit). The helpers were extracted from
    // Draw's foreach for testability — Draw calls them; tests drive them
    // directly with deterministic synthesized inputs. This is the same
    // pattern WalkEntities follows (extracted from Draw, tested in isolation).
    //
    // The tests cover spec §7.2 #11 (static populate + reuse) and #12
    // (animated bypass), plus a multi-MeshRef regression test that would
    // have caught the bug fixed in commit 00fa8ae (per-MeshRef Populate
    // overwrites earlier batches because the cache is keyed by entity.Id).

    /// <summary>
    /// Helper: constructs a CachedBatch with stable group-key inputs so the
    /// hit-path test can verify membership. Mirrors the shape ClassifyBatches
    /// produces under the collector pattern.
    /// </summary>
    private static CachedBatch MakeCachedBatch(
        uint ibo, uint firstIndex, int indexCount, ulong texHandle, Matrix4x4? restPose = null)
    {
        var key = new GroupKey(
            Ibo: ibo,
            FirstIndex: firstIndex,
            BaseVertex: 0,
            IndexCount: indexCount,
            BindlessTextureHandle: texHandle,
            TextureLayer: 0,
            Translucency: TranslucencyKind.Opaque);
        return new CachedBatch(key, texHandle, restPose ?? Matrix4x4.Identity);
    }

    [Fact]
    public void Draw_StaticEntity_PopulatesCacheOnFirstFrameAndHitsOnSecond()
    {
        // Spec §7.2 test #11.
        // Drives Draw's populate + cache-hit algorithm through the production
        // static helpers. Verifies that:
        //   1. First "frame": cache is empty → populate fires once at the
        //      end-of-loop final flush (entity.Id=100 has 2 batches).
        //   2. Second "frame": cache.TryGet(100) hits → ApplyCacheHit appends
        //      cached batches to a fresh _groups dict without re-populating.
        //      cache.Count stays at 1 (Populate is idempotent via overwrite,
        //      but the hit-path doesn't re-populate at all).
        var cache = new EntityClassificationCache();
        var scratch = new List<CachedBatch>();

        Assert.Equal(0, cache.Count);

        // Frame 1: simulate one foreach iteration producing 2 batches for
        // entity 100 in landblock 0xA9B40000. With no prior tracker, the
        // entity-change flush is a no-op. ClassifyBatches' collector adds
        // to scratch. The end-of-loop FinalFlushPopulate commits.
        const uint EntityId = 100;
        const uint LandblockId = 0xA9B40000u;

        // First MeshRef contributes 2 batches (mimics ClassifyBatches output).
        scratch.Add(MakeCachedBatch(ibo: 1, firstIndex: 0, indexCount: 6, texHandle: 0xAA));
        scratch.Add(MakeCachedBatch(ibo: 1, firstIndex: 6, indexCount: 6, texHandle: 0xBB));

        uint? populateEntityId = null;
        uint populateLandblockId = 0u;
        // First-tuple boundary check: no flush, sets the tracker.
        (populateEntityId, populateLandblockId) = WbDrawDispatcher.MaybeFlushOnEntityChange(
            populateEntityId, populateLandblockId, EntityId, cache, scratch);
        // After ClassifyBatches the loop sets the tracker (matching Draw).
        populateEntityId = EntityId;
        populateLandblockId = LandblockId;

        // End-of-loop final flush — this is where the cache populates.
        WbDrawDispatcher.FinalFlushPopulate(populateEntityId, populateLandblockId, cache, scratch);

        // First-frame post-conditions: 1 cache entry, 2 batches in it.
        Assert.Equal(1, cache.Count);
        Assert.True(cache.TryGet(EntityId, LandblockId, out var entry));
        Assert.NotNull(entry);
        Assert.Equal(2, entry!.Batches.Length);
        Assert.Equal(0xAAul, entry.Batches[0].BindlessTextureHandle);
        Assert.Equal(0xBBul, entry.Batches[1].BindlessTextureHandle);

        // Frame 2: cache hit. ApplyCacheHit walks the cached batches and
        // appends RestPose * entityWorld to a per-frame group dict.
        // Production code: this is the !isAnimated && _cache.TryGet branch
        // at the top of the per-entity loop body in Draw.
        var groups = new Dictionary<GroupKey, List<Matrix4x4>>();
        void AppendInstance(GroupKey k, Matrix4x4 m)
        {
            if (!groups.TryGetValue(k, out var list))
            {
                list = new List<Matrix4x4>();
                groups[k] = list;
            }
            list.Add(m);
        }

        Assert.True(cache.TryGet(EntityId, LandblockId, out var entryHit));
        Assert.NotNull(entryHit);
        var entityWorld = Matrix4x4.CreateTranslation(new Vector3(10f, 20f, 30f));
        WbDrawDispatcher.ApplyCacheHit(entryHit!, entityWorld, AppendInstance);

        // Cache state stable — Populate didn't fire on the hit path.
        Assert.Equal(1, cache.Count);

        // Both groups received exactly one matrix each (the entity is one
        // instance contributing once per cached batch).
        Assert.Equal(2, groups.Count);
        foreach (var (_, list) in groups)
            Assert.Single(list);

        // Matrix composition is RestPose * entityWorld (NOT the reverse).
        // RestPose is Matrix4x4.Identity for the synthesized batches, so the
        // appended matrix must equal entityWorld.
        foreach (var (_, list) in groups)
            Assert.Equal(entityWorld, list[0]);
    }

    [Fact]
    public void Draw_AnimatedEntity_DoesNotPopulateCache()
    {
        // Spec §7.2 test #12.
        // Animated entities take the slow path with collector=null: their
        // ClassifyBatches output is NOT routed into _populateScratch and the
        // populate-tracking locals stay null. Result: the cache is never
        // populated for animated entities, and FinalFlushPopulate is a no-op.
        //
        // This test models that flow: scratch stays empty, populateEntityId
        // stays null, FinalFlushPopulate fires but commits nothing.
        var cache = new EntityClassificationCache();
        var scratch = new List<CachedBatch>();

        const uint AnimatedId = 7;
        const uint LandblockId = 0xA9B40000u;
        var animatedSet = new HashSet<uint> { AnimatedId };

        // Even when the entity has MeshRefs that would produce batches, the
        // animated-set membership means collector=null in Draw — scratch
        // stays empty and the tracker stays null. Simulating that here:
        // we do NOT add to scratch and we do NOT set populateEntityId.
        bool isAnimated = animatedSet.Contains(AnimatedId);
        Assert.True(isAnimated);

        uint? populateEntityId = null;
        uint populateLandblockId = 0u;
        // Boundary check still runs but is a no-op — tracker is null.
        (populateEntityId, populateLandblockId) = WbDrawDispatcher.MaybeFlushOnEntityChange(
            populateEntityId, populateLandblockId, AnimatedId, cache, scratch);

        // For animated entities, Draw does NOT set populateEntityId after
        // ClassifyBatches (the `if (collector is not null)` guard).
        // populateEntityId stays null.

        // End-of-loop flush — no-op for animated-only iterations.
        WbDrawDispatcher.FinalFlushPopulate(populateEntityId, populateLandblockId, cache, scratch);

        // Cache should never be populated for animated entities.
        Assert.Equal(0, cache.Count);
        Assert.False(cache.TryGet(AnimatedId, LandblockId, out _));
    }

    [Fact]
    public void Draw_MultiMeshRefStaticEntity_PopulatesAllBatchesIntoSingleCacheEntry()
    {
        // Regression test for the bug fixed at commit 00fa8ae:
        //
        //   Task 9's first attempt called _cache.Populate per-(entity,
        //   MeshRefIndex) tuple, but the cache is keyed by entity.Id. For
        //   multi-MeshRef entities (multi-part Setup buildings, statues,
        //   NPCs), each iteration's Populate OVERWROTE the previous one
        //   — only the LAST MeshRef's batches survived in the cache. After
        //   the fix, Populate fires once per entity at the entity boundary
        //   (or end-of-loop), with all MeshRefs' batches accumulated into
        //   _populateScratch.
        //
        // This test simulates a 3-MeshRef static entity where each MeshRef
        // contributes 2 batches (total = 6). It walks through Draw's loop
        // structure tuple-by-tuple, calling MaybeFlushOnEntityChange before
        // each tuple's classification and FinalFlushPopulate at end-of-loop.
        // Asserts the cache entry holds ALL 6 batches, not just the last 2.
        //
        // If the per-MeshRef Populate bug were reintroduced, this test would
        // see Batches.Length == 2 (last MeshRef only).
        var cache = new EntityClassificationCache();
        var scratch = new List<CachedBatch>();

        const uint EntityId = 200;
        const uint LandblockId = 0xA9B40000u;
        const int MeshRefCount = 3;
        const int BatchesPerMeshRef = 2;
        const int ExpectedTotalBatches = MeshRefCount * BatchesPerMeshRef;

        uint? populateEntityId = null;
        uint populateLandblockId = 0u;

        // Simulate Draw's foreach over _walkScratch. _walkScratch yields
        // (entity, MeshRefIndex, landblockId) — all MeshRefs of one entity
        // are contiguous because the walk emits them in entity-order.
        for (int meshRefIdx = 0; meshRefIdx < MeshRefCount; meshRefIdx++)
        {
            // Boundary check: same entity across all 3 iterations, so this
            // never fires the flush. populateEntityId stays as is (null on
            // first iter; EntityId on subsequent iters after we set it).
            (populateEntityId, populateLandblockId) = WbDrawDispatcher.MaybeFlushOnEntityChange(
                populateEntityId, populateLandblockId, EntityId, cache, scratch);

            // Mimic ClassifyBatches' collector output for THIS MeshRef:
            // 2 batches with distinct (ibo, firstIndex, texHandle) so the
            // ordering can be verified post-hoc.
            for (int b = 0; b < BatchesPerMeshRef; b++)
            {
                ulong texHandle = (ulong)(0x100 + meshRefIdx * BatchesPerMeshRef + b);
                scratch.Add(MakeCachedBatch(
                    ibo: (uint)(meshRefIdx + 1),
                    firstIndex: (uint)(b * 6),
                    indexCount: 6,
                    texHandle: texHandle));
            }

            // After ClassifyBatches, Draw sets the tracker (matching the
            // `if (collector is not null)` block at line 482-486 in
            // WbDrawDispatcher.Draw).
            populateEntityId = EntityId;
            populateLandblockId = LandblockId;
        }

        // End-of-loop final flush. Without this call (or if Populate fired
        // per-tuple inside the loop), the cache would only hold the last
        // 2 batches — exactly the bug class from commit 00fa8ae.
        WbDrawDispatcher.FinalFlushPopulate(populateEntityId, populateLandblockId, cache, scratch);

        // Assertions: ONE cache entry with ALL 6 batches in MeshRef order.
        Assert.Equal(1, cache.Count);
        Assert.True(cache.TryGet(EntityId, LandblockId, out var entry));
        Assert.NotNull(entry);
        Assert.Equal(EntityId, entry!.EntityId);
        Assert.Equal(LandblockId, entry.LandblockHint);

        // KEY ASSERTION: Batches.Length == sum across MeshRefs (6),
        // NOT just the last MeshRef's batch count (2).
        Assert.Equal(ExpectedTotalBatches, entry.Batches.Length);

        // Per-batch ordering check: batches arrived in MeshRef order, so
        // texture handles run 0x100..0x105 in the order they were appended.
        for (int i = 0; i < ExpectedTotalBatches; i++)
            Assert.Equal((ulong)(0x100 + i), entry.Batches[i].BindlessTextureHandle);

        // After flush, scratch is cleared so the next entity starts fresh.
        Assert.Empty(scratch);
    }

    [Fact]
    public void ApplyCacheHit_PerTupleAmplification_DoesNotOccur()
    {
        // Regression test for the bug fixed at the commit landing alongside
        // this test: Task 10's first attempt called ApplyCacheHit per-(entity,
        // MeshRefIndex) tuple in Draw's foreach, but cachedEntry.Batches is
        // flat across all MeshRefs of the entity. For a 3-MeshRef building on
        // frame 2: 3 tuples × 6 cached batches per call = 18 instances drawn
        // instead of 6. Severe Z-fighting and 3× perf hit on every multi-part
        // static entity (buildings, statues, multi-MeshRef NPCs).
        //
        // This is the symmetric mirror of the Task 9 bug fixed at 00fa8ae —
        // both came from spec §5.2 describing the foreach as per-entity when
        // _walkScratch is per-tuple.
        //
        // The fix: track lastHitEntityId; the cache-hit fast path fires only
        // on the FIRST tuple of each entity. Subsequent tuples of the same
        // entity skip the iteration body via continue.
        //
        // This test simulates the inner-loop logic by directly invoking
        // ApplyCacheHit + AppendInstanceToGroup the way Draw would, with N
        // tuples for the same entity, asserting that groups[key].Count equals
        // the cached batch count (6), NOT N × cached batch count (18).

        // Set up a synthetic cache entry with 6 batches (representing 3
        // MeshRefs × 2 batches each).
        const int CachedBatchCount = 6;
        var cache = new EntityClassificationCache();
        var batches = new CachedBatch[CachedBatchCount];
        for (int i = 0; i < CachedBatchCount; i++)
        {
            batches[i] = MakeCachedBatch(
                ibo: 1u, firstIndex: (uint)i, indexCount: 6, texHandle: (ulong)(0x100 + i));
        }
        cache.Populate(entityId: 100, landblockHint: 0xA9B40000u, batches);

        // Simulate Draw's per-entity loop: 3 tuples for the same entity.
        // Track which entity has already cache-hit (mirrors the production
        // lastHitEntityId pattern).
        var groups = new Dictionary<GroupKey, List<Matrix4x4>>();
        uint? lastHitEntityId = null;
        var entityWorld = Matrix4x4.Identity; // simplest case for assertion clarity
        const uint EntityId = 100;
        const int MeshRefCount = 3;

        void AppendInstance(GroupKey k, Matrix4x4 m)
        {
            if (!groups.TryGetValue(k, out var list))
            {
                list = new List<Matrix4x4>();
                groups[k] = list;
            }
            list.Add(m);
        }

        for (int partIdx = 0; partIdx < MeshRefCount; partIdx++)
        {
            // Skip subsequent tuples of an entity that cache-hit (the fix).
            if (lastHitEntityId == EntityId) continue;

            if (cache.TryGet(EntityId, 0xA9B40000u, out var entry))
            {
                Assert.NotNull(entry);
                WbDrawDispatcher.ApplyCacheHit(entry!, entityWorld, AppendInstance);
                lastHitEntityId = EntityId;
            }
        }

        // Assertion: each group's matrix count equals the cached batches matching
        // that key, NOT (cached batches × MeshRef count). Here each batch has a
        // unique key, so each group has exactly 1 matrix.
        int totalMatrices = 0;
        foreach (var (_, matrices) in groups) totalMatrices += matrices.Count;
        Assert.Equal(CachedBatchCount, totalMatrices);  // 6, NOT 18

        // Sanity: 6 distinct keys (one per cached batch since FirstIndex differs).
        Assert.Equal(CachedBatchCount, groups.Count);
    }
}