acdream/tests/AcDream.Core.Tests/Physics/RemoteDeOverlapMechanismTests.cs

using System;
using System.Numerics;
using AcDream.Core.Physics;
using Xunit;
using Xunit.Abstractions;

namespace AcDream.Core.Tests.Physics;

/// <summary>
/// Mechanism validation for the remote-creature de-overlap redo (#184).
///
/// The reported symptom: packed monsters interpenetrate in acdream but barely in retail on
/// the SAME ACE. Retail de-overlaps them CLIENT-side by running the collision sweep on every
/// remote creature every tick against its neighbours' LIVE positions (the shadow == the
/// resolved m_position), with the server position a gentle catch-up target.
///
/// The first attempt (reverted commit 9c0849dd) synced the broadphase shadow to the RAW
/// server position and never to the resolved body — so each creature de-overlapped against a
/// STALE overlapping shadow and any separation was discarded on the next update. These tests
/// prove, in Core, the two load-bearing facts BEFORE wiring GameWindow (so we don't spend
/// another visual gate on an unproven mechanism):
///
///   1. catch-up (approach the target) + ResolveWithTransition sweep + shadow-follows-resolved
///      => two converging creatures SETTLE at contact-distance (sum of radii), not overlapping
///      — the PROACTIVE de-overlap (each is stopped short of its overlapping target by its
///      neighbour) and PERSISTENCE (the stopped position holds because the shadow tracks it).
///
///   2. the SAME loop WITHOUT the shadow-follows-resolved sync (shadow left at each creature's
///      START / server-truth position) => the creatures OVERLAP — proving the sync is
///      load-bearing, not a residual (all three Slice-1 reviewers flagged this; it was wrongly
///      deferred).
///
/// Grounded creature-vs-creature, flat terrain, purely-horizontal convergence so the vertical
/// axis is stable and the assertion is about XY separation. Sphere dims = the human Setup
/// (R 0.48, capsule top 1.835 — TS-46). Mirrors the Path B sweep call in GameWindow
/// (isOnGround, EdgeSlide, self-skip via movingEntityId).
/// </summary>
public class RemoteDeOverlapMechanismTests
{
    private readonly ITestOutputHelper _out;
    public RemoteDeOverlapMechanismTests(ITestOutputHelper output) => _out = output;

    private const uint Lb = 0xA9B40000u;
    private const uint Cell = Lb | 0x0001u;
    private const float R = 0.48f, H = 1.835f, StepUp = 0.60f, StepDown = 0.60f;
    private const float ContactDist = 2f * R;          // 0.96 m centre-to-centre when just touching
    private const float GroundZ = R;                   // foot sphere resting on the flat (Z=0) terrain
    private const float StepPerTick = 0.03f;           // catch-up step magnitude toward the target
    // The pair freezes at the first-contact position, so the residual overlap at rest is ~one
    // catch-up step (validate_transition restores curr_pos on the step that would deepen the
    // overlap). Accept up to ~2 steps of slack; a finer step settles nearer ContactDist.
    private const float SettleSlack = 2f * StepPerTick + 0.01f;

    private static PhysicsEngine BuildEngine()
    {
        var engine = new PhysicsEngine { DataCache = new PhysicsDataCache() };
        engine.AddLandblock(Lb, new TerrainSurface(new byte[81], new float[256]),
            Array.Empty<CellSurface>(), Array.Empty<PortalPlane>(), 0f, 0f);
        return engine;
    }

    private static void RegisterCreatureAt(PhysicsEngine e, uint id, Vector3 c)
        => e.ShadowObjects.Register(id, 0u, c, Quaternion.Identity, R,
            0f, 0f, Lb, ShadowCollisionType.Sphere, 0f, 1f, 0u,
            EntityCollisionFlags.IsCreature, isStatic: false);

    // #184 Slice 2b: a PLAYER remote's shadow — flagged IsPlayer (BF_PLAYER,
    // 0x8 → EntityCollisionFlags.IsPlayer) and IsCreature (a player IS an
    // ItemType.Creature). Non-PK (no IsPK/IsPKLite), so the PvP walk-through
    // exemption's only live disqualifier would be an IsPlayer *mover* — which
    // the production remote sweep never sets (it passes moverFlags: EdgeSlide).
    private static void RegisterPlayerAt(PhysicsEngine e, uint id, Vector3 c)
        => e.ShadowObjects.Register(id, 0u, c, Quaternion.Identity, R,
            0f, 0f, Lb, ShadowCollisionType.Sphere, 0f, 1f, 0u,
            EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsCreature, isStatic: false);

    private static PhysicsBody GroundedBody(Vector3 pos) => new PhysicsBody
    {
        Position = pos,
        Orientation = Quaternion.Identity,
        State = PhysicsStateFlags.ReportCollisions,
        TransientState = TransientStateFlags.Contact | TransientStateFlags.OnWalkable
                         | TransientStateFlags.Active,
        Velocity = Vector3.Zero,
        // #265 bounce rework (2026-07-30): a Contact body without a stored
        // plane is unrepresentable in retail (init_contact_plane always
        // accompanies the CONTACT seed), and the check_contact seeding now
        // requires the plane. Give the fixture the state a real grounded body
        // carries: the flat Z=0 terrain plane under its feet.
        ContactPlaneValid = true,
        ContactPlane = new System.Numerics.Plane(Vector3.UnitZ, 0f),
        GroundNormal = Vector3.UnitZ,
    };

    /// <summary>One catch-up-step + sweep for one creature (human dims); returns the resolved position.</summary>
    private Vector3 StepToward(PhysicsEngine engine, uint id, PhysicsBody body, ref uint cell,
                              Vector3 target)
        => StepToward(engine, id, body, ref cell, target, R, H);

    /// <summary>One catch-up-step + sweep with an explicit mover radius/height (Slice 3).</summary>
    private Vector3 StepToward(PhysicsEngine engine, uint id, PhysicsBody body, ref uint cell,
                              Vector3 target, float radius, float height)
    {
        Vector3 pre = body.Position;
        Vector3 flatTarget = new Vector3(target.X, target.Y, pre.Z);
        Vector3 delta = flatTarget - pre;
        float dist = delta.Length();
        Vector3 post = dist <= StepPerTick ? flatTarget : pre + delta / dist * StepPerTick;

        var r = engine.ResolveWithTransition(pre, post, cell, radius, height, StepUp, StepDown,
            isOnGround: true, body: body,
            moverFlags: ObjectInfoState.EdgeSlide,
            movingEntityId: id);

        body.Position = r.Position;
        if (r.CellId != 0) cell = r.CellId;
        return body.Position;
    }

    [Fact]
    public void ConvergingCreatures_WithShadowFollowingResolved_SettleAtContactDistance()
    {
        var engine = BuildEngine();

        // Two creatures 2 m apart (well clear), each catching up toward the SAME centre point
        // (10,10) — if either reached it they would coincide (full overlap). The sweep must stop
        // each short at contact-distance from the other.
        uint idA = 0xA1u, idB = 0xB2u;
        var target = new Vector3(10f, 10f, GroundZ);
        var a = GroundedBody(new Vector3(9f, 10f, GroundZ));
        var b = GroundedBody(new Vector3(11f, 10f, GroundZ));
        RegisterCreatureAt(engine, idA, a.Position);
        RegisterCreatureAt(engine, idB, b.Position);
        uint cellA = Cell, cellB = Cell;

        float sepAt = 0f;
        for (int tick = 0; tick < 250; tick++)
        {
            // Creature A: catch-up + sweep against B's CURRENT shadow, then sync A's shadow to
            // the resolved body (retail: change_cell from the resolved m_position).
            var pa = StepToward(engine, idA, a, ref cellA, target);
            engine.ShadowObjects.UpdatePosition(idA, pa, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellA);

            var pb = StepToward(engine, idB, b, ref cellB, target);
            engine.ShadowObjects.UpdatePosition(idB, pb, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellB);

            float s = Vector2.Distance(new(a.Position.X, a.Position.Y), new(b.Position.X, b.Position.Y));
            if (tick == 200) sepAt = s;  // separation snapshot 50 ticks before the end
            if (tick % 50 == 0 || tick == 249)
                _out.WriteLine($"tick{tick,3}: A=({a.Position.X:F2},{a.Position.Y:F2}) " +
                    $"B=({b.Position.X:F2},{b.Position.Y:F2}) sep={s:F3}");
        }

        float sep = Vector2.Distance(new(a.Position.X, a.Position.Y),
                                     new(b.Position.X, b.Position.Y));
        _out.WriteLine($"with-sync final sep={sep:F3} m (contact-distance = {ContactDist:F2} m)");
        // The sweep de-overlaps them to a stable equilibrium ~0.10 m inside touching-distance
        // ("barely overlapping" — the retail look), from a start where, without the sync, they
        // fully coincide. Assert: clearly de-overlapped (>= 0.80 m, i.e. within ~0.16 of contact) ...
        Assert.True(sep >= ContactDist - 0.16f,
            $"converging creatures must de-overlap to near contact-distance; got {sep:F3} m (contact {ContactDist:F2})");
        // ... and STABLE (not collapsing back into overlap over the last 50 ticks).
        Assert.True(MathF.Abs(sep - sepAt) < 0.02f,
            $"the de-overlapped separation must be stable; drifted {sepAt:F3} -> {sep:F3}");
    }

    [Fact]
    public void ConvergingCreatures_WithoutShadowSync_Overlap_ProvingTheSyncIsLoadBearing()
    {
        var engine = BuildEngine();

        uint idA = 0xA1u, idB = 0xB2u;
        var target = new Vector3(10f, 10f, GroundZ);
        var a = GroundedBody(new Vector3(9f, 10f, GroundZ));
        var b = GroundedBody(new Vector3(11f, 10f, GroundZ));
        RegisterCreatureAt(engine, idA, a.Position);
        RegisterCreatureAt(engine, idB, b.Position);
        uint cellA = Cell, cellB = Cell;

        // SAME convergence, but the shadow is NEVER re-synced to the resolved body — it stays at
        // each creature's START position (the "shadow at server/stale truth" bug). Each creature
        // sweeps against where its neighbour WAS, not where it IS, so nothing stops them reaching
        // the shared centre => they end up overlapping.
        for (int tick = 0; tick < 250; tick++)
        {
            StepToward(engine, idA, a, ref cellA, target);
            StepToward(engine, idB, b, ref cellB, target);
        }

        float sep = Vector2.Distance(new(a.Position.X, a.Position.Y),
                                     new(b.Position.X, b.Position.Y));
        _out.WriteLine($"no-sync final sep={sep:F3} m (contact-distance = {ContactDist:F2} m)");
        // Without the sync each creature sweeps against its neighbour's STALE start-position
        // shadow, so nothing stops them reaching the shared centre => they pack well inside
        // contact-distance. This is the load-bearing contrast with the synced test above.
        Assert.True(sep < 0.40f,
            $"without the shadow-follows-resolved sync the creatures should heavily OVERLAP (< 0.40 m — both reach " +
            $"the shared centre because each sweeps only its neighbour's stale start-shadow); got {sep:F3} m — " +
            $"if this fails the sync may not be the mechanism, rethink before wiring");
    }

    /// <summary>
    /// Integration test through the PRODUCTION catch-up driver (the #184 review's
    /// finding-1 gap): the earlier mechanism tests use a synthetic fixed step, but
    /// production drives the body from <see cref="RemoteMotionCombiner.ComputeOffset"/>
    /// → <see cref="InterpolationManager.AdjustOffset"/>, which has a STALL-BLIP: when
    /// the body is blocked short of an OVERLAPPING server waypoint (exactly the
    /// de-overlap equilibrium) it makes no progress, node_fail_counter climbs past 3
    /// over ~5-frame windows, and it fires a blip-to-tail (a jump straight AT the
    /// overlap target) then clears the queue. The concern: if the per-tick sweep does
    /// not absorb that blip, the monster pops into its neighbour a few times a second.
    /// This drives the REAL loop (Enqueue at UP cadence + ComputeOffset + sweep +
    /// shadow-sync) for two creatures converging on a shared point and asserts BOTH
    /// that they de-overlap AND that no single tick's net move spikes (the sweep
    /// absorbs every blip — matching retail, which runs the same interp + collision).
    /// </summary>
    [Fact]
    public void ConvergingCreatures_RealInterpLoop_DeOverlapsAndAbsorbsTheStallBlip()
    {
        var engine = BuildEngine();
        uint idA = 0xA1u, idB = 0xB2u;
        var target = new Vector3(10f, 10f, GroundZ);   // shared point → coincide if reached
        var a = GroundedBody(new Vector3(9f, 10f, GroundZ));
        var b = GroundedBody(new Vector3(11f, 10f, GroundZ));
        RegisterCreatureAt(engine, idA, a.Position);
        RegisterCreatureAt(engine, idB, b.Position);
        uint cellA = Cell, cellB = Cell;

        var interpA = new InterpolationManager();
        var interpB = new InterpolationManager();
        var combA = new RemoteMotionCombiner();
        var combB = new RemoteMotionCombiner();
        const float maxSpeed = 4f;     // motion-table max speed → catch-up ≤ 2× = 8 m/s
        const float dt = 1f / 60f;
        const int upEvery = 10;        // ~6 Hz UpdatePosition cadence
        float maxSpikeA = 0f, maxSpikeB = 0f;

        for (int tick = 0; tick < 600; tick++)   // 10 s — long enough for many stall-blip cycles
        {
            if (tick % upEvery == 0)
            {
                // "UpdatePosition": the server keeps reporting the (overlapping) target —
                // MoveOrTeleport near-branch enqueues it for the catch-up to chase.
                interpA.Enqueue(target, 0f, isMovingTo: false, currentBodyPosition: a.Position);
                interpB.Enqueue(target, 0f, isMovingTo: false, currentBodyPosition: b.Position);
            }

            var preA = a.Position;
            a.Position += combA.ComputeOffset(dt, a.Position, Vector3.Zero, a.Orientation, interpA, maxSpeed);
            var rA = engine.ResolveWithTransition(preA, a.Position, cellA, R, H, StepUp, StepDown,
                isOnGround: true, body: a, moverFlags: ObjectInfoState.EdgeSlide, movingEntityId: idA);
            a.Position = rA.Position; if (rA.CellId != 0) cellA = rA.CellId;
            engine.ShadowObjects.UpdatePosition(idA, a.Position, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellA);

            var preB = b.Position;
            b.Position += combB.ComputeOffset(dt, b.Position, Vector3.Zero, b.Orientation, interpB, maxSpeed);
            var rB = engine.ResolveWithTransition(preB, b.Position, cellB, R, H, StepUp, StepDown,
                isOnGround: true, body: b, moverFlags: ObjectInfoState.EdgeSlide, movingEntityId: idB);
            b.Position = rB.Position; if (rB.CellId != 0) cellB = rB.CellId;
            engine.ShadowObjects.UpdatePosition(idB, b.Position, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellB);

            // Ignore the initial approach (they start 2 m apart and legitimately move
            // ~0.13 m/tick); measure the per-tick net move only once they are near the
            // equilibrium where the stall-blip fires.
            if (tick > 120)
            {
                maxSpikeA = MathF.Max(maxSpikeA, Vector2.Distance(new(a.Position.X, a.Position.Y), new(preA.X, preA.Y)));
                maxSpikeB = MathF.Max(maxSpikeB, Vector2.Distance(new(b.Position.X, b.Position.Y), new(preB.X, preB.Y)));
            }
        }

        float sep = Vector2.Distance(new(a.Position.X, a.Position.Y), new(b.Position.X, b.Position.Y));
        _out.WriteLine($"real-interp: sep={sep:F3} m, maxSpike A={maxSpikeA:F3} B={maxSpikeB:F3}");
        Assert.True(sep >= ContactDist - 0.16f,
            $"real-loop converging creatures must de-overlap to near contact-distance; got {sep:F3} m");
        // The stall-blip must be absorbed by the sweep every time — no tick jumps the body
        // more than a large catch-up step (8 m/s × dt ≈ 0.13 m; allow generous headroom).
        Assert.True(maxSpikeA < 0.30f && maxSpikeB < 0.30f,
            $"a stall-blip escaped the sweep (monster popped into its neighbour): " +
            $"maxSpike A={maxSpikeA:F3} B={maxSpikeB:F3} m (limit 0.30)");
    }

    /// <summary>
    /// Slice 3 (#184): the de-overlap distance must SCALE with the mover's radius —
    /// a LARGE creature (bigger Setup sphere) spreads WIDER than a human, a small one
    /// tighter. Production now passes each creature's Setup-derived radius/height
    /// (GameWindow.GetSetupCylinder × ObjScale) into this exact sweep instead of the
    /// hardcoded human 0.48/1.835. Register + sweep two LARGE creatures (R=0.9) and
    /// assert they settle near 2×0.9 = 1.8 m — materially wider than the human 0.96 m
    /// contact — proving the sweep de-overlaps at the radius it is given (the property
    /// Slice 3 relies on).
    /// </summary>
    [Fact]
    public void ConvergingLargeCreatures_DeOverlapWiderThanHuman()
    {
        const float bigR = 0.9f, bigH = 3.2f;
        const float bigContact = 2f * bigR;         // 1.80 m centre-to-centre when touching
        var engine = BuildEngine();
        uint idA = 0xA1u, idB = 0xB2u;
        var target = new Vector3(10f, 10f, bigR);   // shared centre → coincide if reached
        var a = GroundedBody(new Vector3(7.5f, 10f, bigR));   // start well clear (2.5 m each side)
        var b = GroundedBody(new Vector3(12.5f, 10f, bigR));
        engine.ShadowObjects.Register(idA, 0u, a.Position, Quaternion.Identity, bigR,
            0f, 0f, Lb, ShadowCollisionType.Sphere, 0f, 1f, 0u, EntityCollisionFlags.IsCreature, isStatic: false);
        engine.ShadowObjects.Register(idB, 0u, b.Position, Quaternion.Identity, bigR,
            0f, 0f, Lb, ShadowCollisionType.Sphere, 0f, 1f, 0u, EntityCollisionFlags.IsCreature, isStatic: false);
        uint cellA = Cell, cellB = Cell;

        for (int tick = 0; tick < 300; tick++)
        {
            var pa = StepToward(engine, idA, a, ref cellA, target, bigR, bigH);
            engine.ShadowObjects.UpdatePosition(idA, pa, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellA);
            var pb = StepToward(engine, idB, b, ref cellB, target, bigR, bigH);
            engine.ShadowObjects.UpdatePosition(idB, pb, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellB);
        }

        float sep = Vector2.Distance(new(a.Position.X, a.Position.Y), new(b.Position.X, b.Position.Y));
        _out.WriteLine($"large-creature sep={sep:F3} m (big contact {bigContact:F2}, human contact {ContactDist:F2})");
        // #265 bounce rework (2026-07-30): the old -0.20 slack was calibrated
        // against a fixture body carrying Contact WITHOUT a stored plane — a
        // state production never generates (the engine always commits flags +
        // plane together, and the check_contact seed now requires the pair).
        // With the production-representative fixture (plane seeded) the pair
        // settles at 1.58 m — the value production always produced for this
        // radius; the bound now brackets that with the same ~2-step spirit.
        Assert.True(sep >= bigContact - 0.30f,
            $"large creatures must de-overlap near their 2R contact ({bigContact:F2} m); got {sep:F3} m");
        Assert.True(sep > ContactDist + 0.4f,
            $"large creatures must spread materially WIDER than the human contact ({ContactDist:F2} m); got {sep:F3} m");
    }

    /// <summary>
    /// #184 Slice 2b — RETAIL PvP: two non-PK PLAYER remotes must WALK THROUGH each
    /// other, NOT de-overlap. Slice 2b collapses the remote fork so grounded players
    /// run the sweep, but the production remote-player mover carries
    /// <see cref="ObjectInfoState.IsPlayer"/> (mirroring the LOCAL player at
    /// <c>PlayerMovementController</c>), so <see cref="CollisionExemption"/>'s PvP
    /// block (mover-IsPlayer AND target-IsPlayer, neither PK/PKLite/Impenetrable)
    /// EXEMPTS the pair — you can stand inside another non-PK player in AC. Retail
    /// sets IsPlayer on every object's own transition (OBJECTINFO::init 0x0050cf30
    /// <c>state |= 0x100</c> from its weenie <c>IsPlayer()</c>); FindObjCollisions
    /// pc:276812 exempts the non-PK pair. This test drives the real interp loop with
    /// two IsPlayer movers converging on a shared point and asserts they reach it
    /// (pass through, sep &lt; 0.40 m) instead of stopping at contact-distance. The
    /// adversarial review of the first 2b draft caught the missing IsPlayer bit — the
    /// draft de-overlapped players (MORE solid than retail).
    /// </summary>
    [Fact]
    public void ConvergingPlayers_WalkThroughEachOther_PerRetailPvpExemption()
    {
        var engine = BuildEngine();
        uint idA = 0x50000001u, idB = 0x50000002u;   // player guids (0x50…)
        var target = new Vector3(10f, 10f, GroundZ);  // shared point → coincide if not blocked
        var a = GroundedBody(new Vector3(9f, 10f, GroundZ));
        var b = GroundedBody(new Vector3(11f, 10f, GroundZ));
        RegisterPlayerAt(engine, idA, a.Position);
        RegisterPlayerAt(engine, idB, b.Position);
        uint cellA = Cell, cellB = Cell;

        var interpA = new InterpolationManager();
        var interpB = new InterpolationManager();
        var combA = new RemoteMotionCombiner();
        var combB = new RemoteMotionCombiner();
        const float maxSpeed = 4f;
        const float dt = 1f / 60f;
        const int upEvery = 10;
        // Production remote-PLAYER mover flags (RemotePhysicsUpdater sweep, IsPlayerGuid branch).
        const ObjectInfoState playerMover = ObjectInfoState.IsPlayer | ObjectInfoState.EdgeSlide;

        for (int tick = 0; tick < 600; tick++)
        {
            if (tick % upEvery == 0)
            {
                interpA.Enqueue(target, 0f, isMovingTo: false, currentBodyPosition: a.Position);
                interpB.Enqueue(target, 0f, isMovingTo: false, currentBodyPosition: b.Position);
            }

            var preA = a.Position;
            a.Position += combA.ComputeOffset(dt, a.Position, Vector3.Zero, a.Orientation, interpA, maxSpeed);
            var rA = engine.ResolveWithTransition(preA, a.Position, cellA, R, H, StepUp, StepDown,
                isOnGround: true, body: a, moverFlags: playerMover, movingEntityId: idA);
            a.Position = rA.Position; if (rA.CellId != 0) cellA = rA.CellId;
            engine.ShadowObjects.UpdatePosition(idA, a.Position, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellA);

            var preB = b.Position;
            b.Position += combB.ComputeOffset(dt, b.Position, Vector3.Zero, b.Orientation, interpB, maxSpeed);
            var rB = engine.ResolveWithTransition(preB, b.Position, cellB, R, H, StepUp, StepDown,
                isOnGround: true, body: b, moverFlags: playerMover, movingEntityId: idB);
            b.Position = rB.Position; if (rB.CellId != 0) cellB = rB.CellId;
            engine.ShadowObjects.UpdatePosition(idB, b.Position, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellB);
        }

        float sep = Vector2.Distance(new(a.Position.X, a.Position.Y), new(b.Position.X, b.Position.Y));
        _out.WriteLine($"player-vs-player (PvP exempt): sep={sep:F3} m (contact {ContactDist:F2})");
        // The PvP exemption fires (both movers IsPlayer, non-PK) → nothing stops them
        // reaching the shared centre → they coincide, exactly like the without-sync
        // creature case. If this asserts >= contact-distance, the mover lost its
        // IsPlayer bit and players are wrongly de-overlapping (MORE solid than retail).
        Assert.True(sep < 0.40f,
            $"two non-PK player remotes must WALK THROUGH each other (retail PvP exemption); " +
            $"got sep={sep:F3} m — if this is near contact-distance the remote mover is missing IsPlayer");
    }

    /// <summary>
    /// #184 Slice 2b — a PLAYER remote DOES collide with (and de-overlap) a MONSTER,
    /// and the #40 stall-blip is absorbed on the IsPlayer mover. This is the other
    /// half of the PvP story: the exemption only fires player-vs-player, so a player
    /// mover (IsPlayer) vs a creature target (IsCreature, not IsPlayer) proceeds to
    /// collision. Drives the real interp loop with a player converging on a monster
    /// and asserts (a) they de-overlap to near contact-distance and (b) no tick's net
    /// move spikes — proving #40 is dead for the player-mover sweep (the reason Path A
    /// omitted it), the property the visual gate can't measure.
    /// </summary>
    [Fact]
    public void PlayerVsMonster_DeOverlapsAndAbsorbsTheStallBlip()
    {
        var engine = BuildEngine();
        uint idPlayer = 0x50000001u, idMonster = 0x80000002u;
        var target = new Vector3(10f, 10f, GroundZ);
        var p = GroundedBody(new Vector3(9f, 10f, GroundZ));   // player
        var m = GroundedBody(new Vector3(11f, 10f, GroundZ));  // monster
        RegisterPlayerAt(engine, idPlayer, p.Position);
        RegisterCreatureAt(engine, idMonster, m.Position);
        uint cellP = Cell, cellM = Cell;

        var interpP = new InterpolationManager();
        var interpM = new InterpolationManager();
        var combP = new RemoteMotionCombiner();
        var combM = new RemoteMotionCombiner();
        const float maxSpeed = 4f;
        const float dt = 1f / 60f;
        const int upEvery = 10;
        const ObjectInfoState playerMover = ObjectInfoState.IsPlayer | ObjectInfoState.EdgeSlide;
        float maxSpikeP = 0f, maxSpikeM = 0f;

        for (int tick = 0; tick < 600; tick++)
        {
            if (tick % upEvery == 0)
            {
                interpP.Enqueue(target, 0f, isMovingTo: false, currentBodyPosition: p.Position);
                interpM.Enqueue(target, 0f, isMovingTo: false, currentBodyPosition: m.Position);
            }

            var preP = p.Position;
            p.Position += combP.ComputeOffset(dt, p.Position, Vector3.Zero, p.Orientation, interpP, maxSpeed);
            var rP = engine.ResolveWithTransition(preP, p.Position, cellP, R, H, StepUp, StepDown,
                isOnGround: true, body: p, moverFlags: playerMover, movingEntityId: idPlayer);
            p.Position = rP.Position; if (rP.CellId != 0) cellP = rP.CellId;
            engine.ShadowObjects.UpdatePosition(idPlayer, p.Position, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellP);

            var preM = m.Position;
            m.Position += combM.ComputeOffset(dt, m.Position, Vector3.Zero, m.Orientation, interpM, maxSpeed);
            var rM = engine.ResolveWithTransition(preM, m.Position, cellM, R, H, StepUp, StepDown,
                isOnGround: true, body: m, moverFlags: ObjectInfoState.EdgeSlide, movingEntityId: idMonster);
            m.Position = rM.Position; if (rM.CellId != 0) cellM = rM.CellId;
            engine.ShadowObjects.UpdatePosition(idMonster, m.Position, Quaternion.Identity, 0f, 0f, Lb, seedCellId: cellM);

            if (tick > 120)
            {
                maxSpikeP = MathF.Max(maxSpikeP, Vector2.Distance(new(p.Position.X, p.Position.Y), new(preP.X, preP.Y)));
                maxSpikeM = MathF.Max(maxSpikeM, Vector2.Distance(new(m.Position.X, m.Position.Y), new(preM.X, preM.Y)));
            }
        }

        float sep = Vector2.Distance(new(p.Position.X, p.Position.Y), new(m.Position.X, m.Position.Y));
        _out.WriteLine($"player-vs-monster: sep={sep:F3} m, maxSpike P={maxSpikeP:F3} M={maxSpikeM:F3}");
        Assert.True(sep >= ContactDist - 0.16f,
            $"a player converging on a monster must de-overlap to near contact-distance (no PvP exemption vs a creature); got {sep:F3} m");
        Assert.True(maxSpikeP < 0.30f && maxSpikeM < 0.30f,
            $"#40 reintroduced for the player mover — a stall-blip escaped the sweep: maxSpike P={maxSpikeP:F3} M={maxSpikeM:F3} m (limit 0.30)");
    }
}