using Godot;
using System.Collections.Generic;

// Body step 2 — the player IS a muscle-driven ragdoll (Nimso's Project TX, research/nimso-project-tx.md
// §2): every bone drives ITSELF toward the True Body pose through torque-capped joint motors
// (never world-pinned), the hands are additionally pulled to the controllers by a force-capped
// guide that reacts into the chest, and "ground control" holds the pelvis up and carries the body
// after the head. Input-agnostic: a driver (PlayerRig in-game, RagdollSpike headless) solves the
// TrueBodySolver, then calls Step(); the camera stays 1:1 and the driver moves its play space by
// OriginCorrection() when the body is blocked (push-back) or stands on higher/lower ground.
//
// Bodies (boxes, origin = centre of mass): pelvis (+ the not-yet-physical legs' mass), chest,
// head (incl. neck), and per arm upper arm, forearm, hand. No light intermediate links: a 0.6 kg
// neck or 0.8 kg clavicle squeezed between two strong motors oscillates under Box3D's
// one-iteration-per-substep solver (measured: RagdollSpike, 2026-10-02).
// Joints (fork Generic6DOFJoint3D mapping, research/godot-box3d-6dof.md):
//   spine, neck, wrist = BALL (cone/twist limits), spine/neck with muscle motors;
//   shoulder = DRIVE chest→upper arm: linear motor holds the humeral head on the (moving) girdle
//     point, angular motor = the shoulder muscle (no cone limit — Box3D's motor joint has none);
//   elbow = HingeJoint3D (0..150°) with a muscle motor;
//   hand guide = DRIVE chest→hand; balance = DRIVE world anchor→pelvis (angular only).
// All motors are PDs written as target velocities each tick; the force/torque limits are the strength.
public partial class PlayerRagdoll : Node3D
{
    public const float BodyMass = 60f;                 // Nimso's character is 60 kg
    // Muscles: PD gain (1/s), max commanded spin (rad/s), torque cap (N·m).
    public const float SpineKp = 15f, SpineTorque = 400f;
    public const float NeckKp = 20f, NeckTorque = 60f;
    // Asymmetric by direction (Nimso; human strength tables): pulling the arm DOWN (lats/back —
    // pull-ups, hanging, climbing) is roughly twice as strong as raising it (front delts — a weight
    // held out front), and elbow flexion (biceps) beats extension.
    public const float ShoulderKp = 20f, ShoulderTorque = 35f, ShoulderPullTorque = 100f;
    public const float ElbowKp = 20f, ElbowTorque = 45f, ElbowFlexTorque = 70f;
    public const float MuscleMaxSpin = 15f;
    // Feed-forwards are derivatives of tracking: low-pass them or tracking jitter (±1–2 mm per
    // sample) becomes hundreds of newtons of buzz (RagdollSpike REAL input mode).
    public const float MuscleFeedForwardTau = 0.05f;   // s
    public const float GroundFeedForwardTau = 0.12f;   // s
    public const float GirdleKp = 30f, GirdleForce = 2000f;  // shoulder drive, linear part
    // Hand guide (chest→hand drive): pulls the hand onto the controller; capped so the arm,
    // not the guide, carries real loads.
    public const float HandGuideKp = 40f, HandGuideForce = 40f, HandGuideMaxSpeed = 10f;
    // STRUCTURAL strength: force along the arm (straight arm pressing on a ledge, hanging from a
    // bar, carrying a bucket at your side) goes through bone and is strong; force across the arm
    // needs muscle torque and stays at HandGuideForce. Cap = lerp(base, structural, alignment).
    // Two arms ≈ 1.2 × body weight → two hands mantle/pull up, one can't (Nimso).
    public const float HandStructuralForce = 400f;
    public const float StructuralDeadband = 0.03f;
    // Nimso's one global strength value: scales every muscle and the hand guides (menu slider).
    public static float Strength = 1f;
    public const float HandGuideAngularKp = 40f, HandGuideTorque = 25f;
    // Ground control (world-space support on the pelvis, only while grounded).
    public const float GroundFrequency = 4f;           // Hz
    public const float GroundDamping = 1f;
    public const float LegStrength = 4.5f;             // max support, × body weight (real legs brake a landing at 3–5 g; 2× sank ~0.4 m)
    public const float GroundFriction = 1f;            // horizontal force ≤ this × support
    public const float GroundProbeExtra = 0.3f;        // ground counts while within hip height + this
    public const float MaxStepHeight = 0.35f;
    public const float MaxRiseSpeed = 1.2f;            // m/s the legs lift the body toward its stance
    public const float MaxSlopeDegrees = 35f;          // steeper ground gives no support (Nimso: "slopes beyond an angle can't be run up")          // higher ground under the hips isn't stood on unless the body got up there
    public const float BracedAirControl = 150f;        // N of horizontal hip carry while airborne but braced on the hands (mantling)
    public const float CrawlCarry = 300f;
    public const float BlockedPush = 150f;             // N horizontal leg push while the hips are held back              // N: the torso lies on the floor in a crawl; knees/hands would push (not simulated)
    public const float TorsoFriction = 0.3f;           // clothing on floor: lets a crawl slide
    public const float LimbFriction = 0.25f;
    // Lower-body collider: a box from the hips down to MaxStepHeight above the feet, so knee-high
    // obstacles block walking (no walking over tables) while lower steps pass under it. It slides
    // up into the torso when crouching and switches off when crawling (Nimso: legs don't snag).
    public const float LegsMass = 6f;
    public const float LegsKp = 30f, LegsForce = 3000f, LegsTorque = 400f;
    public const float LegsOffAbove = 0.25f;           // collider disabled once slid this far above the hips
    public const float MantleLift = 0.03f;             // braced + hips this far above the support goal = being lifted (legs tuck)
    public const float UntuckDelay = 0.3f;             // s the tuck must be unneeded before the collider returns
    // Balance: an implicit torque-capped motor from a world anchor to the pelvis (explicit torques
    // let the arms' reaction torques set the whole trunk swinging).
    public const float BalanceKp = 10f;
    public const float BalanceTorque = 800f;
    public const float AirBalanceShare = 0.3f;
    // Play-space reconciliation.
    public const float CameraRadius = 0.03f;           // sphere swept from the head's eye to the camera
    public const float FreeSlack = 0.12f;              // camera leading the body's eye by more than this = body held back: stick stops pushing that way
    public const float ViewDeadzone = 0.02f;           // m the head may lead the body before the view is eased back onto it
    public const float ViewFollowTau = 0.04f;          // s
    public const float MaxViewLead = 0.04f;            // hard: the head box reaches 2 cm in front of the eye
    public const float FloorRecenterTau = 1f;          // s
    public const float HeadGuideFrequency = 5f;        // Hz
    public const float HeadGuideMass = 15f;            // kg effective (head + part of the trunk)
    public const float HeadGuideForce = 250f;          // N
    public const float ResetDistance = 1.5f;           // pelvis this far from its target → snap the whole body to the pose
    // Geometry.
    public const float LumbarFraction = 0.35f;         // lumbar joint height above the hips, × trunk
    public const float PelvisLeanShare = 0.5f;         // share of the crouch lean taken by the hips
    public static readonly Vector3 HeadCenterFromEye = new(0f, -0.005f, 0.08f);   // crown ~10 cm above the eyes, chin ~11 cm below (was 12.5 / 8.5)
    public static readonly Vector3 HeadSize = new(0.16f, 0.21f, 0.2f);
    public static readonly Vector3 HandSize = new(0.035f, 0.09f, 0.12f);
    public const float WristOffset = TrueBodySolver.WristOffset;
    public const float ElbowMaxFlex = 2.6f;            // rad (~150°)
    // Sign of HingeJoint3D's angle/motor for a positive rotation of the child about the joint's
    // +Z (measured by RagdollSpike's "hinge sign" phase).
    public static float HingeSign = 1f;

    // Collision layers (AGENTS.md): 3 body, 4 hands, 5 arms.
    private const uint BodyLayer = 0b100, HandLayer = 0b1000, ArmLayer = 0b10000;
    private const uint WorldMask = 0b1;

    private sealed class Ball
    {
        public Generic6DofJoint3D Joint;
        public RigidBody3D Parent, Child;
        public Quaternion FrameA, FrameB;   // joint frame relative to each body
        public float Kp;
        public Quaternion PrevTarget;
        public bool HasPrev;
        public Vector3 Ff;
    }

    private sealed class Drive
    {
        public Generic6DofJoint3D Joint;
        public Quaternion FrameA, FrameB;
        public Vector3 AnchorA, AnchorB;    // anchor point, parent-local / child-local
        public Quaternion PrevTarget;
        public bool HasPrev;
        public Vector3 Ff;
    }

    private sealed class Arm
    {
        public RigidBody3D Upper, Fore, Hand;
        public Ball Wrist;
        public HingeJoint3D Elbow;
        public Drive Shoulder, Guide;
        public float UpperLen, ForeLen;
        public float PrevFlexTarget = -1f;
        public float FlexFf;
        public float GuideCap = HandGuideForce;
        public float ShoulderCap = ShoulderTorque, ElbowCap = ElbowTorque;
        public PhysicsBody3D Held;
    }

    public RigidBody3D Pelvis { get; private set; }
    public RigidBody3D Chest { get; private set; }
    public RigidBody3D Head { get; private set; }
    public bool Built { get; private set; }
    public bool Grounded { get; private set; }
    public float GroundY => _footY;
    public bool Braced { get; private set; }             // a hand touches or holds static world
    public bool LegsSolid => _legsSolid;
    public bool Lifted => _lifted;
    // Horizontal direction in which the body is held back from the camera (zero if not): the stick
    // must not keep moving the play space that way.
    public Vector3 HeldBack { get; private set; }
    // Really climbing: a hand holds static world, or the arms are lifting the body (mantle).
    public bool Climbing => Built && (_arms[0].Held is StaticBody3D || _arms[1].Held is StaticBody3D || _lifted);
    public RigidBody3D Legs { get; private set; }
    public Vector3 Eye => Head.GlobalTransform * _eyeLocal;
    public BodyProfile Profile { get; private set; }
    public int Snaps { get; private set; }               // safety resets (NaN / too far) — should stay 0

    private readonly Arm[] _arms = { new(), new() };
    private readonly List<RigidBody3D> _bodies = new();
    private Ball _spine, _neck;
    private Vector3 _neckBaseToEye;                       // head frame, canonical (neck straight)
    private float _trunk, _lumbar, _neckLen;
    private Vector3 _eyeLocal;
    private Vector3 _pelvisCenterLocal, _chestCenterLocal, _headCenterLocal; // from each body's proximal joint, in its basis
    private readonly Dictionary<RigidBody3D, Transform3D> _targets = new();
    private readonly Vector3[] _girdle = new Vector3[2];  // True Body shoulder offset, chest frame
    private float _footY;
    private float _supportGoalY;
    private RigidBody3D _groundBody;
    private Vector3 _groundPoint;
    private float _tuckFree, _dt;
    private PhysicsShapeQueryParameters3D _legsQuery;

    private bool LegsSpaceFree(Vector3 center, Basis basis)
    {
        _legsQuery ??= new PhysicsShapeQueryParameters3D { Shape = Legs.GetChild<CollisionShape3D>(0).Shape, CollisionMask = WorldMask };
        _legsQuery.Transform = new Transform3D(basis, center);
        return GetWorld3D().DirectSpaceState.IntersectShape(_legsQuery, 1).Count == 0;
    }
    private Vector3 _goalVelocity;

    private static float LowPass(float dt, float tau) => 1f - Mathf.Exp(-dt / tau);
    private bool _lifted;
    private float _legsHeight;
    private Drive _legsDrive;
    private bool _legsSolid = true;
    private ProceduralLegs _legVisuals;
    private float _appliedStrength = 1f;
    private Vector3 _pelvisTargetPrev;
    private bool _havePelvisPrev;

    public RigidBody3D Hand(int side) => Built ? _arms[side].Hand : null;
    public RigidBody3D UpperArm(int side) => Built ? _arms[side].Upper : null;
    public RigidBody3D Forearm(int side) => Built ? _arms[side].Fore : null;
    public IReadOnlyList<RigidBody3D> Bodies => _bodies;

    // ---------------------------------------------------------------- build

    public void Build(BodyProfile p, TrueBodySolver pose, Transform3D left, Transform3D right)
    {
        Clear();
        Profile = p;
        _trunk = TrueBodySolver.TrunkRatio * p.Stature;
        _lumbar = LumbarFraction * _trunk;
        _neckLen = Mathf.Max(0.05f, p.ShoulderDrop + TrueBodySolver.EyeToNeckPivot.Y - TrueBodySolver.NeckBaseAboveShoulders);
        float m = BodyMass;
        float s = p.Stature / 1.75f;

        // Canonical pose (standing, level head, arms straight down-forward-out) in frame C at the
        // current pelvis: joint cones/twists are centred on it and the elbow hinge's zero is a
        // straight arm. The body is then moved onto the live pose.
        Transform3D c = new(pose.YawBasis, pose.PelvisTarget);
        Vector3 hip = Vector3.Zero;
        Vector3 lumbar = new(0f, _lumbar, 0f);
        Vector3 upperChest = new(0f, _trunk, 0f);
        Vector3 neckBase = upperChest + new Vector3(0f, TrueBodySolver.NeckBaseAboveShoulders,
            -(TrueBodySolver.ShouldersBehindEyes - TrueBodySolver.EyeToNeckPivot.Z));
        Vector3 neckPivot = neckBase + new Vector3(0f, _neckLen, 0f);
        _eyeLocal = -TrueBodySolver.EyeToNeckPivot;   // head body origin is set to its centre below
        Vector3 headCenterFromPivot = _eyeLocal + HeadCenterFromEye;

        Vector3 pelvisSize = new(0.17f * p.Stature, _lumbar + 0.06f, 0.18f * s);
        _pelvisCenterLocal = new Vector3(0f, _lumbar * 0.5f - 0.03f, 0f);
        float chestHeight = _trunk - _lumbar + TrueBodySolver.NeckBaseAboveShoulders;
        Vector3 chestSize = new(2f * p.ShoulderHalfWidth * 0.75f, chestHeight, 0.2f * s);
        _chestCenterLocal = new Vector3(0f, chestHeight * 0.5f, 0f);
        _headCenterLocal = headCenterFromPivot;
        _eyeLocal -= headCenterFromPivot;             // eye relative to the head body's centre

        Color skin = new(0.85f, 0.72f, 0.6f), shirt = new(0.3f, 0.45f, 0.7f), pants = new(0.25f, 0.27f, 0.32f);
        Pelvis = MakeBody("Pelvis", new BoxShape3D { Size = pelvisSize }, (0.142f + 0.139f + 0.322f) * m - LegsMass, BodyLayer, WorldMask, pants,
            c * new Transform3D(Basis.Identity, hip + _pelvisCenterLocal), friction: TorsoFriction);
        float standingHip = pose.PelvisTarget.Y - (pose.Eye.Y - p.EyeHeight);
        _legsHeight = Mathf.Max(0.2f, standingHip - MaxStepHeight);
        Legs = MakeBody("Legs", new BoxShape3D { Size = new Vector3(0.28f * s, _legsHeight, 0.2f * s) }, LegsMass, BodyLayer, WorldMask, pants,
            c * new Transform3D(Basis.Identity, hip - new Vector3(0f, _legsHeight * 0.5f, 0f)), visible: false);
        // Explicit inertia: the collider is switched off when tucked, and a body without an
        // enabled shape has no inertia — its 400 N·m drive then tumbled the whole pelvis
        // (owner 2.1 "body spinning" when crouched).
        Legs.Inertia = new Vector3(_legsHeight * _legsHeight + 0.04f, 0.28f * 0.28f + 0.04f, _legsHeight * _legsHeight + 0.08f) * (LegsMass / 12f);
        _footY = pose.Eye.Y - p.EyeHeight;
        Chest = MakeBody("Chest", new BoxShape3D { Size = chestSize }, 0.216f * m, BodyLayer, WorldMask, shirt,
            c * new Transform3D(Basis.Identity, lumbar + _chestCenterLocal), friction: TorsoFriction);
        Head = MakeBody("Head", new BoxShape3D { Size = HeadSize * s }, 0.081f * m, BodyLayer, WorldMask, skin,
            c * new Transform3D(Basis.Identity, neckPivot + headCenterFromPivot), friction: TorsoFriction);
        // The neck: a box hanging under the head, same body.
        Vector3 neckCenter = (neckBase + neckPivot) * 0.5f - (neckPivot + headCenterFromPivot);
        Head.AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = new Vector3(0.09f * s, _neckLen, 0.09f * s) }, Position = neckCenter });
        MeshInstance3D neckMesh = new() { Mesh = new BoxMesh { Size = new Vector3(0.09f * s, _neckLen, 0.09f * s) }, Position = neckCenter };
        neckMesh.SetSurfaceOverrideMaterial(0, new StandardMaterial3D { AlbedoColor = skin, Roughness = 0.6f });
        Head.AddChild(neckMesh);
        _neckBaseToEye = neckPivot - TrueBodySolver.EyeToNeckPivot - neckBase;

        // World anchor for the balance motor (no shape, linear motor force 0 = position free).
        StaticBody3D anchor = new() { Name = "BalanceAnchor", CollisionLayer = 0, CollisionMask = 0 };
        AddChild(anchor);
        _balance = MakeDrive(anchor, Pelvis, Pelvis.GlobalTransform, 0f, BalanceTorque);
        _legsDrive = MakeDrive(Pelvis, Legs, c * new Transform3D(Basis.Identity, hip), LegsForce, LegsTorque);
        _legVisuals = new ProceduralLegs { Name = "LegVisuals" };
        AddChild(_legVisuals);
        _legVisuals.Setup(Pelvis, -_pelvisCenterLocal, p.Stature, pants, new Color(0.15f, 0.15f, 0.17f));
        _balanceCap = BalanceTorque;

        _spine = MakeBall(Pelvis, Chest, c * lumbar, c.Basis * TwistAlong(Vector3.Up), Mathf.DegToRad(60f), Mathf.DegToRad(35f), SpineKp, SpineTorque);
        _neck = MakeBall(Chest, Head, c * neckBase, c.Basis * TwistAlong(Vector3.Up), Mathf.DegToRad(75f), Mathf.DegToRad(85f), NeckKp, NeckTorque);

        for (int side = 0; side < 2; side++)
        {
            Arm a = _arms[side];
            float sign = side == 0 ? -1f : 1f;
            a.UpperLen = p.UpperArm;
            a.ForeLen = p.Forearm;
            Vector3 shoulder = upperChest + new Vector3(sign * p.ShoulderHalfWidth, 0f, 0f);
            Vector3 dir = new Vector3(sign * 0.42f, -0.74f, -0.47f).Normalized();
            Vector3 reference = Vector3.Down * 2f + Vector3.Back * 0.8f + new Vector3(sign * 0.3f, 0f, 0f);
            Vector3 pole = (reference - dir * reference.Dot(dir)).Normalized();
            Vector3 hinge = pole.Cross(dir).Normalized();
            Basis bone = BoneBasis(dir, hinge);
            Vector3 elbow = shoulder + dir * a.UpperLen;
            Vector3 wrist = elbow + dir * a.ForeLen;
            Vector3 handX = (Vector3.Right - dir * dir.X).Normalized();
            Basis handBasis = new(handX, (-dir).Cross(handX), -dir);
            Vector3 handOrigin = wrist + dir * WristOffset;
            string n = side == 0 ? "Left" : "Right";

            a.Upper = MakeBody(n + "UpperArm", new BoxShape3D { Size = new Vector3(0.085f * s, a.UpperLen, 0.085f * s) }, 0.028f * m, ArmLayer, WorldMask, shirt,
                c * new Transform3D(bone, shoulder + dir * (a.UpperLen * 0.5f)), friction: LimbFriction);
            a.Fore = MakeBody(n + "Forearm", new BoxShape3D { Size = new Vector3(0.07f * s, a.ForeLen - 0.01f, 0.07f * s) }, 0.016f * m, ArmLayer, WorldMask, skin,
                c * new Transform3D(bone, elbow + dir * (a.ForeLen * 0.5f)), friction: LimbFriction);
            a.Hand = MakeBody(n + "Hand", new BoxShape3D { Size = HandSize }, 0.006f * m, HandLayer, WorldMask | HandLayer,
                side == 0 ? new Color(0.9f, 0.3f, 0.35f) : new Color(0.3f, 0.5f, 0.9f), c * new Transform3D(handBasis, handOrigin), friction: LimbFriction);
            a.Hand.ContinuousCd = true;
            a.Hand.ContactMonitor = true;
            a.Hand.MaxContactsReported = 8;

            a.Shoulder = MakeDrive(Chest, a.Upper, c * new Transform3D(Basis.Identity, shoulder), GirdleForce, ShoulderTorque);
            a.Elbow = MakeHinge(a.Upper, a.Fore, c * elbow, c.Basis * new Basis(dir, hinge.Cross(dir), hinge));
            a.Wrist = MakeBall(a.Fore, a.Hand, c * wrist, c.Basis * TwistAlong(dir), Mathf.DegToRad(80f), Mathf.DegToRad(90f), 0f, 0f);
            a.Guide = MakeDrive(Chest, a.Hand, c * new Transform3D(handBasis, handOrigin), HandGuideForce, HandGuideTorque);
        }

        Built = true;
        _havePelvisPrev = false;
        _appliedStrength = 1f;
        _legsSolid = true;
        ApplyStrength();
        SnapToPose(pose, left, right);
        GD.Print($"PlayerRagdoll: built for stature {p.Stature:F2} m (arm {p.UpperArm:F3}+{p.Forearm:F3} m, {_bodies.Count} bodies)");
    }

    public void Clear()
    {
        foreach (Node child in GetChildren())
            child.QueueFree();
        foreach (Node child in GetChildren())
            RemoveChild(child);
        _bodies.Clear();
        _targets.Clear();
        for (int i = 0; i < 2; i++)
            _arms[i] = new Arm();
        Built = false;
    }

    // Twist axis (joint X) along `dir`.
    private static Basis TwistAlong(Vector3 dir)
    {
        Vector3 helper = Mathf.Abs(dir.Y) < 0.9f ? Vector3.Up : Vector3.Right;
        Vector3 z = dir.Cross(helper).Normalized();
        return new Basis(dir, z.Cross(dir), z);
    }

    // Segment basis: Y along the bone, X = elbow hinge axis.
    private static Basis BoneBasis(Vector3 boneDir, Vector3 hinge)
    {
        Vector3 x = (hinge - boneDir * hinge.Dot(boneDir)).Normalized();
        return new Basis(x, boneDir, x.Cross(boneDir));
    }

    // NOTE (Box3D wrapper): friction is read only when the body's shapes are created — the material
    // must be set before AddChild (setting it later silently does nothing).
    private RigidBody3D MakeBody(string name, Shape3D shape, float mass, uint layer, uint mask, Color color, Transform3D at, bool visible = true, float friction = 1f)
    {
        RigidBody3D body = new() { Name = name, Mass = mass, CollisionLayer = layer, CollisionMask = mask, Transform = at, ContactMonitor = true, MaxContactsReported = 4,
            PhysicsMaterialOverride = new PhysicsMaterial { Friction = friction } };
        body.AddChild(new CollisionShape3D { Shape = shape });
        if (shape is BoxShape3D box && layer != 0 && visible)
        {
            MeshInstance3D mesh = new() { Mesh = new BoxMesh { Size = box.Size } };
            mesh.SetSurfaceOverrideMaterial(0, new StandardMaterial3D { AlbedoColor = color, Roughness = 0.6f });
            body.AddChild(mesh);
        }
        AddChild(body);
        _bodies.Add(body);
        return body;
    }

    private static Quaternion Rot(Node3D n) => n.GlobalTransform.Basis.Orthonormalized().GetRotationQuaternion();

    // Configure BEFORE AddChild (the node pushes everything to the server on enter-tree).
    private Ball MakeBall(RigidBody3D parent, RigidBody3D child, Vector3 at, Basis frame, float cone, float twist, float kp, float torque)
    {
        Generic6DofJoint3D j = new() { Transform = new Transform3D(frame, at), NodeA = parent.GetPath(), NodeB = child.GetPath(), ExcludeNodesFromCollision = true };
        j.SetParamX(Generic6DofJoint3D.Param.AngularLowerLimit, -twist);
        j.SetParamX(Generic6DofJoint3D.Param.AngularUpperLimit, twist);
        foreach (var set in new System.Action<Generic6DofJoint3D.Param, float>[] { j.SetParamY, j.SetParamZ })
        {
            set(Generic6DofJoint3D.Param.AngularLowerLimit, -cone);
            set(Generic6DofJoint3D.Param.AngularUpperLimit, cone);
        }
        if (torque > 0f)
        {
            j.SetFlagX(Generic6DofJoint3D.Flag.EnableMotor, true);
            j.SetFlagY(Generic6DofJoint3D.Flag.EnableMotor, true);
            j.SetFlagZ(Generic6DofJoint3D.Flag.EnableMotor, true);
            j.SetParamX(Generic6DofJoint3D.Param.AngularMotorForceLimit, torque);
            j.SetParamY(Generic6DofJoint3D.Param.AngularMotorForceLimit, torque);
            j.SetParamZ(Generic6DofJoint3D.Param.AngularMotorForceLimit, torque);
        }
        AddChild(j);
        Quaternion jq = frame.Orthonormalized().GetRotationQuaternion();
        return new Ball { Joint = j, Parent = parent, Child = child, FrameA = Rot(parent).Inverse() * jq, FrameB = Rot(child).Inverse() * jq, Kp = kp };
    }

    private Drive MakeDrive(PhysicsBody3D parent, RigidBody3D child, Transform3D at, float force, float torque)
    {
        Generic6DofJoint3D j = new() { Transform = at, NodeA = parent.GetPath(), NodeB = child.GetPath(), ExcludeNodesFromCollision = true };
        foreach (var flag in new System.Action<Generic6DofJoint3D.Flag, bool>[] { j.SetFlagX, j.SetFlagY, j.SetFlagZ })
        {
            flag(Generic6DofJoint3D.Flag.EnableLinearLimit, false);
            flag(Generic6DofJoint3D.Flag.EnableAngularLimit, false);
            flag(Generic6DofJoint3D.Flag.EnableLinearMotor, true);
            flag(Generic6DofJoint3D.Flag.EnableMotor, true);
        }
        foreach (var param in new System.Action<Generic6DofJoint3D.Param, float>[] { j.SetParamX, j.SetParamY, j.SetParamZ })
        {
            param(Generic6DofJoint3D.Param.LinearMotorForceLimit, force);
            param(Generic6DofJoint3D.Param.AngularMotorForceLimit, torque);
        }
        AddChild(j);
        Quaternion jq = at.Basis.Orthonormalized().GetRotationQuaternion();
        return new Drive
        {
            Joint = j,
            FrameA = Rot(parent).Inverse() * jq,
            FrameB = Rot(child).Inverse() * jq,
            AnchorA = parent.GlobalTransform.AffineInverse() * at.Origin,
            AnchorB = child.GlobalTransform.AffineInverse() * at.Origin,
        };
    }

    private HingeJoint3D MakeHinge(RigidBody3D parent, RigidBody3D child, Vector3 at, Basis frame)
    {
        HingeJoint3D j = new() { Transform = new Transform3D(frame, at), NodeA = parent.GetPath(), NodeB = child.GetPath(), ExcludeNodesFromCollision = true };
        float lo = HingeSign > 0f ? 0f : -ElbowMaxFlex;
        float hi = HingeSign > 0f ? ElbowMaxFlex : 0f;
        j.SetFlag(HingeJoint3D.Flag.UseLimit, true);
        j.SetParam(HingeJoint3D.Param.LimitLower, lo);
        j.SetParam(HingeJoint3D.Param.LimitUpper, hi);
        j.SetFlag(HingeJoint3D.Flag.EnableMotor, true);
        j.SetParam(HingeJoint3D.Param.MotorMaxImpulse, ElbowTorque);
        AddChild(j);
        return j;
    }

    // ---------------------------------------------------------------- pose targets

    // World transform every body should have in the True Body pose. The trunk is rebuilt top-down
    // through the physical chain (upper chest → lumbar → hips) so a leaning chest doesn't drag
    // the head off the camera; the hips take part of the crouch lean (hip hinge).
    private void ComputeTargets(TrueBodySolver s, Transform3D left, Transform3D right, bool physicalArms = false, float dt = 0f)
    {
        Basis chest = s.ChestBasis;
        // Hips take half of a crouch lean, all of it in a crawl (keeps them off the floor).
        float share = Mathf.Lerp(PelvisLeanShare, 1f, Mathf.Clamp((s.Lean - TrueBodySolver.ChestLeanMax) / 0.5f, 0f, 1f));
        Basis pelvis = s.YawBasis * new Basis(Vector3.Right, -s.Lean * share);
        Vector3 lumbar = s.UpperChest - chest.Y * (_trunk - _lumbar);
        Vector3 hip = lumbar - pelvis.Y * _lumbar;
        _targets[Pelvis] = new Transform3D(pelvis, hip + pelvis * _pelvisCenterLocal);
        _targets[Chest] = new Transform3D(chest, lumbar + chest * _chestCenterLocal);
        _targets[Legs] = new Transform3D(s.YawBasis, hip - Vector3.Up * (_legsHeight * 0.5f));  // snap pose only; LegsDrive steps it
        // Head + neck pivot at the neck base: orient like the camera, tilted by the small extra
        // rotation that puts the body's eye on the camera (the True Body bends the neck at two points).
        Vector3 eyeDir = s.HeadBasis * _neckBaseToEye;
        Vector3 wantDir = s.Eye - s.NeckBase;
        Quaternion tilt = eyeDir.LengthSquared() > 1e-8f && wantDir.LengthSquared() > 1e-8f
            ? new Quaternion(eyeDir.Normalized(), wantDir.Normalized()) : Quaternion.Identity;
        Basis head = new Basis(tilt) * s.HeadBasis;
        _targets[Head] = new Transform3D(head, s.NeckBase + head * (_neckBaseToEye - _eyeLocal));

        for (int side = 0; side < 2; side++)
        {
            Arm a = _arms[side];
            Vector3 sh = s.Shoulder[side], el = s.Elbow[side], wr = s.Wrist[side];
            Vector3 armDir = wr - sh;
            armDir = armDir.LengthSquared() > 1e-8f ? armDir.Normalized() : chest * Vector3.Down;
            Vector3 hinge = s.Pole[side].Cross(armDir);
            hinge = hinge.LengthSquared() > 1e-8f ? hinge.Normalized() : chest.X;
            Vector3 up = (el - sh).Normalized();
            Vector3 fore = (wr - el).Normalized();
            _girdle[side] = chest.Transposed() * (sh - (s.UpperChest + chest.X * ((side == 0 ? -1f : 1f) * Profile.ShoulderHalfWidth)));
            _targets[a.Upper] = new Transform3D(BoneBasis(up, hinge), sh + up * (a.UpperLen * 0.5f));
            _targets[a.Fore] = new Transform3D(BoneBasis(fore, hinge), el + fore * (a.ForeLen * 0.5f));
            _targets[a.Hand] = (side == 0 ? left : right).Orthonormalized();

            if (physicalArms)
            {
                // Re-aim the arm from the PHYSICAL shoulder. Horizontally the body may be held away from
                // the camera (pressed against a ledge, leaning over a table): the arm reaches for the
                // real hand. Vertically a sagging body keeps its arm pose (the aim point moves with
                // the shoulder), so straight real arms stay straight virtual arms — pillars that hold
                // you up when crawling or press you up a ledge (the hand guide adds the rest).
                // Only a horizontal TRANSLATION of the reach: solved in the chest TARGET frame (the
                // muscles stay local). Aiming in the physical chest frame made the shoulder muscles
                // feed chest rotation back into the chest — a slow twist that tracking jitter
                // excited into "chest slipping out of control" (owner playtest 2.1).
                Transform3D chestPhys = Chest.GlobalTransform.Orthonormalized();
                Vector3 shPhys = chestPhys * (a.Shoulder.AnchorA + _girdle[side]);
                Vector3 held = shPhys - sh;
                held.Y = 0f;
                Transform3D ctrl = _targets[a.Hand];
                ArmSolver solver = _armSolvers[side];
                solver.Solve(sh, ctrl * new Vector3(0f, 0f, WristOffset) - held, chest, chest.X * (side == 0 ? -1f : 1f),
                    ctrl.Basis, a.UpperLen, a.ForeLen, dt);
                Vector3 d = solver.Wrist - sh;
                Vector3 h = d.LengthSquared() > 1e-8f ? solver.Pole.Cross(d.Normalized()) : chest.X;
                h = h.LengthSquared() > 1e-8f ? h.Normalized() : chest.X;
                Vector3 u = (solver.Elbow - sh).Normalized(), f = (solver.Wrist - solver.Elbow).Normalized();
                _targets[a.Upper] = new Transform3D(BoneBasis(u, h), sh + u * (a.UpperLen * 0.5f));
                _targets[a.Fore] = new Transform3D(BoneBasis(f, h), solver.Elbow + f * (a.ForeLen * 0.5f));
            }
        }
    }

    private readonly ArmSolver[] _armSolvers = { new(), new() };

    public Transform3D Target(RigidBody3D body) => _targets.TryGetValue(body, out Transform3D t) ? t : body.GlobalTransform;

    // ---------------------------------------------------------------- per tick

    public void Step(TrueBodySolver s, Transform3D left, Transform3D right, Vector3 leftVelocity, Vector3 rightVelocity, float floorY, float dt)
    {
        if (!Built)
            return;
        _dt = dt;
        _floorYForHead = floorY;
        ComputeTargets(s, left, right, physicalArms: true, dt);
        foreach (RigidBody3D b in _bodies)
        {
            if (BodyMath.ResetIfNonFinite(b, Target(b), "PlayerRagdoll"))
            {
                Snaps++;
                SnapToPose(s, left, right);
                return;
            }
        }

        Transform3D pelvisTarget = _targets[Pelvis];
        if (Pelvis.GlobalPosition.DistanceTo(pelvisTarget.Origin) > ResetDistance)
        {
            GD.Print("PlayerRagdoll: body too far from the pose, snapped to it");
            Snaps++;
            SnapToPose(s, left, right);
            return;
        }

        Muscle(_spine, _targets[Pelvis].Basis, _targets[Chest].Basis, dt);
        Muscle(_neck, _targets[Chest].Basis, _targets[Head].Basis, dt);
        if (Strength != _appliedStrength)
            ApplyStrength();
        Braced = false;
        for (int side = 0; side < 2; side++)
        {
            Arm a = _arms[side];
            ShoulderDrive(a, side, dt);
            ElbowMuscle(a, dt);
            HandGuide(a, _targets[a.Hand], side == 0 ? leftVelocity : rightVelocity);
            Braced |= IsBraced(a);
        }
        GroundControl(pelvisTarget, floorY, dt);
        HeadGuide(s, dt);
        LegsDrive(s);
        _legVisuals.Tick(_footY, Grounded, !_legsSolid, s.YawBasis, dt);
    }

    // Muscle = PD on the joint's relative rotation toward the relative rotation of the two TARGET
    // bases (a local pose, not a world pin), written as the motor's target velocity in frame A.
    // Feed-forward = the target's own relative spin, so a moving pose isn't chased with a lag of
    // speed/Kp.
    private static void Muscle(Ball b, Basis parentTarget, Basis childTarget, float dt)
    {
        Quaternion current = (Rot(b.Parent) * b.FrameA).Inverse() * (Rot(b.Child) * b.FrameB);
        Quaternion target = (parentTarget.Orthonormalized().GetRotationQuaternion() * b.FrameA).Inverse()
            * (childTarget.Orthonormalized().GetRotationQuaternion() * b.FrameB);
        Vector3 raw = b.HasPrev && dt > 0f ? (BodyMath.RotationError(target, b.PrevTarget) / dt).LimitLength(MuscleMaxSpin) : Vector3.Zero;
        b.Ff = b.Ff.Lerp(raw, LowPass(dt, MuscleFeedForwardTau));
        b.PrevTarget = target;
        b.HasPrev = true;
        Vector3 w = (b.Ff + BodyMath.RotationError(target, current) * b.Kp).LimitLength(MuscleMaxSpin);
        SetAngularVelocity(b.Joint, w);
    }

    private static void SetAngularVelocity(Generic6DofJoint3D j, Vector3 w)
    {
        j.SetParamX(Generic6DofJoint3D.Param.AngularMotorTargetVelocity, w.X);
        j.SetParamY(Generic6DofJoint3D.Param.AngularMotorTargetVelocity, w.Y);
        j.SetParamZ(Generic6DofJoint3D.Param.AngularMotorTargetVelocity, w.Z);
    }

    private static void SetLinearVelocity(Generic6DofJoint3D j, Vector3 v)
    {
        j.SetParamX(Generic6DofJoint3D.Param.LinearMotorTargetVelocity, v.X);
        j.SetParamY(Generic6DofJoint3D.Param.LinearMotorTargetVelocity, v.Y);
        j.SetParamZ(Generic6DofJoint3D.Param.LinearMotorTargetVelocity, v.Z);
    }

    // Elbow flexion (0 = straight) about the upper arm's hinge axis.
    public float ElbowFlex(int side)
    {
        Arm a = _arms[side];
        Basis u = a.Upper.GlobalTransform.Basis.Orthonormalized();
        Vector3 f = a.Fore.GlobalTransform.Basis.Orthonormalized().Y;
        return Mathf.Atan2(u.X.Dot(u.Y.Cross(f)), u.Y.Dot(f));
    }

    private void ElbowMuscle(Arm a, float dt)
    {
        int side = a == _arms[0] ? 0 : 1;
        Vector3 up = _targets[a.Upper].Basis.Y, fore = _targets[a.Fore].Basis.Y;
        float target = Mathf.Acos(Mathf.Clamp(up.Dot(fore), -1f, 1f));
        float raw = a.PrevFlexTarget >= 0f && dt > 0f ? Mathf.Clamp((target - a.PrevFlexTarget) / dt, -MuscleMaxSpin, MuscleMaxSpin) : 0f;
        a.FlexFf = Mathf.Lerp(a.FlexFf, raw, LowPass(dt, MuscleFeedForwardTau));
        a.PrevFlexTarget = target;
        float w = Mathf.Clamp(a.FlexFf + (target - ElbowFlex(side)) * ElbowKp, -MuscleMaxSpin, MuscleMaxSpin);
        a.Elbow.SetParam(HingeJoint3D.Param.MotorTargetVelocity, HingeSign * w);
        float cap = (w > 0f ? ElbowFlexTorque : ElbowTorque) * Mathf.Max(0.05f, Strength);
        if (Mathf.Abs(cap - a.ElbowCap) > 0.5f)
        {
            a.ElbowCap = cap;
            a.Elbow.SetParam(HingeJoint3D.Param.MotorMaxImpulse, cap);
        }
    }

    // Head guide (Boneworks/BIMOS drive the head to the HMD): while standing on the ground, a force-
    // capped PD pulls the character's eye onto the camera, so voluntary head motion (squats, leans,
    // quick looks) doesn't leave the body lagging — a lag the riding view would otherwise turn into
    // a permanent play-space offset. Off while airborne/climbing (no levitating by the head) or
    // bracing on the hands (mantling, pushing off).
    private void HeadGuide(TrueBodySolver s, float dt)
    {
        Vector3 goal = s.Eye + Vector3.Up * (_footY - _floorYForHead);
        Vector3 raw = _haveHeadPrev && dt > 0f ? (goal - _headGoalPrev) / dt : Vector3.Zero;
        _headGoalPrev = goal;
        _haveHeadPrev = true;
        if (raw.Length() > 10f)
            raw = Vector3.Zero;
        _headGoalVelocity = _headGoalVelocity.Lerp(raw, LowPass(dt, GroundFeedForwardTau));
        // Only on your feet: hands bracing (mantling, pushing yourself) or crawling move the body by
        // other means, and the view simply follows it.
        if (!Grounded || Climbing || Braced)
            return;
        float w = Mathf.Tau * HeadGuideFrequency;
        Vector3 force = ((goal - Eye) * (w * w) + (_headGoalVelocity - Head.LinearVelocity) * (2f * w)) * HeadGuideMass;
        Head.ApplyCentralForce(force.LimitLength(HeadGuideForce));
    }

    private Vector3 _headGoalPrev, _headGoalVelocity;
    private bool _haveHeadPrev;
    private float _floorYForHead;

    private void ApplyStrength()
    {
        _appliedStrength = Strength;
        float k = Mathf.Max(0.05f, Strength);
        SetAngularCap(_spine.Joint, SpineTorque * k);
        SetAngularCap(_neck.Joint, NeckTorque * k);
        foreach (Arm a in _arms)
        {
            SetAngularCap(a.Shoulder.Joint, ShoulderTorque * k);
            a.ShoulderCap = ShoulderTorque * k;
            a.ElbowCap = ElbowTorque * k;
            SetAngularCap(a.Guide.Joint, HandGuideTorque * k);
            a.Elbow.SetParam(HingeJoint3D.Param.MotorMaxImpulse, ElbowTorque * k);
        }
    }

    private static void SetAngularCap(Generic6DofJoint3D j, float cap)
    {
        j.SetParamX(Generic6DofJoint3D.Param.AngularMotorForceLimit, cap);
        j.SetParamY(Generic6DofJoint3D.Param.AngularMotorForceLimit, cap);
        j.SetParamZ(Generic6DofJoint3D.Param.AngularMotorForceLimit, cap);
    }

    private static void SetLinearCap(Generic6DofJoint3D j, float cap)
    {
        j.SetParamX(Generic6DofJoint3D.Param.LinearMotorForceLimit, cap);
        j.SetParamY(Generic6DofJoint3D.Param.LinearMotorForceLimit, cap);
        j.SetParamZ(Generic6DofJoint3D.Param.LinearMotorForceLimit, cap);
    }

    // A hand pressing on or holding static world (a ledge, a wall, a grabbed bar) — not the floor:
    // hands on the floor while crawling don't lift the body.
    private static bool IsBraced(Arm a)
    {
        if (a.Held is StaticBody3D)
            return true;
        foreach (Node3D n in a.Hand.GetCollidingBodies())
            if (n is StaticBody3D && !n.IsInGroup(PropGrabber.NoGrabGroup))
                return true;
        return false;
    }

    // The lower-body collider hangs from the actual hips, its bottom never below a step above the
    // feet; once it would have to slide far up into the torso (deep crouch, crawl) it stops colliding.
    private void LegsDrive(TrueBodySolver s)
    {
        Transform3D pelvis = Pelvis.GlobalTransform;
        Vector3 hip = pelvis * (-_pelvisCenterLocal);
        float minCenter = _footY + MaxStepHeight + _legsHeight * 0.5f;
        float center = Mathf.Max(hip.Y - _legsHeight * 0.5f, minCenter);
        // Tuck (collider off, folded into the pelvis): deep crouch/crawl, or the hands bracing to
        // lift the body (mantle, climbing — the knees come up). It STAYS tucked until it has been
        // unneeded for a while AND its untucked spot is free: re-enabling inside a ledge face is
        // what shoved the body off again and again (owner 2.2 "legs trying to get off the
        // ground and instantly coming back").
        bool climbingHold = _arms[0].Held is StaticBody3D || _arms[1].Held is StaticBody3D;
        _lifted = Braced && (Pelvis.GlobalPosition.Y > _supportGoalY + MantleLift || climbingHold);
        bool deep = center + _legsHeight * 0.5f >= hip.Y + LegsOffAbove;
        bool wantTuck = deep || _lifted || (!_legsSolid && Braced);
        _tuckFree = wantTuck ? 0f : _tuckFree + _dt;
        Vector3 untucked = new(hip.X, center, hip.Z);
        bool solid = _legsSolid ? !wantTuck : _tuckFree > UntuckDelay && LegsSpaceFree(untucked, s.YawBasis);
        if (solid != _legsSolid)
        {
            _legsSolid = solid;
            // The shape itself is disabled: Box3D keeps existing contacts when only the layer changes.
            Legs.GetChild<CollisionShape3D>(0).SetDeferred(CollisionShape3D.PropertyName.Disabled, !solid);
        }
        if (!solid)
            center = hip.Y;   // tucked into the pelvis, out of the way
        Vector3 desired = new(hip.X, center, hip.Z);
        Quaternion frameA = Rot(Pelvis) * _legsDrive.FrameA;
        SetLinearVelocity(_legsDrive.Joint, frameA.Inverse() * ((desired - Legs.GlobalPosition) * LegsKp));
        Quaternion target = (Rot(Pelvis) * _legsDrive.FrameA).Inverse() * (s.YawBasis.GetRotationQuaternion() * _legsDrive.FrameB);
        Quaternion current = frameA.Inverse() * (Rot(Legs) * _legsDrive.FrameB);
        SetAngularVelocity(_legsDrive.Joint, (BodyMath.RotationError(target, current) * LegsKp).LimitLength(MuscleMaxSpin));
    }

    // Shoulder = one drive: its linear motor keeps the humeral head on the chest's shoulder point
    // shifted by the True Body girdle offset (shrug/protraction), its angular motor is the
    // shoulder muscle (same PD + feed-forward as Muscle, parent = chest).
    private void ShoulderDrive(Arm a, int side, float dt)
    {
        Drive d = a.Shoulder;
        Transform3D chest = Chest.GlobalTransform.Orthonormalized();
        Vector3 desired = chest * (d.AnchorA + _girdle[side]);
        Vector3 err = desired - a.Upper.GlobalTransform * d.AnchorB;
        Quaternion frameA = chest.Basis.GetRotationQuaternion() * d.FrameA;
        SetLinearVelocity(d.Joint, frameA.Inverse() * (err * GirdleKp));

        Quaternion current = frameA.Inverse() * (Rot(a.Upper) * d.FrameB);
        // Local muscle: relative to the chest's TARGET pose (never to the physical chest — that
        // closes a rotation feedback loop through the arms).
        Quaternion target = (_targets[Chest].Basis.Orthonormalized().GetRotationQuaternion() * d.FrameA).Inverse()
            * (_targets[a.Upper].Basis.Orthonormalized().GetRotationQuaternion() * d.FrameB);
        Vector3 raw = d.HasPrev && dt > 0f ? (BodyMath.RotationError(target, d.PrevTarget) / dt).LimitLength(MuscleMaxSpin) : Vector3.Zero;
        d.Ff = d.Ff.Lerp(raw, LowPass(dt, MuscleFeedForwardTau));
        d.PrevTarget = target;
        d.HasPrev = true;
        Vector3 spin = (d.Ff + BodyMath.RotationError(target, current) * ShoulderKp).LimitLength(MuscleMaxSpin);
        SetAngularVelocity(d.Joint, spin);
        // Which way is the muscle moving the arm? Rotating the bone direction downward = pulling.
        Vector3 bone = (a.Upper.GlobalTransform.Basis.Y).Normalized();
        Vector3 boneMotion = (frameA * spin).Cross(bone);
        float cap = (boneMotion.Y < 0f ? ShoulderPullTorque : ShoulderTorque) * Mathf.Max(0.05f, Strength);
        if (Mathf.Abs(cap - a.ShoulderCap) > 0.5f)
        {
            a.ShoulderCap = cap;
            SetAngularCap(d.Joint, cap);
        }
    }

    // Force-capped pull of the hand onto the controller, reacting into the chest (arm muscles).
    private void HandGuide(Arm a, Transform3D target, Vector3 targetVelocity)
    {
        Transform3D chest = Chest.GlobalTransform;
        Vector3 anchor = chest * a.Guide.AnchorA;
        Vector3 anchorVelocity = Chest.LinearVelocity + Chest.AngularVelocity.Cross(anchor - Chest.GlobalPosition);
        Vector3 err = target.Origin - a.Hand.GlobalPosition;
        Vector3 desired = (targetVelocity + err * HandGuideKp).LimitLength(HandGuideMaxSpeed);
        Quaternion frameA = Rot(Chest) * a.Guide.FrameA;
        Vector3 shoulder = a.Upper.GlobalTransform * a.Shoulder.AnchorB;
        Vector3 axis = a.Hand.GlobalPosition - shoulder;
        // Axial share of the error, with a 3 cm floor so a nearly-reached target's noisy error
        // direction can't flip a held load into "structural".
        // PUSH only (the hand wants to go further from the shoulder — straight arms pressing on a
        // ledge): bone in compression. Pulling (shortening the arm: pull-ups, climbing) is muscle
        // work — the asymmetric shoulder/elbow muscles do it, and one arm can't lift the body.
        float align = axis.LengthSquared() > 1e-8f ? Mathf.Max(0f, err.Dot(axis.Normalized())) / (err.Length() + StructuralDeadband) : 0f;
        // Structure only helps against something: a hand in free air reaching past its arm's
        // length must not haul the whole body after it.
        if (a.Held == null && a.Hand.GetContactCount() == 0)
            align = 0f;
        float cap = Mathf.Lerp(HandGuideForce, HandStructuralForce, align) * Mathf.Max(0.05f, Strength);
        if (Mathf.Abs(cap - a.GuideCap) > 1f)
        {
            a.GuideCap = cap;
            SetLinearCap(a.Guide.Joint, cap);
        }
        SetLinearVelocity(a.Guide.Joint, frameA.Inverse() * (desired - anchorVelocity));
        // Angular: hand rotation onto the controller rotation, as a relative spin vs the chest.
        Vector3 rotErr = BodyMath.RotationError(target.Basis.GetRotationQuaternion(), Rot(a.Hand));
        Vector3 wWorld = (rotErr * HandGuideAngularKp).LimitLength(MuscleMaxSpin * 2f) - Chest.AngularVelocity;
        SetAngularVelocity(a.Guide.Joint, frameA.Inverse() * wWorld);
    }

    // "Leg strength": a stiff, mass-scaled support holding the pelvis at its pose height above the
    // ground and carrying it after the head horizontally (friction-limited), plus an upright/yaw
    // balance motor (world anchor → pelvis). No ground → no support (falls); balance weakens in the air.
    private void GroundControl(Transform3D target, float floorY, float dt)
    {
        Vector3 pos = Pelvis.GlobalPosition;
        float hipAboveFloor = target.Origin.Y - floorY;
        var query = PhysicsRayQueryParameters3D.Create(pos, pos + Vector3.Down * (hipAboveFloor + GroundProbeExtra + 0.5f), WorldMask);
        Godot.Collections.Dictionary hit = GetWorld3D().DirectSpaceState.IntersectRay(query);
        bool wasGrounded = Grounded;
        Grounded = false;
        _groundBody = null;
        if (hit.Count > 0)
        {
            Vector3 point = (Vector3)hit["position"];
            Vector3 normal = (Vector3)hit["normal"];
            _groundBody = GodotObject.InstanceFromId((ulong)hit["collider_id"]) as RigidBody3D;  // Box3D: "collider" is null
            _groundPoint = point;
            bool inReach = pos.Y - point.Y < hipAboveFloor + GroundProbeExtra;
            bool steep = normal.Y <= Mathf.Cos(Mathf.DegToRad(MaxSlopeDegrees));
            if (inReach && steep && wasGrounded && point.Y <= _footY + MaxStepHeight)
            {
                // The foot of a too-steep slope: keep standing on the ground we're on (the legs
                // collider stops us); dropping support here would collapse the body.
                Grounded = true;
            }
            else if (inReach && !steep)
            {
                // Step rule: ground more than a step above the feet is only stood on once the
                // hips are already well above it (climbed/mantled/landed there) — otherwise it's an
                // obstacle the legs collider holds us back from, and the feet stay where they were.
                // Higher ground climbed onto must be under the hips' back edge too (not just a
                // pelvis centre hanging over the lip).
                bool clear = point.Y <= _footY + MaxStepHeight || GroundBehindAt(pos, point.Y);
                bool reachable = point.Y <= _footY + MaxStepHeight
                    || ((!wasGrounded || _lifted || pos.Y - point.Y > 0.6f * hipAboveFloor) && clear);
                Grounded = true;
                if (reachable)
                    _footY = point.Y;
            }
        }

        // Feed-forward from the GOAL (pose + ground offset): when the play-space floor follows the
        // ground, the pose target and the ground offset move by the same amount in opposite
        // directions — differentiating the pose alone would launch the body.
        Vector3 goalNow = target.Origin + Vector3.Up * (_footY - floorY);
        Vector3 rawVelocity = _havePelvisPrev && dt > 0f ? (goalNow - _pelvisTargetPrev) / dt : Vector3.Zero;
        _pelvisTargetPrev = goalNow;
        _havePelvisPrev = true;
        if (rawVelocity.Length() > 10f)
            rawVelocity = Vector3.Zero;
        _goalVelocity = _goalVelocity.Lerp(rawVelocity, LowPass(dt, GroundFeedForwardTau));
        // Held back = the hips can't get to where the pose wants them (knee-high box, table edge,
        // wall). Measured at the hips, not the head: the body tips over an obstacle, so the head
        // may be right where the camera is while the hips are stuck.
        Vector3 hipLag = new(target.Origin.X - pos.X, 0f, target.Origin.Z - pos.Z);
        HeldBack = Grounded && hipLag.Length() > FreeSlack ? hipLag.Normalized() : Vector3.Zero;
        Vector3 targetVelocity = _goalVelocity;

        float weight = BodyMass * 9.8f;
        if (Grounded)
        {
            // Pose heights are relative to the tracking floor; stand on the ground actually under us.
            Vector3 goal = target.Origin + Vector3.Up * (GroundY - floorY);
            _supportGoalY = goal.Y;
            float w = Mathf.Tau * GroundFrequency;
            Vector3 accel = (goal - pos) * (w * w) + (targetVelocity - Pelvis.LinearVelocity) * (2f * GroundDamping * w);
            Vector3 force = accel * BodyMass;
            // Vertical as a rate-limited velocity servo: full leg strength brakes a landing, but
            // the legs raise the body at most MaxRiseSpeed — they can push, not pull, so a hard push
            // toward a higher stance (onto a ledge) would otherwise overshoot into a jump.
            float riseVelocity = Mathf.Clamp((goal.Y - pos.Y) * w, -3f, MaxRiseSpeed) + targetVelocity.Y;
            float verticalAccel = (riseVelocity - Pelvis.LinearVelocity.Y) * (2f * GroundDamping * w);
            float support = Mathf.Clamp(verticalAccel * BodyMass + weight, 0f, LegStrength * weight);
            float carry = Mathf.Max(GroundFriction * support, _legsSolid ? 0f : CrawlCarry);
            // Hips held back by an obstacle: lean on it, don't bulldoze — full leg force tipped the
            // body over knee-high boxes and steep slopes (and the riding view tipped with it).
            if (HeldBack != Vector3.Zero)
                carry = Mathf.Min(carry, BlockedPush);
            Vector3 horizontal = new Vector3(force.X, 0f, force.Z).LimitLength(carry);
            Pelvis.ApplyCentralForce(horizontal + Vector3.Up * support);
            // Newton's third law: standing on something movable (seesaw, plank, crate) pushes it
            // down — the feet aren't physical, so the reaction is applied where the ground ray hit.
            if (_groundBody != null && IsInstanceValid(_groundBody))
                _groundBody.ApplyForce(-(horizontal + Vector3.Up * support), _groundPoint - _groundBody.GlobalPosition);
        }
        else if (Braced)
        {
            // Hanging/mantling on the hands: the hips still drift after the head a little, so you can
            // shift your weight over a ledge you're pressing yourself up onto.
            Vector3 lead = target.Origin - pos;
            lead.Y = 0f;
            Vector3 v = Pelvis.LinearVelocity;
            v.Y = 0f;
            Pelvis.ApplyCentralForce(((lead * 20f - v * 8f) * BodyMass).LimitLength(BracedAirControl));
        }

        Vector3 rotErr = BodyMath.RotationError(target.Basis.GetRotationQuaternion(), Rot(Pelvis));
        Vector3 spin = (rotErr * BalanceKp).LimitLength(MuscleMaxSpin);
        SetAngularVelocity(_balance.Joint, _balance.FrameA.Inverse() * spin);  // anchor = world frame
        float cap = BalanceTorque * (Grounded ? 1f : AirBalanceShare);
        if (cap != _balanceCap)
        {
            _balanceCap = cap;
            _balance.Joint.SetParamX(Generic6DofJoint3D.Param.AngularMotorForceLimit, cap);
            _balance.Joint.SetParamY(Generic6DofJoint3D.Param.AngularMotorForceLimit, cap);
            _balance.Joint.SetParamZ(Generic6DofJoint3D.Param.AngularMotorForceLimit, cap);
        }
    }

    private Drive _balance;
    private float _balanceCap;

    // ---------------------------------------------------------------- play space

    // How the driver should move its play-space origin after the last physics step. ONE rule, always,
    // in all directions (Boneworks/BIMOS: the rig rides the body): the camera rides the character's
    // eye. Head motion leads the body by at most ViewDeadzone (eased) / MaxViewLead (hard) — less
    // than the head box reaches in front of the eye, so the view stays inside the head. A sphere
    // swept eye→camera snaps the view back at once when geometry is in the way.
    // This does NOT feed back into the pose: the True Body reads the head relative to the play-space
    // floor (crouch = local head height) and ground control re-bases its goal onto the real ground
    // (footY − floorY), so moving the play space vertically changes neither. (2.3 split height off
    // to a separate floor-follow out of fear of that loop; the two then fought over the same height,
    // and landings left the view hanging above a sinking body — owner 2.4.)
    public Vector3 OriginCorrection(Vector3 camera, float floorY, float dt)
    {
        if (!Built)
            return Vector3.Zero;
        Vector3 eye = Eye;
        Vector3 lead = camera - eye;
        Vector3 correction = Vector3.Zero;
        if (lead.LengthSquared() > 1e-8f)
        {
            _cameraQuery ??= new PhysicsShapeQueryParameters3D { Shape = new SphereShape3D { Radius = CameraRadius }, CollisionMask = WorldMask };
            _cameraQuery.Transform = new Transform3D(Basis.Identity, eye);
            _cameraQuery.Motion = lead;
            float[] fractions = GetWorld3D().DirectSpaceState.CastMotion(_cameraQuery);
            float safe = fractions.Length > 0 ? fractions[0] : 1f;
            if (safe < 1f)
                correction = -lead * (1f - safe);
            Vector3 rest = lead + correction;
            if (rest.Length() > ViewDeadzone)
                correction -= rest * (1f - ViewDeadzone / rest.Length()) * (1f - Mathf.Exp(-dt / ViewFollowTau));
            rest = lead + correction;
            if (rest.Length() > MaxViewLead)
                correction -= rest * (1f - MaxViewLead / rest.Length());
        }
        // Slow re-centering of the play-space floor onto the real ground while standing: removes the
        // small height offsets the riding view picks up (a transient lag, a landing). The pose and the
        // head guide are re-based on the real ground, so this only brings the camera onto the eye —
        // the same direction as the follow above, never against it.
        if (Grounded)
            correction.Y += (_footY - floorY) * (1f - Mathf.Exp(-dt / FloorRecenterTau));
        // The pose target moves with the play space: a correction is not intended motion, so it
        // must not show up in ground control's feed-forward (that loop ran away at 7 m/s).
        _pelvisTargetPrev += correction;
        _headGoalPrev += correction;
        return correction;
    }

    private PhysicsShapeQueryParameters3D _cameraQuery;

    // Second ground ray 12 cm behind the hips (toward the body's back): ground at least `groundY`.
    private bool GroundBehindAt(Vector3 pelvisPos, float groundY)
    {
        Vector3 back = Pelvis.GlobalTransform.Basis.Z;
        back.Y = 0f;
        back = back.LengthSquared() > 1e-6f ? back.Normalized() * 0.12f : Vector3.Zero;
        var query = PhysicsRayQueryParameters3D.Create(pelvisPos + back, pelvisPos + back + Vector3.Down * 3f, WorldMask);
        Godot.Collections.Dictionary hit = GetWorld3D().DirectSpaceState.IntersectRay(query);
        return hit.Count > 0 && ((Vector3)hit["position"]).Y > groundY - 0.05f;
    }

    public void TeleportBy(Transform3D delta)
    {
        if (!Built)
            return;
        foreach (RigidBody3D b in _bodies)
        {
            b.GlobalTransform = delta * b.GlobalTransform;
            b.LinearVelocity = delta.Basis * b.LinearVelocity;
            b.AngularVelocity = delta.Basis * b.AngularVelocity;
        }
        _pelvisTargetPrev = delta * _pelvisTargetPrev;
        _footY += delta.Origin.Y;
        foreach (RigidBody3D b in _bodies)
            b.ResetPhysicsInterpolation();
    }

    public void SnapToPose(TrueBodySolver s, Transform3D left, Transform3D right)
    {
        ComputeTargets(s, left, right);
        foreach (RigidBody3D b in _bodies)
        {
            b.GlobalTransform = Target(b);
            b.LinearVelocity = Vector3.Zero;
            b.AngularVelocity = Vector3.Zero;
        }
        // The hands sit at the wrist end of the solved arm, not at a possibly unreachable controller.
        for (int side = 0; side < 2; side++)
        {
            Arm a = _arms[side];
            Basis hb = _targets[a.Hand].Basis;
            a.Hand.GlobalTransform = new Transform3D(hb, s.Wrist[side] - hb * new Vector3(0f, 0f, WristOffset));
        }
        _havePelvisPrev = false;
        foreach (RigidBody3D b in _bodies)
            b.ResetPhysicsInterpolation();
    }

    // Collision exceptions between a held prop and that arm's segments (its hand is excluded by
    // the grab weld itself) — a forearm wrapped around a held object must not fight it.
    public void SetHeld(int side, PhysicsBody3D prop)
    {
        if (!Built)
            return;
        Arm a = _arms[side];
        if (a.Held != null && IsInstanceValid(a.Held) && a.Held != _arms[1 - side].Held)
        {
            a.Upper.RemoveCollisionExceptionWith(a.Held);
            a.Fore.RemoveCollisionExceptionWith(a.Held);
        }
        a.Held = prop;
        if (prop != null)
        {
            a.Upper.AddCollisionExceptionWith(prop);
            a.Fore.AddCollisionExceptionWith(prop);
        }
    }
}