concinnity_core/physics/

system.rs

// The rigid-body simulation driver. An internal system (not a declarable
// asset): a schedule constructs one when the world declares a `PhysicsConfig`,
// a `RigidBody`, a `PropBody`, or a `TriggerVolume`, reading the optional
// `PhysicsConfig` for the floor / terrain.

use alloc::boxed::Box;
use alloc::collections::{BTreeMap, BTreeSet};
use alloc::vec::Vec;

use crate::physics::{
    BodyHandle, CharacterCapsule, CharacterMoveInput, ColliderShape, ContactHit, GRAVITY,
    PhysicsBudget, SensorCrossing, SimConfig, Simulation,
};

use crate::components::{
    BodyDynamics, Camera3D, Collider, ContactEvent, Held, PhysicsConfig, PhysicsJoint, Pickup,
    RigidBody, Transform, TriggerFilter, TriggerVolume, VolumeEvent,
};
use crate::ecs::asset_id::AssetId;
use crate::ecs::{
    Entity, EntityByName, EventCursor, MenuActive, PipelineContext, ScheduleMode, SimTiming,
    StepResult, System, WorldPhysicsBudget,
};
use crate::math::{cos, sin, sqrt};

use super::budget::DriverCapacities;
use super::contacts::{ContactBatch, ContactGate};
use super::convert::{collider_shape, joint_spec};
use super::fanout::{PhysicsFanout, SerialFanout};
use super::index::SortedMap;
use super::interp::PointInterp;
use super::layers::{LAYER_CHARACTER, LAYER_PROP, LAYER_TRIGGER, LAYER_WORLD, LayerTable};
use super::props::{PropBodies, PropCollSnap, STATIC_FRICTION};
use super::terrain::{TerrainParams, build_heightfield, build_heightfield_collider};

// Reach distance for picking up a Prop, in world units.
const PICKUP_REACH: f32 = 3.0;
// Minimum facing dot product (~60-degree cone) for a pickup.
const PICKUP_MIN_DOT: f32 = 0.5;
// Distance ahead of the camera a carried prop hovers.
const HOLD_DISTANCE: f32 = 1.8;
// Drop of a carried prop below eye level.
const HOLD_DROP: f32 = 0.35;
// Launch speed applied to a prop when it is dropped/thrown.
const THROW_SPEED: f32 = 6.0;

/// Rigid-body simulation behavior. Constructed internally by `World::start`
/// from the world's `PhysicsConfig`; never a declarable asset.
///
/// [`new`](PhysicsSystem::new) steps the simulation on the calling thread; a
/// host lends it threads through [`with_fanout`](PhysicsSystem::with_fanout).
#[derive(Debug)]
pub struct PhysicsSystem {
    // Camera eye Y at spawn; the flat-floor fallback derives nothing from it,
    // but it seeds a sensible fallback camera position.
    floor_y: f32,
    // Terrain parameters. None when terrain_subdivisions == 0 (flat floor).
    terrain: Option<TerrainParams>,
    // Reference to a `ProceduralMesh` asset whose `heightfield` generator
    // drives the physics collider. Resolved against the live component list
    // at `init`. Takes precedence over `terrain` when both are set.
    terrain_mesh: Option<AssetId>,
    // World-space Y offset applied to whichever terrain source is active
    // (procedural noise or heightfield mesh). Matches the rendering Prop's
    // `position[1]`.
    terrain_offset_y: f32,
    // The simulation, built in init() and sized from the world's budget.
    world: Option<Simulation>,
    // The player capsule, when the world has a Camera3D + RigidBody.
    player: Option<PlayerPhysics>,
    // One capsule per root-motion character rig (see `super::rig`).
    rigs: Vec<super::rig::RigPhysics>,
    // One entry per Prop that carries a collider.
    props: PropBodies,
    // Reader cursor over the `RootMotionEvent` event queue.
    root_cursor: EventCursor,
    // Scratch for the per-tick scan for freshly spawned collider-bearing
    // entities, refilled every step.
    new_props: Vec<(Entity, PropCollSnap)>,
    // Per-step drain scratch, reused so the event handoffs never reallocate.
    motion_scratch: Vec<crate::components::RootMotionEvent>,
    contact_scratch: Vec<ContactHit>,
    sensor_scratch: Vec<SensorCrossing>,
    // Index into `props` of the prop currently being carried.
    held: Option<usize>,
    // Sensor tag -> the TriggerVolume it senses for, with its filter. Tags are
    // the volume's AssetId, stamped into the sensor collider's user_data.
    sensor_filters: SortedMap<u64, (AssetId, TriggerFilter)>,
    // Layer-name resolution and the collide matrix from the PhysicsConfig.
    layers: LayerTable,
    // Minimum contact impulse for publishing a ContactEvent.
    contact_min_impulse: f32,
    // Strongest contact per body pair across the frame's ticks.
    contact_batch: ContactBatch,
    // Per-pair refractory so sustained contact reports once per impact.
    contact_gate: ContactGate,
    // Bodies the world authored room for beyond the ones it declares.
    spawn_headroom: u32,
    // Hard ceiling on live bodies: the simulation's whole reservation, so a
    // spawn past it is refused rather than silently declined by a full pool.
    // Resolved at init, along with the reservation itself.
    body_cap: u32,
    // Where a step's independent work runs. `SerialFanout` until a host lends
    // its own.
    fanout: Box<dyn PhysicsFanout>,
}

// Runtime physics state for the player camera capsule.
#[derive(Debug)]
struct PlayerPhysics {
    handle: BodyHandle,
    // The capsule each tick's move is resolved against. Its dimensions come
    // from the RigidBody at init and never change afterwards.
    shape: CharacterCapsule,
    // Camera eye Y minus capsule-centre Y.
    eye_offset: f32,
    // False for a free-flying camera (no RigidBody): no gravity, no jump.
    has_gravity: bool,
    gravity_scale: f32,
    jump_height: f32,
    // Current vertical velocity (world units/second).
    vy: f32,
    // Whether the capsule rested on a surface last tick.
    grounded: bool,
    // Authoritative simulated capsule centre with its render blend snapshots.
    center: PointInterp,
    // The eye position written back last frame. A Camera3D position that
    // differs was moved externally (free-fly, a teleport) and is adopted with
    // no blend across the jump.
    written_eye: Option<[f32; 3]>,
}

impl PhysicsSystem {
    // Number of rigid bodies the physics world holds. Test-only observable for
    // the body-reaping path.
    #[cfg(test)]
    fn physics_body_count(&self) -> usize {
        self.world.as_ref().map_or(0, |w| w.body_count())
    }

    // Number of colliders the physics world holds. Test-only observable for
    // the spawn/despawn leak checks.
    #[cfg(test)]
    fn physics_collider_count(&self) -> usize {
        self.world.as_ref().map_or(0, |w| w.collider_count())
    }

    /// Build the simulation from the world's `PhysicsConfig` (floor / terrain).
    /// Bodies and colliders are added from the ECS in [`System::init`].
    pub fn new(config: PhysicsConfig) -> Self {
        let terrain = if config.terrain_subdivisions > 0 {
            Some(TerrainParams {
                half_width: config.terrain_half_width,
                half_depth: config.terrain_half_depth,
                subdivisions: config.terrain_subdivisions,
                amplitude: config.terrain_amplitude,
                offset_y: config.terrain_offset_y,
            })
        } else {
            None
        };
        Self {
            floor_y: config.floor_y,
            terrain,
            terrain_mesh: config.terrain_mesh,
            terrain_offset_y: config.terrain_offset_y,
            world: None,
            player: None,
            rigs: Vec::new(),
            props: PropBodies::default(),
            root_cursor: EventCursor::default(),
            new_props: Vec::new(),
            motion_scratch: Vec::new(),
            contact_scratch: Vec::new(),
            sensor_scratch: Vec::new(),
            held: None,
            sensor_filters: SortedMap::default(),
            layers: LayerTable::new(&config),
            contact_min_impulse: config.contact_min_impulse.max(0.0),
            contact_batch: ContactBatch::default(),
            contact_gate: ContactGate::default(),
            spawn_headroom: config.spawn_headroom,
            body_cap: 0,
            fanout: Box::new(SerialFanout),
        }
    }

    /// Run the step's independent work through `fanout` rather than on the
    /// calling thread. What a step hands out and the order its results load
    /// back in are the simulation's; a fan-out decides only where the work
    /// runs, so this cannot change the state a tick lands on.
    pub fn with_fanout(mut self, fanout: Box<dyn PhysicsFanout>) -> Self {
        self.fanout = fanout;
        self
    }

    // The world's body budget: the record cook shipped, or, when no record
    // shipped, the same derivation over the loaded components with the
    // headroom its `PhysicsConfig` authored. Only a shipped record is checked
    // against the live world.
    //
    // The two headrooms can differ: a shipped one is already raised to cover
    // the spawners whose cadence bounds their population, while a directly
    // constructed World gets only what its config authored, since nothing
    // counted its spawners.
    //
    // Whichever it came from, the budget is the whole reservation: the
    // simulation is sized from it and never grows, so its cap is the ceiling
    // spawns are refused against.
    fn resolve_budget(&self, ctx: &PipelineContext) -> PhysicsBudget {
        let scan = super::budget::scan_counts(ctx);
        let Some(record) = ctx.resource::<WorldPhysicsBudget>().map(|b| b.0) else {
            return PhysicsBudget::derive(&scan, self.spawn_headroom);
        };
        let shipped = super::budget::budget_of(&record);
        debug_assert_eq!(
            shipped,
            PhysicsBudget::derive(&scan, record.spawn_headroom),
            "the shipped physics budget does not match the loaded world"
        );
        shipped
    }

    // Size every container the driver holds per body from the budget, once,
    // before anything is built.
    fn reserve(&mut self, budget: &PhysicsBudget) {
        let caps = DriverCapacities::derive(budget);
        self.props = PropBodies::with_capacity(&caps);
        self.rigs = Vec::with_capacity(caps.rigs);
        self.new_props = Vec::with_capacity(caps.new_props);
        self.motion_scratch = Vec::with_capacity(caps.root_motions);
        self.contact_scratch = Vec::with_capacity(caps.contacts);
        self.sensor_scratch = Vec::with_capacity(caps.sensor_crossings);
        self.contact_batch = ContactBatch::with_capacity(caps.contact_pairs);
        self.contact_gate = ContactGate::with_capacity(caps.contact_pairs);
        self.sensor_filters = SortedMap::with_capacity(caps.sensor_filters);
    }

    // Build one body per collider-bearing entity from its per-instance
    // components (Transform + Collider + optional BodyDynamics + the Pickup
    // tag), keying `body_handles` by AssetId (via the name index's inverse)
    // so the joint wiring resolves.
    fn build_prop_bodies(
        &mut self,
        ctx: &PipelineContext,
        world: &mut Simulation,
        body_handles: &mut BTreeMap<AssetId, BodyHandle>,
    ) {
        let entity_name: BTreeMap<Entity, AssetId> = ctx
            .resource::<EntityByName>()
            .map(|n| n.0.iter().map(|(&id, &e)| (e, id)).collect())
            .unwrap_or_default();
        let pickup: BTreeSet<Entity> = ctx.query_with_entity::<Pickup>().map(|(e, _)| e).collect();
        let dynamics: BTreeMap<Entity, BodyDynamics> = ctx
            .query_with_entity::<BodyDynamics>()
            .map(|(e, b)| (e, *b))
            .collect();
        let snaps: Vec<(Entity, PropCollSnap)> = ctx
            .join2::<Collider, Transform>()
            .map(|(entity, collider, transform)| {
                (
                    entity,
                    PropCollSnap {
                        shape: collider_shape(&collider.0, transform.scale),
                        layer: collider.0.layer.clone(),
                        position: transform.position,
                        rotation_deg: transform.rotation_deg,
                        pickup: pickup.contains(&entity),
                        dynamics: dynamics.get(&entity).copied(),
                    },
                )
            })
            .collect();

        for (entity, snap) in snaps {
            let Some(handle) = self.props.add(&self.layers, world, entity, snap) else {
                continue;
            };
            if let Some(&id) = entity_name.get(&entity) {
                body_handles.insert(id, handle);
            }
        }
    }
}

impl System for PhysicsSystem {
    fn init(&mut self, ctx: &mut PipelineContext) {
        // Before anything is built, and before the joint wiring below drains
        // the column the scan counts.
        let budget = self.resolve_budget(ctx);
        self.body_cap = budget.body_cap();
        self.reserve(&budget);

        // The simulation reserves the whole budget here, so nothing on the
        // step path allocates and a body past the reservation is refused
        // rather than grown into.
        let mut world = Simulation::new(
            SimConfig {
                gravity: GRAVITY,
                ..SimConfig::default()
            },
            budget.body_cap() as usize,
        );
        world.set_contact_min_impulse(self.contact_min_impulse, SimTiming::TICK_DT);
        // The step's per-worker scratch, reserved from the schedule this world
        // will run under. A serial schedule reserves one worker's worth, which
        // is what a simulation that is never lent threads keeps.
        world.reserve_workers(
            self.fanout
                .worker_count(ScheduleMode::current(ctx.resources)),
        );
        let world_mask = self.layers.mask(LAYER_WORLD);

        // floor: heightfield-mesh-driven, procedural noise, or flat slab
        let mut floor_built = false;
        if let Some(mesh_id) = self.terrain_mesh {
            let mesh_snap = ctx
                .query::<crate::components::ProceduralMesh>()
                .find(|m| m.asset_id == mesh_id)
                .cloned();
            // Anything else (missing asset, wrong generator, a collider that
            // fails to build) leaves `floor_built` false and falls through to
            // the flat-slab fallback below.
            if let Some(m) = mesh_snap
                && m.generator == "heightfield"
                && build_heightfield_collider(
                    &mut world,
                    &m,
                    self.terrain_offset_y,
                    world_mask,
                    ctx,
                )
                .is_ok()
            {
                floor_built = true;
            }
        }
        if !floor_built {
            if let Some(terrain) = self.terrain.clone() {
                build_heightfield(&mut world, &terrain, world_mask);
            } else {
                // A large thin slab whose top face sits at Y = 0.
                world.add_fixed(
                    &ColliderShape::Cuboid {
                        half_extents: [500.0, 5.0, 500.0],
                    },
                    [0.0, -5.0, 0.0],
                    [0.0; 3],
                    STATIC_FRICTION,
                    world_mask,
                );
            }
        }

        // Sensor regions: one fixed sensor body per TriggerVolume, tagged with
        // the volume's AssetId so step's crossing drain maps back to it.
        let trigger_mask = self.layers.mask(LAYER_TRIGGER);
        let volumes: Vec<TriggerVolume> = ctx.query::<TriggerVolume>().cloned().collect();
        for volume in &volumes {
            let shape = collider_shape(&volume.collider, [1.0; 3]);
            let tag = u64::from(volume.asset_id.0);
            if world
                .add_sensor(
                    &shape,
                    volume.position,
                    volume.rotation_deg,
                    tag,
                    trigger_mask,
                )
                .is_none()
            {
                continue;
            }
            self.sensor_filters
                .insert(tag, (volume.asset_id, volume.detects));
        }

        // Prop name -> BodyHandle, populated alongside `self.prop_bodies`.
        // Joints resolve their `body_a`/`body_b` references through this map.
        let mut body_handles: BTreeMap<AssetId, BodyHandle> = BTreeMap::new();
        self.build_prop_bodies(ctx, &mut world, &mut body_handles);

        // joints
        // Each PhysicsJoint references one or two Props by AssetId. Cross-reference
        // validation already guarantees the Prop exists; here we additionally
        // require the Prop to own a collider (and therefore a body). A PhysicsJoint
        // with body_b empty anchors body_a to a hidden static body created on
        // demand at the world-space `anchor_b`.
        let joints: Vec<PhysicsJoint> = ctx.drain::<PhysicsJoint>();
        for joint in joints {
            let Some(body_a_id) = joint.body_a else {
                continue;
            };
            let Some(handle_a) = body_handles.get(&body_a_id).copied() else {
                continue;
            };
            let handle_b = if let Some(body_b_id) = joint.body_b {
                match body_handles.get(&body_b_id).copied() {
                    Some(h) => h,
                    None => {
                        continue;
                    }
                }
            } else {
                // Static world anchor at anchor_b. Sub-millimetre ball so it
                // takes effectively no space in the broad phase.
                let anchor = world.add_fixed(
                    &ColliderShape::Ball { radius: 0.001 },
                    joint.anchor_b,
                    [0.0; 3],
                    0.0,
                    world_mask,
                );
                match anchor {
                    Some(handle) => handle,
                    None => continue,
                }
            };
            // When body_b is the implicit world anchor, the anchor sits at the
            // origin of that hidden body, not at the authored offset.
            let anchor_b = if joint.body_b.is_some() {
                joint.anchor_b
            } else {
                [0.0, 0.0, 0.0]
            };
            if !world.add_joint(
                handle_a,
                handle_b,
                joint.anchor_a,
                anchor_b,
                joint_spec(&joint),
            ) {
                continue;
            }
        }
        // player capsule for the Camera3D
        // Every first-person camera is collided as a capsule. A RigidBody
        // upgrades it from a free-flying spectator to a grounded,
        // gravity-bound character. A third-person camera (a controller with
        // a `follow` block) gets no capsule: it is a virtual orbit around the
        // followed character, whose own rig capsule is the collided body.
        let camera_pos = ctx
            .query::<Camera3D>()
            .next()
            .filter(|c| {
                c.controller
                    .as_ref()
                    .is_none_or(|ctrl| ctrl.follow.is_none())
            })
            .map(|c| c.position);
        if let Some(cam_pos) = camera_pos {
            let rb_opt = ctx.query::<RigidBody>().next().cloned();
            let has_gravity = rb_opt.is_some();
            let rb = rb_opt.unwrap_or_default();
            if self.floor_y == 0.0 {
                self.floor_y = cam_pos[1];
            }
            let radius = rb.capsule_radius.max(0.05);
            let half_height = ((rb.capsule_height * 0.5) - radius).max(0.05);
            // a grounded character's eye sits at the capsule top; a flying
            // camera's capsule is centred on the eye.
            let eye_offset = if has_gravity {
                (rb.capsule_height * 0.5).max(radius + 0.05)
            } else {
                0.0
            };
            let center = [cam_pos[0], cam_pos[1] - eye_offset, cam_pos[2]];
            world.configure_character(rb.max_slope_deg, rb.step_height, has_gravity);
            let handle = world.add_character(
                half_height,
                radius,
                center,
                self.layers.mask(LAYER_CHARACTER),
            );
            self.player = handle.map(|handle| PlayerPhysics {
                handle,
                shape: CharacterCapsule::new(half_height, radius),
                eye_offset,
                has_gravity,
                gravity_scale: rb.gravity_scale.max(0.0),
                jump_height: rb.jump_height.max(0.0),
                vy: 0.0,
                grounded: true,
                center: PointInterp::new(center),
                written_eye: None,
            });
        }

        // Kinematic capsules for the root-motion character rigs published by
        // GraphicsSystem (which ran init first this tick).
        super::rig::init_rigs(
            &mut world,
            ctx,
            self.layers.mask(LAYER_CHARACTER),
            &mut self.rigs,
        );

        // Published from the built world: the simulation's own storage is only
        // knowable once it is reserved.
        super::budget::publish_reservation(crate::memory::ledger(), &budget, &world);
        self.world = Some(world);
    }

    fn step(&mut self, ctx: &mut PipelineContext) -> StepResult {
        // Freeze while a menu is open: skip the solve and the write-back so the
        // world truly pauses (and an external editor's edits to simulated
        // Transforms are not stomped by a stale blend). The simulation clock
        // holds its accumulator across the pause, so resuming costs one normal
        // frame. The flag is published by whichever system owns the menu, which
        // runs first in the table, so it reflects this same tick.
        if ctx.resource::<MenuActive>().is_some_and(|m| m.0) {
            return StepResult::Continue;
        }

        if self.world.is_none() {
            return StepResult::Continue;
        }

        // The frame's fixed-tick budget and render blend factor. Absent (a
        // directly-stepped world with no App), every step runs exactly one tick
        // and writes the freshly simulated state.
        let timing = ctx.resource::<SimTiming>().copied().unwrap_or_default();

        // snapshot reads (released before any query_mut below)
        let (cam_pos, cam_yaw, cam_pitch, desired_move, jump_req, interact_req) = ctx
            .query::<Camera3D>()
            .next()
            .map(|c| {
                (
                    c.position,
                    c.yaw,
                    c.pitch,
                    c.desired_move,
                    c.jump_requested,
                    c.interact_requested,
                )
            })
            .unwrap_or(([0.0, self.floor_y, 0.0], 0.0, 0.0, [0.0; 3], false, false));

        // camera-space basis vectors
        let fwd_flat = [-sin(cam_yaw), 0.0, -cos(cam_yaw)];
        let fwd_full = [
            -(sin(cam_yaw) * cos(cam_pitch)),
            -sin(cam_pitch),
            -(cos(cam_yaw) * cos(cam_pitch)),
        ];

        // Whichever pool the schedule names, read once for the frame. Both
        // land in the same place; only how long the step takes differs.
        let mode = ScheduleMode::current(ctx.resources);

        let world = self.world.as_mut().expect("world checked above");

        // Reap bodies whose entity was despawned before the step, keeping
        // self.held a valid index into the compacted list.
        self.held = self
            .props
            .reap(world, self.held, |entity| ctx.is_alive(entity));

        // Adopt collider-bearing entities that appeared since init (runtime
        // spawns): each gets a body at its spawn transform, with its pose
        // snapshots seeded there so the render blend starts clean. The scan
        // materializes into scratch because adopting one mutates the tracked
        // set the scan itself reads.
        self.new_props.clear();
        self.new_props.extend(
            ctx.join2::<Collider, Transform>()
                .filter(|(entity, _, _)| {
                    !self.props.is_tracked(*entity) && !self.props.is_refused(*entity)
                })
                .map(|(entity, collider, transform)| {
                    (
                        entity,
                        PropCollSnap {
                            shape: collider_shape(&collider.0, transform.scale),
                            layer: collider.0.layer.clone(),
                            position: transform.position,
                            rotation_deg: transform.rotation_deg,
                            pickup: false,
                            dynamics: None,
                        },
                    )
                }),
        );
        for (entity, mut snap) in self.new_props.drain(..) {
            snap.pickup = ctx.get::<Pickup>(entity).is_some();
            snap.dynamics = ctx.get::<BodyDynamics>(entity).copied();
            self.props
                .adopt(&self.layers, world, entity, snap, self.body_cap);
        }

        // pickup / drop on the interact edge; held_changed carries the entity to
        // toggle the Held tag on in the write-back.
        let mut held_changed: Option<(Entity, bool)> = None;
        if interact_req {
            if let Some(held_idx) = self.held.take() {
                // drop: hand the prop back to dynamic simulation with a throw.
                let pp = self.props.get(held_idx).expect("held index is valid");
                let throw = [
                    fwd_full[0] * THROW_SPEED,
                    fwd_full[1] * THROW_SPEED + 1.0,
                    fwd_full[2] * THROW_SPEED,
                ];
                world.make_dynamic(pp.handle, throw);
                held_changed = Some((pp.entity, false));
            } else {
                // pickup: nearest carriable prop within reach the player faces.
                // Entity positions for the reach test, read from the Transform
                // column only on the interact edge (not every frame).
                let entity_positions: BTreeMap<Entity, [f32; 3]> = ctx
                    .query_with_entity::<Transform>()
                    .map(|(e, t)| (e, t.position))
                    .collect();
                let mut best: Option<(f32, usize)> = None;
                for (idx, pp) in self.props.iter().enumerate() {
                    if !pp.pickup {
                        continue;
                    }
                    let pos = entity_positions.get(&pp.entity).copied().unwrap_or(cam_pos);
                    let dx = pos[0] - cam_pos[0];
                    let dz = pos[2] - cam_pos[2];
                    let dist = sqrt(dx * dx + dz * dz);
                    if dist >= PICKUP_REACH || dist <= 0.0 {
                        continue;
                    }
                    let dot = (fwd_flat[0] * dx + fwd_flat[2] * dz) / dist;
                    if dot > PICKUP_MIN_DOT && best.is_none_or(|(d, _)| dist < d) {
                        best = Some((dist, idx));
                    }
                }
                if let Some((_, idx)) = best {
                    let pp = self.props.get(idx).expect("scanned index is valid");
                    world.make_kinematic(pp.handle);
                    held_changed = Some((pp.entity, true));
                    self.held = Some(idx);
                }
            }
        }

        // Adopt an externally moved camera (free-fly, a teleport): a position
        // that differs from the eye written back last frame was not ours, so
        // the capsule snaps to it with no blend across the jump.
        if let Some(player) = self.player.as_mut()
            && player.written_eye != Some(cam_pos)
        {
            player
                .center
                .snap([cam_pos[0], cam_pos[1] - player.eye_offset, cam_pos[2]]);
        }

        // The carried prop's hover point in front of the camera, refreshed
        // from this frame's camera pose.
        let hold_pos = [
            cam_pos[0] + fwd_full[0] * HOLD_DISTANCE,
            cam_pos[1] + fwd_full[1] * HOLD_DISTANCE - HOLD_DROP,
            cam_pos[2] + fwd_full[2] * HOLD_DISTANCE,
        ];

        // Root-motion displacements published since last frame, applied on the
        // frame's first tick. Rig capsules whose entity moved externally snap
        // before any tick runs.
        super::rig::drain_motions_into(ctx, &mut self.root_cursor, &mut self.motion_scratch);
        super::rig::sync_rigs(ctx, &mut self.rigs);

        for tick in 0..timing.ticks {
            let dt = timing.tick_dt;

            // carried prop hovers in front of the camera
            if let Some(prop) = self.held.and_then(|idx| self.props.get(idx)) {
                world.set_kinematic_translation(prop.handle, hold_pos);
            }

            // move the player capsule
            if let Some(player) = self.player.as_mut() {
                if player.has_gravity {
                    if tick == 0 && jump_req && player.grounded && player.jump_height > 0.0 {
                        player.vy = sqrt(2.0 * GRAVITY * player.gravity_scale * player.jump_height);
                    }
                    player.vy -= GRAVITY * player.gravity_scale * dt;
                }

                let center = player.center.current();
                let desired = [desired_move[0] * dt, player.vy * dt, desired_move[2] * dt];
                let moved = world.move_character(
                    &player.shape,
                    &CharacterMoveInput {
                        center,
                        desired,
                        dt,
                        exclude: player.handle,
                        mask: self.layers.mask(LAYER_CHARACTER),
                    },
                );
                let new_center = [
                    center[0] + moved.translation[0],
                    center[1] + moved.translation[1],
                    center[2] + moved.translation[2],
                ];
                world.set_kinematic_translation(player.handle, new_center);

                player.grounded = moved.grounded;
                if moved.grounded && player.vy < 0.0 {
                    player.vy = 0.0;
                }
                player.center.push(new_center);
            }

            // move the root-motion character rig capsules
            super::rig::tick_rigs(
                world,
                ctx,
                &mut self.rigs,
                if tick == 0 { &self.motion_scratch } else { &[] },
                dt,
                GRAVITY,
                self.layers.mask(LAYER_CHARACTER),
            );

            // advance the simulation
            self.fanout.step(world, dt, mode);

            // batch the tick's contact hits (strongest per pair this frame)
            self.contact_gate.advance_tick();
            world.drain_contact_hits_into(&mut self.contact_scratch);
            for hit in self.contact_scratch.drain(..) {
                self.contact_batch.add(hit);
            }

            // record the tick's dynamic prop poses for the render blend
            self.props.record_tick_poses(world);
        }

        // answer the IK ground probes and the follow camera's occlusion probe
        super::probes::step_probes(
            world,
            ctx,
            &self.rigs,
            self.layers
                .query_mask(LAYER_CHARACTER, &[LAYER_WORLD, LAYER_PROP]),
        );

        // publish the frame's contact events: one per body pair that passed
        // the impulse threshold, gated by the per-pair refractory. `a` is
        // always a prop entity; a hit whose sides both lack one (terrain,
        // capsules) has no consumer-visible subject and is dropped.
        for hit in self.contact_batch.drain() {
            let event = match (self.props.entity_of(hit.a), self.props.entity_of(hit.b)) {
                (Some(a), b) => ContactEvent {
                    a,
                    b,
                    point: hit.point,
                    normal: hit.normal,
                    impulse: hit.impulse,
                },
                (None, Some(b)) => ContactEvent {
                    a: b,
                    b: None,
                    point: hit.point,
                    normal: [-hit.normal[0], -hit.normal[1], -hit.normal[2]],
                    impulse: hit.impulse,
                },
                (None, None) => continue,
            };
            if self.contact_gate.admit(&hit) {
                ctx.events_mut::<ContactEvent>().send(event);
            }
        }

        // publish the sensor boundary crossings that pass their volume's
        // filter. A crossing whose body was removed this same step has no
        // `other` to classify, so only an `any` volume reports it.
        world.drain_sensor_crossings_into(&mut self.sensor_scratch);
        for crossing in self.sensor_scratch.drain(..) {
            let Some(&(volume, filter)) = self.sensor_filters.get(&crossing.tag) else {
                continue;
            };
            let passes = match filter {
                TriggerFilter::Player => crossing.other.is_some_and(|h| {
                    self.player.as_ref().is_some_and(|p| p.handle == h)
                        || self.rigs.iter().any(|r| r.handle == h)
                }),
                TriggerFilter::Props => crossing
                    .other
                    .is_some_and(|h| self.props.entity_of(h).is_some()),
                TriggerFilter::Any => true,
            };
            if passes {
                ctx.events_mut::<VolumeEvent>().send(VolumeEvent {
                    volume,
                    entered: crossing.entered,
                });
            }
        }

        // Write each dynamic prop's blended pose back to its Transform:
        // positions lerped, rotations slerped as quaternions, with the Euler
        // decomposition happening only here at the write boundary.
        let alpha = timing.alpha;
        for &(entity, pos, rot) in self.props.sample_poses(alpha) {
            if let Some(t) = ctx.get_mut::<Transform>(entity) {
                t.position = pos;
                t.rotation_deg = rot;
            }
        }
        if let Some((entity, is_held)) = held_changed {
            if is_held {
                if ctx.get::<Held>(entity).is_none() {
                    ctx.insert(entity, Held);
                }
            } else {
                ctx.remove::<Held>(entity);
            }
        }

        // write the blended camera position + view matrix
        let mut grounded = true;
        if let Some(player) = self.player.as_mut() {
            let center = player.center.sample(alpha);
            let eye = [center[0], center[1] + player.eye_offset, center[2]];
            player.written_eye = Some(eye);
            grounded = player.grounded;
            for camera in ctx.query_mut::<Camera3D>() {
                camera.position = eye;
                camera.view_matrix =
                    crate::gfx::camera::view_matrix(camera.position, camera.yaw, camera.pitch);
            }
        }

        // publish grounded state for jump gating
        for body in ctx.query_mut::<RigidBody>() {
            body.is_grounded = grounded;
        }

        // write the blended rig positions for the render follow
        super::rig::publish_rigs(ctx, &mut self.rigs, alpha);

        StepResult::Continue
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use alloc::string::ToString;
    use alloc::vec;

    use crate::components::{CameraController, CharacterRig, FollowController, PropCollider};
    use crate::ecs::SkinnedMeshHandle;
    use crate::physics::budget::{record_of, scan_counts};
    use crate::physics::test_world::TestWorld;

    // Make a spawned prop dynamic, exactly as the load-time PropBody
    // decomposition would.
    fn make_dynamic(world: &mut TestWorld, entity: Entity) {
        world.components.insert_typed(entity, ball_dynamics());
    }

    fn controlled_camera() -> Camera3D {
        Camera3D {
            fov_y_degrees: 75.0,
            near: 0.05,
            far: 200.0,
            view_matrix: [[0.0; 4]; 4],
            position: [0.0, 1.0, 0.0],
            yaw: 0.0,
            pitch: 0.0,
            desired_move: [0.0; 3],
            jump_requested: false,
            interact_requested: false,
            controller: Some(CameraController::default()),
        }
    }

    // A third-person camera is a virtual orbit: no player capsule is created
    // for it. (Regression: the spectator capsule spawned at the camera eye
    // overlapped the followed rig's capsule and squeezed it through the
    // floor.) A first-person camera keeps its capsule. The schedule gate that
    // builds the system at all is covered by the engine's schedule tests.
    #[test]
    fn third_person_camera_gets_no_player_capsule() {
        // Third-person (follow) camera: a virtual orbit, so no player capsule.
        let mut world = TestWorld::new();
        let mut camera = controlled_camera();
        camera.controller = Some(CameraController {
            follow: Some(FollowController {
                target: Some(SkinnedMeshHandle(1)),
                ..FollowController::default()
            }),
            ..CameraController::default()
        });
        world.components.push_typed(camera);
        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());
        assert!(physics.player.is_none(), "no capsule for the orbit camera");

        // First-person camera keeps its capsule.
        let mut world = TestWorld::new();
        world.components.push_typed(controlled_camera());
        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());
        assert!(
            physics.player.is_some(),
            "first-person camera keeps its capsule"
        );
    }

    fn ball_dynamics() -> BodyDynamics {
        BodyDynamics {
            mass: 1.0,
            friction: 0.5,
            linear_damping: 0.0,
            ..Default::default()
        }
    }

    // A camera looking down -Z with the interact field latched on, so the
    // PhysicsSystem (which reads Camera3D.interact_requested) triggers a pickup.
    fn interacting_camera(position: [f32; 3]) -> Camera3D {
        Camera3D {
            interact_requested: true,
            controller: None,
            position,
            ..controlled_camera()
        }
    }

    // The simulated pose is written back to the prop's Transform. With no
    // `SimTiming` published, each step runs exactly one fixed tick.
    #[test]
    fn dynamic_prop_writes_transform() {
        let id = AssetId(1);
        let mut world = TestWorld::new();
        let entity = world.spawn_prop(id, [0.0, 5.0, 0.0], false);
        make_dynamic(&mut world, entity);

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());
        for _ in 0..10 {
            physics.step(&mut world.ctx());
        }

        let transform_y = world.components.get::<Transform>(entity).unwrap().position[1];
        assert!(
            transform_y < 5.0,
            "the simulated pose falls the Transform (y={transform_y})"
        );
    }

    // A menu freezes the solve: while `MenuActive(true)` is published the body
    // does not fall, and clearing it resumes the fall from where it froze.
    // (The App-level simulation clock additionally holds its accumulator
    // across the pause, so a live run resumes without a catch-up burst.)
    #[test]
    fn menu_active_freezes_then_resumes_physics() {
        let id = AssetId(1);
        let mut world = TestWorld::new();
        let entity = world.spawn_prop(id, [0.0, 5.0, 0.0], false);
        make_dynamic(&mut world, entity);

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());

        // Paused: the body stays put however many frames pass.
        world.resources.insert(MenuActive(true));
        for _ in 0..5 {
            physics.step(&mut world.ctx());
        }
        let y_paused = world.components.get::<Transform>(entity).unwrap().position[1];
        assert!(
            (y_paused - 5.0).abs() < 1e-3,
            "the body must not fall while a menu is active (y={y_paused})"
        );

        // Resumed: the body falls again.
        world.resources.insert(MenuActive(false));
        for _ in 0..5 {
            physics.step(&mut world.ctx());
        }
        let y_resumed = world.components.get::<Transform>(entity).unwrap().position[1];
        assert!(
            y_resumed < y_paused - 1e-3,
            "the body must fall once the menu closes (y={y_resumed})"
        );
    }

    // A zero-tick frame (the accumulator has not crossed a tick) advances
    // nothing; the written Transform blends between the last two ticks by the
    // frame's alpha.
    #[test]
    fn zero_tick_frames_blend_between_the_last_two_ticks() {
        let id = AssetId(1);
        let mut world = TestWorld::new();
        let entity = world.spawn_prop(id, [0.0, 5.0, 0.0], false);
        make_dynamic(&mut world, entity);

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());

        let tick = |ticks, alpha| SimTiming {
            ticks,
            tick_dt: SimTiming::TICK_DT,
            alpha,
        };
        world.resources.insert(tick(1, 1.0));
        physics.step(&mut world.ctx());
        let y_prev = world.components.get::<Transform>(entity).unwrap().position[1];
        physics.step(&mut world.ctx());
        let y_curr = world.components.get::<Transform>(entity).unwrap().position[1];
        assert!(y_curr < y_prev, "the body falls tick over tick");

        // No tick, alpha 0: the write-back returns to the previous tick's pose.
        world.resources.insert(tick(0, 0.0));
        physics.step(&mut world.ctx());
        let y_alpha0 = world.components.get::<Transform>(entity).unwrap().position[1];
        assert!(
            (y_alpha0 - y_prev).abs() < 1e-6,
            "alpha 0 samples the previous tick"
        );

        // No tick, alpha 0.5: halfway between the two ticks.
        world.resources.insert(tick(0, 0.5));
        physics.step(&mut world.ctx());
        let y_mid = world.components.get::<Transform>(entity).unwrap().position[1];
        let expected = (y_prev + y_curr) * 0.5;
        assert!(
            (y_mid - expected).abs() < 1e-6,
            "alpha 0.5 blends the tick poses (y={y_mid}, expected {expected})"
        );
    }

    // The same number of fixed ticks produces bit-identical world state
    // however they are grouped into frames: 30 fps frames (two ticks each)
    // against 120 fps frames (a tick every other frame).
    #[test]
    fn tick_grouping_does_not_change_the_outcome() {
        let run = |frames: &[u32]| -> ([f32; 3], [f32; 3]) {
            let id = AssetId(1);
            let mut world = TestWorld::new();
            let entity = world.spawn_prop(id, [0.3, 5.0, 0.1], false);
            make_dynamic(&mut world, entity);

            let mut physics = PhysicsSystem::new(PhysicsConfig::default());
            physics.init(&mut world.ctx());
            for &ticks in frames {
                world.resources.insert(SimTiming {
                    ticks,
                    tick_dt: SimTiming::TICK_DT,
                    alpha: 1.0,
                });
                physics.step(&mut world.ctx());
            }
            let t = world.components.get::<Transform>(entity).unwrap();
            (t.position, t.rotation_deg)
        };

        // One simulated second: 30 frames of 2 ticks vs 120 frames alternating
        // 0 and 1 ticks. Both run exactly 60 fixed ticks.
        let thirty: Vec<u32> = core::iter::repeat_n(2, 30).collect();
        let one_twenty: Vec<u32> = (0..120).map(|i| i % 2).collect();
        assert_eq!(
            run(&thirty),
            run(&one_twenty),
            "the fixed-tick outcome must not depend on frame grouping"
        );
    }

    // Despawning a decomposed prop reaps its body, so it stops simulating
    // (and colliding) once its entity is gone.
    #[test]
    fn despawning_a_prop_reaps_its_physics_body() {
        let id = AssetId(1);
        let mut world = TestWorld::new();
        let ball = world.spawn_prop(id, [0.0, 5.0, 0.0], false);
        make_dynamic(&mut world, ball);

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());

        // Settle so the body is live and falling.
        for _ in 0..2 {
            physics.step(&mut world.ctx());
        }
        let before = physics.physics_body_count();

        // Despawn the ball (stand-in for GraphicsSystem) and step: PhysicsSystem
        // reaps the orphaned body.
        world.components.despawn(ball);
        physics.step(&mut world.ctx());
        let after = physics.physics_body_count();
        assert_eq!(after, before - 1, "the despawned prop's body was removed");

        // The sim keeps running cleanly with the body gone (no further removals).
        physics.step(&mut world.ctx());
        assert_eq!(
            physics.physics_body_count(),
            after,
            "no further bodies removed"
        );
    }

    // Picking up a carriable prop tags its entity with Held.
    #[test]
    fn pickup_sets_held_tag() {
        let id = AssetId(1);
        let mut world = TestWorld::new();
        let carriable = world.spawn_prop(id, [0.0, 1.0, -2.0], true);
        make_dynamic(&mut world, carriable);
        world
            .components
            .push_typed(interacting_camera([0.0, 1.0, 0.0]));

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());

        physics.step(&mut world.ctx());

        assert_eq!(
            world.ctx().query::<Held>().count(),
            1,
            "pickup inserts the Held tag on the entity"
        );
    }

    // A collider-bearing entity that appears after init (a runtime spawn) is
    // adopted on the next step: it gets a body and falls like an authored one.
    // The headroom is what reserves the body for it: the simulation is sized
    // once at init and a spawn past the reservation is refused.
    #[test]
    fn runtime_spawned_prop_gets_a_body_and_falls() {
        let mut world = TestWorld::new();
        let mut physics = PhysicsSystem::new(PhysicsConfig {
            spawn_headroom: 1,
            ..PhysicsConfig::default()
        });
        physics.init(&mut world.ctx());
        let baseline = physics.physics_body_count();

        let spawned = world.spawn_prop(AssetId(7), [0.0, 5.0, 0.0], false);
        make_dynamic(&mut world, spawned);
        physics.step(&mut world.ctx());
        assert_eq!(
            physics.physics_body_count(),
            baseline + 1,
            "the spawned entity got a body on its first step"
        );
        for _ in 0..30 {
            physics.step(&mut world.ctx());
        }
        let y = world.components.get::<Transform>(spawned).unwrap().position[1];
        assert!(y < 4.5, "the spawned body falls (y = {y})");
        for _ in 0..300 {
            physics.step(&mut world.ctx());
        }
        let y = world.components.get::<Transform>(spawned).unwrap().position[1];
        assert!(
            (y - 0.5).abs() < 0.1,
            "the spawned ball rests on the flat floor (y = {y})"
        );
    }

    // Spawn, despawn, and respawn leave no bodies or colliders behind: a
    // reaped body hands its slot back, so one body's worth of headroom covers
    // any number of rounds.
    #[test]
    fn spawn_despawn_respawn_cycle_is_leak_free() {
        let mut world = TestWorld::new();
        let mut physics = PhysicsSystem::new(PhysicsConfig {
            spawn_headroom: 1,
            ..PhysicsConfig::default()
        });
        physics.init(&mut world.ctx());
        let bodies = physics.physics_body_count();
        let colliders = physics.physics_collider_count();

        for round in 0..3 {
            let spawned = world.spawn_prop(AssetId(100 + round), [0.0, 3.0, 0.0], false);
            make_dynamic(&mut world, spawned);
            physics.step(&mut world.ctx());
            assert_eq!(physics.physics_body_count(), bodies + 1, "round {round}");
            world.components.despawn(spawned);
            physics.step(&mut world.ctx());
            assert_eq!(
                physics.physics_body_count(),
                bodies,
                "round {round} reaped the body"
            );
            assert_eq!(
                physics.physics_collider_count(),
                colliders,
                "round {round} reaped the collider"
            );
        }
    }

    // A world whose shipped budget was counted from the same content it holds
    // passes the init assert, and reserves the cap that budget implies.
    #[test]
    fn a_shipped_budget_matching_the_world_is_adopted() {
        let mut world = TestWorld::new();
        let entity = world.spawn_prop(AssetId(1), [0.0, 3.0, 0.0], false);
        make_dynamic(&mut world, entity);
        world.components.push_typed(controlled_camera());

        // The record cook would have written for this world: one dynamic prop,
        // the floor, the player capsule, and room for two spawns.
        let counts = scan_counts(&world.ctx());
        let budget = PhysicsBudget::derive(&counts, 2);
        assert_eq!(budget.dynamic, 1);
        assert_eq!(budget.kinematic, 1, "the first-person camera capsule");
        world
            .resources
            .insert(WorldPhysicsBudget(record_of(&budget)));

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());
        assert_eq!(physics.body_cap, budget.body_cap());
        assert_eq!(
            physics.physics_body_count(),
            budget.body_total() as usize,
            "init built exactly the bodies the budget reserved"
        );
    }

    // A world with no shipped budget (built in memory, as every test world is)
    // reserves from what it holds plus the headroom its config authored, and
    // caps spawns there. The cap used to be left open for such a world, on the
    // grounds that nothing had counted its spawns; a fixed-capacity simulation
    // cannot honour that -- a spawn past the reservation gets no body either
    // way, and the cap is what turns a silently declined one into a refusal
    // naming the knob to raise.
    #[test]
    fn a_world_without_a_shipped_budget_reserves_from_what_it_holds() {
        let mut world = TestWorld::new();
        let entity = world.spawn_prop(AssetId(1), [0.0, 3.0, 0.0], false);
        make_dynamic(&mut world, entity);

        let mut physics = PhysicsSystem::new(PhysicsConfig {
            spawn_headroom: 4,
            ..PhysicsConfig::default()
        });
        physics.init(&mut world.ctx());

        assert_eq!(
            physics.physics_body_count(),
            2,
            "the floor and the one authored prop"
        );
        assert_eq!(physics.body_cap, 2 + 4, "plus the authored headroom");
    }

    // Past the cap, a spawned prop gets no body and the world keeps stepping.
    // The refusal happens once: the next tick's scan passes over the entity
    // rather than re-refusing it.
    #[test]
    fn a_spawn_past_the_shipped_budget_is_refused() {
        let mut world = TestWorld::new();
        let authored = world.spawn_prop(AssetId(1), [0.0, 3.0, 0.0], false);
        make_dynamic(&mut world, authored);

        // A budget with no headroom: the floor and the one authored prop.
        let counts = scan_counts(&world.ctx());
        let budget = PhysicsBudget::derive(&counts, 0);
        world
            .resources
            .insert(WorldPhysicsBudget(record_of(&budget)));

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());
        let full = physics.physics_body_count();
        assert_eq!(full, budget.body_cap() as usize, "the budget is spent");

        let spawned = world.spawn_prop(AssetId(2), [0.0, 6.0, 0.0], false);
        make_dynamic(&mut world, spawned);
        physics.step(&mut world.ctx());
        assert_eq!(physics.physics_body_count(), full, "no body was built");
        assert!(physics.props.is_refused(spawned));

        // Stepping on keeps the refusal: the entity is skipped, not retried,
        // and the authored prop carries on simulating.
        for _ in 0..10 {
            physics.step(&mut world.ctx());
        }
        assert_eq!(physics.physics_body_count(), full);
        let refused_y = world.components.get::<Transform>(spawned).unwrap().position[1];
        assert_eq!(refused_y, 6.0, "a refused prop is not simulated at all");
        let live_y = world
            .components
            .get::<Transform>(authored)
            .unwrap()
            .position[1];
        assert!(live_y < 3.0, "the authored prop still falls");
    }

    // A rig capsule authored standing exactly on the floor stays there: it
    // neither sinks tick after tick nor is lifted off it. The driver used to
    // spawn one a fingernail above its authored position, because the
    // controller ignored a hit a downward move started already touching and
    // the capsule sank a little every frame; the controller now separates
    // along the contact normal instead, so the authored position is the one
    // that holds.
    #[test]
    fn a_rig_capsule_spawned_on_the_floor_neither_sinks_nor_rises() {
        let identity = [
            [1.0, 0.0, 0.0, 0.0],
            [0.0, 1.0, 0.0, 0.0],
            [0.0, 0.0, 1.0, 0.0],
            [0.0, 0.0, 0.0, 1.0],
        ];
        let mut world = TestWorld::new();
        world.components.push_typed(CharacterRig::new(
            SkinnedMeshHandle(1),
            0,
            identity,
            0.6,
            0.3,
        ));

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());
        let rig_y = |world: &mut TestWorld| {
            world
                .ctx()
                .query::<CharacterRig>()
                .next()
                .expect("the rig is there")
                .position[1]
        };
        assert_eq!(
            rig_y(&mut world),
            0.0,
            "the capsule spawns at its authored position, unlifted"
        );

        for _ in 0..30 {
            physics.step(&mut world.ctx());
        }
        let settled = rig_y(&mut world);
        for _ in 0..300 {
            physics.step(&mut world.ctx());
        }
        let held = rig_y(&mut world);

        assert!(
            settled.abs() < 0.01,
            "the capsule stayed on the floor (y = {settled})"
        );
        assert!(
            (held - settled).abs() < 1.0e-4,
            "and stopped moving ({settled} -> {held})"
        );
    }

    // A hard landing publishes one ContactEvent naming the prop; resting
    // afterwards stays silent.
    #[test]
    fn contact_event_fires_on_impact_and_not_at_rest() {
        let id = AssetId(1);
        let mut world = TestWorld::new();
        let entity = world.spawn_prop(id, [0.0, 5.0, 0.0], false);
        make_dynamic(&mut world, entity);

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());

        let mut cursor = EventCursor::default();
        let mut impacts: Vec<ContactEvent> = Vec::new();
        for _ in 0..120 {
            physics.step(&mut world.ctx());
            let ctx = world.ctx();
            if let Some(events) = ctx.events::<ContactEvent>() {
                impacts.extend(events.read(&mut cursor).copied());
            }
        }
        assert_eq!(impacts.len(), 1, "one landing, one event: {impacts:?}");
        let impact = impacts[0];
        assert_eq!(impact.a, entity);
        assert_eq!(impact.b, None, "the floor slab has no entity");
        assert!(
            impact.impulse > 3.0 && impact.impulse < 50.0,
            "impulse {} out of the plausible landing range",
            impact.impulse
        );

        // Settled: hundreds of resting ticks publish nothing further.
        for _ in 0..300 {
            physics.step(&mut world.ctx());
            let ctx = world.ctx();
            if let Some(events) = ctx.events::<ContactEvent>() {
                assert_eq!(
                    events.read(&mut cursor).count(),
                    0,
                    "resting contact must not publish events"
                );
            }
        }
    }

    // A sensor region reports a prop crossing it, in and then out, naming the
    // volume that saw it.
    #[test]
    fn a_trigger_volume_reports_a_prop_crossing_it() {
        let volume_id = AssetId(9);
        let mut world = TestWorld::new();
        world.components.push_typed(TriggerVolume {
            asset_id: volume_id,
            position: [0.0, 3.0, 0.0],
            rotation_deg: [0.0; 3],
            collider: PropCollider {
                shape: "cuboid".to_string(),
                half_extents: [1.0, 0.5, 1.0],
                ..Default::default()
            },
            detects: TriggerFilter::Props,
        });
        let ball = world.spawn_prop(AssetId(1), [0.0, 6.0, 0.0], false);
        make_dynamic(&mut world, ball);

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());

        let mut cursor = EventCursor::default();
        let mut crossings: Vec<VolumeEvent> = Vec::new();
        for _ in 0..180 {
            physics.step(&mut world.ctx());
            let ctx = world.ctx();
            if let Some(events) = ctx.events::<VolumeEvent>() {
                crossings.extend(events.read(&mut cursor).copied());
            }
        }
        assert_eq!(crossings.len(), 2, "in, then out: {crossings:?}");
        assert!(crossings.iter().all(|c| c.volume == volume_id));
        assert!(crossings[0].entered, "the ball entered first");
        assert!(!crossings[1].entered, "and left afterwards");
    }

    // A joint anchored to the world holds its body up: the hidden anchor body
    // the driver mints for it is part of the reservation, and the prop hangs
    // off it instead of falling.
    #[test]
    fn a_world_anchored_joint_holds_its_prop_up() {
        let bob_id = AssetId(1);
        let mut world = TestWorld::new();
        let bob = world.spawn_prop(bob_id, [1.0, 4.0, 0.0], false);
        make_dynamic(&mut world, bob);
        world.components.push_typed(PhysicsJoint {
            asset_id: AssetId(2),
            kind: "spherical".to_string(),
            body_a: Some(bob_id),
            body_b: None,
            // The bob's own centre hangs one unit from the anchor point.
            anchor_a: [-1.0, 0.0, 0.0],
            anchor_b: [0.0, 4.0, 0.0],
            ..Default::default()
        });

        let mut physics = PhysicsSystem::new(PhysicsConfig::default());
        physics.init(&mut world.ctx());
        for _ in 0..240 {
            physics.step(&mut world.ctx());
        }

        let position = world.components.get::<Transform>(bob).unwrap().position;
        let reach =
            ((position[0]).powi(2) + (position[1] - 4.0).powi(2) + (position[2]).powi(2)).sqrt();
        assert!(
            (reach - 1.0).abs() < 0.05,
            "the bob hangs one unit from the anchor, at {position:?} ({reach})"
        );
        assert!(
            position[1] > 2.5,
            "and is held up rather than falling ({position:?})"
        );
    }

    // The config's no_collide pairs reach the built colliders: a prop on a
    // layer that ignores `world` falls straight through the floor.
    #[test]
    fn no_collide_config_lets_a_layered_prop_fall_through_the_floor() {
        let config = PhysicsConfig {
            layers: vec!["ghost".to_string()],
            no_collide: vec![["ghost".to_string(), "world".to_string()]],
            ..PhysicsConfig::default()
        };

        let mut world = TestWorld::new();
        let entity = world.spawn_prop(AssetId(1), [0.0, 2.0, 0.0], false);
        make_dynamic(&mut world, entity);
        world
            .components
            .get_mut::<Collider>(entity)
            .unwrap()
            .0
            .layer = "ghost".to_string();

        let mut physics = PhysicsSystem::new(config);
        physics.init(&mut world.ctx());
        for _ in 0..240 {
            physics.step(&mut world.ctx());
        }
        let y = world.components.get::<Transform>(entity).unwrap().position[1];
        assert!(
            y < -10.0,
            "the ghost-layer prop fell through the floor (y = {y})"
        );
    }
}