dreamwell_engine/

game_object.rs

// game_object.rs — Portable, serializable game object and scene model.
//
// Like Unity's GameObject + MonoBehaviour pattern, adapted for Dreamwell's
// GPU-driven meshlet/particle/procedural rendering and ESCG (Event-Sourced
// Compute Graph) backend. Every object is an empty container that gains
// visual identity via MeshBinding and behavior via ComponentSlots.
//
// CODESPEC §2.3: Enums as tagged unions for MeshBinding, ComponentKind.
// CODESPEC §3.1: Scene stores objects contiguously for cache-friendly iteration.
// CODESPEC §0.1: All logic is pure — no RNG, no time, no I/O.

use serde::{Deserialize, Serialize};

/// Maximum scene objects per GameObjectScene. Prevents unbounded GPU buffer growth.
pub const MAX_SCENE_OBJECTS: usize = 65536;

/// Maximum components per GameObject.
pub const MAX_COMPONENTS_PER_OBJECT: usize = 32;

// ── Transform ──────────────────────────────────────────────────────────

/// 3D transform: position, rotation (quaternion), scale.
/// Column-major model matrix computed on demand via `to_matrix()`.
#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
pub struct Transform {
    pub position: [f32; 3],
    /// Quaternion `[x, y, z, w]`. Identity = `[0, 0, 0, 1]`.
    pub rotation: [f32; 4],
    pub scale: [f32; 3],
}

impl Default for Transform {
    fn default() -> Self {
        Self {
            position: [0.0; 3],
            rotation: [0.0, 0.0, 0.0, 1.0],
            scale: [1.0, 1.0, 1.0],
        }
    }
}

impl Transform {
    /// Compute the 4×4 column-major model matrix from TRS.
    pub fn to_matrix(&self) -> [f32; 16] {
        let [qx, qy, qz, qw] = self.rotation;
        let [sx, sy, sz] = self.scale;
        let [px, py, pz] = self.position;

        let x2 = qx + qx;
        let y2 = qy + qy;
        let z2 = qz + qz;
        let xx = qx * x2;
        let xy = qx * y2;
        let xz = qx * z2;
        let yy = qy * y2;
        let yz = qy * z2;
        let zz = qz * z2;
        let wx = qw * x2;
        let wy = qw * y2;
        let wz = qw * z2;

        [
            (1.0 - (yy + zz)) * sx,
            (xy + wz) * sx,
            (xz - wy) * sx,
            0.0,
            (xy - wz) * sy,
            (1.0 - (xx + zz)) * sy,
            (yz + wx) * sy,
            0.0,
            (xz + wy) * sz,
            (yz - wx) * sz,
            (1.0 - (xx + yy)) * sz,
            0.0,
            px,
            py,
            pz,
            1.0,
        ]
    }

    /// Validate: finite position, unit quaternion (±1%), positive finite scale.
    pub fn validate(&self) -> Result<(), String> {
        if self.position.iter().any(|p| !p.is_finite()) {
            return Err("transform_invalid_position:contains NaN or Inf".into());
        }
        let [qx, qy, qz, qw] = self.rotation;
        let len_sq = qx * qx + qy * qy + qz * qz + qw * qw;
        if !len_sq.is_finite() || (len_sq - 1.0).abs() > 0.01 {
            return Err(format!(
                "transform_invalid_rotation:quaternion length² {len_sq} not ≈1.0"
            ));
        }
        if self.scale.iter().any(|&s| s <= 0.0 || !s.is_finite()) {
            return Err("transform_invalid_scale:must be positive and finite".into());
        }
        Ok(())
    }
}

// ── Mesh binding ───────────────────────────────────────────────────────

/// Primitive mesh type. Maps 1:1 to `dreamwell_gpu::primitives::PrimitiveShape`.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum PrimitiveKind {
    Cube,
    Sphere,
    Cylinder,
    Cone,
    Plane,
    Torus,
    Capsule,
    Pyramid,
    Wedge,
}

impl PrimitiveKind {
    pub const ALL: &'static [PrimitiveKind] = &[
        Self::Cube,
        Self::Sphere,
        Self::Cylinder,
        Self::Cone,
        Self::Plane,
        Self::Torus,
        Self::Capsule,
        Self::Pyramid,
        Self::Wedge,
    ];

    pub fn name(self) -> &'static str {
        match self {
            Self::Cube => "Cube",
            Self::Sphere => "Sphere",
            Self::Cylinder => "Cylinder",
            Self::Cone => "Cone",
            Self::Plane => "Plane",
            Self::Torus => "Torus",
            Self::Capsule => "Capsule",
            Self::Pyramid => "Pyramid",
            Self::Wedge => "Wedge",
        }
    }
}

/// What mesh to render for this object.
#[derive(Debug, Clone, PartialEq, Default, Serialize, Deserialize)]
pub enum MeshBinding {
    /// No mesh — invisible container for components only.
    #[default]
    None,
    /// Built-in primitive with pre-computed meshlets. Color is RGBA linear.
    Primitive { kind: PrimitiveKind, color: [f32; 4] },
    /// Custom mesh referenced by asset key (resolved at load time).
    Custom { asset_key: String },
}

// ── Component system ───────────────────────────────────────────────────

/// Component kind — typed categories of attachable game logic.
/// Analogous to Unity MonoBehaviour categories, mapped to Dreamwell's
/// ESCG backend systems (physics compute, particles, Waymark scripts).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum ComponentKind {
    /// Rigid-body physics (integrates with PhysicsGpu simulation).
    Physics,
    /// Collision shape (box, sphere, capsule, mesh).
    Collider,
    /// Waymark script — event-driven logic from content packs.
    /// Properties carry the pack-defined fields (like C# serialized fields).
    WaymarkScript,
    /// Audio source.
    Audio,
    /// Point, spot, or directional light.
    Light,
    /// Particle emitter (integrates with PhysicsGpu emitter pool).
    Particle,
    /// Camera override (when active, replaces scene camera).
    Camera,
    /// Trigger volume — fires ESCG events on enter/exit.
    Trigger,
    /// Animation controller (integrates with GpuTweenContext).
    Animation,
    /// DreamMatter particle attachment — dissolve/materialize particle effect.
    /// When attached, the GPU spawns particles from this object's mesh surface.
    /// Properties: emission_rate, lifetime, particle_count, effect_preset.
    DreamMatter,
    /// FBX animation source — skeletal animation from imported FBX binary.
    /// Properties: fbx_asset_key, clip_name, loop_mode, playback_speed.
    FbxAnimation,
    /// User-defined via Waymark content packs.
    Custom,
}

/// A single attached component with typed kind and Waymark-style property bag.
/// Properties are key-value pairs that the ESCG runtime evaluates per-tick.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ComponentSlot {
    pub kind: ComponentKind,
    pub enabled: bool,
    /// Properties bag: Waymark script fields, physics params, emitter config, etc.
    /// Serialized as JSON for portability across editor / runtime / server.
    pub properties: serde_json::Value,
}

impl ComponentSlot {
    pub fn new(kind: ComponentKind) -> Self {
        Self {
            kind,
            enabled: true,
            properties: serde_json::Value::Object(serde_json::Map::new()),
        }
    }

    /// Set a named property. Bounded: max 256 properties per component.
    pub fn set_property(&mut self, key: &str, value: serde_json::Value) -> Result<(), String> {
        if let serde_json::Value::Object(ref mut map) = self.properties {
            if map.len() >= 256 && !map.contains_key(key) {
                return Err("component_property_limit:max 256 properties".into());
            }
            map.insert(key.to_string(), value);
            Ok(())
        } else {
            Err("component_properties_not_object:must be a JSON object".into())
        }
    }

    pub fn get_property(&self, key: &str) -> Option<&serde_json::Value> {
        self.properties.get(key)
    }
}

// ── GameObject ─────────────────────────────────────────────────────────

/// A game object — empty container that gains visual and behavioral identity
/// through `MeshBinding` and `ComponentSlot`s.
///
/// - `id`: deterministic u64, no UUIDs or RNG.
/// - `transform`: inline hot data, accessed every frame.
/// - `components`: bounded `Vec<ComponentSlot>` (max 32).
/// - Fully serializable for scene save/load and network replication.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct GameObject {
    pub id: u64,
    pub name: String,
    pub transform: Transform,
    pub mesh: MeshBinding,
    pub visible: bool,
    pub parent_id: Option<u64>,
    pub components: Vec<ComponentSlot>,
    /// Property tags (e.g., "isFlammable", "isDestructible"). Drive physics
    /// heuristics, DreamMatter behavior, and event emission at runtime.
    #[serde(default, skip_serializing_if = "Vec::is_empty")]
    pub property_tags: Vec<String>,
    /// Topology layer index (0=Universe, 9=Point). Determines which gameplay
    /// channel this object belongs to. GPU quantum culling uses this to render
    /// only the active layer's content.
    #[serde(default = "default_topology_layer")]
    pub topology_layer: u8,
}

fn default_topology_layer() -> u8 {
    9
}

impl GameObject {
    /// Create a new empty game object with default transform and no mesh.
    pub fn new(id: u64, name: String) -> Self {
        Self {
            id,
            name,
            transform: Transform::default(),
            mesh: MeshBinding::None,
            visible: true,
            parent_id: None,
            components: Vec::new(),
            property_tags: Vec::new(),
            topology_layer: 9,
        }
    }

    /// Create with a primitive mesh (default gray color).
    pub fn with_primitive(id: u64, name: String, kind: PrimitiveKind) -> Self {
        Self {
            mesh: MeshBinding::Primitive {
                kind,
                color: [0.7, 0.7, 0.7, 1.0],
            },
            ..Self::new(id, name)
        }
    }

    /// Attach a component. Returns error if `MAX_COMPONENTS_PER_OBJECT` exceeded.
    pub fn add_component(&mut self, slot: ComponentSlot) -> Result<(), String> {
        if self.components.len() >= MAX_COMPONENTS_PER_OBJECT {
            return Err(format!(
                "game_object_component_limit:max {MAX_COMPONENTS_PER_OBJECT} components"
            ));
        }
        self.components.push(slot);
        Ok(())
    }

    /// Remove the first component matching `kind`. Returns true if found.
    pub fn remove_component(&mut self, kind: ComponentKind) -> bool {
        if let Some(pos) = self.components.iter().position(|c| c.kind == kind) {
            self.components.swap_remove(pos);
            true
        } else {
            false
        }
    }

    pub fn get_component(&self, kind: ComponentKind) -> Option<&ComponentSlot> {
        self.components.iter().find(|c| c.kind == kind)
    }

    pub fn get_component_mut(&mut self, kind: ComponentKind) -> Option<&mut ComponentSlot> {
        self.components.iter_mut().find(|c| c.kind == kind)
    }

    /// Check whether this object has any component of the given kind.
    pub fn has_component(&self, kind: ComponentKind) -> bool {
        self.components.iter().any(|c| c.kind == kind)
    }

    /// Validate data integrity.
    pub fn validate(&self) -> Result<(), String> {
        self.transform.validate()?;
        if self.name.len() > 256 {
            return Err("game_object_name_too_long:max 256 chars".into());
        }
        if self.components.len() > MAX_COMPONENTS_PER_OBJECT {
            return Err(format!(
                "game_object_too_many_components:{} > {MAX_COMPONENTS_PER_OBJECT}",
                self.components.len()
            ));
        }
        Ok(())
    }
}

// ── GameObjectScene ────────────────────────────────────────────────────

/// A bounded scene of game objects with hierarchy support.
/// Serializable as JSON for `.wm` scene files, editor snapshots, and replication.
#[derive(Debug, Clone, Default, Serialize, Deserialize)]
pub struct GameObjectScene {
    pub name: String,
    pub objects: Vec<GameObject>,
    next_id: u64,
}

impl GameObjectScene {
    pub fn new(name: String) -> Self {
        Self {
            name,
            objects: Vec::new(),
            next_id: 0,
        }
    }

    /// Spawn an empty game object. Returns its ID.
    pub fn spawn(&mut self, name: String) -> Result<u64, String> {
        if self.objects.len() >= MAX_SCENE_OBJECTS {
            return Err(format!("scene_object_limit:max {MAX_SCENE_OBJECTS}"));
        }
        let id = self.next_id;
        self.next_id = self.next_id.saturating_add(1);
        self.objects.push(GameObject::new(id, name));
        Ok(id)
    }

    /// Spawn a game object with a primitive mesh. Returns its ID.
    pub fn spawn_primitive(&mut self, name: String, kind: PrimitiveKind) -> Result<u64, String> {
        if self.objects.len() >= MAX_SCENE_OBJECTS {
            return Err(format!("scene_object_limit:max {MAX_SCENE_OBJECTS}"));
        }
        let id = self.next_id;
        self.next_id = self.next_id.saturating_add(1);
        self.objects.push(GameObject::with_primitive(id, name, kind));
        Ok(id)
    }

    pub fn find(&self, id: u64) -> Option<&GameObject> {
        self.objects.iter().find(|o| o.id == id)
    }

    pub fn find_mut(&mut self, id: u64) -> Option<&mut GameObject> {
        self.objects.iter_mut().find(|o| o.id == id)
    }

    /// Despawn an object by ID. Clears dangling parent references.
    pub fn despawn(&mut self, id: u64) -> bool {
        let Some(pos) = self.objects.iter().position(|o| o.id == id) else {
            return false;
        };
        self.objects.swap_remove(pos);
        for obj in &mut self.objects {
            if obj.parent_id == Some(id) {
                obj.parent_id = None;
            }
        }
        true
    }

    pub fn len(&self) -> usize {
        self.objects.len()
    }

    pub fn is_empty(&self) -> bool {
        self.objects.is_empty()
    }

    /// Validate entire scene.
    pub fn validate(&self) -> Result<(), String> {
        if self.objects.len() > MAX_SCENE_OBJECTS {
            return Err(format!(
                "scene_object_limit:{} > {MAX_SCENE_OBJECTS}",
                self.objects.len()
            ));
        }
        for obj in &self.objects {
            obj.validate().map_err(|e| format!("object '{}': {}", obj.name, e))?;
        }
        Ok(())
    }

    /// Root objects (no parent).
    pub fn roots(&self) -> impl Iterator<Item = &GameObject> {
        self.objects.iter().filter(|o| o.parent_id.is_none())
    }

    /// Children of a given parent.
    pub fn children_of(&self, parent_id: u64) -> impl Iterator<Item = &GameObject> {
        self.objects.iter().filter(move |o| o.parent_id == Some(parent_id))
    }

    /// Propagate transforms through the parent-child hierarchy.
    /// Each child's world matrix = parent_world * child_local.
    /// Stores results indexed by object position in `self.objects`.
    ///
    /// Uses topological BFS from roots — O(N) single pass, each object
    /// computed exactly once. Parents are always processed before children.
    pub fn propagate_transforms(&self) -> Vec<[f32; 16]> {
        use crate::transform::mat4_mul;

        let len = self.objects.len();
        let mut world: Vec<[f32; 16]> =
            vec![[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,]; len];
        if len == 0 {
            return world;
        }

        // Build id → index lookup and children adjacency list in one pass.
        let mut id_to_idx: std::collections::HashMap<u64, usize> = std::collections::HashMap::with_capacity(len);
        let mut children: Vec<Vec<usize>> = vec![Vec::new(); len];
        let mut roots: Vec<usize> = Vec::new();

        for (i, obj) in self.objects.iter().enumerate() {
            id_to_idx.insert(obj.id, i);
        }
        for (i, obj) in self.objects.iter().enumerate() {
            if let Some(pid) = obj.parent_id {
                if let Some(&pi) = id_to_idx.get(&pid) {
                    children[pi].push(i);
                } else {
                    roots.push(i); // orphaned parent ref → treat as root
                }
            } else {
                roots.push(i);
            }
        }

        // BFS from roots — parents always processed before children.
        let mut queue = std::collections::VecDeque::with_capacity(len);
        for &ri in &roots {
            world[ri] = self.objects[ri].transform.to_matrix();
            queue.push_back(ri);
        }

        while let Some(idx) = queue.pop_front() {
            for &ci in &children[idx] {
                let local = self.objects[ci].transform.to_matrix();
                world[ci] = mat4_mul(&world[idx], &local);
                queue.push_back(ci);
            }
        }

        world
    }

    /// Count of visible objects with a mesh binding.
    pub fn visible_mesh_count(&self) -> usize {
        self.objects
            .iter()
            .filter(|o| o.visible && !matches!(o.mesh, MeshBinding::None))
            .count()
    }
}

// ── Tests ──────────────────────────────────────────────────────────────

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn transform_default_identity() {
        let t = Transform::default();
        assert_eq!(t.position, [0.0; 3]);
        assert_eq!(t.rotation, [0.0, 0.0, 0.0, 1.0]);
        assert_eq!(t.scale, [1.0, 1.0, 1.0]);
    }

    #[test]
    fn transform_identity_matrix() {
        let t = Transform::default();
        let m = t.to_matrix();
        #[rustfmt::skip]
        let expected = [
            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,
        ];
        for (a, b) in m.iter().zip(expected.iter()) {
            assert!((a - b).abs() < 1e-6, "matrix mismatch: {a} vs {b}");
        }
    }

    #[test]
    fn transform_translation_in_matrix() {
        let t = Transform {
            position: [1.0, 2.0, 3.0],
            ..Default::default()
        };
        let m = t.to_matrix();
        assert_eq!(m[12], 1.0);
        assert_eq!(m[13], 2.0);
        assert_eq!(m[14], 3.0);
    }

    #[test]
    fn transform_scale_in_matrix() {
        let t = Transform {
            scale: [2.0, 3.0, 4.0],
            ..Default::default()
        };
        let m = t.to_matrix();
        assert!((m[0] - 2.0).abs() < 1e-6);
        assert!((m[5] - 3.0).abs() < 1e-6);
        assert!((m[10] - 4.0).abs() < 1e-6);
    }

    #[test]
    fn transform_validate_ok() {
        assert!(Transform::default().validate().is_ok());
    }

    #[test]
    fn transform_validate_zero_scale() {
        let t = Transform {
            scale: [0.0, 1.0, 1.0],
            ..Default::default()
        };
        assert!(t.validate().is_err());
    }

    #[test]
    fn transform_validate_nan_position() {
        let t = Transform {
            position: [f32::NAN, 0.0, 0.0],
            ..Default::default()
        };
        assert!(t.validate().is_err());
    }

    #[test]
    fn transform_validate_non_unit_quat() {
        let t = Transform {
            rotation: [1.0, 1.0, 1.0, 1.0],
            ..Default::default()
        };
        assert!(t.validate().is_err());
    }

    #[test]
    fn game_object_new_defaults() {
        let obj = GameObject::new(0, "Test".into());
        assert_eq!(obj.id, 0);
        assert!(obj.visible);
        assert!(obj.components.is_empty());
        assert_eq!(obj.mesh, MeshBinding::None);
        assert_eq!(obj.parent_id, None);
    }

    #[test]
    fn game_object_with_primitive() {
        let obj = GameObject::with_primitive(1, "Cube".into(), PrimitiveKind::Cube);
        assert!(matches!(
            obj.mesh,
            MeshBinding::Primitive {
                kind: PrimitiveKind::Cube,
                ..
            }
        ));
    }

    #[test]
    fn game_object_add_remove_component() {
        let mut obj = GameObject::new(0, "T".into());
        obj.add_component(ComponentSlot::new(ComponentKind::Physics)).unwrap();
        assert!(obj.has_component(ComponentKind::Physics));
        assert!(obj.remove_component(ComponentKind::Physics));
        assert!(!obj.has_component(ComponentKind::Physics));
    }

    #[test]
    fn game_object_component_limit() {
        let mut obj = GameObject::new(0, "T".into());
        for _ in 0..MAX_COMPONENTS_PER_OBJECT {
            obj.add_component(ComponentSlot::new(ComponentKind::Custom)).unwrap();
        }
        assert!(obj.add_component(ComponentSlot::new(ComponentKind::Custom)).is_err());
    }

    #[test]
    fn component_slot_properties() {
        let mut slot = ComponentSlot::new(ComponentKind::WaymarkScript);
        slot.set_property("speed", serde_json::json!(5.0)).unwrap();
        slot.set_property("health", serde_json::json!(100)).unwrap();
        assert_eq!(slot.get_property("speed"), Some(&serde_json::json!(5.0)));
        assert_eq!(slot.get_property("missing"), None);
    }

    #[test]
    fn scene_spawn_and_find() {
        let mut scene = GameObjectScene::new("Test".into());
        let id = scene.spawn("Player".into()).unwrap();
        assert_eq!(scene.len(), 1);
        assert_eq!(scene.find(id).unwrap().name, "Player");
    }

    #[test]
    fn scene_spawn_primitive() {
        let mut scene = GameObjectScene::new("Test".into());
        let id = scene.spawn_primitive("Cube".into(), PrimitiveKind::Cube).unwrap();
        let obj = scene.find(id).unwrap();
        assert!(matches!(
            obj.mesh,
            MeshBinding::Primitive {
                kind: PrimitiveKind::Cube,
                ..
            }
        ));
    }

    #[test]
    fn scene_despawn_clears_parent_refs() {
        let mut scene = GameObjectScene::new("Test".into());
        let parent = scene.spawn("Parent".into()).unwrap();
        let child = scene.spawn("Child".into()).unwrap();
        scene.find_mut(child).unwrap().parent_id = Some(parent);
        scene.despawn(parent);
        assert_eq!(scene.find(child).unwrap().parent_id, None);
    }

    #[test]
    fn scene_hierarchy() {
        let mut scene = GameObjectScene::new("Test".into());
        let p = scene.spawn("Parent".into()).unwrap();
        let c1 = scene.spawn("C1".into()).unwrap();
        let c2 = scene.spawn("C2".into()).unwrap();
        scene.find_mut(c1).unwrap().parent_id = Some(p);
        scene.find_mut(c2).unwrap().parent_id = Some(p);
        assert_eq!(scene.roots().count(), 1);
        assert_eq!(scene.children_of(p).count(), 2);
    }

    #[test]
    fn scene_serialize_roundtrip() {
        let mut scene = GameObjectScene::new("TestScene".into());
        let id = scene.spawn_primitive("Cube".into(), PrimitiveKind::Cube).unwrap();
        scene.find_mut(id).unwrap().transform.position = [1.0, 2.0, 3.0];

        let mut slot = ComponentSlot::new(ComponentKind::WaymarkScript);
        slot.set_property("speed", serde_json::json!(5.0)).unwrap();
        scene.find_mut(id).unwrap().add_component(slot).unwrap();

        let json = serde_json::to_string_pretty(&scene).unwrap();
        let restored: GameObjectScene = serde_json::from_str(&json).unwrap();

        assert_eq!(restored.name, "TestScene");
        assert_eq!(restored.len(), 1);
        assert_eq!(restored.objects[0].transform.position, [1.0, 2.0, 3.0]);
        assert_eq!(restored.objects[0].components[0].kind, ComponentKind::WaymarkScript);
    }

    #[test]
    fn scene_validate_ok() {
        let mut scene = GameObjectScene::new("Ok".into());
        scene.spawn_primitive("Cube".into(), PrimitiveKind::Cube).unwrap();
        assert!(scene.validate().is_ok());
    }

    #[test]
    fn scene_validate_bad_transform() {
        let mut scene = GameObjectScene::new("Bad".into());
        let id = scene.spawn("Bad".into()).unwrap();
        scene.find_mut(id).unwrap().transform.scale = [0.0, 1.0, 1.0];
        assert!(scene.validate().is_err());
    }

    #[test]
    fn primitive_kind_all_count() {
        assert_eq!(PrimitiveKind::ALL.len(), 9);
    }

    #[test]
    fn mesh_binding_default_is_none() {
        assert_eq!(MeshBinding::default(), MeshBinding::None);
    }

    #[test]
    fn game_object_validate_name_too_long() {
        let obj = GameObject::new(0, "x".repeat(257));
        assert!(obj.validate().is_err());
    }

    #[test]
    fn visible_mesh_count() {
        let mut scene = GameObjectScene::new("Test".into());
        scene.spawn_primitive("A".into(), PrimitiveKind::Cube).unwrap();
        scene.spawn("Empty".into()).unwrap(); // no mesh
        let id = scene.spawn_primitive("Hidden".into(), PrimitiveKind::Sphere).unwrap();
        scene.find_mut(id).unwrap().visible = false;
        assert_eq!(scene.visible_mesh_count(), 1);
    }

    #[test]
    fn propagate_transforms_root_only() {
        let mut scene = GameObjectScene::new("Test".into());
        let id = scene.spawn("A".into()).unwrap();
        scene.find_mut(id).unwrap().transform.position = [1.0, 2.0, 3.0];
        let world = scene.propagate_transforms();
        assert_eq!(world.len(), 1);
        assert_eq!(world[0][12], 1.0);
        assert_eq!(world[0][13], 2.0);
        assert_eq!(world[0][14], 3.0);
    }

    #[test]
    fn propagate_transforms_parent_child() {
        let mut scene = GameObjectScene::new("Test".into());
        let p = scene.spawn("Parent".into()).unwrap();
        scene.find_mut(p).unwrap().transform.position = [10.0, 0.0, 0.0];
        let c = scene.spawn("Child".into()).unwrap();
        scene.find_mut(c).unwrap().transform.position = [5.0, 0.0, 0.0];
        scene.find_mut(c).unwrap().parent_id = Some(p);

        let world = scene.propagate_transforms();
        // Child world position = parent(10,0,0) + child(5,0,0) = (15,0,0)
        assert!((world[1][12] - 15.0).abs() < 1e-5);
    }

    #[test]
    fn propagate_transforms_empty_scene() {
        let scene = GameObjectScene::new("Empty".into());
        let world = scene.propagate_transforms();
        assert!(world.is_empty());
    }

    #[test]
    fn next_id_monotonic() {
        let mut scene = GameObjectScene::new("Test".into());
        let a = scene.spawn("A".into()).unwrap();
        let b = scene.spawn("B".into()).unwrap();
        assert_eq!(a, 0);
        assert_eq!(b, 1);
        scene.despawn(a);
        let c = scene.spawn("C".into()).unwrap();
        assert_eq!(c, 2); // never reuses
    }
}