feuerwerk/src/runtime/World.ts

import { ENTITY_HANDLE, isHandleEnabled, type PointerState, worldMatrixOf } from '../core/graph';
import { newId } from '../core/ids';
import { type Asset, isPrivateVariable, type TilemapAsset } from '../core/model/Asset';
import { sameAtRest, withoutObjects } from '../core/graph/liveObjects';
import { bagWithoutItemTypes, type PropertyBag, setBagValue, type Vec2 } from '../core/model/Properties';
import { isPlaceableType } from '../core/model/AssetTypes';
import { bakeInstances, materializeInstance, objectRef, resolveObjects } from '../core/model/instance';
import { groupNamed, type ObjectTree, treeOrder } from '../core/model/hierarchy';
import { liveTileAt, withTile } from '../core/model/liveTilemap';
import { referenceCandidates } from '../core/model/objectRefs';
import { type Cell, presentTile } from '../core/model/tilemap';
import { descendantsBelow } from '../core/model/walk';
import { findAsset, findAssetOf, findScene, type Project } from '../core/model/Project';
import { createSceneObject, objectName, type SceneObject } from '../core/model/SceneObject';
import { apply, IDENTITY } from '../core/model/matrix';
import { hasTransformProps } from '../core/properties/PropertySchema';
import { carriedTransform, localMatrixOf, transformOf } from '../core/properties/transform';
import { footprintContains } from '../core/properties/bounds';
import { colliderFrame, colliderShape, shapeContains } from '../core/properties/collider';
import { resolveSceneObjectProps } from '../core/properties/resolve';
import type { Entity } from './Entity';
import type { System } from './systems/System';

/** How long (seconds of run time) a cast ray stays drawn, and how many at most. */
export const RAY_SHOWN = 0.25;
const MAX_RAYS = 64;

export interface CastRay {
  from: Vec2;
  to: Vec2;
  hit: boolean;
  time: number;
}

export interface WorldOptions {
  /**
   * Whether a placed container's content may be written into the scene (see `syncFromModel`).
   * Baking is a real, irreversible edit, so the editor run switches it off; its content still runs,
   * but what it writes goes into the instance edits and it cannot destroy itself for good.
   */
  bakeContainers?: boolean;
  /** What a `Load scene` brings along from the world before (#165); given, the scene counts as changed. */
  brought?: BroughtEntities;
}

/**
 * The kept entities of the world before, whose objects the scene holds by now, and the ids of the
 * scene's own objects they stand in for (see `keptObjects.ts`).
 */
export interface BroughtEntities {
  entities: readonly Entity[];
  standIns: ReadonlyMap<string, string>;
}

/** What `syncToModel` wrote: `structure` = objects appeared or vanished, `values` = the rest. */
export interface ModelWrite {
  structure: boolean;
  values: boolean;
}

/**
 * What the model said of an entity when the world last read it or wrote it. `syncToModel` runs every
 * frame and compares against this instead of resolving every object anew (#287), which holds as long
 * as whoever else changes the model has it read again (`syncFromModel`).
 */
interface ModelState {
  asset: Asset;
  x: number;
  y: number;
  z: number;
  props: PropertyBag;
}

const NO_KEYS: readonly string[] = [];

/** The keys whose value in the run no longer says what the model does. */
function driftedKeys(live: PropertyBag, model: ModelState): readonly string[] {
  let keys: string[] | undefined;
  for (const key of Object.keys(live)) {
    const value = live[key];
    const atRest = model.props[key];
    if (atRest === value || !(key in model.props)) continue;
    // What a run holds of an object stays in the run; the scene gets the empty one.
    // A private variable (#257) has no override to hold its value: that stays in the run.
    if (isPrivateVariable(model.asset, key) || sameAtRest(atRest, value)) continue;
    (keys ??= []).push(key);
  }
  return keys ?? NO_KEYS;
}

/**
 * The running game: entities of one scene plus the systems ticking them. Built on the play copy
 * (discarded on stop) or, for the editor run, on the saved model, where its writes stay.
 */
export class World {
  readonly entities: Entity[] = [];
  /** The running scene's own properties – read from the model, and written by an animation over the scene. */
  readonly sceneProps: PropertyBag = {};
  /** What the run itself set on the scene (`setSceneProp`); re-reading the model does not undo it. */
  private readonly runSceneProps: PropertyBag = {};
  /** Set by a behavior (Stop game node); the editor leaves play mode on the next tick. */
  stopRequested = false;
  /** Set by a behavior (Restart game node); the editor restarts the scene on the next tick. */
  restartRequested = false;
  /**
   * Set by a behavior (Load scene node): the scene to swap to once this tick is over, `''` for
   * none asked. Asked twice in a tick, the last one wins – a flag, not a queue, because loading
   * two scenes in a row is not a thing anyone means.
   */
  loadSceneRequested = '';
  /**
   * Set by a behavior (Pause game / Resume game, #167). The world time, the physics and the
   * animations stand still, and only behaviors that run while paused are ticked. Belongs to the
   * world, so the next scene starts unpaused.
   */
  paused = false;
  private frameFrozen = false;
  /**
   * Mouse state in world units, fed by the game view (the only place with the camera
   * transform). `pressed`/`released` last for one tick, like the keyboard edges.
   */
  readonly pointer: PointerState = {
    x: 0,
    y: 0,
    inside: false,
    down: false,
    pressed: false,
    released: false,
    wheel: 0,
  };
  /** Objects the editor is dragging (ids): the physics puts them where the drag does, at rest. */
  held: ReadonlySet<string> = new Set();
  /** Messages logged by behaviors (Log node), newest last. */
  readonly log: string[] = [];
  /** The rays cast lately (#153), for the editor run to draw: where from, where to, whether they met anything. */
  readonly rays: CastRay[] = [];
  /** Until when (run time) an object falls through one-way colliders and tiles – `Ignore one-way` (#155). */
  private readonly ignoringOneWay = new Map<Entity, number>();
  private readonly systems: System[] = [];
  /** Told what each system took of a tick, in ms, when set – the editor's performance overlay (#275). */
  timeSystem: ((system: System, ms: number) => void) | null = null;
  private elapsed = 0;
  /** Set when `spawn` added an object; reported (and cleared) by the next `syncToModel`. */
  private structureChanged = false;
  /** Children a Destroy moved up to their grandparent; written into the scene by `syncToModel`. */
  private readonly movedUp = new Set<Entity>();
  /** Set when an entity left the run; the next `syncToModel` drops its object. */
  private entitiesGone = false;
  private readonly modelStates = new WeakMap<Entity, ModelState>();
  private tilesChanged = false;
  /** Entities of the world before, by object id, until the scene's object takes each one up. */
  private readonly arriving = new Map<string, Entity>();
  /** Objects this scene did not build because one brought along is it already, by their id. */
  private readonly standIns = new Map<string, Entity>();

  private constructor(
    readonly project: Project,
    readonly sceneId: string,
    private readonly bakeContainers: boolean,
  ) {}

  /** Builds a world from the given scene. Throws when the scene does not exist. */
  static instantiate(project: Project, sceneId: string, options: WorldOptions = {}): World {
    if (!findScene(project, sceneId)) throw new Error(`Scene ${sceneId} not found`);
    const world = new World(project, sceneId, options.bakeContainers ?? true);
    const brought = options.brought;
    for (const entity of brought?.entities ?? []) world.arriving.set(entity.objectId, entity);
    world.syncFromModel();
    world.arriving.clear();
    if (!brought) return world;
    for (const [id, standIn] of brought.standIns) {
      const entity = world.entities.find((e) => e.objectId === standIn);
      if (entity) world.standIns.set(id, entity);
    }
    world.structureChanged = true;
    return world;
  }

  /** Seconds since the world started. */
  get time(): number {
    return this.elapsed;
  }

  get backgroundColor(): number {
    const background = this.sceneProps.backgroundColor;
    return typeof background === 'number' ? background : 0x000000;
  }

  /** Makes the entities match the scene; props are updated in place so systems holding a bag stay current. */
  syncFromModel(): void {
    const scene = findScene(this.project, this.sceneId);
    if (!scene) return;
    // Baking spells a placed container's content out as stored objects, which is what lets a
    // behavior destroy one of them for good. Play does it (on its throwaway copy); the editor run
    // must not, so there the content stays resolved and `objectRef` writes into the instance edits.
    if (this.bakeContainers && bakeInstances(this.project, scene)) this.structureChanged = true;
    Object.assign(this.sceneProps, scene.props, this.runSceneProps);
    const objects = new Map(resolveObjects(this.project, scene).map((o) => [o.id, o]));
    const keep = new Set<Entity>();
    for (const object of objects.values()) {
      const asset = findAsset(this.project, object.assetId);
      if (!asset || !isPlaceableType(asset.type)) continue;
      keep.add(this.materialize(object, asset));
    }
    for (let i = this.entities.length - 1; i >= 0; i--)
      if (!keep.has(this.entities[i]!)) this.entities.splice(i, 1);
    // Link parents once every entity exists (storage order is not guaranteed to be parent-first).
    for (const entity of this.entities) {
      const object = objects.get(entity.objectId);
      entity.parent = object?.parentId ? (this.findByObjectId(object.parentId) ?? null) : null;
    }
  }

  /** Remembers a ray for a moment, forgetting those cast longer ago than `RAY_SHOWN`. */
  noteRay(from: Vec2, to: Vec2, hit: boolean): void {
    const since = this.time - RAY_SHOWN;
    const kept = this.rays.filter((ray) => ray.time >= since);
    this.rays.splice(0, this.rays.length, ...kept.slice(-(MAX_RAYS - 1)), { from, to, hit, time: this.time });
  }

  /** Lets the object, and anything below it, fall through one-way colliders for `seconds` from now. */
  ignoreOneWay(entity: Entity, seconds: number): void {
    this.ignoringOneWay.set(entity, this.time + Math.max(0, seconds));
  }

  /** Whether one-way colliders let the entity through right now, asked of it or any object above it. */
  ignoresOneWay(entity: Entity): boolean {
    for (let e: Entity | null = entity; e; e = e.parent) {
      const until = this.ignoringOneWay.get(e);
      if (until !== undefined && until > this.time) return true;
    }
    return false;
  }

  /** Whether a drag holds the object: itself, an object above it, or its rigidbody is dragged. */
  holds(entity: Entity): boolean {
    if (this.held.size === 0) return false;
    for (let e: Entity | null = entity; e; e = e.parent) if (this.held.has(e.objectId)) return true;
    return this.childrenOf(entity).some((c) => c.type === 'rigidbody' && this.held.has(c.objectId));
  }

  /** The asset of a placed tilemap; undefined for anything else. */
  private tilemapAssetOf(entity: Entity): TilemapAsset | undefined {
    return entity.type === 'tilemap' ? findAssetOf(this.project, entity.assetId, 'tilemap') : undefined;
  }

  isTilemap(entity: Entity): boolean {
    return this.tilemapAssetOf(entity) !== undefined;
  }

  /**
   * The tile in a cell of a placed tilemap as the run left it (see `model/liveTilemap.ts`), `''` for
   * a cell naming a tile the project does not have, as it is drawn.
   */
  tileAt(entity: Entity, layer: string, cell: Cell): string | null {
    const asset = this.tilemapAssetOf(entity);
    const tile = asset ? liveTileAt(asset, entity.tileEdits, entity.props, layer, cell) : null;
    return tile === null ? null : presentTile(this.project, tile);
  }

  /** Sets a cell of this one placement for the rest of the run; `''` empties it. */
  setTile(entity: Entity, layer: string, cell: Cell, tileId: string): boolean {
    const asset = this.tilemapAssetOf(entity);
    if (!asset || (tileId !== '' && !findAssetOf(this.project, tileId, 'tile'))) return false;
    const edits = withTile(asset, entity.tileEdits, entity.props, layer, cell, tileId);
    if (!edits) return false;
    if (edits !== entity.tileEdits) this.tilesChanged = true;
    entity.tileEdits = edits;
    return true;
  }

  /**
   * Whether a tile was set since the last ask. The editor run needs it: a changed cell reaches no
   * model, so nothing else would tell the views to draw the map again.
   */
  takeTileChange(): boolean {
    const changed = this.tilesChanged;
    this.tilesChanged = false;
    return changed;
  }

  /** Sets a scene property for the rest of the run; the scene in the model keeps its own. */
  setSceneProp(key: string, value: PropertyBag[string]): void {
    this.sceneProps[key] = value;
    this.runSceneProps[key] = value;
  }

  /** The entity for a scene object, created when missing; position, name and props re-read. */
  private materialize(object: SceneObject, asset: Asset): Entity {
    let entity = this.entities.find((e) => e.objectId === object.id);
    const arrived = entity ? undefined : this.arriving.get(object.id);
    if (arrived) {
      // It comes as the run left it: what the scene says of it is only what the next write compares
      // against, so the run's values reach the scene instead of the scene's replacing them.
      this.entities.push(arrived);
      arrived.assetId = asset.id;
      arrived.type = asset.type;
      arrived.name = objectName(object, asset);
      this.modelStates.set(arrived, { asset, ...object.position, props: this.modelProps(asset, object) });
      return arrived;
    }
    if (!entity) {
      entity = {
        [ENTITY_HANDLE]: true,
        id: `e-${object.id}`,
        objectId: object.id,
        assetId: asset.id,
        type: asset.type,
        name: '',
        x: 0,
        y: 0,
        z: 0,
        props: {},
        parent: null,
      };
      if (asset.type === 'collider') {
        const collider = entity;
        collider.wornFrame = () => colliderFrame(this.project, collider.props);
      }
      this.entities.push(entity);
    }
    entity.assetId = asset.id;
    entity.type = asset.type;
    entity.name = objectName(object, asset);
    entity.x = object.position.x;
    entity.y = object.position.y;
    entity.z = object.position.z;
    const resolved = this.modelProps(asset, object);
    this.modelStates.set(entity, { asset, ...object.position, props: resolved });
    for (const key of Object.keys(entity.props)) if (!(key in resolved)) delete entity.props[key];
    // The scene only ever got the objects' empty places, so a value that differs by them alone stays.
    // A private value never reaches the scene, so the scene's is only where it starts.
    for (const [key, value] of Object.entries(resolved)) {
      if (isPrivateVariable(asset, key) && key in entity.props) continue;
      if (!sameAtRest(value, entity.props[key])) entity.props[key] = value;
    }
    return entity;
  }

  /** An object's values as a run holds them. */
  private modelProps(asset: Asset, object: SceneObject): PropertyBag {
    return bagWithoutItemTypes(resolveSceneObjectProps(this.project, asset, object));
  }

  /**
   * Adds the object to the scene and creates only its entity, leaving the others alone (they may have
   * moved this tick already). Position is local to `parent`, world when there is none.
   */
  spawn(
    assetId: string,
    position: { x: number; y: number; z: number },
    parent: Entity | null,
  ): Entity | null {
    const scene = findScene(this.project, this.sceneId);
    const asset = findAsset(this.project, assetId);
    if (!scene || !asset || !isPlaceableType(asset.type)) return null;
    const parentEntity = parent && this.entities.includes(parent) ? parent : null;
    const object = createSceneObject(newId(), asset.id, position, parentEntity?.objectId ?? '');
    // Spawned at the top level, it is listed in a group named after its asset, not as a row of its own.
    if (!parentEntity) object.groupId = groupNamed(scene, asset.name, newId).id;
    scene.objects.push(object);
    // A container brings its content along (parented below it, in template order).
    const created = asset.type === 'container' ? materializeInstance(this.project, scene, object) : [];
    scene.objects.push(...created);
    const entity = this.materialize(object, asset);
    entity.parent = parentEntity;
    for (const child of created) {
      const childAsset = findAsset(this.project, child.assetId);
      if (!childAsset || !isPlaceableType(childAsset.type)) continue;
      this.materialize(child, childAsset).parent = this.findByObjectId(child.parentId) ?? null;
    }
    this.structureChanged = true;
    return entity;
  }

  /**
   * Writes the entities' state into the scene: positions, property values that differ from
   * what the model resolves (stored as overrides, so the inspector marks them) and removed
   * entities (their objects are dropped – only stored ones, see `WorldOptions.bakeContainers`).
   */
  syncToModel(): ModelWrite {
    const scene = findScene(this.project, this.sceneId);
    if (!scene) return { structure: false, values: false };
    let values = false;
    const spawned = this.structureChanged;
    this.structureChanged = false;
    // Moved up by a Destroy that kept them. Only a stored object can say so: a container's content
    // has its place in the template, which is why the editor run cannot destroy it for good either.
    for (const entity of this.movedUp) {
      const object = scene.objects.find((o) => o.id === entity.objectId);
      if (object && this.entities.includes(entity)) object.parentId = entity.parent?.objectId ?? '';
    }
    const reparented = this.movedUp.size > 0;
    this.movedUp.clear();
    const dropped = this.entitiesGone && this.dropGone(scene);
    this.entitiesGone = false;
    for (const entity of this.entities) {
      // A dragged object's place belongs to the drag, which reaches the entity on the next re-read.
      if (this.held.has(entity.objectId)) continue;
      const model = this.modelStates.get(entity);
      if (!model) continue;
      const moved = model.x !== entity.x || model.y !== entity.y || model.z !== entity.z;
      const drifted = driftedKeys(entity.props, model);
      if (!moved && drifted.length === 0) continue;
      // Only now, because a ref for resolved content costs a resolve of its own – and because a
      // direct write to such an object would land in the freshly built copy and be lost.
      const ref = objectRef(this.project, scene, entity.objectId);
      if (!ref) continue;
      if (moved) {
        ref.setPosition({ x: entity.x, y: entity.y, z: entity.z });
        model.x = entity.x;
        model.y = entity.y;
        model.z = entity.z;
      }
      if (drifted.length > 0) {
        // Read again rather than patched: an override may bring rows of its own (a sprite's shader).
        // A container's content is a copy made before the write, so the written values go on top.
        const overrides = { ...ref.object.overrides };
        for (const key of drifted) {
          const value = withoutObjects(entity.props[key]);
          ref.setOverride(key, value);
          setBagValue(overrides, key, value);
        }
        model.props = this.modelProps(model.asset, { ...ref.object, overrides });
      }
      values = true;
    }
    return { structure: spawned || reparented || dropped, values };
  }

  /** Drops the objects whose entity left the run; true when there was one. */
  private dropGone(scene: ObjectTree): boolean {
    // Looked up once for the whole pass: asking the list per object grows with the square of the
    // entities (#284). The first of an id counts, as `findByObjectId` has it.
    const alive = new Set(this.entities.map((e) => e.objectId));
    const assets = new Map<string, Asset>();
    for (const asset of this.project.assets) if (!assets.has(asset.id)) assets.set(asset.id, asset);
    const before = scene.objects.length;
    for (let i = scene.objects.length - 1; i >= 0; i--) {
      const object = scene.objects[i]!;
      const asset = assets.get(object.assetId);
      if (asset && isPlaceableType(asset.type) && !alive.has(object.id)) scene.objects.splice(i, 1);
    }
    return scene.objects.length !== before;
  }

  /** Registers a system; it starts immediately and is ticked in registration order. */
  addSystem(system: System): this {
    this.systems.push(system);
    system.start?.(this);
    return this;
  }

  /** Whether the frame being ticked is a paused one – read once when it began, see `tick`. */
  get frozen(): boolean {
    return this.frameFrozen;
  }

  tick(dt: number): void {
    // A pause asked for halfway through a frame starts with the next one, so every behavior of
    // this frame sees the same world.
    this.frameFrozen = this.paused;
    if (!this.frameFrozen) this.elapsed += dt;
    const timed = this.timeSystem;
    for (const system of this.systems) {
      if (this.frameFrozen && !system.runsWhilePaused) continue;
      if (!timed) {
        system.update(this, dt);
        continue;
      }
      const start = performance.now();
      system.update(this, dt);
      timed(system, performance.now() - start);
    }
    // The pointer edges belong to exactly one tick, whatever the systems did with them.
    this.dropPointerEdges();
  }

  /** Forgets a pending click edge without running a tick (used while paused). */
  dropPointerEdges(): void {
    this.pointer.pressed = false;
    this.pointer.released = false;
    this.pointer.wheel = 0;
  }

  /** Game view: where the mouse is (world units) and whether it is over the view at all. */
  movePointer(x: number, y: number, inside: boolean): void {
    this.pointer.x = x;
    this.pointer.y = y;
    this.pointer.inside = inside;
  }

  /** Game view: the wheel turned by `notches`; what turns within one tick adds up. */
  turnWheel(notches: number): void {
    this.pointer.wheel += notches;
  }

  /** Game view: the mouse button changed. Sets the edge flag the current tick reports. */
  setPointerDown(down: boolean): void {
    if (down === this.pointer.down) return;
    this.pointer.down = down;
    if (down) this.pointer.pressed = true;
    else this.pointer.released = true;
  }

  /** Stops the systems and drops every entity; the world is unusable afterwards. */
  destroy(): void {
    for (const system of this.systems) system.stop?.(this);
    this.systems.length = 0;
    this.entities.length = 0;
  }

  /**
   * Whether a world point lies on an entity: on the colliders of its subtree when it has
   * any, otherwise on the box of a sprite or text in it. Disabled parts never answer, so a
   * hidden screen cannot swallow clicks meant for what is drawn behind it.
   */
  hitTest(entity: Entity, x: number, y: number): boolean {
    const point = { x, y };
    const parts = [entity, ...this.descendantsOf(entity)].filter(
      (e) => isHandleEnabled(e) && this.insideClipAreas(e, point),
    );
    const colliders = parts.filter((e) => e.type === 'collider');
    if (colliders.length > 0)
      return colliders.some((c) =>
        shapeContains(colliderShape(c.props, worldMatrixOf(c), c.wornFrame?.()), point),
      );
    return parts.some(
      (e) =>
        (e.type === 'sprite' || e.type === 'text') &&
        footprintContains(e.type, e.props, worldMatrixOf(e), point),
    );
  }

  /** Whether the point is inside every clip area above the entity (#260): what is cut off is not hit. */
  private insideClipAreas(entity: Entity, point: Vec2): boolean {
    for (let at = entity.parent; at; at = at.parent)
      if (at.type === 'clip' && !footprintContains('clip', at.props, worldMatrixOf(at), point)) return false;
    return true;
  }

  childrenOf(entity: Entity): Entity[] {
    return this.entities.filter((e) => e.parent === entity);
  }

  descendantsOf(entity: Entity): Entity[] {
    return descendantsBelow(
      entity,
      (parent) => this.childrenOf(parent),
      (child) => child,
    );
  }

  /** An object the scene left out for one brought along (#165) answers as that one. */
  findByObjectId(objectId: string): Entity | undefined {
    const own = this.entities.find((e) => e.objectId === objectId);
    if (own) return own;
    const standIn = this.standIns.get(objectId);
    return standIn && this.entities.includes(standIn) ? standIn : undefined;
  }

  /** The entity a reference on `from` names (#258): its own container placement's copy first. */
  findReferenced(from: Entity, ref: string): Entity | undefined {
    for (const id of referenceCandidates(from.objectId, ref)) {
      const found = this.findByObjectId(id);
      if (found) return found;
    }
    return undefined;
  }

  /** The first entity with that name; an object without a name of its own carries its asset's name. */
  findByName(name: string): Entity | undefined {
    return this.entities.find((e) => e.name === name);
  }

  /** The nearest entity with that name below `parent`, level by level – a child before a grandchild. */
  childByName(parent: Entity, name: string): Entity | undefined {
    for (
      let level = this.childrenOf(parent);
      level.length > 0;
      level = level.flatMap((e) => this.childrenOf(e))
    ) {
      const found = level.find((e) => e.name === name);
      if (found) return found;
    }
    return undefined;
  }

  /**
   * The camera the game is seen through: of the cameras that are switched on (itself and every
   * parent, see `properties/enabled.ts`) the first one in hierarchy order. Undefined while none
   * is on, and then nothing is rendered – switching one off is how a scene hands the view over.
   */
  activeCamera(): Entity | undefined {
    const on = this.entities.filter((e) => e.type === 'camera' && isHandleEnabled(e));
    if (on.length < 2) return on[0];
    // Ranked only when the order can decide something, since this runs per frame.
    const scene = findScene(this.project, this.sceneId);
    const order = scene ? treeOrder(scene).map(({ object }) => object.id) : [];
    const rank = (camera: Entity) => {
      const index = order.indexOf(camera.objectId);
      return index < 0 ? order.length : index;
    };
    return on.reduce((first, camera) => (rank(camera) < rank(first) ? camera : first));
  }

  /** Removes an entity and, recursively, everything parented to it. */
  remove(entity: Entity): void {
    for (const child of this.childrenOf(entity)) this.remove(child);
    const index = this.entities.indexOf(entity);
    if (index < 0) return;
    this.entities.splice(index, 1);
    this.entitiesGone = true;
  }

  /**
   * Removes only the entity: its children move up to its parent (top level when it had none) and
   * stay where they are in the world. Its behaviors, its rigidbody and its joints are part of it rather
   * than things it carries, so they go with it – moved up they would drive the parent instead.
   * Answers the children that moved up.
   */
  removeKeepingChildren(entity: Entity): Entity[] {
    if (!this.entities.includes(entity)) return [];
    const own = localMatrixOf(entity, transformOf(entity.props));
    const kept: Entity[] = [];
    for (const child of this.childrenOf(entity)) {
      if (child.type === 'behavior' || child.type === 'rigidbody' || child.type === 'joint') continue;
      const at = apply(own, child);
      child.x = at.x;
      child.y = at.y;
      child.z += entity.z;
      const before = transformOf(child.props);
      const carried = carriedTransform(before, own, IDENTITY);
      if (hasTransformProps(child.type) && carried !== before) {
        child.props.rotation = carried.rotation;
        child.props.scale = carried.scale;
        child.props.mirrorX = carried.mirrorX;
        child.props.mirrorY = carried.mirrorY;
      }
      child.parent = entity.parent;
      this.movedUp.add(child);
      kept.push(child);
    }
    this.remove(entity);
    return kept;
  }
}