Add examples

* Fix bulk_insert with moving archetypes

* Reword message

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 ## Unreleased
 
+### Added
+- Added signals that allow listening to relation part of pairs in signals.
+
+### Changed
+- `OnRemove` hooks so that they are allowed to move entity's archetype even during deletion.
+
+## 0.8.0
+
 ### Added
 - `jecs.Exclusive` trait for making exclusive relationships.
 

addons/ob.luau

@@ -160,6 +160,10 @@ local function monitors_new<T...>(
 	end
 
 	local function removed(entity: jecs.Entity, component: jecs.Id)
+		local r = jecs.record(world, entity)
+		if not archetypes[r.archetype.id] then
+			return
+		end
 		local EcsOnRemove = jecs.OnRemove :: jecs.Id
 		if callback ~= nil then
 			callback(entity, EcsOnRemove)

benches/general.luau

@@ -199,6 +199,7 @@ do
 			end
 
 			local q = world:query(A, B, C, D)
+			q:archetypes()
 			START()
 			for id in q do
 			end

benches/query.luau

@@ -11,15 +11,22 @@ end
 local jecs = require("@jecs")
 local mirror = require("@mirror")
 
-type i53 = number
-
 do
 	TITLE(testkit.color.white_underline("Jecs query"))
-	local ecs = jecs.world()
+	local ecs = jecs.world() :: jecs.World
 	do
 		TITLE("one component in common")
 
-		local function view_bench(world: jecs.World, A: i53, B: i53, C: i53, D: i53, E: i53, F: i53, G: i53, H: i53)
+		local function view_bench(world: jecs.World,
+			A: jecs.Id,
+			B: jecs.Id,
+			C: jecs.Id,
+			D: jecs.Id,
+			E: jecs.Id,
+			F: jecs.Id,
+			G: jecs.Id,
+			H: jecs.Id
+		)
 			BENCH("1 component", function()
 				for _ in world:query(A) do
 				end
@@ -131,11 +138,21 @@ end
 
 do
 	TITLE(testkit.color.white_underline("Mirror query"))
-	local ecs = mirror.World.new()
+	local ecs = mirror.World.new() :: jecs.World
 	do
 		TITLE("one component in common")
 
-		local function view_bench(world: jecs.World, A: i53, B: i53, C: i53, D: i53, E: i53, F: i53, G: i53, H: i53)
+		local function view_bench(
+			world: mirror.World,
+			A: jecs.Id,
+			B: jecs.Id,
+			C: jecs.Id,
+			D: jecs.Id,
+			E: jecs.Id,
+			F: jecs.Id,
+			G: jecs.Id,
+			H: jecs.Id
+		)
 			BENCH("1 component", function()
 				for _ in world:query(A) do
 				end

docs/api/world.md

@@ -355,46 +355,9 @@ This operation is the same as calling:
 world:target(entity, jecs.ChildOf, 0)
 ```
 
-## contains
-
-Checks if an entity or component (id) exists in the world.
-
-```luau
-function World:contains(
-    entity: Entity,
-): boolean
-```
-
-Example:
-
-::: code-group
-
-```luau [luau]
-local entity = world:entity()
-print(world:contains(entity))
-print(world:contains(1))
-print(world:contains(2))
-
--- Outputs:
--- true
--- true
--- false
-```
-
-```ts [typescript]
-const entity = world.entity();
-print(world.contains(entity));
-print(world.contains(1));
-print(world.contains(2));
-
-// Outputs:
-// true
-// true
-// false
-```
-
 :::
 
+
 ## remove
 
 Removes a component (ID) from an entity
@@ -460,11 +423,11 @@ Example:
 
 ```luau [luau]
 local entity = world:entity()
-print(world:has(entity))
+print(world:contains(entity))
 
 world:delete(entity)
 
-print(world:has(entity))
+print(world:contains(entity))
 
 -- Outputs:
 -- true
@@ -601,7 +564,7 @@ print(retrievedParent === parent) // true
 
 ## contains
 
-Checks if an entity exists and is alive in the world.
+Checks if an entity or component (id) exists and is alive in the world.
 
 ```luau
 function World:contains(
@@ -640,12 +603,26 @@ function World:exists(
 
 ## cleanup
 
-Cleans up deleted entities and their associated data. This is automatically called by jecs, but can be called manually if needed.
+Cleans up empty archetypes.
 
 ```luau
 function World:cleanup(): void
 ```
 
+:::info
+It is recommended to profile the optimal interval you should cleanup because it varies completely from game to game.
+
+Here are a couple of reasons from Sander Mertens:
+	- some applications are memory constrained, so any wasted memory on empty
+	  archetypes has to get cleaned up
+	- many archetypes can get created during game startup but aren't used later
+	  on, so it would be wasteful to keep them around
+	- empty archetypes can slow queries down, especially if there are many more
+	  empty ones than non-empty ones
+	- if the total number of component permutations (/relationships) is too
+	  high, you have no choice but to periodically cleanup empty archetypes
+:::
+
 Example:
 
 ::: code-group

examples/luau/entities/basics.luau

@@ -1,15 +1,19 @@
 local jecs = require("@jecs")
-local world = jecs.World.new()
+local world = jecs.world()
 
-local Position = world:component()
+local Position = world:component() :: jecs.Id<vector>
 local Walking = world:component()
-local Name = world:component()
+local Name = world:component() :: jecs.Id<string>
+
+local function name(e: jecs.Entity): string
+	return assert(world:get(e, Name))
+end
 
 -- Create an entity with name Bob
 local bob = world:entity()
 
 -- The set operation finds or creates a component, and sets it.
-world:set(bob, Position, Vector3.new(10, 20, 30))
+world:set(bob, Position, vector.create(10, 20, 30))
 -- Name the entity Bob
 world:set(bob, Name, "Bob")
 -- The add operation adds a component without setting a value. This is
@@ -18,15 +22,16 @@ world:add(bob, Walking)
 
 -- Get the value for the Position component
 local pos = world:get(bob, Position)
-print(`\{{pos.X}, {pos.Y}, {pos.Z}\}`)
+assert(pos)
+print(`\{{pos.x}, {pos.y}, {pos.z}\}`)
 
 -- Overwrite the value of the Position component
-world:set(bob, Position, Vector3.new(40, 50, 60))
+world:set(bob, Position, vector.create(40, 50, 60))
 
 local alice = world:entity()
 -- Create another named entity
 world:set(alice, Name, "Alice")
-world:set(alice, Position, Vector3.new(10, 20, 30))
+world:set(alice, Position, vector.create(10, 20, 30))
 world:add(alice, Walking)
 
 -- Remove tag
@@ -34,7 +39,7 @@ world:remove(alice, Walking)
 
 -- Iterate all entities with Position
 for entity, p in world:query(Position) do
-	print(`{entity}: \{{p.X}, {p.Y}, {p.Z}\}`)
+	print(`{name(entity)}: \{{p.x}, {p.y}, {p.z}\}`)
 end
 
 -- Output:

examples/luau/queries/archetypes/targets.luau

@@ -0,0 +1,60 @@
+-- Using world:target is the recommended way to grab the target in a wildcard
+-- query. However the random access can add up in very hot paths. Accessing its
+-- column can drastically improve performance, especially when there are
+-- multiple adjacent pairs.
+
+local jecs = require("@jecs")
+local pair = jecs.pair
+local __ = jecs.Wildcard
+
+local world = jecs.world()
+
+local Likes = world:entity()
+local function name(e, name: string): string
+	if name then
+		world:set(e, jecs.Name, name)
+		return name
+	end
+	return assert(world:get(e, jecs.Name))
+end
+
+local e1 = world:entity()
+name(e1, "e1")
+local e2 = world:entity()
+name(e2, "e2")
+local e3 = world:entity()
+name(e3, "e3")
+
+world:add(e1, pair(Likes, e2))
+world:add(e1, pair(Likes, e3))
+
+local likes = jecs.component_record(world, pair(Likes, __))
+assert(likes)
+
+local likes_cr = likes.records
+local likes_counts = likes.counts
+
+local archetypes = world:query(pair(Likes, __)):archetypes()
+
+for _, archetype in archetypes do
+	local types = archetype.types
+
+	-- Get the starting index which is what the (R, *) alias is at
+	local wc = likes_cr[archetype.id]
+	local count = likes_counts[archetype.id]
+
+	local entities = archetype.entities
+	for i = #entities, 1, -1 do
+		-- It is generally a good idea to iterate backwards on arrays if you
+		-- need to delete the iterated entity to prevent iterator invalidation
+		local entity = entities[i]
+		for cr = wc, wc + count - 1 do
+			local person = jecs.pair_second(world, types[cr])
+			print(`entity ${entity} likes ${person}`)
+		end
+	end
+end
+
+-- Output:
+--  entity $273 likes $275
+--  entity $273 likes $274

examples/luau/queries/archetypes/visibility_cascades.luau

@@ -0,0 +1,44 @@
+-- To get the most out of performance, you can lift the inner loop of queries to
+-- the system in which you can do archetype-specific optimizations like finding
+-- the parent once per archetype rather than per entity.
+
+local jecs = require("@jecs")
+local pair = jecs.pair
+local ChildOf = jecs.ChildOf
+local __ = jecs.Wildcard
+
+local world = jecs.world()
+
+local Position = world:component() :: jecs.Id<vector>
+local Visible = world:entity()
+
+local parent = world:entity()
+world:set(parent, Position, vector.zero)
+world:add(parent, Visible)
+
+local child = world:entity()
+world:set(child, Position, vector.one)
+world:add(child, pair(ChildOf, parent))
+
+local parents = jecs.component_record(world, pair(ChildOf, __))
+assert(parents)
+
+local parent_cr = parents.records
+
+local archetypes = world:query(Position, pair(ChildOf, __)):archetypes()
+
+for _, archetype in archetypes do
+	local types = archetype.types
+	local p = jecs.pair_second(world, types[parent_cr[archetype.id]])
+	if world:has(p, Visible) then
+		local columns = archetype.columns_map
+		local positions = columns[Position]
+	  	for row, entity in archetype.entities do
+			local pos = positions[row]
+			print(`Child ${entity} of ${p} is visible at {pos}`)
+		end
+	end
+end
+
+-- Output:
+--  Child $274 of $273 is visibile at 1,1,1

examples/luau/queries/basics.luau

@@ -1,5 +1,5 @@
 local jecs = require("@jecs")
-local world = jecs.World.new()
+local world = jecs.world()
 
 local Position = world:component()
 local Velocity = world:component()

examples/luau/queries/changetracking.luau

@@ -1,19 +1,16 @@
 local jecs = require("@jecs")
 local pair = jecs.pair
 
-local world = jecs.World.new()
+local world = jecs.world()
-local Name = world:component()
+local Name = world:component() :: jecs.Id<string>
 
-local function named(ctr, name)
+
-	local e = ctr(world)
+local function name(e: jecs.Entity): string
-	world:set(e, Name, name)
+	return assert(world:get(e, Name))
-	return e
-end
-local function name(e)
-	return world:get(e, Name)
 end
 
-local Position = named(world.component, "Position") :: jecs.Entity<vector>
+local Position = world:component() :: jecs.Id<vector>
+world:set(Position, Name, "Position")
 local Previous = jecs.Rest
 
 local added = world
@@ -29,10 +26,14 @@ local removed = world
 	:cached()
 
 
-local e1 = named(world.entity, "e1")
+local e1 = world:entity()
+world:set(e1, Name, "e1")
 world:set(e1, Position, vector.create(10, 20, 30))
-local e2 = named(world.entity, "e2")
+
+local e2 = world:entity()
+world:set(e2, Name, "e2")
 world:set(e2, Position, vector.create(10, 20, 30))
+
 for entity, p in added do
 	print(`Added {name(entity)}: \{{p.x}, {p.y}, {p.z}}`)
 	world:set(entity, pair(Previous, Position), p)
@@ -40,10 +41,10 @@ end
 
 world:set(e1, Position, vector.create(999, 999, 1998))
 
-for _, archetype in changed:archetypes() do
+for entity, new, old in changed do
 	if new ~= old then
-		print(`{name(e)}'s Position changed from \{{old.x}, {old.y}, {old.z}\} to \{{new.x}, {new.y}, {new.z}\}`)
+		print(`{name(entity)}'s Position changed from \{{old.x}, {old.y}, {old.z}\} to \{{new.x}, {new.y}, {new.z}\}`)
-		world:set(e, pair(Previous, Position), new)
+		world:set(entity, pair(Previous, Position), new)
 	end
 end
 

examples/luau/queries/spatial_grids.luau

@@ -3,32 +3,47 @@ local pair = jecs.pair
 local ChildOf = jecs.ChildOf
 local __ = jecs.Wildcard
 local Name = jecs.Name
-local world = jecs.World.new()
+local world = jecs.world()
 
-type Id<T = nil> = number & { __T: T }
+local Voxel = world:component() :: jecs.Id
-local Voxel = world:component() :: Id
+local Position = world:component() :: jecs.Id<vector>
-local Position = world:component() :: Id<Vector3>
+local Perception = world:component() :: jecs.Id<{
-local Perception = world:component() :: Id<{
 	range: number,
 	fov: number,
-	dir: Vector3,
+	dir: vector,
 }>
-local PrimaryPart = world:component() :: Id<Part>
+type part = {
+	Position: vector
+}
+local PrimaryPart = world:component() :: jecs.Id<part>
 
-local local_player = game:GetService("Players").LocalPlayer
+local local_player = {
+	Character = {
+		PrimaryPart = {
+			Position = vector.create(50, 0, 30)
+		}
+	}
+}
+local workspace = {
+	CurrentCamera = {
+		CFrame = {
+			LookVector = vector.create(0, 0, -1)
+		}
+	}
+}
 
-local function distance(a: Vector3, b: Vector3)
+local function distance(a: vector, b: vector)
-	return (b - a).Magnitude
+	return vector.magnitude((b - a))
 end
 
-local function is_in_fov(a: Vector3, b: Vector3, forward_dir: Vector3, fov_angle: number)
+local function is_in_fov(a: vector, b: vector, forward_dir: vector, fov_angle: number)
 	local to_target = b - a
 
-	local forward_xz = Vector3.new(forward_dir.X, 0, forward_dir.Z).Unit
+	local forward_xz = vector.normalize(vector.create(forward_dir.x, 0, forward_dir.z))
-	local to_target_xz = Vector3.new(to_target.X, 0, to_target.Z).Unit
+	local to_target_xz = vector.normalize(vector.create(to_target.x, 0, to_target.z))
 
-	local angle_to_target = math.deg(math.atan2(to_target_xz.Z, to_target_xz.X))
+	local angle_to_target = math.deg(math.atan2(to_target_xz.z, to_target_xz.x))
-	local forward_angle = math.deg(math.atan2(forward_xz.Z, forward_xz.X))
+	local forward_angle = math.deg(math.atan2(forward_xz.z, forward_xz.z))
 
 	local angle_difference = math.abs(forward_angle - angle_to_target)
 
@@ -42,7 +57,7 @@ end
 local map = {}
 local grid = 50
 
-local function add_to_voxel(source: number, position: Vector3, prev_voxel_id: number?)
+local function add_to_voxel(source: jecs.Entity, position: vector, prev_voxel_id: jecs.Entity?)
 	local hash = position // grid
 	local voxel_id = map[hash]
 	if not voxel_id then
@@ -79,7 +94,7 @@ local function update_camera_direction(dt: number)
 end
 
 local function perceive_enemies(dt: number)
-	local it = world:query(Perception, Position, PrimaryPart)
+	local it = world:query(Perception, Position, PrimaryPart):iter()
 	-- There is only going to be one entity matching the query
 	local e, self_perception, self_position, self_primary_part = it()
 
@@ -93,28 +108,28 @@ local function perceive_enemies(dt: number)
 
 		if is_in_fov(self_position, target_position, self_perception.dir, self_perception.fov) then
 			local p = target_position
-			print(`Entity {world:get(e, Name)} can see target {world:get(enemy, Name)} at ({p.X}, {p.Y}, {p.Z})`)
+			print(`Entity {world:get(e, Name)} can see target {world:get(enemy, Name)} at ({p.x}, {p.y}, {p.z})`)
 		end
 	end
 end
 
 local player = world:entity()
 world:set(player, Perception, {
-	range = 100,
+	range = 200,
 	fov = 90,
-	dir = Vector3.new(1, 0, 0),
+	dir = vector.create(1, 0, 0),
 })
 world:set(player, Name, "LocalPlayer")
-local primary_part = (local_player.Character :: Model).PrimaryPart :: Part
+local primary_part = local_player.Character.PrimaryPart
 world:set(player, PrimaryPart, primary_part)
-world:set(player, Position, Vector3.zero)
+world:set(player, Position, vector.zero)
 
 local enemy = world:entity()
 world:set(enemy, Name, "Enemy $1")
-world:set(enemy, Position, Vector3.new(50, 0, 20))
+world:set(enemy, Position, vector.create(50, 0, 20))
 
 add_to_voxel(player, primary_part.Position)
-add_to_voxel(enemy, world)
+add_to_voxel(enemy, assert(world:get(enemy, Position)))
 
 local dt = 1 / 60
 reconcile_client_owned_assembly_to_voxel(dt)

jecs.d.ts

@@ -7,10 +7,14 @@ export type Entity<TData = unknown> = number & {
 	readonly __type_TData: TData;
 };
 
+type TagDiscriminator = {
+	readonly __nominal_Tag: unique symbol;
+};
+
 /**
  * An entity with no associated data when used as a component
  */
-export type Tag = Entity<undefined>;
+export type Tag = Entity<TagDiscriminator>;
 
 /**
  * A pair of entities:
@@ -26,12 +30,12 @@ export type Pair<P = unknown, O = unknown> = number & {
  * An `Id` can be either a single Entity or a Pair of Entities.
  * By providing `TData`, you can specifically require an Id that yields that type.
  */
-export type Id<TData = unknown> = Entity<TData> | Pair<TData, unknown> | Pair<undefined, TData>;
+export type Id<TData = unknown> = Entity<TData> | Pair<TData, unknown> | Pair<TagDiscriminator, TData>;
 
 export type InferComponent<E> = E extends Entity<infer D>
 	? D
 	: E extends Pair<infer P, infer O>
-	? P extends undefined
+	? P extends TagDiscriminator
 		? O
 		: P
 	: never;
@@ -43,22 +47,30 @@ type InferComponents<A extends Id[]> = { [K in keyof A]: InferComponent<A[K]> };
 type ArchetypeId = number;
 export type Column<T> = T[];
 
-export type Archetype<T extends unknown[]> = {
+export type Archetype<T extends Id[]> = {
 	id: number;
-	types: number[];
+	types: Entity[];
 	type: string;
-	entities: number[];
+	entities: Entity[];
 	columns: Column<unknown>[];
-	columns_map: Record<Id, Column<T[number]>>
+	columns_map: { [K in T[number]]: Column<InferComponent<K>> };
 };
 
-type Iter<T extends unknown[]> = IterableFunction<LuaTuple<[Entity, ...T]>>;
+type IterFn<T extends Id[]> = IterableFunction<LuaTuple<[Entity, ...InferComponents<T>]>>;
+type Iter<T extends Id[]> = IterFn<T> & {
+	/**
+	 * This isn't callable
+	 * @hidden
+	 * @deprecated
+	 */
+	(): never
+};
 
-export type CachedQuery<T extends unknown[]> = {
+export type CachedQuery<T extends Id[]> = {
 	/**
 	 * Returns an iterator that produces a tuple of [Entity, ...queriedComponents].
 	 */
-	iter(): Iter<T>;
+	iter(): IterFn<T>;
 
 	/**
 	 * Returns the matched archetypes of the query
@@ -67,11 +79,11 @@ export type CachedQuery<T extends unknown[]> = {
 	archetypes(): Archetype<T>[];
 } & Iter<T>;
 
-export type Query<T extends unknown[]> = {
+export type Query<T extends Id[]> = {
 	/**
 	 * Returns an iterator that produces a tuple of [Entity, ...queriedComponents].
 	 */
-	iter(): Iter<T>;
+	iter(): IterFn<T>;
 
 	/**
 	 * Creates and returns a cached version of this query for efficient reuse.
@@ -119,7 +131,7 @@ export class World {
 	 * @returns An entity (Tag) with no data.
 	 */
 	entity(): Tag;
-	entity<T extends Entity>(id: T): InferComponent<T> extends undefined ? Tag : T;
+	entity<T extends Entity>(id: T): T;
 
 	/**
 	 * Creates a new entity in the first 256 IDs, typically used for static
@@ -148,7 +160,7 @@ export class World {
 	 * @param entity The target entity.
 	 * @param component The component (or tag) to add.
 	 */
-	add<C>(entity: Entity, component: undefined extends InferComponent<C> ? C : Id<undefined>): void;
+	add<C>(entity: Entity, component: TagDiscriminator extends InferComponent<C> ? C : Id<TagDiscriminator>): void;
 
 	/**
 	 * Installs a hook on the given component.
@@ -172,6 +184,11 @@ export class World {
 	 */
 	cleanup(): void;
 
+	/**
+	 * Removes all instances of specified component
+	 */
+	// purge<T>(component: Id<T>): void
+
 	/**
 	 * Clears all components and relationships from the given entity, but
 	 * does not delete the entity from the world.
@@ -246,11 +263,11 @@ export class World {
 	 * @param components The list of components to query.
 	 * @returns A Query object to iterate over results.
 	 */
-	query<T extends Id[]>(...components: T): Query<InferComponents<T>>;
+	query<T extends Id[]>(...components: T): Query<T>;
 
-	added<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>, value: T) => void): () => void
+	added<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>, value: T) => void): () => void;
-	changed<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>, value: T) => void): () => void
+	changed<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>, value: T) => void): () => void;
-	removed<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>) => void): () => void
+	removed<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>) => void): () => void;
 }
 
 export function world(): World;
@@ -297,11 +314,11 @@ export function ECS_PAIR_FIRST(pair: Pair): number;
 export function ECS_PAIR_SECOND(pair: Pair): number;
 
 type StatefulHook = Entity<<T>(e: Entity<T>, id: Id<T>, data: T) => void> & {
-	readonly __nominal_StatefulHook: unique symbol,
+	readonly __nominal_StatefulHook: unique symbol;
-}
+};
 type StatelessHook = Entity<<T>(e: Entity<T>, id: Id<T>) => void> & {
-	readonly __nominal_StatelessHook: unique symbol,
+	readonly __nominal_StatelessHook: unique symbol;
-}
+};
 
 export declare const OnAdd: StatefulHook;
 export declare const OnRemove: StatelessHook;
@@ -319,12 +336,27 @@ export declare const Exclusive: Tag;
 export declare const Rest: Entity;
 
 export type ComponentRecord = {
-	records: Map<Id, number>,
+	records: Map<Id, number>;
-	counts: Map<Id, number>,
+	counts: Map<Id, number>;
-	size: number,
+	size: number;
-}
+};
 
-export function component_record(world: World, id: Id): ComponentRecord
+export function component_record(world: World, id: Id): ComponentRecord;
 
-export function bulk_insert<const C extends Id[]>(world: World, entity: Entity, ids: C, values: InferComponents<C>): void
+type TagToUndefined<T> = T extends TagDiscriminator ? undefined : T
-export function bulk_remove(world: World, entity: Entity, ids: Id[]): void
+
+export function bulk_insert<const C extends Id[]>(
+	world: World,
+	entity: Entity,
+	ids: C,
+	values: { [K in keyof C]: TagToUndefined<InferComponent<C[K]>> },
+): void;
+export function bulk_remove(world: World, entity: Entity, ids: Id[]): void;
+
+export type EntityRecord<T extends Id[]> = {
+	archetype: Archetype<T>,
+	row: number,
+	dense: number,
+};
+
+export function record<T extends Id[] = []>(world: World, entity: Entity): EntityRecord<T>;

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "@rbxts/jecs",
-	"version": "0.9.0-rc.7",
+	"version": "0.9.0-rc.12",
 	"description": "Stupidly fast Entity Component System",
 	"main": "jecs.luau",
 	"repository": {

wally.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ukendio/jecs"
-version = "0.9.0-rc.7"
+version = "0.9.0-rc.12"
 registry = "https://github.com/UpliftGames/wally-index"
 realm = "shared"
 license = "MIT"

werewitch.luau

@@ -1,66 +0,0 @@
-local jecs = require("@jecs")
-
-local world = jecs.world()
-local pair = jecs.pair
-
-local IsA = world:entity()
-
-local traits = {
-	IsA = IsA
-}
-
-world:set(IsA, jecs.OnAdd, function(component, id)
-	local second = jecs.pair_second(world, id)
-	assert(second ~= component, "circular")
-
-	local is_tag = jecs.is_tag(world, second)
-	world:added(component, function(entity, _, value)
-		if is_tag then
-			world:add(entity, second)
-		else
-			world:set(entity, second, value)
-		end
-	end)
-
-	world:removed(component, function(entity)
-		world:remove(entity, second)
-	end)
-
-	if not is_tag then
-		world:changed(component, function(entity, _, value)
-			world:set(entity, second, value)
-		end)
-	end
-
-	local r = jecs.record(world, second) :: jecs.Record
-	local archetype = r.archetype
-	if not archetype then
-		return
-	end
-	local types = archetype.types
-
-	for _, id in types do
-		if jecs.is_tag(world, id) then
-			world:add(component, id)
-		else
-			local metadata = world:get(second, id)
-			if not world:has(component, id) then
-				world:set(component, id, metadata)
-			end
-		end
-	end
-
-	-- jecs.bulk_insert(world, component, ids, values)
-end)
-
-local Witch = world:entity()
-local Werewolf = world:entity()
-
-local WereWitch = world:entity()
-world:add(WereWitch, pair(IsA, Witch))
-world:add(WereWitch, pair(IsA, Werewolf))
-
-local e = world:entity()
-world:add(e, WereWitch)
-print(world:has(e, pair(IsA, Witch))) -- false
-print(world:has(e, Witch)) -- true