Implement world:targets() as a valid method

src/jecs.luau

@@ -194,6 +194,7 @@ type world = {
 	entity: (self: world, id: i53?) -> i53,
 	component: (self: world) -> i53,
 	target: (self: world, id: i53, relation: i53, index: number?) -> i53?,
+	targets: (self: world, id: i53, relation: i53) -> () -> i53?,
 	delete: (self: world, id: i53) -> (),
 	add: (self: world, id: i53, component: i53) -> (),
 	set: (self: world, id: i53, component: i53, data: any) -> (),
@@ -253,6 +254,13 @@ export type World = {
 		index: number?
 	) -> Entity<unknown>?,
 
+	-- Gets an iterator for all viable targets of a relationship
+	targets: <T>(
+		self: World,
+		id: Entity<T> | number,
+		relation: ecs_entity_t<Component>
+	) -> () -> Id,
+
 	--- Deletes an entity and all it's related components and relationships.
 	delete: <T>(self: World, id: Entity<T>) -> (),
 
@@ -3302,6 +3310,50 @@ local function world_new(DEBUG: boolean?)
 		    ECS_PAIR_SECOND(nth))
 	end
 
+	local NOOP = NOOP :: () -> i53
+
+	local function world_targets(world: world, entity: i53, relation: i53): () -> i53?
+		local record = entity_index_try_get_unsafe(entity)
+		if not record then
+			return NOOP
+		end
+
+		local archetype = record.archetype
+		if not archetype then
+			return NOOP
+		end
+
+		local r = ECS_PAIR(relation, EcsWildcard)
+		local ct_idx = world.component_index
+		local idr = ct_idx[r]
+
+		if not idr then
+			return NOOP
+		end
+
+		local archetype_id = archetype.id
+		local count = idr.counts[archetype_id]
+		if not count then
+			return NOOP
+		end
+
+		local nth = idr.records[archetype_id]
+		local end_count = nth + count
+
+		local archetype_types = archetype.types
+		local sparse_array = entity_index.sparse_array
+		local dense_array = entity_index.dense_array
+
+		return function(): i53?
+			if nth == end_count then
+				return nil
+			end
+			local target = dense_array[sparse_array[ECS_PAIR_SECOND(archetype_types[nth])].dense]
+			nth += 1
+			return target
+		end
+	end
+
 	local function world_parent(world: world, entity: i53): i53?
 		return world_target(world, entity, EcsChildOf, 0)
 	end
@@ -3730,6 +3782,7 @@ local function world_new(DEBUG: boolean?)
 	world.get = world_get :: any
 	world.has = world_has :: any
 	world.target = world_target
+	world.targets = world_targets
 	world.parent = world_parent
 	world.contains = world_contains
 	world.exists = world_exists

test/tests.luau

@@ -3077,7 +3077,7 @@ TEST("world:set()", function()
 	end
 end)
 
-TEST("world:target", function()
+TEST("world:target()", function()
 	do CASE "nth index"
 		local world = jecs.world()
 		local A = world:component()
@@ -3178,6 +3178,130 @@ TEST("world:target", function()
 	end
 end)
 
+TEST("world:targets()", function()
+	do CASE "should find single relation"
+		local world = jecs.world()
+
+		local Alice = world:entity()
+		local Bob = world:entity()
+		local Likes = world:entity()
+
+		world:add(Alice, jecs.pair(Likes, Bob))
+
+		local i = 0
+		for target in world:targets(Alice, Likes) do
+			i += 1
+			CHECK(target == Bob)
+		end
+		CHECK(i == 1)
+	end
+	do CASE "basic iteration"
+		local world = jecs.world()
+
+		local ROOT = world:entity()
+		local e1 = world:entity()
+		local targets = {}
+
+		for i = 1, 10 do
+			local target = world:entity()
+			targets[i] = target
+			world:add(e1, pair(ROOT, target))
+		end
+
+		local i = 0
+		for target in world:targets(e1, ROOT) do
+			i += 1
+			CHECK(targets[i] == target)
+		end
+
+		CHECK(i == 10)
+	end
+	do CASE "multiple iterations"
+		local world = jecs.world()
+
+		local ROOT = world:entity()
+		local OTHER = world:entity()
+
+		local e = world:entity()
+
+		local root_targets = {}
+		local other_targets = {}
+
+		for i = 1, 5 do
+			local t = world:entity()
+			root_targets[i] = t
+			world:add(e, pair(ROOT, t))
+		end
+
+		for i = 1, 3 do
+			local t = world:entity()
+			other_targets[i] = t
+			world:add(e, pair(OTHER, t))
+		end
+
+		local i = 0
+		for target in world:targets(e, ROOT) do
+			i += 1
+			CHECK(root_targets[i] == target)
+		end
+		CHECK(i == 5)
+
+		local j = 0
+		for target in world:targets(e, OTHER) do
+			j += 1
+			CHECK(other_targets[j] == target)
+		end
+		CHECK(j == 3)
+	end
+	do CASE "empty iterator"
+		local world = jecs.world()
+
+		local ROOT = world:entity()
+		local OTHER = world:entity()
+
+		local e = world:entity()
+
+		world:add(e, pair(ROOT, world:entity()))
+
+		local count = 0
+		for _ in world:targets(e, OTHER) do
+			count += 1
+		end
+
+		CHECK(count == 0)
+	end
+	do CASE "should ignore deleted targets"
+		local world = jecs.world()
+
+		local ROOT = world:entity()
+		local e = world:entity()
+
+		local alive = {}
+		local dead = {}
+
+		for i = 1, 3 do
+			local t = world:entity()
+			alive[#alive + 1] = t
+			world:add(e, pair(ROOT, t))
+		end
+
+		for i = 1, 2 do
+			local t = world:entity()
+			dead[#dead + 1] = t
+			world:add(e, pair(ROOT, t))
+			world:delete(t)
+		end
+
+		local count = 0
+		for t in world:targets(e, ROOT) do
+			count += 1
+			CHECK(t ~= nil)
+		end
+
+		CHECK(count == #alive)
+	end
+end)
+
 TEST("#adding a recycled target", function()
     local world = jecs.world()
     local R = world:component()