diff --git a/src/init.luau b/src/init.luau
index 1eb67d29..a97d3696 100644
--- a/src/init.luau
+++ b/src/init.luau
@@ -148,7 +148,7 @@ local function ECS_GENERATION_INC(e: i53)
 			return id
 		end
 
-		return ECS_COMBINE(id, next_gen) + flags
+		return ECS_COMBINE(id, next_gen)
 	end
 	return ECS_COMBINE(e, 1)
 end
@@ -1683,13 +1683,13 @@ function World.new()
 	self.ROOT_ARCHETYPE = archetype_create(self, {}, "")
 
 	for i = 1, HI_COMPONENT_ID do
-		local e = entity_index_new_id(entity_index, i)
+		local e = entity_index_new_id(entity_index)
 		world_add(self, e, EcsComponent)
 	end
 
 	for i = HI_COMPONENT_ID + 1, EcsRest do
 		-- Initialize built-in components
-		entity_index_new_id(entity_index, i)
+		entity_index_new_id(entity_index)
 	end
 
 	world_add(self, EcsName, EcsComponent)
@@ -1885,4 +1885,6 @@ return {
 	entity_index_try_get = entity_index_try_get,
 	entity_index_try_get_any = entity_index_try_get_any,
 	entity_index_is_alive = entity_index_is_alive,
+	entity_index_remove = entity_index_remove,
+	entity_index_new_id = entity_index_new_id
 }
diff --git a/test/gen.luau b/test/gen.luau
new file mode 100644
index 00000000..19d1244f
--- /dev/null
+++ b/test/gen.luau
@@ -0,0 +1,192 @@
+type i53 = number
+type i24 = number
+
+type Ty = { i53 }
+type ArchetypeId = number
+
+type Column = { any }
+
+type Map<K, V> = { [K]: V }
+
+type GraphEdge = {
+	from: Archetype,
+	to: Archetype?,
+	prev: GraphEdge?,
+	next: GraphEdge?,
+	id: number,
+}
+
+type GraphEdges = Map<i53, GraphEdge>
+
+type GraphNode = {
+	add: GraphEdges,
+	remove: GraphEdges,
+	refs: GraphEdge,
+}
+
+type ArchetypeRecord = {
+	count: number,
+	column: number,
+}
+
+export type Archetype = {
+	id: number,
+	node: GraphNode,
+	types: Ty,
+	type: string,
+	entities: { number },
+	columns: { Column },
+	records: { ArchetypeRecord },
+}
+type Record = {
+	archetype: Archetype,
+	row: number,
+	dense: i24,
+}
+
+type EntityIndex = {
+	dense_array: Map<i24, i53>,
+	sparse_array: Map<i53, Record>,
+	sparse_count: number,
+	alive_count: number,
+	max_id: number
+}
+
+
+local ECS_PAIR_FLAG =                      0x8
+local ECS_ID_FLAGS_MASK =                 0x10
+local ECS_ENTITY_MASK =     bit32.lshift(1, 24)
+local ECS_GENERATION_MASK = bit32.lshift(1, 16)
+
+-- HIGH 24 bits LOW 24 bits
+local function ECS_GENERATION(e: i53): i24
+	return if e > ECS_ENTITY_MASK then (e // ECS_ID_FLAGS_MASK) % ECS_GENERATION_MASK else 0
+end
+
+local function ECS_COMBINE(source: number, target: number): i53
+	return (source * 268435456) + (target * ECS_ID_FLAGS_MASK)
+end
+
+local function ECS_GENERATION_INC(e: i53)
+	if e > ECS_ENTITY_MASK then
+		local flags = e // ECS_ID_FLAGS_MASK
+		local id = flags // ECS_ENTITY_MASK
+		local generation = flags % ECS_GENERATION_MASK
+
+		local next_gen = generation + 1
+		if next_gen > ECS_GENERATION_MASK then
+			return id
+		end
+
+		return ECS_COMBINE(id, next_gen) + flags
+	end
+	return ECS_COMBINE(e, 1)
+end
+local function ECS_ENTITY_T_LO(e: i53): i24
+	return if e > ECS_ENTITY_MASK then (e // ECS_ID_FLAGS_MASK) // ECS_ENTITY_MASK else e
+end
+
+
+local function entity_index_try_get_any(entity_index: EntityIndex, entity: number): Record?
+	local r = entity_index.sparse_array[ECS_ENTITY_T_LO(entity)]
+	if not r then
+		return nil
+	end
+
+	if not r or r.dense == 0 then
+		return nil
+	end
+
+	return r
+end
+
+local function entity_index_try_get(entity_index: EntityIndex, entity: number): Record?
+	local r = entity_index_try_get_any(entity_index, entity)
+	if r then
+		local r_dense = r.dense
+		if r_dense > entity_index.alive_count then
+			return nil
+		end
+		if entity_index.dense_array[r_dense] ~= entity then
+			return nil
+		end
+	end
+	return r
+end
+
+local function entity_index_get_alive(entity_index: EntityIndex,
+	entity: number): number
+
+	local r = entity_index_try_get_any(entity_index, entity)
+	if r then
+		return entity_index.dense_array[r.dense]
+	end
+	return 0
+end
+
+local function entity_index_remove(entity_index: EntityIndex, entity: number)
+	local r = entity_index_try_get(entity_index, entity)
+	if not r then
+		return
+	end
+	local dense_array = entity_index.dense_array
+	local index_of_deleted_entity = r.dense
+	local last_entity_alive_at_index = entity_index.alive_count
+	entity_index.alive_count -= 1
+
+	local last_alive_entity = dense_array[last_entity_alive_at_index]
+	local r_swap = entity_index_try_get_any(
+		entity_index, last_alive_entity) :: Record
+	r_swap.dense = index_of_deleted_entity
+	r.archetype = nil :: any
+	r.row = nil :: any
+	r.dense = last_entity_alive_at_index
+
+	dense_array[index_of_deleted_entity] = last_alive_entity
+	dense_array[last_entity_alive_at_index] = ECS_GENERATION_INC(entity)
+end
+
+local function entity_index_new_id(entity_index: EntityIndex, data): i53
+	local dense_array = entity_index.dense_array
+	if entity_index.alive_count ~= #dense_array then
+		entity_index.alive_count += 1
+		local id = dense_array[entity_index.alive_count]
+		return id
+	end
+	entity_index.max_id +=1
+	local id = entity_index.max_id
+	entity_index.alive_count += 1
+
+	dense_array[entity_index.alive_count] = id
+	entity_index.sparse_array[id] = {
+		dense = entity_index.alive_count,
+		archetype = data
+	} :: Record
+
+	entity_index.sparse_count += 1
+
+	return id
+end
+
+local function entity_index_is_alive(entity_index: EntityIndex, entity: number)
+	return entity_index_try_get(entity_index, entity) ~= nil
+end
+
+local eidx = {
+	alive_count = 0,
+	max_id = 0,
+	sparse_array = {} :: { Record },
+	sparse_count = 0,
+	dense_array = {} :: { i53 }
+}
+local e1v0 = entity_index_new_id(eidx, "e1v0")
+local e2v0 = entity_index_new_id(eidx, "e2v0")
+local e3v0 = entity_index_new_id(eidx, "e3v0")
+local e4v0 = entity_index_new_id(eidx, "e4v0")
+local e5v0 = entity_index_new_id(eidx, "e5v0")
+local t = require("@testkit")
+local tprint = t.print
+entity_index_remove(eidx, e5v0)
+local e5v1 = entity_index_new_id(eidx, "e5v1")
+entity_index_remove(eidx, e2v0)
+tprint(eidx)
diff --git a/test/lol.luau b/test/lol.luau
new file mode 100644
index 00000000..cbf50dd2
--- /dev/null
+++ b/test/lol.luau
@@ -0,0 +1,157 @@
+local c = {
+	white_underline = function(s: any)
+		return `\27[1;4m{s}\27[0m`
+	end,
+
+	white = function(s: any)
+		return `\27[37;1m{s}\27[0m`
+	end,
+
+	green = function(s: any)
+		return `\27[32;1m{s}\27[0m`
+	end,
+
+	red = function(s: any)
+		return `\27[31;1m{s}\27[0m`
+	end,
+
+	yellow = function(s: any)
+		return `\27[33;1m{s}\27[0m`
+	end,
+
+	red_highlight = function(s: any)
+		return `\27[41;1;30m{s}\27[0m`
+	end,
+
+	green_highlight = function(s: any)
+		return `\27[42;1;30m{s}\27[0m`
+	end,
+
+	gray = function(s: any)
+		return `\27[30;1m{s}\27[0m`
+	end,
+}
+
+
+local ECS_PAIR_FLAG =                      0x8
+local ECS_ID_FLAGS_MASK =                 0x10
+local ECS_ENTITY_MASK =     bit32.lshift(1, 24)
+local ECS_GENERATION_MASK = bit32.lshift(1, 16)
+
+type i53 = number
+type i24 = number
+
+local function ECS_ENTITY_T_LO(e: i53): i24
+	return if e > ECS_ENTITY_MASK then (e // ECS_ID_FLAGS_MASK) // ECS_ENTITY_MASK else e
+end
+
+local function ECS_GENERATION(e: i53): i24
+	return if e > ECS_ENTITY_MASK then (e // ECS_ID_FLAGS_MASK) % ECS_GENERATION_MASK else 0
+end
+
+local ECS_ID = ECS_ENTITY_T_LO
+
+local function ECS_COMBINE(source: number, target: number): i53
+	return (source * 268435456) + (target * ECS_ID_FLAGS_MASK)
+end
+
+local function ECS_GENERATION_INC(e: i53)
+	if e > ECS_ENTITY_MASK then
+		local flags = e // ECS_ID_FLAGS_MASK
+		local id = flags // ECS_ENTITY_MASK
+		local generation = flags % ECS_GENERATION_MASK
+
+		local next_gen = generation + 1
+		if next_gen > ECS_GENERATION_MASK then
+			return id
+		end
+
+		return ECS_COMBINE(id, next_gen) + flags
+	end
+	return ECS_COMBINE(e, 1)
+end
+
+local function bl()
+	print("")
+end
+
+local function pe(e)
+	local gen = ECS_GENERATION(e)
+	return c.green(`e{ECS_ID(e)}`)..c.yellow(`v{gen}`)
+end
+
+local function dprint(tbl: {  [number]: number })
+	bl()
+	print("--------")
+	for i, e in tbl do
+		print("| "..pe(e).." |")
+		print("--------")
+	end
+	bl()
+end
+
+local max_id = 0
+local alive_count = 0
+local dense = {}
+local sparse = {}
+local function alloc()
+	if alive_count ~= #dense then
+		alive_count += 1
+		print("*recycled", pe(dense[alive_count]))
+		return dense[alive_count]
+	end
+	max_id += 1
+	local id = max_id
+	alive_count += 1
+	dense[alive_count] = id
+	sparse[id] = {
+		dense = alive_count
+	}
+	print("*allocated", pe(id))
+	return id
+end
+
+local function remove(entity)
+	local id = ECS_ID(entity)
+	local r = sparse[id]
+	local index_of_deleted_entity = r.dense
+	local last_entity_alive_at_index = alive_count -- last entity alive
+	alive_count -= 1
+	local last_alive_entity = dense[last_entity_alive_at_index]
+	local r_swap = sparse[ECS_ID(last_alive_entity)]
+	r_swap.dense = r.dense
+	r.dense = last_entity_alive_at_index
+	dense[index_of_deleted_entity] = last_alive_entity
+	dense[last_entity_alive_at_index] = ECS_GENERATION_INC(entity)
+end
+
+local function alive(e)
+	local r = sparse[ECS_ID(e)]
+
+	return dense[r.dense] == e
+end
+
+local function pa(e)
+	print(`{pe(e)} is {if alive(e) then "alive" else "not alive"}`)
+end
+
+local tprint = require("@testkit").print
+local e1v0 = alloc()
+local e2v0 = alloc()
+local e3v0 = alloc()
+local e4v0 = alloc()
+local e5v0 = alloc()
+pa(e1v0)
+pa(e4v0)
+remove(e5v0)
+pa(e5v0)
+
+local e5v1 = alloc()
+pa(e5v0)
+pa(e5v1)
+pa(e2v0)
+print(ECS_ID(e2v0))
+
+dprint(dense)
+remove(e2v0)
+dprint(dense)
diff --git a/test/tests.luau b/test/tests.luau
index ee02770a..efa86cd9 100644
--- a/test/tests.luau
+++ b/test/tests.luau
@@ -191,6 +191,36 @@ TEST("world:entity()", function()
 		CHECK(ecs_pair_first(world, pair) == e2)
 		CHECK(ecs_pair_second(world, pair) == e3)
 	end
+
+	do CASE "Recycling"
+		local world = world_new()
+		local e = world:entity()
+		world:delete(e)
+		local e1 = world:entity()
+		world:delete(e1)
+		local e2 = world:entity()
+		CHECK(ECS_ID(e2) == e)
+		CHECK(ECS_GENERATION(e2) == 2)
+		CHECK(world:contains(e2))
+		CHECK(not world:contains(e1))
+		CHECK(not world:contains(e))
+	end
+
+	do CASE "Recycling max generation"
+		local world = world_new()
+		local pin = jecs.Rest + 1
+		for i = 1, 2^16-1 do
+			local e = world:entity()
+			world:delete(e)
+		end
+		local e = world:entity()
+		CHECK(ECS_ID(e) == pin)
+		CHECK(ECS_GENERATION(e) == 2^16-1)
+		world:delete(e)
+		e = world:entity()
+		CHECK(ECS_ID(e) == pin)
+		CHECK(ECS_GENERATION(e) == 0)
+	end
 end)
 
 TEST("world:set()", function()