From 4db44476a978c86e1be5bc82c286b3aa4123741d Mon Sep 17 00:00:00 2001
From: Ukendio <ukendio@gmail.com>
Date: Tue, 9 Dec 2025 20:34:05 +0100
Subject: [PATCH] Only delete archetypes when completely invalidated

---
 src/jecs.luau   | 29 ++++++++++-----------
 test/tests.luau | 67 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+), 14 deletions(-)

diff --git a/src/jecs.luau b/src/jecs.luau
index 4e4b266..25dec1c 100755
--- a/src/jecs.luau
+++ b/src/jecs.luau
@@ -3084,17 +3084,6 @@ local function world_new()
 
 		local archetype = record.archetype
 
-		if archetype then
-			for _, id in archetype.types do
-				local idr = component_index[id]
-				local on_remove = idr.on_remove
-				if on_remove then
-					on_remove(entity, id, true)
-				end
-			end
-			archetype_delete(world, record.archetype, record.row)
-		end
-
 		local component_index = world.component_index
 		local archetypes = world.archetypes
 		local tgt = ECS_PAIR(EcsWildcard, entity)
@@ -3163,6 +3152,7 @@ local function world_new()
 				local idr_t_archetype = archetypes[archetype_id]
 				local idr_t_types = idr_t_archetype.types
 				local entities = idr_t_archetype.entities
+				local will_delete_archetype = false
 
 				for _, id in idr_t_types do
 					if not ECS_IS_PAIR(id) then
@@ -3173,6 +3163,7 @@ local function world_new()
 					if object ~= entity then
 						continue
 					end
+					will_delete_archetype = true
 					local id_record = component_index[id]
 					local flags = id_record.flags
 					local flags_delete_mask = bit32.btest(flags, ECS_ID_DELETE)
@@ -3197,10 +3188,10 @@ local function world_new()
 						end
 					end
 				end
-			end
 
-			for archetype_id in archetype_ids do
-				archetype_destroy(world, archetypes[archetype_id])
+				if will_delete_archetype then
+					archetype_destroy(world, idr_t_archetype)
+				end
 			end
 		end
 
@@ -3252,6 +3243,16 @@ local function world_new()
 			end
 		end
 
+		if archetype then
+			for _, id in archetype.types do
+				local cr = component_index[id]
+				local on_remove = cr.on_remove
+				if on_remove then
+					on_remove(entity, id, true)
+				end
+			end
+			archetype_delete(world, record.archetype, record.row)
+		end
 
 		local dense = record.dense
 		local i_swap = entity_index.alive_count
diff --git a/test/tests.luau b/test/tests.luau
index a552b93..13d8c60 100755
--- a/test/tests.luau
+++ b/test/tests.luau
@@ -24,6 +24,73 @@ type Id<T=unknown> = jecs.Id<T>
 local entity_visualiser = require("@modules/entity_visualiser")
 local dwi = entity_visualiser.stringify
 
+FOCUS()
+TEST("reproduce idr_t nil archetype bug", function()
+	local world = jecs.world()
+
+	local cts = {
+	    Humanoid = world:component(),
+	    Animator = world:component(),
+	    VelocitizeAnimationWeight = world:component(),
+	}
+
+	local char = world:entity()
+
+	-- REMOVING ONE OF THESE THESE OFFSETS i BY +1
+	world:set(char, cts.Humanoid, 0)
+	world:set(char, cts.Animator, 0)
+	--
+
+	world:added(cts.Humanoid, function() end) -- REMOVING THIS OFFSETS i BY +1 TOO
+	world:removed(cts.Animator, function(entity, id)
+	    local r = jecs.record(world, entity)
+	    local src = r.archetype
+
+	    --REMOVING THIS jecs.archetype_traverse_remove CALL STOPS IT FROM HAPPENING
+	    local dst = src and jecs.archetype_traverse_remove(world, id, src)
+	end)
+
+	local batches = 10
+	local batchSize = 20
+
+	local trackedEntities: { [number]: { parentId: number? } } = {}
+
+	for batch = 1, batches do
+	    for i = 1, batchSize do
+	        local root = world:entity()
+	        world:add(root, jecs.pair(jecs.ChildOf, char))
+
+	        -- Removing animator from trackEntity1 causes it to stop happening
+	        local trackEntity1 = world:entity()
+	        world:set(trackEntity1, cts.Animator, 0)
+	        world:add(trackEntity1, jecs.pair(jecs.ChildOf, root))
+	        trackedEntities[trackEntity1] = { parentId = root }
+
+	        -- Removing animator from trackEntity2 causes it to happen less frequently
+	        local trackEntity2 = world:entity()
+	        world:set(trackEntity2, cts.Animator, 0)
+	        world:add(trackEntity2, jecs.pair(jecs.ChildOf, root))
+	        trackedEntities[trackEntity2] = { parentId = root }
+
+	        -- Removing this, but keeping Animator on the other 2 causes it to stop happening
+	        world:set(trackEntity1, cts.VelocitizeAnimationWeight, 0)
+
+	        world:delete(root)
+
+	        for entityId, info in trackedEntities do
+	            if world:contains(entityId) and not world:parent(entityId :: any) then
+	                print(`bugged entity found: {entityId}`)
+	                print(`original parent: {info.parentId}`)
+	                print(`batch = {batch}, i = {i}`)
+	                print("==========================================")
+	                trackedEntities[entityId] = nil
+					world:deletee(entityId)
+	            end
+	        end
+	    end
+	end
+end)
+
 TEST("Ensure archetype edges get cleaned", function()
 	local A = jecs.component()
 	local B = jecs.component()