Add backwards iteration (#61)

* Iterate backwards

* Should test for removing as well

* Add test

* Add faíling test

* Add to changelog

CHANGELOG.md

@@ -10,12 +10,16 @@ The format is based on [Keep a Changelog][kac], and this project adheres to
 
 ## [Unreleased]
 
+### Changed
+
+- Iterator now goes backwards instead to prevent common cases of iterator invalidation
+
 ## [0.2.1] - 2024-07-06
 
 ### Added
 
-* Added `jecs.Component` built-in component which will be added to ids created with `world:component()`.
-    * used to find every component id with `query(jecs.Component)
+- Added `jecs.Component` built-in component which will be added to ids created with `world:component()`.
+    - Used to find every component id with `query(jecs.Component)
 
 ## [0.2.0] - 2024-07-03
 

src/init.luau

@@ -184,13 +184,9 @@ local function nextEntityId(entityIndex: EntityIndex, index: i24): i53
 	return id
 end
 
-local function transitionArchetype(
-	entityIndex: EntityIndex,
-	to: Archetype,
-	destinationRow: i24,
-	from: Archetype,
-	sourceRow: i24
-)
+local function transitionArchetype(entityIndex: EntityIndex, to: Archetype,
+	destinationRow: i24, from: Archetype, sourceRow: i24)
+
 	local columns = from.columns
 	local sourceEntities = from.entities
 	local destinationEntities = to.entities
@@ -690,11 +686,11 @@ local function preparedQuery(compatibleArchetypes: { Archetype },
 
 	local queryOutput = {}
 
-	local i = 1
+	local entities = archetype.entities 
+	local i = #entities
 
 	local function queryNext(): ...any
-		local entityId = archetype.entities[i]
-
+		local entityId = entities[i]
 		while entityId == nil do 
 			lastArchetype += 1
 			archetype = compatibleArchetypes[lastArchetype]
@@ -703,12 +699,13 @@ local function preparedQuery(compatibleArchetypes: { Archetype },
 				return
 			end
 
-			i = 1
-			entityId = archetype.entities[1]
+			entities = archetype.entities
+			i = #entities
+			entityId = entities[i]
 		end
 
 		local row = i
-		i+=1
+		i-=1
 
 		local columns = archetype.columns
 		local tr = indices[lastArchetype]
@@ -788,9 +785,10 @@ local function preparedQuery(compatibleArchetypes: { Archetype },
 	
 	local it = {
 		__iter = function() 
-			i = 1
 			lastArchetype = 1
 			archetype = compatibleArchetypes[1]
+			entities = archetype.entities
+			i = #entities
 
 			return queryNext
 		end,

tests/world.luau

@@ -25,30 +25,23 @@ local N = 10
 type World = jecs.WorldShim
 
 TEST("world", function()
-	do CASE("should be iterable")
+	do CASE("should find every component id")
 		local world = jecs.World.new() :: World
 		local A = world:component()
 		local B = world:component()
-		local eA = world:entity()
-		world:set(eA, A, true)
-		local eB = world:entity()
-		world:set(eB, B, true)
-		local eAB = world:entity()
-		world:set(eAB, A, true)
-		world:set(eAB, B, true)
+		world:entity()
+		world:entity()
+		world:entity()
 
-		for id, data in world do
-			if id == eA then
-				CHECK(data[A] == true)
-				CHECK(data[B] == nil)
-			elseif id == eB then
-				CHECK(data[A] == nil)
-				CHECK(data[B] == true)
-			elseif id == eAB then
-				CHECK(data[A] == true)
-				CHECK(data[B] == true)
+		local count = 0
+		for componentId in world:query(jecs.Component) do
+			if componentId ~= A and componentId ~= B then 
+				error("found entity")
 			end
+			count += 1
 		end
+
+		CHECK(count == 2)
 	end
 
 	do CASE("should remove its components") 
@@ -395,6 +388,70 @@ TEST("world", function()
 		end
 		CHECK(count == 2)
 	end
+
+	do CASE "should be able to add/remove matching entity during iteration" 
+		local world = jecs.World.new()
+		local Name = world:component()
+		for i = 1, 5 do 
+			local e = world:entity()
+			world:set(e, Name, tostring(e))
+		end
+		local count = 0
+		for id, name in world:query(Name) do 
+			count += 1
+			CHECK(id == tonumber(name))
+
+			world:remove(id, Name)
+			local e = world:entity()
+			world:set(e, Name, tostring(e))
+		end
+		CHECK(count == 5)
+	end
+	
+	do CASE "should allow adding a matching entity during iteration"
+		local world = jecs.World.new()
+		local A = world:component()
+		local B = world:component()
+
+		local e1 = world:entity()
+		local e2 = world:entity()
+		world:add(e1, A)
+		world:add(e2, A)
+		world:add(e2, B)
+
+		local count = 0
+		for id in world:query(A) do 
+			local e = world:entity()
+			world:add(e, A)
+			world:add(e, B)
+			count += 1
+		end
+
+		CHECK(count == 3)
+	end
+
+
+	do CASE "should not iterate same entity when adding component"
+		local world = jecs.World.new()
+		local A = world:component()
+		local B = world:component()
+
+		local e1 = world:entity()
+		local e2 = world:entity()
+		world:add(e1, A)
+		world:add(e2, A)
+		world:add(e2, B)
+
+		local count = 0
+		for id in world:query(A) do 
+			world:add(id, B)
+
+			count += 1
+		end
+
+		print(count)
+		CHECK(count == 2)
+	end
 end)
 
 FINISH()