Add test with ChangeTracker

tests/world.luau

@@ -512,6 +512,218 @@ TEST("world", function()
 		local query = world:query(B)
 		CHECK(query == query:without())
 	end
+
+	do CASE "should not find any entities"
+		local world = jecs.World.new()
+
+		local Hello = world:component()
+		local Bob = world:component()
+
+		local helloBob = world:entity()
+		world:add(helloBob, jecs.pair(Hello, Bob))
+		world:add(helloBob, Bob)
+
+		local withoutCount = 0
+		for _ in world
+		    :query(jecs.pair(Hello, Bob))
+			:without(Bob)
+		do
+			withoutCount += 1
+		end
+
+		CHECK(withoutCount == 0)
+	end
+
+	do CASE "should allow change tracking"
+	    local world = jecs.World.new()
+        local Previous = world:component()
+
+        local ChangeTracker = {}
+        ChangeTracker.__index = ChangeTracker
+
+        function ChangeTracker.new(component)
+            return setmetatable({
+                addedComponents = {}, -- Map<Entity, T>
+                removedComponents = {}, -- Vec<Entity>
+                component = component,
+                previous = jecs.pair(Previous, component),
+                isTrivial = nil,
+            }, ChangeTracker)
+        end
+
+        local function shallowEq(a, b)
+            for k, v in a do
+                if b[k] ~= v then
+                    return false
+                end
+            end
+            return true
+        end
+
+        function ChangeTracker.track(tracker, world, fn)
+            local added = false
+            local removed = false
+
+            local addedComponents = tracker.addedComponents
+            local removedComponents = tracker.removedComponents
+            local component = tracker.component
+            local previous = tracker.previous
+            local isTrivial = tracker.isTrivial
+
+            local changes = {}
+            function changes:added()
+                added = true
+                local q = world:query(component):without(previous)
+                return function()
+                    local id, data = q:next()
+                    if not id then
+                        return nil
+                    end
+
+                    if isTrivial == nil then
+                        isTrivial = typeof(data) ~= "table"
+                        tracker.isTrivial = isTrivial
+                    end
+
+                    if not isTrivial then
+                        data = table.clone(data)
+                    end
+
+                    addedComponents[id] = data
+                    return id, data
+                end
+            end
+
+            function changes:changed()
+                local q = world:query(component, previous)
+
+                return function()
+                    local id, new, old = q:next()
+                    while true do
+                        if not id then
+                            return nil
+                        end
+
+                        if not isTrivial then
+                            if not shallowEq(new, old) then
+                                break
+                            end
+                        elseif new ~= old then
+                            break
+                        end
+
+                        id, new, old = q:next()
+                    end
+
+                    print("nil?", id)
+                    addedComponents[id] = new
+
+                    return id, old, new
+                end
+            end
+
+            function changes:removed()
+                removed = true
+
+                local q = world:query(tracker.previous):without(tracker.component)
+                return function()
+                    local id = q:next()
+                    if id then
+                        table.insert(removedComponents, id)
+                    end
+                    return id
+                end
+            end
+
+            fn(changes)
+            if not added then
+                for _ in changes:added() do
+                end
+            end
+
+            if not removed then
+                for _ in changes:removed() do
+                end
+            end
+
+            for e, data in addedComponents do
+                world:set(e, previous, if isTrivial then data else table.clone(data))
+            end
+
+            for _, e in removedComponents do
+                world:remove(e, previous)
+            end
+        end
+
+        local Test = world:component()
+        local TestTracker = ChangeTracker.new(Test)
+
+		local e = world:entity()
+		world:set(e, Test, { foo = 11 })
+    	for e, test in world:query(Test) do
+    	    test.foo = test.foo + 1
+    	end
+
+        TestTracker:track(world, function(changes)
+            local added = 0
+            local changed = 0
+            local removed = 0
+      		for e, data in changes.added() do
+                added+=1
+      		end
+      		for e, old, new in changes.changed() do
+                changed+=1
+      		end
+      		for e in changes.removed() do
+                removed+=1
+      		end
+            CHECK(added == 1)
+            CHECK(changed == 0)
+            CHECK(removed == 0)
+        end)
+
+        for e, test in world:query(Test) do
+       	    test.foo = test.foo + 1
+       	end
+
+        TestTracker:track(world, function(changes)
+            local added = 0
+            local changed = 0
+            local removed = 0
+      		for e, data in changes.added() do
+                added+=1
+      		end
+      		for e, old, new in changes.changed() do
+                changed+=1
+      		end
+      		for e in changes.removed() do
+                removed+=1
+      		end
+            CHECK(added == 0)
+            CHECK(changed == 1)
+            CHECK(removed == 0)
+        end)
+
+        world:remove(e, Test)
+
+        TestTracker:track(world, function(changes)
+            local added = 0
+            local changed = 0
+            local removed = 0
+      		for e, data in changes.added() do
+                added+=1
+      		end
+      		for e, old, new in changes.changed() do
+                changed+=1
+      		end
+      		for e in changes.removed() do
+                removed+=1
+      		end
+            CHECK(added == 0)
+            CHECK(changed == 0)
+            CHECK(removed == 1)
+        end)
+    end
 end)
 
 FINISH()