From 70d0b9ec8278d594f4b612547e54ca47eb05312c Mon Sep 17 00:00:00 2001
From: waynemwashuma <94756970+waynemwashuma@users.noreply.github.com>
Date: Tue, 30 Sep 2025 11:41:45 +0300
Subject: [PATCH 1/7] Add `Entity.generation`

---
 src/ecs/entities/entity.js | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/ecs/entities/entity.js b/src/ecs/entities/entity.js
index 2511a7f7..8ef08824 100644
--- a/src/ecs/entities/entity.js
+++ b/src/ecs/entities/entity.js
@@ -6,11 +6,19 @@ export class Entity {
    */
   index
 
+  /**
+   * @readonly
+   * @type {number}
+   */
+  generation
+
   /**
    * @param {number} index
+   * @param {number} generation
    */
-  constructor(index){
+  constructor(index, generation){
     this.index = index
+    this.generation = generation
   }
 
   /**

From c8ab338d3a04384f70fb65ca62a671d719e04d2a Mon Sep 17 00:00:00 2001
From: waynemwashuma <94756970+waynemwashuma@users.noreply.github.com>
Date: Tue, 30 Sep 2025 11:43:40 +0300
Subject: [PATCH 2/7] Update methods of `Entity` to consider entity generation

---
 src/ecs/entities/entity.js | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/ecs/entities/entity.js b/src/ecs/entities/entity.js
index 8ef08824..3001934f 100644
--- a/src/ecs/entities/entity.js
+++ b/src/ecs/entities/entity.js
@@ -1,3 +1,5 @@
+import { packInto64Int, unpackFrom64Int } from '../../algorithms/packnumber.js'
+
 export class Entity {
 
   /**
@@ -25,14 +27,28 @@ export class Entity {
    * @param {Entity} other
    */
   equals(other){
-    return this.index === other.index
+    return (
+      this.index === other.index &&
+      this.generation === other.generation
+    )
   }
 
   /**
    * @returns {EntityId}
    */
   id(){
-    return this.index
+    const { index, generation } = this
+
+    return packInto64Int(index, generation)
+  }
+
+  /**
+   * @param {EntityId} id
+   */
+  static from(id){
+    const [index, generation] = unpackFrom64Int(id)
+
+    return new Entity(index, generation)
   }
 }
 

From aaa2e55c19c6a20be1a1ed7ea0e5ed79d30c7150 Mon Sep 17 00:00:00 2001
From: waynemwashuma <94756970+waynemwashuma@users.noreply.github.com>
Date: Tue, 30 Sep 2025 11:45:16 +0300
Subject: [PATCH 3/7] Update `World`

---
 src/ecs/entities/entities.js | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/src/ecs/entities/entities.js b/src/ecs/entities/entities.js
index d7909ff1..9bdb924f 100644
--- a/src/ecs/entities/entities.js
+++ b/src/ecs/entities/entities.js
@@ -3,11 +3,4 @@ import { DenseList } from '../../datastructures/index.js'
 import { EntityLocation } from './location.js'
 
 /** @augments {DenseList<EntityLocation>} */
-export class Entities extends DenseList {
-  reserve() {
-    return this.push(new EntityLocation(
-      /** @type {ArchetypeId}*/(-1),
-      /** @type {TableRow}*/(-1)
-    ))
-  }
-}
\ No newline at end of file
+export class Entities extends DenseList {}
\ No newline at end of file

From 417a76f65c5f6858e1e991bb78f55f4a1ebeaf4f Mon Sep 17 00:00:00 2001
From: waynemwashuma <94756970+waynemwashuma@users.noreply.github.com>
Date: Tue, 30 Sep 2025 12:19:45 +0300
Subject: [PATCH 4/7] Add `EntityLocation.generation`

---
 src/ecs/entities/entitycell.js |  8 +++++++-
 src/ecs/entities/location.js   |  5 +++++
 src/ecs/registry.js            | 27 +++++++++++++++++++--------
 3 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/src/ecs/entities/entitycell.js b/src/ecs/entities/entitycell.js
index c7d99fbc..a076294c 100644
--- a/src/ecs/entities/entitycell.js
+++ b/src/ecs/entities/entitycell.js
@@ -38,8 +38,14 @@ export class EntityCell {
    */
   constructor(world, entity) {
     const entities = world.getEntities()
+    const location = entities.get(entity.index)
+
+    if(location && location.generation === entity.generation){
+      this.location = location
+    } else {
+      this.location = new EntityLocation()
+    }
 
-    this.location = entities.get(entity.index) || new EntityLocation()
     this.tables = world.getTables()
     this.archetypes = world.getArchetypes()
     this.entity = entity
diff --git a/src/ecs/entities/location.js b/src/ecs/entities/location.js
index 273ffeb8..67131118 100644
--- a/src/ecs/entities/location.js
+++ b/src/ecs/entities/location.js
@@ -12,6 +12,11 @@ export class EntityLocation {
    */
   index
 
+  /**
+   * @type {number}
+   */
+  generation = 0
+
   /**
    * @type {ArchetypeId}
    */
diff --git a/src/ecs/registry.js b/src/ecs/registry.js
index 81fe13d4..57a82f5c 100644
--- a/src/ecs/registry.js
+++ b/src/ecs/registry.js
@@ -134,13 +134,20 @@ export class World {
    */
   spawn(components) {
     const entityIndex = this.entities.reserve()
+    let location = this.entities.get(entityIndex)
 
-    // SAFETY: the entity was reserved in this function so we know its there.
-    const location = /** @type {EntityLocation}*/ (this.entities.get(entityIndex))
+    if(!location){
+      const newLocation = new EntityLocation()
+
+      this.entities.set(entityIndex, newLocation)
+      location = newLocation
+    }
+
+    location.generation += 1
 
     // SAFETY:Object constructors can be casted from `Function` to `Constructor`
-    const newIds = (components.map((c) => typeid( /** @type {Constructor} */ (c.constructor))))
-    const entity = new Entity(entityIndex)
+    const newIds = (components.map((c) => typeid( /** @type {Constructor} */(c.constructor))))
+    const entity = new Entity(entityIndex, location.generation)
 
     newIds.push(typeid(Entity))
     components.push(entity)
@@ -166,7 +173,7 @@ export class World {
   insert(entity, components) {
     const location = this.entities.get(entity.index)
 
-    if (!location) {
+    if (!location || location.generation !== entity.generation) {
       return
     }
 
@@ -212,7 +219,7 @@ export class World {
   remove(entity, components) {
     const location = this.entities.get(entity.index)
     
-    if (!location) {
+    if (!location || location.generation !== entity.generation) {
       return
     }
 
@@ -265,7 +272,9 @@ export class World {
   despawn(entity) {
     const location = this.entities.get(entity.index)
 
-    if (!location) return
+    if (!location || location.generation !== entity.generation){
+      return
+    }
     
     const { archetypeId, tableId, index } = location
     const archetype = this.archetypes.get(archetypeId)
@@ -303,7 +312,9 @@ export class World {
   get(entity, type) {
     const location = this.entities.get(entity.index)
 
-    if (!location) return null
+    if (!location || location.generation !== entity.generation) {
+      return null
+    }
 
     const { tableId, index } = location
     const table = this.tables.get(tableId)

From 4a5588dc153c325ca686e6d9644dc11e193b9834 Mon Sep 17 00:00:00 2001
From: waynemwashuma <94756970+waynemwashuma@users.noreply.github.com>
Date: Tue, 30 Sep 2025 12:20:20 +0300
Subject: [PATCH 5/7] Update `Query`

---
 src/ecs/query/query.js | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/ecs/query/query.js b/src/ecs/query/query.js
index ea4894fc..a493d7db 100644
--- a/src/ecs/query/query.js
+++ b/src/ecs/query/query.js
@@ -107,10 +107,10 @@ export class Query {
    */
   get(entity) {
     const { world, descriptors, tableIds } = this
-    const entities = world.getEntities()
-    const location = entities.get(entity.index)
+    const cell = world.getEntity(entity)
+    const {location} = cell
 
-    if (!location) return null
+    if (!cell.exists()) return null
 
     const { tableId, index } = location
     const table = world.getTables().get(tableId)

From f597646c48d4258b2d4d43d3d2934d989633c016 Mon Sep 17 00:00:00 2001
From: waynemwashuma <94756970+waynemwashuma@users.noreply.github.com>
Date: Tue, 30 Sep 2025 12:21:35 +0300
Subject: [PATCH 6/7] Add and update unit tests

---
 src/ecs/tests/entities/entitycell.test.js |  4 +--
 src/ecs/tests/query.test.js               |  4 +--
 src/ecs/tests/world.test.js               | 35 +++++++++++++++++++++++
 3 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/src/ecs/tests/entities/entitycell.test.js b/src/ecs/tests/entities/entitycell.test.js
index 2fdd7db1..65ec7c4f 100644
--- a/src/ecs/tests/entities/entitycell.test.js
+++ b/src/ecs/tests/entities/entitycell.test.js
@@ -28,8 +28,8 @@ describe("Testing `EntityCell`", () => {
     const cell1 = world.getEntity(entity1)
     const cell2 = world.getEntity(entity2)
 
-    deepStrictEqual(cell1.id(), new Entity(0))
-    deepStrictEqual(cell2.id(), new Entity(1))
+    deepStrictEqual(cell1.id(), new Entity(0, 1))
+    deepStrictEqual(cell2.id(), new Entity(1, 1))
   })
 
   test('`EntityCell` tests entity existence correctly.', () => {
diff --git a/src/ecs/tests/query.test.js b/src/ecs/tests/query.test.js
index bc58a801..b915a0dd 100644
--- a/src/ecs/tests/query.test.js
+++ b/src/ecs/tests/query.test.js
@@ -75,7 +75,7 @@ describe("Testing `Query`", () => {
   test('query for single component, specific entity', () => {
     const world = createWorld()
     const query = new Query(world, [A])
-    const entity = new Entity(0)
+    const entity = new Entity(0, 1)
     const components = query.get(entity)
     assert(components)
 
@@ -89,7 +89,7 @@ describe("Testing `Query`", () => {
   test('query for multiple components, specific entity', () => {
     const world = createWorld()
     const query = new Query(world, [A, B, C])
-    const entity = new Entity(0)
+    const entity = new Entity(0, 1)
     const components = query.get(entity)
 
     assert(components)
diff --git a/src/ecs/tests/world.test.js b/src/ecs/tests/world.test.js
index be0622cc..772cadd4 100644
--- a/src/ecs/tests/world.test.js
+++ b/src/ecs/tests/world.test.js
@@ -28,6 +28,26 @@ describe("Testing `World`", () => {
     deepStrictEqual(components, [typeid(A), typeid(B), typeid(C), typeid(Entity)])
   })
 
+  test('Entity generation starts at one.', () => {
+    const world = new World()
+    const entity = world.spawn([new A(), new B(), new C()])
+
+    deepStrictEqual(entity,new Entity(0,1))
+  })
+
+  test('Entity generation is incremented.', () => {
+    const world = new World()
+    const entity1 = world.spawn([new A(), new B(), new C()])
+    world.despawn(entity1)
+    const entity2 = world.spawn([new A(), new B(), new C()])
+    world.despawn(entity2)
+    const entity3 = world.spawn([new A(), new B(), new C()])
+
+    deepStrictEqual(entity1,new Entity(0,1))
+    deepStrictEqual(entity2,new Entity(0,2))
+    deepStrictEqual(entity3,new Entity(0,3))
+  })
+
   test('Entity is despawned correctly from a world.', () => {
     const world = new World()
     const entity = world.spawn([new A(), new B(), new C()])
@@ -38,6 +58,21 @@ describe("Testing `World`", () => {
     deepStrictEqual(components, [])
   })
 
+  test('Invalidated entity cannot be accessed on same index', () => {
+    const world = new World()
+    const entity1 = world.spawn([new A()])
+    world.despawn(entity1)
+    const entity2 = world.spawn([new A()])
+
+    const cell1 = world.getEntity(entity1)
+    const cell2 = world.getEntity(entity2)
+  
+    deepStrictEqual(entity1,new Entity(0,1))
+    deepStrictEqual(entity2,new Entity(0,2))
+    deepStrictEqual(cell1.exists(), false)
+    deepStrictEqual(cell2.exists(), true)
+  })
+
   test('Components are inserted into an entity.', () => {
     const world = new World()
     const entity = world.spawn([])

From 7aed7a1ffbbda32a1105257bbfe86c9f4d13acaa Mon Sep 17 00:00:00 2001
From: waynemwashuma <94756970+waynemwashuma@users.noreply.github.com>
Date: Tue, 7 Oct 2025 12:34:09 +0300
Subject: [PATCH 7/7] Lint files

---
 src/ecs/query/query.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/ecs/query/query.js b/src/ecs/query/query.js
index a493d7db..36e34334 100644
--- a/src/ecs/query/query.js
+++ b/src/ecs/query/query.js
@@ -108,7 +108,7 @@ export class Query {
   get(entity) {
     const { world, descriptors, tableIds } = this
     const cell = world.getEntity(entity)
-    const {location} = cell
+    const { location } = cell
 
     if (!cell.exists()) return null