Add benchmarks and optimize

Store all data in slices rather than maps.

component.go

@@ -4,10 +4,10 @@ import (
 	"sync"
 )
 
-var componentMutex sync.RWMutex
+var componentMutex sync.Mutex
 
 // ComponentID is a component identifier. Each Component is assigned a unique ID
-// via NextComponentID, and implements a ComponentID method which returns the ID.
+// via NewComponentID, and implements a ComponentID method returning its ID.
 type ComponentID int
 
 // Component represents data for an entity, and how it interacts with the world.
@@ -15,31 +15,32 @@ type Component interface {
 	ComponentID() ComponentID
 }
 
-var nextComponentID ComponentID
+var maxComponentID ComponentID
+
+// NewComponentID returns the next available ComponentID.
+func NewComponentID() ComponentID {
+	mutex.Lock()
+	defer mutex.Unlock()
+
+	entityMutex.Lock()
+	defer entityMutex.Unlock()
 
-// NextComponentID returns the next available ComponentID.
-func NextComponentID() ComponentID {
 	componentMutex.Lock()
 	defer componentMutex.Unlock()
 
-	nextComponentID++
-	return nextComponentID
+	maxComponentID++
+
+	for i := Entity(1); i < maxEntityID; i++ {
+		gameComponents[i] = append(gameComponents[i], nil)
+	}
+
+	return maxComponentID
 }
 
 // AddComponent adds a Component to an Entity.
-func (entity EntityID) AddComponent(component Component) {
-	componentMutex.Lock()
-	defer componentMutex.Unlock()
-
-	if wasRemoved(entity) {
-		return
-	}
-
+func (entity Entity) AddComponent(component Component) {
 	componentID := component.ComponentID()
 
-	if gameComponents[entity] == nil {
-		gameComponents[entity] = make(map[ComponentID]interface{})
-	}
 	gameComponents[entity][componentID] = component
 
 	entityMutex.Lock()
@@ -48,10 +49,7 @@ func (entity EntityID) AddComponent(component Component) {
 }
 
 // Component gets a Component of an Entity.
-func (entity EntityID) Component(componentID ComponentID) interface{} {
-	componentMutex.RLock()
-	defer componentMutex.RUnlock()
-
+func (entity Entity) Component(componentID ComponentID) interface{} {
 	components := gameComponents[entity]
 	if components == nil {
 		return nil

component_test.go

@@ -0,0 +1,49 @@
+package gohan
+
+import "testing"
+
+var testComponentID = NewComponentID()
+
+type testComponent struct {
+	X, Y float64
+}
+
+func (t testComponent) ComponentID() ComponentID {
+	return testComponentID
+}
+
+func BenchmarkComponent(b *testing.B) {
+	e := NewEntity()
+
+	e.AddComponent(&testComponent{
+		X: 108,
+		Y: 0,
+	})
+
+	b.StopTimer()
+	b.ResetTimer()
+	b.ReportAllocs()
+	b.StartTimer()
+
+	for i := 0; i < b.N; i++ {
+		_ = e.Component(testComponentID)
+	}
+}
+
+func BenchmarkAddComponent(b *testing.B) {
+	e := NewEntity()
+
+	c := &testComponent{
+		X: 108,
+		Y: 0,
+	}
+
+	b.StopTimer()
+	b.ResetTimer()
+	b.ReportAllocs()
+	b.StartTimer()
+
+	for i := 0; i < b.N; i++ {
+		e.AddComponent(c)
+	}
+}

doc.go

@@ -1,6 +1,11 @@
 /*
 Package gohan provides an Entity Component System framework for Ebiten.
 
+An example game is available at /examples/twinstick which may be built by
+executing the following command (in /examples/twinstick):
+
+  go build -tags example .
+
 Entity
 
 A general-purpose object, which consists of a unique ID, starting with 1.

entity.go

@@ -5,25 +5,36 @@ import (
 	"time"
 )
 
-// EntityID is an entity identifier.
-type EntityID int
+// Entity is an entity identifier.
+type Entity int
 
-var nextEntityID EntityID
+var maxEntityID Entity
 
 var entityMutex sync.Mutex
 
-// NextEntityID returns the next available EntityID.
-func NextEntityID() EntityID {
+// NewEntity returns a new (or previously removed and cleared) Entity. Because
+// Gohan reuses removed Entity IDs, a previously removed ID may be returned.
+func NewEntity() Entity {
 	entityMutex.Lock()
 	defer entityMutex.Unlock()
 
-	nextEntityID++
-	allEntities = append(allEntities, nextEntityID)
-	return nextEntityID
+	if len(availableEntityIDs) > 0 {
+		id := availableEntityIDs[0]
+		availableEntityIDs = availableEntityIDs[1:]
+		allEntities = append(allEntities, id)
+		return id
+	}
+
+	maxEntityID++
+	allEntities = append(allEntities, maxEntityID)
+	gameComponents = append(gameComponents, make([]interface{}, maxComponentID+1))
+	return maxEntityID
 }
 
-// RemoveEntity removes the provided Entity, and all of its components.
-func RemoveEntity(entity EntityID) {
+// Remove removes the provided Entity's components, causing it to no longer be
+// handled by any system.  Because Gohan reuses removed EntityIDs, applications
+// must also remove any internal references to the removed Entity.
+func (entity Entity) Remove() {
 	entityMutex.Lock()
 	defer entityMutex.Unlock()
 
@@ -36,18 +47,6 @@ func RemoveEntity(entity EntityID) {
 	}
 }
 
-func wasRemoved(entity EntityID) bool {
-	entityMutex.Lock()
-	defer entityMutex.Unlock()
-
-	for _, e := range allEntities {
-		if e == entity {
-			return false
-		}
-	}
-	return true
-}
-
 var numEntities int
 var numEntitiesT time.Time
 

entity_test.go

@@ -0,0 +1,60 @@
+package gohan
+
+import (
+	"testing"
+	"time"
+)
+
+func TestActiveEntities(t *testing.T) {
+	t.Parallel()
+
+	active := ActiveEntities()
+	if active != 0 {
+		t.Fatalf("expected 0 active entities, got %d", active)
+	}
+
+	wait()
+	active = ActiveEntities()
+	if active != 0 {
+		t.Fatalf("expected 0 active entities, got %d", active)
+	}
+
+	// Create entity.
+	e1 := NewEntity()
+
+	wait()
+	active = ActiveEntities()
+	if active != 1 {
+		t.Fatalf("expected 1 active entities, got %d", active)
+	}
+
+	// Create entity.
+	e2 := NewEntity()
+
+	wait()
+	active = ActiveEntities()
+	if active != 2 {
+		t.Fatalf("expected 2 active entities, got %d", active)
+	}
+
+	e1.Remove()
+
+	wait()
+	active = ActiveEntities()
+	if active != 1 {
+		t.Fatalf("expected 1 active entities, got %d", active)
+	}
+
+	e2.Remove()
+
+	wait()
+	active = ActiveEntities()
+	if active != 0 {
+		t.Fatalf("expected 0 active entities, got %d", active)
+	}
+}
+
+// wait causes the program to wait long enough to expire all duration-based caches.
+func wait() {
+	time.Sleep(2 * time.Second)
+}

examples/twinstick/component/bullet.go

@@ -10,13 +10,13 @@ import (
 type BulletComponent struct {
 }
 
-var BulletComponentID = gohan.NextComponentID()
+var BulletComponentID = gohan.NewComponentID()
 
 func (p *BulletComponent) ComponentID() gohan.ComponentID {
 	return BulletComponentID
 }
 
-func Bullet(e gohan.EntityID) *BulletComponent {
+func Bullet(e gohan.Entity) *BulletComponent {
 	c, ok := e.Component(BulletComponentID).(*BulletComponent)
 	if !ok {
 		return nil

examples/twinstick/component/position.go

@@ -11,13 +11,13 @@ type PositionComponent struct {
 	X, Y float64
 }
 
-var PositionComponentID = gohan.NextComponentID()
+var PositionComponentID = gohan.NewComponentID()
 
 func (p *PositionComponent) ComponentID() gohan.ComponentID {
 	return PositionComponentID
 }
 
-func Position(e gohan.EntityID) *PositionComponent {
+func Position(e gohan.Entity) *PositionComponent {
 	c, ok := e.Component(PositionComponentID).(*PositionComponent)
 	if !ok {
 		return nil

examples/twinstick/component/velocity.go

@@ -11,13 +11,13 @@ type VelocityComponent struct {
 	X, Y float64
 }
 
-var VelocityComponentID = gohan.NextComponentID()
+var VelocityComponentID = gohan.NewComponentID()
 
 func (c *VelocityComponent) ComponentID() gohan.ComponentID {
 	return VelocityComponentID
 }
 
-func Velocity(e gohan.EntityID) *VelocityComponent {
+func Velocity(e gohan.Entity) *VelocityComponent {
 	c, ok := e.Component(VelocityComponentID).(*VelocityComponent)
 	if !ok {
 		return nil

examples/twinstick/component/weapon.go

@@ -20,13 +20,13 @@ type WeaponComponent struct {
 	BulletSpeed float64
 }
 
-var WeaponComponentID = gohan.NextComponentID()
+var WeaponComponentID = gohan.NewComponentID()
 
 func (p *WeaponComponent) ComponentID() gohan.ComponentID {
 	return WeaponComponentID
 }
 
-func Weapon(e gohan.EntityID) *WeaponComponent {
+func Weapon(e gohan.Entity) *WeaponComponent {
 	c, ok := e.Component(WeaponComponentID).(*WeaponComponent)
 	if !ok {
 		return nil

examples/twinstick/entity/bullet.go

@@ -8,8 +8,8 @@ import (
 	"code.rocketnine.space/tslocum/gohan/examples/twinstick/component"
 )
 
-func NewBullet(x, y, xSpeed, ySpeed float64) gohan.EntityID {
-	bullet := gohan.NextEntityID()
+func NewBullet(x, y, xSpeed, ySpeed float64) gohan.Entity {
+	bullet := gohan.NewEntity()
 
 	bullet.AddComponent(&component.PositionComponent{
 		X: x,

examples/twinstick/entity/player.go

@@ -11,8 +11,8 @@ import (
 	"code.rocketnine.space/tslocum/gohan/examples/twinstick/component"
 )
 
-func NewPlayer() gohan.EntityID {
-	player := gohan.NextEntityID()
+func NewPlayer() gohan.Entity {
+	player := gohan.NewEntity()
 
 	// Set position to -1,-1 to indicate the player has not been assigned a
 	// position yet. We will place the player in the center of the screen when

examples/twinstick/game/game.go

@@ -20,7 +20,7 @@ import (
 type game struct {
 	w, h int
 
-	player gohan.EntityID
+	player gohan.Entity
 
 	op *ebiten.DrawImageOptions
 
@@ -62,7 +62,7 @@ func (g *game) Layout(outsideWidth, outsideHeight int) (int, int) {
 		g.w, g.h = w, h
 		g.movementSystem.ScreenW, g.movementSystem.ScreenH = float64(w), float64(h)
 
-		position := g.player.Component(component.PositionComponentID).(*component.PositionComponent)
+		position := component.Position(g.player)
 		if position.X == -1 && position.Y == -1 {
 			position.X, position.Y = float64(g.w)/2-16, float64(g.h)/2-16
 		}

examples/twinstick/system/draw_bullets.go

@@ -20,19 +20,19 @@ func NewDrawBulletsSystem() *DrawBulletsSystem {
 	}
 }
 
-func (s *DrawBulletsSystem) Matches(entity gohan.EntityID) bool {
+func (s *DrawBulletsSystem) Matches(entity gohan.Entity) bool {
 	position := entity.Component(component.PositionComponentID)
 	bullet := entity.Component(component.BulletComponentID)
 
 	return position != nil && bullet != nil
 }
 
-func (s *DrawBulletsSystem) Update(_ gohan.EntityID) error {
+func (s *DrawBulletsSystem) Update(_ gohan.Entity) error {
 	return gohan.ErrSystemWithoutUpdate
 }
 
-func (s *DrawBulletsSystem) Draw(entity gohan.EntityID, screen *ebiten.Image) error {
-	position := entity.Component(component.PositionComponentID).(*component.PositionComponent)
+func (s *DrawBulletsSystem) Draw(entity gohan.Entity, screen *ebiten.Image) error {
+	position := component.Position(entity)
 
 	s.op.GeoM.Reset()
 	s.op.GeoM.Translate(-16, -16)

examples/twinstick/system/draw_player.go

@@ -11,27 +11,27 @@ import (
 )
 
 type drawPlayerSystem struct {
-	player gohan.EntityID
+	player gohan.Entity
 	op     *ebiten.DrawImageOptions
 }
 
-func NewDrawPlayerSystem(player gohan.EntityID) *drawPlayerSystem {
+func NewDrawPlayerSystem(player gohan.Entity) *drawPlayerSystem {
 	return &drawPlayerSystem{
 		player: player,
 		op:     &ebiten.DrawImageOptions{},
 	}
 }
 
-func (s *drawPlayerSystem) Matches(entity gohan.EntityID) bool {
+func (s *drawPlayerSystem) Matches(entity gohan.Entity) bool {
 	return entity == s.player
 }
 
-func (s *drawPlayerSystem) Update(_ gohan.EntityID) error {
+func (s *drawPlayerSystem) Update(_ gohan.Entity) error {
 	return gohan.ErrSystemWithoutUpdate
 }
 
-func (s *drawPlayerSystem) Draw(entity gohan.EntityID, screen *ebiten.Image) error {
-	position := entity.Component(component.PositionComponentID).(*component.PositionComponent)
+func (s *drawPlayerSystem) Draw(entity gohan.Entity, screen *ebiten.Image) error {
+	position := component.Position(entity)
 
 	s.op.GeoM.Reset()
 	s.op.GeoM.Translate(position.X-16, position.Y-16)

examples/twinstick/system/input_fire.go

@@ -18,16 +18,16 @@ func angle(x1, y1, x2, y2 float64) float64 {
 }
 
 type fireInputSystem struct {
-	player gohan.EntityID
+	player gohan.Entity
 }
 
-func NewFireInputSystem(player gohan.EntityID) *fireInputSystem {
+func NewFireInputSystem(player gohan.Entity) *fireInputSystem {
 	return &fireInputSystem{
 		player: player,
 	}
 }
 
-func (_ *fireInputSystem) Matches(e gohan.EntityID) bool {
+func (_ *fireInputSystem) Matches(e gohan.Entity) bool {
 	weapon := e.Component(component.WeaponComponentID)
 
 	return weapon != nil
@@ -48,14 +48,14 @@ func (s *fireInputSystem) fire(weapon *component.WeaponComponent, position *comp
 	_ = bullet
 }
 
-func (s *fireInputSystem) Update(_ gohan.EntityID) error {
-	weapon := s.player.Component(component.WeaponComponentID).(*component.WeaponComponent)
+func (s *fireInputSystem) Update(_ gohan.Entity) error {
+	weapon := component.Weapon(s.player)
 
 	if weapon.Ammo <= 0 {
 		return nil
 	}
 
-	position := s.player.Component(component.PositionComponentID).(*component.PositionComponent)
+	position := component.Position(s.player)
 
 	if ebiten.IsMouseButtonPressed(ebiten.MouseButtonLeft) {
 		cursorX, cursorY := ebiten.CursorPosition()
@@ -99,6 +99,6 @@ func (s *fireInputSystem) Update(_ gohan.EntityID) error {
 	return nil
 }
 
-func (_ *fireInputSystem) Draw(_ gohan.EntityID, _ *ebiten.Image) error {
+func (_ *fireInputSystem) Draw(_ gohan.Entity, _ *ebiten.Image) error {
 	return gohan.ErrSystemWithoutDraw
 }

examples/twinstick/system/input_move.go

@@ -10,21 +10,21 @@ import (
 )
 
 type movementInputSystem struct {
-	player gohan.EntityID
+	player gohan.Entity
 }
 
-func NewMovementInputSystem(player gohan.EntityID) *movementInputSystem {
+func NewMovementInputSystem(player gohan.Entity) *movementInputSystem {
 	return &movementInputSystem{
 		player: player,
 	}
 }
 
-func (s *movementInputSystem) Matches(e gohan.EntityID) bool {
+func (s *movementInputSystem) Matches(e gohan.Entity) bool {
 	return e == s.player
 }
 
-func (s *movementInputSystem) Update(e gohan.EntityID) error {
-	velocity := s.player.Component(component.VelocityComponentID).(*component.VelocityComponent)
+func (s *movementInputSystem) Update(e gohan.Entity) error {
+	velocity := component.Velocity(s.player)
 	if ebiten.IsKeyPressed(ebiten.KeyA) {
 		velocity.X -= 0.5
 		if velocity.X < -5 {
@@ -52,6 +52,6 @@ func (s *movementInputSystem) Update(e gohan.EntityID) error {
 	return nil
 }
 
-func (s *movementInputSystem) Draw(_ gohan.EntityID, _ *ebiten.Image) error {
+func (s *movementInputSystem) Draw(_ gohan.Entity, _ *ebiten.Image) error {
 	return gohan.ErrSystemWithoutDraw
 }

examples/twinstick/system/input_profile.go

@@ -15,21 +15,21 @@ import (
 )
 
 type profileSystem struct {
-	player     gohan.EntityID
+	player     gohan.Entity
 	cpuProfile *os.File
 }
 
-func NewProfileSystem(player gohan.EntityID) *profileSystem {
+func NewProfileSystem(player gohan.Entity) *profileSystem {
 	return &profileSystem{
 		player: player,
 	}
 }
 
-func (s *profileSystem) Matches(e gohan.EntityID) bool {
+func (s *profileSystem) Matches(e gohan.Entity) bool {
 	return e == s.player
 }
 
-func (s *profileSystem) Update(e gohan.EntityID) error {
+func (s *profileSystem) Update(e gohan.Entity) error {
 	if ebiten.IsKeyPressed(ebiten.KeyControl) && inpututil.IsKeyJustPressed(ebiten.KeyP) {
 		if s.cpuProfile == nil {
 			log.Println("CPU profiling started...")
@@ -60,6 +60,6 @@ func (s *profileSystem) Update(e gohan.EntityID) error {
 	return nil
 }
 
-func (s *profileSystem) Draw(_ gohan.EntityID, _ *ebiten.Image) error {
+func (s *profileSystem) Draw(_ gohan.Entity, _ *ebiten.Image) error {
 	return gohan.ErrSystemWithoutDraw
 }

examples/twinstick/system/movement.go

@@ -13,23 +13,23 @@ type MovementSystem struct {
 	ScreenW, ScreenH float64
 }
 
-func (_ *MovementSystem) Matches(entity gohan.EntityID) bool {
-	position := entity.Component(component.PositionComponentID)
-	velocity := entity.Component(component.VelocityComponentID)
+func (_ *MovementSystem) Matches(entity gohan.Entity) bool {
+	position := component.Position(entity)
+	velocity := component.Velocity(entity)
 
 	return position != nil && velocity != nil
 }
 
-func (s *MovementSystem) Update(entity gohan.EntityID) error {
-	position := entity.Component(component.PositionComponentID).(*component.PositionComponent)
-	velocity := entity.Component(component.VelocityComponentID).(*component.VelocityComponent)
+func (s *MovementSystem) Update(entity gohan.Entity) error {
+	position := component.Position(entity)
+	velocity := component.Velocity(entity)
 
 	bullet := entity.Component(component.BulletComponentID)
 
 	// Check for collision.
 	if position.X+velocity.X < 16 {
 		if bullet != nil {
-			gohan.RemoveEntity(entity)
+			entity.Remove()
 			return nil
 		}
 
@@ -37,7 +37,7 @@ func (s *MovementSystem) Update(entity gohan.EntityID) error {
 		velocity.X = 0
 	} else if position.X+velocity.X > s.ScreenW-16 {
 		if bullet != nil {
-			gohan.RemoveEntity(entity)
+			entity.Remove()
 			return nil
 		}
 
@@ -46,7 +46,7 @@ func (s *MovementSystem) Update(entity gohan.EntityID) error {
 	}
 	if position.Y+velocity.Y < 16 {
 		if bullet != nil {
-			gohan.RemoveEntity(entity)
+			entity.Remove()
 			return nil
 		}
 
@@ -54,7 +54,7 @@ func (s *MovementSystem) Update(entity gohan.EntityID) error {
 		velocity.Y = 0
 	} else if position.Y+velocity.Y > s.ScreenH-16 {
 		if bullet != nil {
-			gohan.RemoveEntity(entity)
+			entity.Remove()
 			return nil
 		}
 
@@ -72,6 +72,6 @@ func (s *MovementSystem) Update(entity gohan.EntityID) error {
 	return nil
 }
 
-func (_ *MovementSystem) Draw(_ gohan.EntityID, _ *ebiten.Image) error {
+func (_ *MovementSystem) Draw(_ gohan.Entity, _ *ebiten.Image) error {
 	return gohan.ErrSystemWithoutDraw
 }

examples/twinstick/system/printinfo.go

@@ -14,10 +14,10 @@ import (
 type printInfoSystem struct {
 	img    *ebiten.Image
 	op     *ebiten.DrawImageOptions
-	player gohan.EntityID
+	player gohan.Entity
 }
 
-func NewPrintInfoSystem(player gohan.EntityID) *printInfoSystem {
+func NewPrintInfoSystem(player gohan.Entity) *printInfoSystem {
 	p := &printInfoSystem{
 		img:    ebiten.NewImage(200, 100),
 		op:     &ebiten.DrawImageOptions{},
@@ -27,15 +27,15 @@ func NewPrintInfoSystem(player gohan.EntityID) *printInfoSystem {
 	return p
 }
 
-func (s *printInfoSystem) Matches(e gohan.EntityID) bool {
+func (s *printInfoSystem) Matches(e gohan.Entity) bool {
 	return e == s.player
 }
 
-func (s *printInfoSystem) Update(_ gohan.EntityID) error {
+func (s *printInfoSystem) Update(_ gohan.Entity) error {
 	return gohan.ErrSystemWithoutUpdate
 }
 
-func (s *printInfoSystem) Draw(_ gohan.EntityID, screen *ebiten.Image) error {
+func (s *printInfoSystem) Draw(_ gohan.Entity, screen *ebiten.Image) error {
 	s.img.Clear()
 	ebitenutil.DebugPrint(s.img, fmt.Sprintf("KEY  WASD+MOUSE\nENT  %d\nUPD  %d\nDRA  %d\nTPS  %0.0f\nFPS  %0.0f", gohan.ActiveEntities(), gohan.UpdatedEntities(), gohan.DrawnEntities(), ebiten.CurrentTPS(), ebiten.CurrentFPS()))
 	screen.DrawImage(s.img, s.op)

gohan.go

@@ -12,15 +12,19 @@ import (
 )
 
 var (
-	gameComponents = make(map[EntityID]map[ComponentID]interface{})
+	gameComponents [][]interface{}
 
-	allEntities []EntityID
+	allEntities []Entity
 
-	modifiedEntities []EntityID
-	removedEntities  []EntityID
+	modifiedEntities []Entity
+	removedEntities  []Entity
+
+	// availableEntityIDs is the set of EntityIDs available because they were
+	// removed from the game.
+	availableEntityIDs []Entity
 
 	gameSystems        []System
-	gameSystemEntities [][]EntityID // Slice of entities matching each system.
+	gameSystemEntities [][]Entity // Slice of entities matching each system.
 
 	gameSystemReceivesUpdate []bool
 	gameSystemReceivesDraw   []bool
@@ -39,6 +43,9 @@ var (
 )
 
 func init() {
+	// Pad slices to match IDs starting with 1.
+	gameComponents = append(gameComponents, nil)
+
 	debugEnv := os.Getenv("GOHAN_DEBUG")
 	debugEnv = strings.TrimSpace(debugEnv)
 	debugEnv = strings.ToLower(debugEnv)
@@ -59,7 +66,7 @@ func attachEntitiesToSystem(system System) {
 	// This function is always called on a newly added system.
 	systemID := len(gameSystemEntities) - 1
 
-	for entity := EntityID(0); entity < nextEntityID; entity++ {
+	for entity := Entity(0); entity < maxEntityID; entity++ {
 		if system.Matches(entity) {
 			gameSystemEntities[systemID] = append(gameSystemEntities[systemID], entity)
 
@@ -99,12 +106,9 @@ func propagateEntityChanges() {
 	defer entityMutex.Unlock()
 
 	for _, entity := range removedEntities {
-		delete(gameComponents, entity)
-
+		// Remove from attached systems.
 	REMOVED:
 		for i := range gameSystemEntities {
-			delete(gameComponents, entity)
-
 			for j, e := range gameSystemEntities[i] {
 				if e == entity {
 					gameSystemEntities[i] = append(gameSystemEntities[i][:j], gameSystemEntities[i][j+1:]...)
@@ -112,7 +116,14 @@ func propagateEntityChanges() {
 				}
 			}
 		}
+
+		// Remove components.
+		gameComponents[entity] = make([]interface{}, maxComponentID+1)
 	}
+
+	// Mark EntityIDs as available.
+	availableEntityIDs = append(availableEntityIDs, removedEntities...)
+
 	removedEntities = nil
 
 	for _, entity := range modifiedEntities {

system.go

@@ -14,13 +14,13 @@ import (
 // See ErrSystemWithoutUpdate and ErrSystemWithoutDraw.
 type System interface {
 	// Matches returns whether the provided entity is handled by this system.
-	Matches(entity EntityID) bool
+	Matches(entity Entity) bool
 
 	// Update is called once for each matching entity each time the game state is updated.
-	Update(entity EntityID) error
+	Update(entity Entity) error
 
 	// Draw is called once for each matching entity each time the game is drawn to the screen.
-	Draw(entity EntityID, screen *ebiten.Image) error
+	Draw(entity Entity, screen *ebiten.Image) error
 }
 
 // Special error values.