String methods, no separate walkers

engine/actor.go

@@ -127,3 +127,5 @@ func (a *Actor) Prepare(g *Game) error {
 	a.game = g
 	return nil
 }
+
+func (a *Actor) String() string { return "Actor@" + a.Pos.String() }

engine/camera.go

@@ -90,6 +90,8 @@ func (c *Camera) Scan(visit func(interface{}) error) error {
 	return visit(c.Child)
 }
 
+func (c *Camera) String() string { return "Camera@" + c.Centre.String() }
+
 // Transform returns the camera transform.
 func (c *Camera) Transform() (opts ebiten.DrawImageOptions) {
 	opts.GeoM.Translate(geom.CFloat(c.Centre.Mul(-1)))

engine/container.go

@@ -18,6 +18,8 @@ func (c Container) Scan(visit func(interface{}) error) error {
 	return nil
 }
 
+func (c Container) String() string { return "Container" }
+
 // Register records component in the slice, if parent is the container.
 func (c *Container) Register(component, parent interface{}) error {
 	if parent == c {

engine/drawdag.go

@@ -120,6 +120,8 @@ func (d *DrawDAG) Scan(visit func(interface{}) error) error {
 	return visit(d.Child)
 }
 
+func (d *DrawDAG) String() string { return "DrawDAG" }
+
 // Update checks for any changes to descendants, and updates its internal
 // data structures accordingly.
 func (d *DrawDAG) Update() error {
@@ -325,7 +327,8 @@ func newDAG() *dag {
 	}
 }
 
-func (d *dag) String() string {
+// Dot returns a dot-syntax-like description of the graph.
+func (d *dag) Dot() string {
 	var sb strings.Builder
 	sb.WriteString("digraph {\n")
 	for v, e := range d.out {

engine/drawdfs.go

@@ -58,3 +58,5 @@ func (d *DrawDFS) draw(component interface{}, screen *ebiten.Image, opts ebiten.
 func (d *DrawDFS) Scan(visit func(interface{}) error) error {
 	return visit(d.Child)
 }
+
+func (d *DrawDFS) String() string { return "DrawDFS" }

engine/game.go

@@ -169,46 +169,6 @@ func (g *Game) Scan(visit func(interface{}) error) error {
 	return visit(g.Root)
 }
 
-// PreorderWalk calls visit with every component and its parent, reachable from
-// the  given component via Scan, for as long as visit returns nil. The parent
-// value passed to visit when visiting component will be nil. The parent will be
-// visited before the children.
-func PreorderWalk(component interface{}, visit func(component, parent interface{}) error) error {
-	return preorderWalk(component, nil, visit)
-}
-
-func preorderWalk(component, parent interface{}, visit func(component, parent interface{}) error) error {
-	if err := visit(component, parent); err != nil {
-		return err
-	}
-	if sc, ok := component.(Scanner); ok {
-		return sc.Scan(func(c interface{}) error {
-			return preorderWalk(c, component, visit)
-		})
-	}
-	return nil
-}
-
-// PostorderWalk calls visit with every component and its parent, reachable from
-// the  given component via Scan, for as long as visit returns nil. The parent
-// value passed to visit when visiting component will be nil. The children will
-// be visited before the parent.
-func PostorderWalk(component interface{}, visit func(component, parent interface{}) error) error {
-	return postorderWalk(component, nil, visit)
-}
-
-func postorderWalk(component, parent interface{}, visit func(component, parent interface{}) error) error {
-	if sc, ok := component.(Scanner); ok {
-		scv := func(c interface{}) error {
-			return postorderWalk(c, component, visit)
-		}
-		if err := sc.Scan(scv); err != nil {
-			return err
-		}
-	}
-	return visit(component, parent)
-}
-
 // Load loads a component and all subcomponents recursively.
 // Note that this method does not implement Loader.
 func (g *Game) Load(component interface{}, assets fs.FS) error {
@@ -258,14 +218,9 @@ func (g *Game) LoadAndPrepare(assets fs.FS) error {
 
 	// Build the component databases
 	startBuild := time.Now()
-	g.dbmu.Lock()
+	if err := g.build(); err != nil {
-	g.byID = make(map[string]Identifier)
-	g.byAB = make(map[abKey]map[interface{}]struct{})
-	g.par = make(map[interface{}]interface{})
-	if err := PreorderWalk(g, g.registerOne); err != nil {
 		return err
 	}
-	g.dbmu.Unlock()
 	log.Printf("finished building db in %v", time.Since(startBuild))
 
 	// Prepare all the Preppers
@@ -277,6 +232,15 @@ func (g *Game) LoadAndPrepare(assets fs.FS) error {
 	return nil
 }
 
+func (g *Game) build() error {
+	g.dbmu.Lock()
+	defer g.dbmu.Unlock()
+	g.byID = make(map[string]Identifier)
+	g.byAB = make(map[abKey]map[interface{}]struct{})
+	g.par = make(map[interface{}]interface{})
+	return g.registerRecursive(g, nil)
+}
+
 // Register registers a component into the component database (as the
 // child of a given parent). Passing a nil component or parent is an error.
 // Registering multiple components with the same ID is also an error.
@@ -291,8 +255,19 @@ func (g *Game) Register(component, parent interface{}) error {
 	}
 	g.dbmu.Lock()
 	defer g.dbmu.Unlock()
-	// preorderWalk goes in the right order for registering.
+	return g.registerRecursive(component, parent)
-	return preorderWalk(component, parent, g.registerOne)
+}
+
+func (g *Game) registerRecursive(component, parent interface{}) error {
+	if err := g.registerOne(component, parent); err != nil {
+		return err
+	}
+	if sc, ok := component.(Scanner); ok {
+		return sc.Scan(func(x interface{}) error {
+			return g.registerRecursive(x, component)
+		})
+	}
+	return nil
 }
 
 func (g *Game) registerOne(component, parent interface{}) error {
@@ -335,12 +310,21 @@ func (g *Game) Unregister(component interface{}) {
 		return
 	}
 	g.dbmu.Lock()
-	// postorderWalk goes in the right order for unregistering.
+	g.unregisterRecursive(component)
-	postorderWalk(component, nil, g.unregisterOne)
 	g.dbmu.Unlock()
 }
 
-func (g *Game) unregisterOne(component, _ interface{}) error {
+func (g *Game) unregisterRecursive(component interface{}) {
+	if sc, ok := component.(Scanner); ok {
+		sc.Scan(func(x interface{}) error {
+			g.unregisterRecursive(x)
+			return nil
+		})
+	}
+	g.unregisterOne(component)
+}
+
+func (g *Game) unregisterOne(component interface{}) {
 	// unregister from g.byAB, using g.par to trace the path
 	ct := reflect.TypeOf(component)
 	for _, b := range Behaviours {
@@ -361,9 +345,10 @@ func (g *Game) unregisterOne(component, _ interface{}) error {
 	if id, ok := component.(Identifier); ok && id.Ident() != "" {
 		delete(g.byID, id.Ident())
 	}
-	return nil
 }
 
+func (g *Game) String() string { return "Game" }
+
 // --------- Helper stuff ---------
 
 type abKey struct {

engine/imageref.go

@@ -57,3 +57,5 @@ func (r *ImageRef) Load(assets fs.FS) error {
 	imageCache[assetKey{assets, r.Path}] = r.image
 	return nil
 }
+
+func (r *ImageRef) String() string { return "ImageRef{" + r.Path + "}" }

engine/parallax.go

@@ -43,6 +43,8 @@ func (p *Parallax) Scan(visit func(interface{}) error) error {
 	return visit(p.Child)
 }
 
+func (p *Parallax) String() string { return "Parallax" }
+
 // Transform returns a GeoM translation of Factor * camera.Centre.
 func (p *Parallax) Transform() (opts ebiten.DrawImageOptions) {
 	x, y := geom.CFloat(p.camera.Centre)

engine/prisms.go

@@ -125,6 +125,8 @@ func (m *PrismMap) Scan(visit func(interface{}) error) error {
 	return nil
 }
 
+func (m *PrismMap) String() string { return "PrismMap" }
+
 // Transform retrurns a translation by the draw offset.
 func (m *PrismMap) Transform() (opts ebiten.DrawImageOptions) {
 	opts.GeoM.Translate(geom.CFloat(m.DrawOffset))

engine/repl.go

@@ -88,23 +88,26 @@ func (g *Game) cmdTree(dst io.Writer, argv []string) {
 			return
 		}
 	}
-	PreorderWalk(c, func(w, _ interface{}) error {
+	g.printTreeRecursive(dst, 0, c)
+}
+
+func (g *Game) printTreeRecursive(dst io.Writer, depth int, c interface{}) {
 	indent := ""
-		l := 0
+	if depth > 0 {
-		for p := w; p != c; p = g.par[p] {
+		indent = strings.Repeat("  ", depth-1) + "↳ "
-			l++
 	}
-		if l > 0 {
+	i, ok := c.(Identifier)
-			indent = strings.Repeat("  ", l-1) + "↳ "
-		}
-		i, ok := w.(Identifier)
 	if ok {
-			fmt.Fprintf(dst, "%s%T %q\n", indent, w, i.Ident())
+		fmt.Fprintf(dst, "%s%v %q\n", indent, c, i.Ident())
 	} else {
-			fmt.Fprintf(dst, "%s%T\n", indent, w)
+		fmt.Fprintf(dst, "%s%v\n", indent, c)
 	}
+	if sc, ok := c.(Scanner); ok {
+		sc.Scan(func(x interface{}) error {
+			g.printTreeRecursive(dst, depth+1, x)
 			return nil
 		})
+	}
 }
 
 func (g *Game) cmdQuery(dst io.Writer, argv []string) {

engine/scene.go

@@ -43,6 +43,8 @@ func (s *Scene) Scan(visit func(interface{}) error) error {
 	return visit(s.Child)
 }
 
+func (s *Scene) String() string { return "Scene" }
+
 // SceneRef loads a gzipped, gob-encoded Scene from the asset FS.
 // After Load, Scene is usable.
 // This is mostly useful for scenes that refer to other scenes, e.g.
@@ -83,3 +85,5 @@ func (r *SceneRef) Load(assets fs.FS) error {
 func (r *SceneRef) Save() error {
 	return SaveGobz(r.Scene, filepath.Base(r.Path))
 }
+
+func (r *SceneRef) String() string { return "SceneRef{" + r.Path + "}" }

engine/sheet.go

@@ -57,3 +57,5 @@ func (s *Sheet) SubImage(i int) *ebiten.Image {
 	r := image.Rectangle{p, p.Add(s.CellSize)}
 	return s.Src.Image().SubImage(r).(*ebiten.Image)
 }
+
+func (s *Sheet) String() string { return "Sheet" }