shareable: Avoid unneeded extending images

internal/packing/packing.go

@@ -48,8 +48,8 @@ type Node struct {
 	y      int
 	width  int
 	height int
-
 	used   bool
+
 	parent *Node
 	child0 *Node
 	child1 *Node
@@ -268,3 +268,33 @@ func (p *Page) Extend() bool {
 	p.size = newSize
 	return true
 }
+
+func (n *Node) clone() *Node {
+	if n == nil {
+		return nil
+	}
+	cloned := &Node{
+		x:      n.x,
+		y:      n.y,
+		width:  n.width,
+		height: n.height,
+		used:   n.used,
+		child0: n.child0.clone(),
+		child1: n.child1.clone(),
+	}
+	if cloned.child0 != nil {
+		cloned.child0.parent = cloned
+	}
+	if cloned.child1 != nil {
+		cloned.child1.parent = cloned
+	}
+	return cloned
+}
+
+func (p *Page) Clone() *Page {
+	return &Page{
+		root:    p.root.clone(),
+		size:    p.size,
+		maxSize: p.maxSize,
+	}
+}

internal/shareable/shareable.go

@@ -35,22 +35,39 @@ type backend struct {
 }
 
 func (b *backend) TryAlloc(width, height int) (*packing.Node, bool) {
+	// If the region is allocated without any extention, it's fine.
+	if n := b.page.Alloc(width, height); n != nil {
+		return n, true
+	}
+
+	// Simulate the extending the page and calculate the appropriate page size.
+	page := b.page.Clone()
+	nExtended := 0
 	for {
-		if n := b.page.Alloc(width, height); n != nil {
-			return n, true
+		if !page.Extend() {
+			// The page can't be extended any more. Return as failure.
+			return nil, false
 		}
-		if !b.page.Extend() {
+		nExtended++
+		if n := page.Alloc(width, height); n != nil {
 			break
 		}
-
-		s := b.page.Size()
-		newImg := restorable.NewImage(s, s, false)
-		newImg.DrawImage(b.restorable, 0, 0, s, s, nil, nil, opengl.CompositeModeCopy, graphics.FilterNearest)
-
-		b.restorable.Dispose()
-		b.restorable = newImg
 	}
-	return nil, false
+
+	for i := 0; i < nExtended; i++ {
+		b.page.Extend()
+	}
+	s := b.page.Size()
+	newImg := restorable.NewImage(s, s, false)
+	newImg.DrawImage(b.restorable, 0, 0, s, s, nil, nil, opengl.CompositeModeCopy, graphics.FilterNearest)
+	b.restorable.Dispose()
+	b.restorable = newImg
+
+	n := b.page.Alloc(width, height)
+	if n == nil {
+		panic("not reached")
+	}
+	return n, true
 }
 
 var (