From 86be32dfb5081c520a01e9072d6f8562b0086a2f Mon Sep 17 00:00:00 2001
From: Hajime Hoshi <hajimehoshi@gmail.com>
Date: Thu, 8 Mar 2018 00:18:28 +0900
Subject: [PATCH] packing: Add comments

---
 internal/packing/packing.go      | 24 ++++++++++++------------
 internal/packing/packing_test.go |  9 ++++++++-
 2 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/internal/packing/packing.go b/internal/packing/packing.go
index 0578de3e0..2f9fb6090 100644
--- a/internal/packing/packing.go
+++ b/internal/packing/packing.go
@@ -231,19 +231,28 @@ func (p *Page) Extend() bool {
 	newSize := p.size * 2
 	edgeNodes := []*Node{}
 	abort := errors.New("abort")
-	aborted := false
+	canExtendNodes := true
 	_ = walk(p.root, func(n *Node) error {
 		if n.x+n.width < p.size && n.y+n.height < p.size {
 			return nil
 		}
 		if n.used {
-			aborted = true
+			canExtendNodes = false
 			return abort
 		}
 		edgeNodes = append(edgeNodes, n)
 		return nil
 	})
-	if aborted {
+	if canExtendNodes {
+		for _, n := range edgeNodes {
+			if n.x+n.width == p.size {
+				n.width += newSize - p.size
+			}
+			if n.y+n.height == p.size {
+				n.height += newSize - p.size
+			}
+		}
+	} else {
 		leftUpper := p.root
 		leftLower := &Node{
 			x:      0,
@@ -278,15 +287,6 @@ func (p *Page) Extend() bool {
 		}
 		left.parent = p.root
 		right.parent = p.root
-	} else {
-		for _, n := range edgeNodes {
-			if n.x+n.width == p.size {
-				n.width += newSize - p.size
-			}
-			if n.y+n.height == p.size {
-				n.height += newSize - p.size
-			}
-		}
 	}
 
 	p.size = newSize
diff --git a/internal/packing/packing_test.go b/internal/packing/packing_test.go
index cf2037f81..a33cf5c76 100644
--- a/internal/packing/packing_test.go
+++ b/internal/packing/packing_test.go
@@ -211,6 +211,7 @@ func TestExtend(t *testing.T) {
 	p := &Page{}
 	s := p.Size()
 	p.Alloc(s/2, s/2)
+	// Now Extend extends the page by exntending the existing edge nodes.
 	p.Extend()
 	if p.Alloc(s*3/2, s*2) == nil {
 		t.Fail()
@@ -223,7 +224,7 @@ func TestExtend(t *testing.T) {
 	}
 }
 
-func TestExtend2(t *testing.T) {
+func TestExtendAddingNewNodes(t *testing.T) {
 	p := &Page{}
 	s := p.Size()
 	p.Alloc(s/2, s/2)
@@ -232,7 +233,13 @@ func TestExtend2(t *testing.T) {
 	p.Alloc(s/2, s/2)
 	p.Free(n1)
 	p.Free(n2)
+	// There is an allocation at lower left.
+	// In this case, Extend doesn't extend the existing edge nodes, but
+	// instead adds new nodes.
 	p.Extend()
+	if p.Alloc(s*3/2, s*2) != nil {
+		t.Fail()
+	}
 	if p.Alloc(s, s*2) == nil {
 		t.Fail()
 	}