exp/html: change a node's children from a slice to a linked list.

Also rename Node.{Add,Remove} to Node.{AppendChild,RemoveChild} to
be consistent with the DOM.

benchmark                      old ns/op    new ns/op    delta
BenchmarkParser                  4042040      3749618   -7.23%

benchmark                       old MB/s     new MB/s  speedup
BenchmarkParser                    19.34        20.85    1.08x

BenchmarkParser mallocs per iteration is also:
10495 before / 7992 after

R=andybalholm, r, adg
CC=golang-dev
https://golang.org/cl/6495061

src/pkg/exp/html/node.go

@@ -36,8 +36,8 @@ var scopeMarker = Node{Type: scopeMarkerNode}
 // Similarly, "math" is short for "http://www.w3.org/1998/Math/MathML", and
 // "svg" is short for "http://www.w3.org/2000/svg".
 type Node struct {
-	Parent    *Node
-	Child     []*Node
+	Parent, FirstChild, LastChild, PrevSibling, NextSibling *Node
+
 	Type      NodeType
 	DataAtom  atom.Atom
 	Data      string
@@ -45,48 +45,93 @@ type Node struct {
 	Attr      []Attribute
 }
 
-// Add adds a node as a child of n.
-// It will panic if the child's parent is not nil.
-func (n *Node) Add(child *Node) {
-	if child.Parent != nil {
-		panic("html: Node.Add called for a child Node that already has a parent")
+// InsertBefore inserts newChild as a child of n, immediately before oldChild
+// in the sequence of n's children. oldChild may be nil, in which case newChild
+// is appended to the end of n's children.
+//
+// It will panic if newChild already has a parent or siblings.
+func (n *Node) InsertBefore(newChild, oldChild *Node) {
+	if newChild.Parent != nil || newChild.PrevSibling != nil || newChild.NextSibling != nil {
+		panic("html: InsertBefore called for an attached child Node")
 	}
-	child.Parent = n
-	n.Child = append(n.Child, child)
+	var prev, next *Node
+	if oldChild != nil {
+		prev, next = oldChild.PrevSibling, oldChild
+	} else {
+		prev = n.LastChild
+	}
+	if prev != nil {
+		prev.NextSibling = newChild
+	} else {
+		n.FirstChild = newChild
+	}
+	if next != nil {
+		next.PrevSibling = newChild
+	} else {
+		n.LastChild = newChild
+	}
+	newChild.Parent = n
+	newChild.PrevSibling = prev
+	newChild.NextSibling = next
 }
 
-// Remove removes a node as a child of n.
-// It will panic if the child's parent is not n.
-func (n *Node) Remove(child *Node) {
-	if child.Parent == n {
-		child.Parent = nil
-		for i, m := range n.Child {
-			if m == child {
-				copy(n.Child[i:], n.Child[i+1:])
-				j := len(n.Child) - 1
-				n.Child[j] = nil
-				n.Child = n.Child[:j]
-				return
-			}
-		}
+// AppendChild adds a node c as a child of n.
+//
+// It will panic if c already has a parent or siblings.
+func (n *Node) AppendChild(c *Node) {
+	if c.Parent != nil || c.PrevSibling != nil || c.NextSibling != nil {
+		panic("html: AppendChild called for an attached child Node")
 	}
-	panic("html: Node.Remove called for a non-child Node")
+	last := n.LastChild
+	if last != nil {
+		last.NextSibling = c
+	} else {
+		n.FirstChild = c
+	}
+	n.LastChild = c
+	c.Parent = n
+	c.PrevSibling = last
+}
+
+// RemoveChild removes a node c that is a child of n. Afterwards, c will have
+// no parent and no siblings.
+//
+// It will panic if c's parent is not n.
+func (n *Node) RemoveChild(c *Node) {
+	if c.Parent != n {
+		panic("html: RemoveChild called for a non-child Node")
+	}
+	if n.FirstChild == c {
+		n.FirstChild = c.NextSibling
+	}
+	if c.NextSibling != nil {
+		c.NextSibling.PrevSibling = c.PrevSibling
+	}
+	if n.LastChild == c {
+		n.LastChild = c.PrevSibling
+	}
+	if c.PrevSibling != nil {
+		c.PrevSibling.NextSibling = c.NextSibling
+	}
+	c.Parent = nil
+	c.PrevSibling = nil
+	c.NextSibling = nil
 }
 
 // reparentChildren reparents all of src's child nodes to dst.
 func reparentChildren(dst, src *Node) {
-	for _, n := range src.Child {
-		if n.Parent != src {
-			panic("html: nodes have an inconsistent parent/child relationship")
+	for {
+		child := src.FirstChild
+		if child == nil {
+			break
 		}
-		n.Parent = dst
+		src.RemoveChild(child)
+		dst.AppendChild(child)
 	}
-	dst.Child = append(dst.Child, src.Child...)
-	src.Child = nil
 }
 
 // clone returns a new node with the same type, data and attributes.
-// The clone has no parent and no children.
+// The clone has no parent, no siblings and no children.
 func (n *Node) clone() *Node {
 	m := &Node{
 		Type:     n.Type,

src/pkg/exp/html/node_test.go

@@ -0,0 +1,146 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package html
+
+import (
+	"fmt"
+)
+
+// checkTreeConsistency checks that a node and its descendants are all
+// consistent in their parent/child/sibling relationships.
+func checkTreeConsistency(n *Node) error {
+	return checkTreeConsistency1(n, 0)
+}
+
+func checkTreeConsistency1(n *Node, depth int) error {
+	if depth == 1e4 {
+		return fmt.Errorf("html: tree looks like it contains a cycle")
+	}
+	if err := checkNodeConsistency(n); err != nil {
+		return err
+	}
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		if err := checkTreeConsistency1(c, depth+1); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// checkNodeConsistency checks that a node's parent/child/sibling relationships
+// are consistent.
+func checkNodeConsistency(n *Node) error {
+	if n == nil {
+		return nil
+	}
+
+	nParent := 0
+	for p := n.Parent; p != nil; p = p.Parent {
+		nParent++
+		if nParent == 1e4 {
+			return fmt.Errorf("html: parent list looks like an infinite loop")
+		}
+	}
+
+	nForward := 0
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		nForward++
+		if nForward == 1e6 {
+			return fmt.Errorf("html: forward list of children looks like an infinite loop")
+		}
+		if c.Parent != n {
+			return fmt.Errorf("html: inconsistent child/parent relationship")
+		}
+	}
+
+	nBackward := 0
+	for c := n.LastChild; c != nil; c = c.PrevSibling {
+		nBackward++
+		if nBackward == 1e6 {
+			return fmt.Errorf("html: backward list of children looks like an infinite loop")
+		}
+		if c.Parent != n {
+			return fmt.Errorf("html: inconsistent child/parent relationship")
+		}
+	}
+
+	if n.Parent != nil {
+		if n.Parent == n {
+			return fmt.Errorf("html: inconsistent parent relationship")
+		}
+		if n.Parent == n.FirstChild {
+			return fmt.Errorf("html: inconsistent parent/first relationship")
+		}
+		if n.Parent == n.LastChild {
+			return fmt.Errorf("html: inconsistent parent/last relationship")
+		}
+		if n.Parent == n.PrevSibling {
+			return fmt.Errorf("html: inconsistent parent/prev relationship")
+		}
+		if n.Parent == n.NextSibling {
+			return fmt.Errorf("html: inconsistent parent/next relationship")
+		}
+
+		parentHasNAsAChild := false
+		for c := n.Parent.FirstChild; c != nil; c = c.NextSibling {
+			if c == n {
+				parentHasNAsAChild = true
+				break
+			}
+		}
+		if !parentHasNAsAChild {
+			return fmt.Errorf("html: inconsistent parent/child relationship")
+		}
+	}
+
+	if n.PrevSibling != nil && n.PrevSibling.NextSibling != n {
+		return fmt.Errorf("html: inconsistent prev/next relationship")
+	}
+	if n.NextSibling != nil && n.NextSibling.PrevSibling != n {
+		return fmt.Errorf("html: inconsistent next/prev relationship")
+	}
+
+	if (n.FirstChild == nil) != (n.LastChild == nil) {
+		return fmt.Errorf("html: inconsistent first/last relationship")
+	}
+	if n.FirstChild != nil && n.FirstChild == n.LastChild {
+		// We have a sole child.
+		if n.FirstChild.PrevSibling != nil || n.FirstChild.NextSibling != nil {
+			return fmt.Errorf("html: inconsistent sole child's sibling relationship")
+		}
+	}
+
+	seen := map[*Node]bool{}
+
+	var last *Node
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		if seen[c] {
+			return fmt.Errorf("html: inconsistent repeated child")
+		}
+		seen[c] = true
+		last = c
+	}
+	if last != n.LastChild {
+		return fmt.Errorf("html: inconsistent last relationship")
+	}
+
+	var first *Node
+	for c := n.LastChild; c != nil; c = c.PrevSibling {
+		if !seen[c] {
+			return fmt.Errorf("html: inconsistent missing child")
+		}
+		delete(seen, c)
+		first = c
+	}
+	if first != n.FirstChild {
+		return fmt.Errorf("html: inconsistent first relationship")
+	}
+
+	if len(seen) != 0 {
+		return fmt.Errorf("html: inconsistent forwards/backwards child list")
+	}
+
+	return nil
+}

src/pkg/exp/html/parse.go

@@ -212,7 +212,7 @@ func (p *parser) addChild(n *Node) {
 	if p.shouldFosterParent() {
 		p.fosterParent(n)
 	} else {
-		p.top().Add(n)
+		p.top().AppendChild(n)
 	}
 
 	if n.Type == ElementNode {
@@ -235,7 +235,7 @@ func (p *parser) shouldFosterParent() bool {
 // fosterParent adds a child node according to the foster parenting rules.
 // Section 12.2.5.3, "foster parenting".
 func (p *parser) fosterParent(n *Node) {
-	var table, parent *Node
+	var table, parent, prev *Node
 	var i int
 	for i = len(p.oe) - 1; i >= 0; i-- {
 		if p.oe[i].DataAtom == a.Table {
@@ -254,26 +254,17 @@ func (p *parser) fosterParent(n *Node) {
 		parent = p.oe[i-1]
 	}
 
-	var child *Node
-	for i, child = range parent.Child {
-		if child == table {
-			break
-		}
+	if table != nil {
+		prev = table.PrevSibling
+	} else {
+		prev = parent.LastChild
 	}
-
-	if i > 0 && parent.Child[i-1].Type == TextNode && n.Type == TextNode {
-		parent.Child[i-1].Data += n.Data
+	if prev != nil && prev.Type == TextNode && n.Type == TextNode {
+		prev.Data += n.Data
 		return
 	}
 
-	if i == len(parent.Child) {
-		parent.Add(n)
-	} else {
-		// Insert n into parent.Child at index i.
-		parent.Child = append(parent.Child[:i+1], parent.Child[i:]...)
-		parent.Child[i] = n
-		n.Parent = parent
-	}
+	parent.InsertBefore(n, table)
 }
 
 // addText adds text to the preceding node if it is a text node, or else it
@@ -292,8 +283,8 @@ func (p *parser) addText(text string) {
 	}
 
 	t := p.top()
-	if i := len(t.Child); i > 0 && t.Child[i-1].Type == TextNode {
-		t.Child[i-1].Data += text
+	if n := t.LastChild; n != nil && n.Type == TextNode {
+		n.Data += text
 		return
 	}
 	p.addChild(&Node{
@@ -470,14 +461,14 @@ func initialIM(p *parser) bool {
 			return true
 		}
 	case CommentToken:
-		p.doc.Add(&Node{
+		p.doc.AppendChild(&Node{
 			Type: CommentNode,
 			Data: p.tok.Data,
 		})
 		return true
 	case DoctypeToken:
 		n, quirks := parseDoctype(p.tok.Data)
-		p.doc.Add(n)
+		p.doc.AppendChild(n)
 		p.quirks = quirks
 		p.im = beforeHTMLIM
 		return true
@@ -515,7 +506,7 @@ func beforeHTMLIM(p *parser) bool {
 			return true
 		}
 	case CommentToken:
-		p.doc.Add(&Node{
+		p.doc.AppendChild(&Node{
 			Type: CommentNode,
 			Data: p.tok.Data,
 		})
@@ -712,7 +703,7 @@ func inBodyIM(p *parser) bool {
 		d := p.tok.Data
 		switch n := p.oe.top(); n.DataAtom {
 		case a.Pre, a.Listing:
-			if len(n.Child) == 0 {
+			if n.FirstChild == nil {
 				// Ignore a newline at the start of a <pre> block.
 				if d != "" && d[0] == '\r' {
 					d = d[1:]
@@ -753,7 +744,7 @@ func inBodyIM(p *parser) bool {
 			}
 			body := p.oe[1]
 			if body.Parent != nil {
-				body.Parent.Remove(body)
+				body.Parent.RemoveChild(body)
 			}
 			p.oe = p.oe[:1]
 			p.addElement()
@@ -1128,9 +1119,9 @@ func (p *parser) inBodyEndTagFormatting(tagAtom a.Atom) {
 			}
 			// Step 9.9.
 			if lastNode.Parent != nil {
-				lastNode.Parent.Remove(lastNode)
+				lastNode.Parent.RemoveChild(lastNode)
 			}
-			node.Add(lastNode)
+			node.AppendChild(lastNode)
 			// Step 9.10.
 			lastNode = node
 		}
@@ -1138,20 +1129,20 @@ func (p *parser) inBodyEndTagFormatting(tagAtom a.Atom) {
 		// Step 10. Reparent lastNode to the common ancestor,
 		// or for misnested table nodes, to the foster parent.
 		if lastNode.Parent != nil {
-			lastNode.Parent.Remove(lastNode)
+			lastNode.Parent.RemoveChild(lastNode)
 		}
 		switch commonAncestor.DataAtom {
 		case a.Table, a.Tbody, a.Tfoot, a.Thead, a.Tr:
 			p.fosterParent(lastNode)
 		default:
-			commonAncestor.Add(lastNode)
+			commonAncestor.AppendChild(lastNode)
 		}
 
 		// Steps 11-13. Reparent nodes from the furthest block's children
 		// to a clone of the formatting element.
 		clone := formattingElement.clone()
 		reparentChildren(clone, furthestBlock)
-		furthestBlock.Add(clone)
+		furthestBlock.AppendChild(clone)
 
 		// Step 14. Fix up the list of active formatting elements.
 		if oldLoc := p.afe.index(formattingElement); oldLoc != -1 && oldLoc < bookmark {
@@ -1187,7 +1178,7 @@ func textIM(p *parser) bool {
 		p.oe.pop()
 	case TextToken:
 		d := p.tok.Data
-		if n := p.oe.top(); n.DataAtom == a.Textarea && len(n.Child) == 0 {
+		if n := p.oe.top(); n.DataAtom == a.Textarea && n.FirstChild == nil {
 			// Ignore a newline at the start of a <textarea> block.
 			if d != "" && d[0] == '\r' {
 				d = d[1:]
@@ -1626,7 +1617,7 @@ func inSelectIM(p *parser) bool {
 			}
 		}
 	case CommentToken:
-		p.doc.Add(&Node{
+		p.doc.AppendChild(&Node{
 			Type: CommentNode,
 			Data: p.tok.Data,
 		})
@@ -1684,7 +1675,7 @@ func afterBodyIM(p *parser) bool {
 		if len(p.oe) < 1 || p.oe[0].DataAtom != a.Html {
 			panic("html: bad parser state: <html> element not found, in the after-body insertion mode")
 		}
-		p.oe[0].Add(&Node{
+		p.oe[0].AppendChild(&Node{
 			Type: CommentNode,
 			Data: p.tok.Data,
 		})
@@ -1800,7 +1791,7 @@ func afterAfterBodyIM(p *parser) bool {
 			return inBodyIM(p)
 		}
 	case CommentToken:
-		p.doc.Add(&Node{
+		p.doc.AppendChild(&Node{
 			Type: CommentNode,
 			Data: p.tok.Data,
 		})
@@ -1816,7 +1807,7 @@ func afterAfterBodyIM(p *parser) bool {
 func afterAfterFramesetIM(p *parser) bool {
 	switch p.tok.Type {
 	case CommentToken:
-		p.doc.Add(&Node{
+		p.doc.AppendChild(&Node{
 			Type: CommentNode,
 			Data: p.tok.Data,
 		})
@@ -2068,7 +2059,7 @@ func ParseFragment(r io.Reader, context *Node) ([]*Node, error) {
 		DataAtom: a.Html,
 		Data:     a.Html.String(),
 	}
-	p.doc.Add(root)
+	p.doc.AppendChild(root)
 	p.oe = nodeStack{root}
 	p.resetInsertionMode()
 
@@ -2089,10 +2080,12 @@ func ParseFragment(r io.Reader, context *Node) ([]*Node, error) {
 		parent = root
 	}
 
-	result := parent.Child
-	parent.Child = nil
-	for _, n := range result {
-		n.Parent = nil
+	var result []*Node
+	for c := parent.FirstChild; c != nil; {
+		next := c.NextSibling
+		parent.RemoveChild(c)
+		result = append(result, c)
+		c = next
 	}
 	return result, nil
 }

src/pkg/exp/html/parse_test.go

@@ -177,7 +177,7 @@ func dumpLevel(w io.Writer, n *Node, level int) error {
 		return errors.New("unknown node type")
 	}
 	io.WriteString(w, "\n")
-	for _, c := range n.Child {
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
 		if err := dumpLevel(w, c, level+1); err != nil {
 			return err
 		}
@@ -186,12 +186,12 @@ func dumpLevel(w io.Writer, n *Node, level int) error {
 }
 
 func dump(n *Node) (string, error) {
-	if n == nil || len(n.Child) == 0 {
+	if n == nil || n.FirstChild == nil {
 		return "", nil
 	}
 	var b bytes.Buffer
-	for _, child := range n.Child {
-		if err := dumpLevel(&b, child, 0); err != nil {
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		if err := dumpLevel(&b, c, 0); err != nil {
 			return "", err
 		}
 	}
@@ -267,10 +267,14 @@ func testParseCase(text, want, context string) (err error) {
 			Type: DocumentNode,
 		}
 		for _, n := range nodes {
-			doc.Add(n)
+			doc.AppendChild(n)
 		}
 	}
 
+	if err := checkTreeConsistency(doc); err != nil {
+		return err
+	}
+
 	got, err := dump(doc)
 	if err != nil {
 		return err

src/pkg/exp/html/render.go

@@ -73,7 +73,7 @@ func render1(w writer, n *Node) error {
 	case TextNode:
 		return escape(w, n.Data)
 	case DocumentNode:
-		for _, c := range n.Child {
+		for c := n.FirstChild; c != nil; c = c.NextSibling {
 			if err := render1(w, c); err != nil {
 				return err
 			}
@@ -171,7 +171,7 @@ func render1(w writer, n *Node) error {
 		}
 	}
 	if voidElements[n.Data] {
-		if len(n.Child) != 0 {
+		if n.FirstChild != nil {
 			return fmt.Errorf("html: void element <%s> has child nodes", n.Data)
 		}
 		_, err := w.WriteString("/>")
@@ -182,7 +182,7 @@ func render1(w writer, n *Node) error {
 	}
 
 	// Add initial newline where there is danger of a newline beging ignored.
-	if len(n.Child) > 0 && n.Child[0].Type == TextNode && strings.HasPrefix(n.Child[0].Data, "\n") {
+	if c := n.FirstChild; c != nil && c.Type == TextNode && strings.HasPrefix(c.Data, "\n") {
 		switch n.Data {
 		case "pre", "listing", "textarea":
 			if err := w.WriteByte('\n'); err != nil {
@@ -194,7 +194,7 @@ func render1(w writer, n *Node) error {
 	// Render any child nodes.
 	switch n.Data {
 	case "iframe", "noembed", "noframes", "noscript", "plaintext", "script", "style", "xmp":
-		for _, c := range n.Child {
+		for c := n.FirstChild; c != nil; c = c.NextSibling {
 			if c.Type == TextNode {
 				if _, err := w.WriteString(c.Data); err != nil {
 					return err
@@ -211,7 +211,7 @@ func render1(w writer, n *Node) error {
 			return plaintextAbort
 		}
 	default:
-		for _, c := range n.Child {
+		for c := n.FirstChild; c != nil; c = c.NextSibling {
 			if err := render1(w, c); err != nil {
 				return err
 			}

src/pkg/exp/html/render_test.go

@@ -10,99 +10,144 @@ import (
 )
 
 func TestRenderer(t *testing.T) {
-	n := &Node{
-		Type: ElementNode,
-		Data: "html",
-		Child: []*Node{
-			{
-				Type: ElementNode,
-				Data: "head",
-			},
-			{
-				Type: ElementNode,
-				Data: "body",
-				Child: []*Node{
-					{
-						Type: TextNode,
-						Data: "0<1",
-					},
-					{
-						Type: ElementNode,
-						Data: "p",
-						Attr: []Attribute{
-							{
-								Key: "id",
-								Val: "A",
-							},
-							{
-								Key: "foo",
-								Val: `abc"def`,
-							},
-						},
-						Child: []*Node{
-							{
-								Type: TextNode,
-								Data: "2",
-							},
-							{
-								Type: ElementNode,
-								Data: "b",
-								Attr: []Attribute{
-									{
-										Key: "empty",
-										Val: "",
-									},
-								},
-								Child: []*Node{
-									{
-										Type: TextNode,
-										Data: "3",
-									},
-								},
-							},
-							{
-								Type: ElementNode,
-								Data: "i",
-								Attr: []Attribute{
-									{
-										Key: "backslash",
-										Val: `\`,
-									},
-								},
-								Child: []*Node{
-									{
-										Type: TextNode,
-										Data: "&4",
-									},
-								},
-							},
-						},
-					},
-					{
-						Type: TextNode,
-						Data: "5",
-					},
-					{
-						Type: ElementNode,
-						Data: "blockquote",
-					},
-					{
-						Type: ElementNode,
-						Data: "br",
-					},
-					{
-						Type: TextNode,
-						Data: "6",
-					},
+	nodes := [...]*Node{
+		0: {
+			Type: ElementNode,
+			Data: "html",
+		},
+		1: {
+			Type: ElementNode,
+			Data: "head",
+		},
+		2: {
+			Type: ElementNode,
+			Data: "body",
+		},
+		3: {
+			Type: TextNode,
+			Data: "0<1",
+		},
+		4: {
+			Type: ElementNode,
+			Data: "p",
+			Attr: []Attribute{
+				{
+					Key: "id",
+					Val: "A",
+				},
+				{
+					Key: "foo",
+					Val: `abc"def`,
 				},
 			},
 		},
+		5: {
+			Type: TextNode,
+			Data: "2",
+		},
+		6: {
+			Type: ElementNode,
+			Data: "b",
+			Attr: []Attribute{
+				{
+					Key: "empty",
+					Val: "",
+				},
+			},
+		},
+		7: {
+			Type: TextNode,
+			Data: "3",
+		},
+		8: {
+			Type: ElementNode,
+			Data: "i",
+			Attr: []Attribute{
+				{
+					Key: "backslash",
+					Val: `\`,
+				},
+			},
+		},
+		9: {
+			Type: TextNode,
+			Data: "&4",
+		},
+		10: {
+			Type: TextNode,
+			Data: "5",
+		},
+		11: {
+			Type: ElementNode,
+			Data: "blockquote",
+		},
+		12: {
+			Type: ElementNode,
+			Data: "br",
+		},
+		13: {
+			Type: TextNode,
+			Data: "6",
+		},
 	}
+
+	// Build a tree out of those nodes, based on a textual representation.
+	// Only the ".\t"s are significant. The trailing HTML-like text is
+	// just commentary. The "0:" prefixes are for easy cross-reference with
+	// the nodes array.
+	treeAsText := [...]string{
+		0: `<html>`,
+		1: `.	<head>`,
+		2: `.	<body>`,
+		3: `.	.	"0&lt;1"`,
+		4: `.	.	<p id="A" foo="abc&#34;def">`,
+		5: `.	.	.	"2"`,
+		6: `.	.	.	<b empty="">`,
+		7: `.	.	.	.	"3"`,
+		8: `.	.	.	<i backslash="\">`,
+		9: `.	.	.	.	"&amp;4"`,
+		10: `.	.	"5"`,
+		11: `.	.	<blockquote>`,
+		12: `.	.	<br>`,
+		13: `.	.	"6"`,
+	}
+	if len(nodes) != len(treeAsText) {
+		t.Fatal("len(nodes) != len(treeAsText)")
+	}
+	var stack [8]*Node
+	for i, line := range treeAsText {
+		level := 0
+		for line[0] == '.' {
+			// Strip a leading ".\t".
+			line = line[2:]
+			level++
+		}
+		n := nodes[i]
+		if level == 0 {
+			if stack[0] != nil {
+				t.Fatal("multiple root nodes")
+			}
+			stack[0] = n
+		} else {
+			stack[level-1].AppendChild(n)
+			stack[level] = n
+			for i := level + 1; i < len(stack); i++ {
+				stack[i] = nil
+			}
+		}
+		// At each stage of tree construction, we check all nodes for consistency.
+		for j, m := range nodes {
+			if err := checkNodeConsistency(m); err != nil {
+				t.Fatalf("i=%d, j=%d: %v", i, j, err)
+			}
+		}
+	}
+
 	want := `<html><head></head><body>0&lt;1<p id="A" foo="abc&#34;def">` +
 		`2<b empty="">3</b><i backslash="\">&amp;4</i></p>` +
 		`5<blockquote></blockquote><br/>6</body></html>`
 	b := new(bytes.Buffer)
-	if err := Render(b, n); err != nil {
+	if err := Render(b, nodes[0]); err != nil {
 		t.Fatal(err)
 	}
 	if got := b.String(); got != want {