Source: html.go in package github.com/russross/blackfriday/v2

Blackfriday Markdown Processor Available at http://github.com/russross/blackfriday Copyright © 2011 Russ Ross <russ@russross.com>. Distributed under the Simplified BSD License. See README.md for details.

HTML rendering backend


package blackfriday

import (
	"bytes"
	"fmt"
	"io"
	"regexp"
	"strings"
)

HTMLFlags control optional behavior of HTML renderer.

type HTMLFlags int

HTML renderer configuration options.

const (
	HTMLFlagsNone           HTMLFlags = 0
	SkipHTML                HTMLFlags = 1 << iota // Skip preformatted HTML blocks
	SkipImages                                    // Skip embedded images
	SkipLinks                                     // Skip all links
	Safelink                                      // Only link to trusted protocols
	NofollowLinks                                 // Only link with rel="nofollow"
	NoreferrerLinks                               // Only link with rel="noreferrer"
	NoopenerLinks                                 // Only link with rel="noopener"
	HrefTargetBlank                               // Add a blank target
	CompletePage                                  // Generate a complete HTML page
	UseXHTML                                      // Generate XHTML output instead of HTML
	FootnoteReturnLinks                           // Generate a link at the end of a footnote to return to the source
	Smartypants                                   // Enable smart punctuation substitutions
	SmartypantsFractions                          // Enable smart fractions (with Smartypants)
	SmartypantsDashes                             // Enable smart dashes (with Smartypants)
	SmartypantsLatexDashes                        // Enable LaTeX-style dashes (with Smartypants)
	SmartypantsAngledQuotes                       // Enable angled double quotes (with Smartypants) for double quotes rendering
	SmartypantsQuotesNBSP                         // Enable « French guillemets » (with Smartypants)
	TOC                                           // Generate a table of contents
)

var (
	htmlTagRe = regexp.MustCompile("(?i)^" + htmlTag)
)

const (
	htmlTag = "(?:" + openTag + "|" + closeTag + "|" + htmlComment + "|" +
		processingInstruction + "|" + declaration + "|" + cdata + ")"
	closeTag              = "</" + tagName + "\\s*[>]"
	openTag               = "<" + tagName + attribute + "*" + "\\s*/?>"
	attribute             = "(?:" + "\\s+" + attributeName + attributeValueSpec + "?)"
	attributeValue        = "(?:" + unquotedValue + "|" + singleQuotedValue + "|" + doubleQuotedValue + ")"
	attributeValueSpec    = "(?:" + "\\s*=" + "\\s*" + attributeValue + ")"
	attributeName         = "[a-zA-Z_:][a-zA-Z0-9:._-]*"
	cdata                 = "<!\\[CDATA\\[[\\s\\S]*?\\]\\]>"
	declaration           = "<![A-Z]+" + "\\s+[^>]*>"
	doubleQuotedValue     = "\"[^\"]*\""
	htmlComment           = "<!---->|<!--(?:-?[^>-])(?:-?[^-])*-->"
	processingInstruction = "[<][?].*?[?][>]"
	singleQuotedValue     = "'[^']*'"
	tagName               = "[A-Za-z][A-Za-z0-9-]*"
	unquotedValue         = "[^\"'=<>`\\x00-\\x20]+"
)

HTMLRendererParameters is a collection of supplementary parameters tweaking the behavior of various parts of HTML renderer.

Prepend this text to each relative URL.

Add this text to each footnote anchor, to ensure uniqueness.

Show this text inside the <a> tag for a footnote return link, if the HTML_FOOTNOTE_RETURN_LINKS flag is enabled. If blank, the string <sup>[return]</sup> is used.

If set, add this text to the front of each Heading ID, to ensure uniqueness.

If set, add this text to the back of each Heading ID, to ensure uniqueness.

Increase heading levels: if the offset is 1, <h1> becomes <h2> etc. Negative offset is also valid. Resulting levels are clipped between 1 and 6.

	HeadingLevelOffset int

	Title string // Document title (used if CompletePage is set)
	CSS   string // Optional CSS file URL (used if CompletePage is set)
	Icon  string // Optional icon file URL (used if CompletePage is set)

	Flags HTMLFlags // Flags allow customizing this renderer's behavior
}

HTMLRenderer is a type that implements the Renderer interface for HTML output. Do not create this directly, instead use the NewHTMLRenderer function.

type HTMLRenderer struct {
	HTMLRendererParameters

	closeTag string // how to end singleton tags: either " />" or ">"

Track heading IDs to prevent ID collision in a single generation.

	headingIDs map[string]int

	lastOutputLen int
	disableTags   int

	sr *SPRenderer
}

const (
	xhtmlClose = " />"
	htmlClose  = ">"
)

NewHTMLRenderer creates and configures an HTMLRenderer object, which satisfies the Renderer interface.

configure the rendering engine

	closeTag := htmlClose
	if params.Flags&UseXHTML != 0 {
		closeTag = xhtmlClose
	}

	if params.FootnoteReturnLinkContents == "" {
		params.FootnoteReturnLinkContents = `<sup>[return]</sup>`
	}

	return &HTMLRenderer{
		HTMLRendererParameters: params,

		closeTag:   closeTag,
		headingIDs: make(map[string]int),

		sr: NewSmartypantsRenderer(params.Flags),
	}
}

func isHTMLTag(tag []byte, tagname string) bool {
	found, _ := findHTMLTagPos(tag, tagname)
	return found
}

Look for a character, but ignore it when it's in any kind of quotes, it might be JavaScript

func skipUntilCharIgnoreQuotes(html []byte, start int, char byte) int {
	inSingleQuote := false
	inDoubleQuote := false
	inGraveQuote := false
	i := start
	for i < len(html) {
		switch {
		case html[i] == char && !inSingleQuote && !inDoubleQuote && !inGraveQuote:
			return i
		case html[i] == '\'':
			inSingleQuote = !inSingleQuote
		case html[i] == '"':
			inDoubleQuote = !inDoubleQuote
		case html[i] == '`':
			inGraveQuote = !inGraveQuote
		}
		i++
	}
	return start
}

func findHTMLTagPos(tag []byte, tagname string) (bool, int) {
	i := 0
	if i < len(tag) && tag[0] != '<' {
		return false, -1
	}
	i++
	i = skipSpace(tag, i)

	if i < len(tag) && tag[i] == '/' {
		i++
	}

	i = skipSpace(tag, i)
	j := 0
	for ; i < len(tag); i, j = i+1, j+1 {
		if j >= len(tagname) {
			break
		}

		if strings.ToLower(string(tag[i]))[0] != tagname[j] {
			return false, -1
		}
	}

	if i == len(tag) {
		return false, -1
	}

	rightAngle := skipUntilCharIgnoreQuotes(tag, i, '>')
	if rightAngle >= i {
		return true, rightAngle
	}

	return false, -1
}

func skipSpace(tag []byte, i int) int {
	for i < len(tag) && isspace(tag[i]) {
		i++
	}
	return i
}

a tag begin with '#'

	if link[0] == '#' {
		return true
	}

link begin with '/' but not '//', the second maybe a protocol relative link

	if len(link) >= 2 && link[0] == '/' && link[1] != '/' {
		return true
	}

only the root '/'

	if len(link) == 1 && link[0] == '/' {
		return true
	}

current directory : begin with "./"

	if bytes.HasPrefix(link, []byte("./")) {
		return true
	}

parent directory : begin with "../"

	if bytes.HasPrefix(link, []byte("../")) {
		return true
	}

	return false
}

func (r *HTMLRenderer) ensureUniqueHeadingID(id string) string {
	for count, found := r.headingIDs[id]; found; count, found = r.headingIDs[id] {
		tmp := fmt.Sprintf("%s-%d", id, count+1)

		if _, tmpFound := r.headingIDs[tmp]; !tmpFound {
			r.headingIDs[id] = count + 1
			id = tmp
		} else {
			id = id + "-1"
		}
	}

	if _, found := r.headingIDs[id]; !found {
		r.headingIDs[id] = 0
	}

	return id
}

func (r *HTMLRenderer) addAbsPrefix(link []byte) []byte {
	if r.AbsolutePrefix != "" && isRelativeLink(link) && link[0] != '.' {
		newDest := r.AbsolutePrefix
		if link[0] != '/' {
			newDest += "/"
		}
		newDest += string(link)
		return []byte(newDest)
	}
	return link
}

func appendLinkAttrs(attrs []string, flags HTMLFlags, link []byte) []string {
	if isRelativeLink(link) {
		return attrs
	}
	val := []string{}
	if flags&NofollowLinks != 0 {
		val = append(val, "nofollow")
	}
	if flags&NoreferrerLinks != 0 {
		val = append(val, "noreferrer")
	}
	if flags&NoopenerLinks != 0 {
		val = append(val, "noopener")
	}
	if flags&HrefTargetBlank != 0 {
		attrs = append(attrs, "target=\"_blank\"")
	}
	if len(val) == 0 {
		return attrs
	}
	attr := fmt.Sprintf("rel=%q", strings.Join(val, " "))
	return append(attrs, attr)
}

func isMailto(link []byte) bool {
	return bytes.HasPrefix(link, []byte("mailto:"))
}

func needSkipLink(flags HTMLFlags, dest []byte) bool {
	if flags&SkipLinks != 0 {
		return true
	}
	return flags&Safelink != 0 && !isSafeLink(dest) && !isMailto(dest)
}

func isSmartypantable(node *Node) bool {
	pt := node.Parent.Type
	return pt != Link && pt != CodeBlock && pt != Code
}

func appendLanguageAttr(attrs []string, info []byte) []string {
	if len(info) == 0 {
		return attrs
	}
	endOfLang := bytes.IndexAny(info, "\t ")
	if endOfLang < 0 {
		endOfLang = len(info)
	}
	return append(attrs, fmt.Sprintf("class=\"language-%s\"", info[:endOfLang]))
}

func (r *HTMLRenderer) tag(w io.Writer, name []byte, attrs []string) {
	w.Write(name)
	if len(attrs) > 0 {
		w.Write(spaceBytes)
		w.Write([]byte(strings.Join(attrs, " ")))
	}
	w.Write(gtBytes)
	r.lastOutputLen = 1
}

func footnoteRef(prefix string, node *Node) []byte {
	urlFrag := prefix + string(slugify(node.Destination))
	anchor := fmt.Sprintf(`<a href="#fn:%s">%d</a>`, urlFrag, node.NoteID)
	return []byte(fmt.Sprintf(`<sup class="footnote-ref" id="fnref:%s">%s</sup>`, urlFrag, anchor))
}

func footnoteItem(prefix string, slug []byte) []byte {
	return []byte(fmt.Sprintf(`<li id="fn:%s%s">`, prefix, slug))
}

func footnoteReturnLink(prefix, returnLink string, slug []byte) []byte {
	const format = ` <a class="footnote-return" href="#fnref:%s%s">%s</a>`
	return []byte(fmt.Sprintf(format, prefix, slug, returnLink))
}

func itemOpenCR(node *Node) bool {
	if node.Prev == nil {
		return false
	}
	ld := node.Parent.ListData
	return !ld.Tight && ld.ListFlags&ListTypeDefinition == 0
}

func skipParagraphTags(node *Node) bool {
	grandparent := node.Parent.Parent
	if grandparent == nil || grandparent.Type != List {
		return false
	}
	tightOrTerm := grandparent.Tight || node.Parent.ListFlags&ListTypeTerm != 0
	return grandparent.Type == List && tightOrTerm
}

func cellAlignment(align CellAlignFlags) string {
	switch align {
	case TableAlignmentLeft:
		return "left"
	case TableAlignmentRight:
		return "right"
	case TableAlignmentCenter:
		return "center"
	default:
		return ""
	}
}

func (r *HTMLRenderer) out(w io.Writer, text []byte) {
	if r.disableTags > 0 {
		w.Write(htmlTagRe.ReplaceAll(text, []byte{}))
	} else {
		w.Write(text)
	}
	r.lastOutputLen = len(text)
}

func (r *HTMLRenderer) cr(w io.Writer) {
	if r.lastOutputLen > 0 {
		r.out(w, nlBytes)
	}
}

var (
	nlBytes    = []byte{'\n'}
	gtBytes    = []byte{'>'}
	spaceBytes = []byte{' '}
)

var (
	brTag              = []byte("<br>")
	brXHTMLTag         = []byte("<br />")
	emTag              = []byte("<em>")
	emCloseTag         = []byte("</em>")
	strongTag          = []byte("<strong>")
	strongCloseTag     = []byte("</strong>")
	delTag             = []byte("<del>")
	delCloseTag        = []byte("</del>")
	ttTag              = []byte("<tt>")
	ttCloseTag         = []byte("</tt>")
	aTag               = []byte("<a")
	aCloseTag          = []byte("</a>")
	preTag             = []byte("<pre>")
	preCloseTag        = []byte("</pre>")
	codeTag            = []byte("<code>")
	codeCloseTag       = []byte("</code>")
	pTag               = []byte("<p>")
	pCloseTag          = []byte("</p>")
	blockquoteTag      = []byte("<blockquote>")
	blockquoteCloseTag = []byte("</blockquote>")
	hrTag              = []byte("<hr>")
	hrXHTMLTag         = []byte("<hr />")
	ulTag              = []byte("<ul>")
	ulCloseTag         = []byte("</ul>")
	olTag              = []byte("<ol>")
	olCloseTag         = []byte("</ol>")
	dlTag              = []byte("<dl>")
	dlCloseTag         = []byte("</dl>")
	liTag              = []byte("<li>")
	liCloseTag         = []byte("</li>")
	ddTag              = []byte("<dd>")
	ddCloseTag         = []byte("</dd>")
	dtTag              = []byte("<dt>")
	dtCloseTag         = []byte("</dt>")
	tableTag           = []byte("<table>")
	tableCloseTag      = []byte("</table>")
	tdTag              = []byte("<td")
	tdCloseTag         = []byte("</td>")
	thTag              = []byte("<th")
	thCloseTag         = []byte("</th>")
	theadTag           = []byte("<thead>")
	theadCloseTag      = []byte("</thead>")
	tbodyTag           = []byte("<tbody>")
	tbodyCloseTag      = []byte("</tbody>")
	trTag              = []byte("<tr>")
	trCloseTag         = []byte("</tr>")
	h1Tag              = []byte("<h1")
	h1CloseTag         = []byte("</h1>")
	h2Tag              = []byte("<h2")
	h2CloseTag         = []byte("</h2>")
	h3Tag              = []byte("<h3")
	h3CloseTag         = []byte("</h3>")
	h4Tag              = []byte("<h4")
	h4CloseTag         = []byte("</h4>")
	h5Tag              = []byte("<h5")
	h5CloseTag         = []byte("</h5>")
	h6Tag              = []byte("<h6")
	h6CloseTag         = []byte("</h6>")

	footnotesDivBytes      = []byte("\n<div class=\"footnotes\">\n\n")
	footnotesCloseDivBytes = []byte("\n</div>\n")
)

func headingTagsFromLevel(level int) ([]byte, []byte) {
	if level <= 1 {
		return h1Tag, h1CloseTag
	}
	switch level {
	case 2:
		return h2Tag, h2CloseTag
	case 3:
		return h3Tag, h3CloseTag
	case 4:
		return h4Tag, h4CloseTag
	case 5:
		return h5Tag, h5CloseTag
	}
	return h6Tag, h6CloseTag
}

func (r *HTMLRenderer) outHRTag(w io.Writer) {
	if r.Flags&UseXHTML == 0 {
		r.out(w, hrTag)
	} else {
		r.out(w, hrXHTMLTag)
	}
}

RenderNode is a default renderer of a single node of a syntax tree. For block nodes it will be called twice: first time with entering=true, second time with entering=false, so that it could know when it's working on an open tag and when on close. It writes the result to w. The return value is a way to tell the calling walker to adjust its walk pattern: e.g. it can terminate the traversal by returning Terminate. Or it can ask the walker to skip a subtree of this node by returning SkipChildren. The typical behavior is to return GoToNext, which asks for the usual traversal to the next node.

func (r *HTMLRenderer) RenderNode(w io.Writer, node *Node, entering bool) WalkStatus {
	attrs := []string{}
	switch node.Type {
	case Text:
		if r.Flags&Smartypants != 0 {
			var tmp bytes.Buffer
			escapeHTML(&tmp, node.Literal)
			r.sr.Process(w, tmp.Bytes())
		} else {
			if node.Parent.Type == Link {
				escLink(w, node.Literal)
			} else {
				escapeHTML(w, node.Literal)
			}
		}
	case Softbreak:

TODO: make it configurable via out(renderer.softbreak)

	case Hardbreak:
		if r.Flags&UseXHTML == 0 {
			r.out(w, brTag)
		} else {
			r.out(w, brXHTMLTag)
		}
		r.cr(w)
	case Emph:
		if entering {
			r.out(w, emTag)
		} else {
			r.out(w, emCloseTag)
		}
	case Strong:
		if entering {
			r.out(w, strongTag)
		} else {
			r.out(w, strongCloseTag)
		}
	case Del:
		if entering {
			r.out(w, delTag)
		} else {
			r.out(w, delCloseTag)
		}
	case HTMLSpan:
		if r.Flags&SkipHTML != 0 {
			break
		}
		r.out(w, node.Literal)

mark it but don't link it if it is not a safe link: no smartypants

		dest := node.LinkData.Destination
		if needSkipLink(r.Flags, dest) {
			if entering {
				r.out(w, ttTag)
			} else {
				r.out(w, ttCloseTag)
			}
		} else {
			if entering {
				dest = r.addAbsPrefix(dest)
				var hrefBuf bytes.Buffer
				hrefBuf.WriteString("href=\"")
				escLink(&hrefBuf, dest)
				hrefBuf.WriteByte('"')
				attrs = append(attrs, hrefBuf.String())
				if node.NoteID != 0 {
					r.out(w, footnoteRef(r.FootnoteAnchorPrefix, node))
					break
				}
				attrs = appendLinkAttrs(attrs, r.Flags, dest)
				if len(node.LinkData.Title) > 0 {
					var titleBuff bytes.Buffer
					titleBuff.WriteString("title=\"")
					escapeHTML(&titleBuff, node.LinkData.Title)
					titleBuff.WriteByte('"')
					attrs = append(attrs, titleBuff.String())
				}
				r.tag(w, aTag, attrs)
			} else {
				if node.NoteID != 0 {
					break
				}
				r.out(w, aCloseTag)
			}
		}
	case Image:
		if r.Flags&SkipImages != 0 {
			return SkipChildren
		}
		if entering {
			dest := node.LinkData.Destination
			dest = r.addAbsPrefix(dest)

if options.safe && potentiallyUnsafe(dest) { out(w, `<img src="" alt="`) } else {

				r.out(w, []byte(`<img src="`))
				escLink(w, dest)

}

			}
			r.disableTags++
		} else {
			r.disableTags--
			if r.disableTags == 0 {
				if node.LinkData.Title != nil {
					r.out(w, []byte(`" title="`))
					escapeHTML(w, node.LinkData.Title)
				}
				r.out(w, []byte(`" />`))
			}
		}
	case Code:
		r.out(w, codeTag)
		escapeHTML(w, node.Literal)
		r.out(w, codeCloseTag)
	case Document:
		break
	case Paragraph:
		if skipParagraphTags(node) {
			break
		}

TODO: untangle this clusterfuck about when the newlines need to be added and when not.

			if node.Prev != nil {
				switch node.Prev.Type {
				case HTMLBlock, List, Paragraph, Heading, CodeBlock, BlockQuote, HorizontalRule:
					r.cr(w)
				}
			}
			if node.Parent.Type == BlockQuote && node.Prev == nil {
				r.cr(w)
			}
			r.out(w, pTag)
		} else {
			r.out(w, pCloseTag)
			if !(node.Parent.Type == Item && node.Next == nil) {
				r.cr(w)
			}
		}
	case BlockQuote:
		if entering {
			r.cr(w)
			r.out(w, blockquoteTag)
		} else {
			r.out(w, blockquoteCloseTag)
			r.cr(w)
		}
	case HTMLBlock:
		if r.Flags&SkipHTML != 0 {
			break
		}
		r.cr(w)
		r.out(w, node.Literal)
		r.cr(w)
	case Heading:
		headingLevel := r.HTMLRendererParameters.HeadingLevelOffset + node.Level
		openTag, closeTag := headingTagsFromLevel(headingLevel)
		if entering {
			if node.IsTitleblock {
				attrs = append(attrs, `class="title"`)
			}
			if node.HeadingID != "" {
				id := r.ensureUniqueHeadingID(node.HeadingID)
				if r.HeadingIDPrefix != "" {
					id = r.HeadingIDPrefix + id
				}
				if r.HeadingIDSuffix != "" {
					id = id + r.HeadingIDSuffix
				}
				attrs = append(attrs, fmt.Sprintf(`id="%s"`, id))
			}
			r.cr(w)
			r.tag(w, openTag, attrs)
		} else {
			r.out(w, closeTag)
			if !(node.Parent.Type == Item && node.Next == nil) {
				r.cr(w)
			}
		}
	case HorizontalRule:
		r.cr(w)
		r.outHRTag(w)
		r.cr(w)
	case List:
		openTag := ulTag
		closeTag := ulCloseTag
		if node.ListFlags&ListTypeOrdered != 0 {
			openTag = olTag
			closeTag = olCloseTag
		}
		if node.ListFlags&ListTypeDefinition != 0 {
			openTag = dlTag
			closeTag = dlCloseTag
		}
		if entering {
			if node.IsFootnotesList {
				r.out(w, footnotesDivBytes)
				r.outHRTag(w)
				r.cr(w)
			}
			r.cr(w)
			if node.Parent.Type == Item && node.Parent.Parent.Tight {
				r.cr(w)
			}
			r.tag(w, openTag[:len(openTag)-1], attrs)
			r.cr(w)
		} else {

cr(w) if node.parent.Type != Item { cr(w) }

			if node.Parent.Type == Item && node.Next != nil {
				r.cr(w)
			}
			if node.Parent.Type == Document || node.Parent.Type == BlockQuote {
				r.cr(w)
			}
			if node.IsFootnotesList {
				r.out(w, footnotesCloseDivBytes)
			}
		}
	case Item:
		openTag := liTag
		closeTag := liCloseTag
		if node.ListFlags&ListTypeDefinition != 0 {
			openTag = ddTag
			closeTag = ddCloseTag
		}
		if node.ListFlags&ListTypeTerm != 0 {
			openTag = dtTag
			closeTag = dtCloseTag
		}
		if entering {
			if itemOpenCR(node) {
				r.cr(w)
			}
			if node.ListData.RefLink != nil {
				slug := slugify(node.ListData.RefLink)
				r.out(w, footnoteItem(r.FootnoteAnchorPrefix, slug))
				break
			}
			r.out(w, openTag)
		} else {
			if node.ListData.RefLink != nil {
				slug := slugify(node.ListData.RefLink)
				if r.Flags&FootnoteReturnLinks != 0 {
					r.out(w, footnoteReturnLink(r.FootnoteAnchorPrefix, r.FootnoteReturnLinkContents, slug))
				}
			}
			r.out(w, closeTag)
			r.cr(w)
		}
	case CodeBlock:
		attrs = appendLanguageAttr(attrs, node.Info)
		r.cr(w)
		r.out(w, preTag)
		r.tag(w, codeTag[:len(codeTag)-1], attrs)
		escapeHTML(w, node.Literal)
		r.out(w, codeCloseTag)
		r.out(w, preCloseTag)
		if node.Parent.Type != Item {
			r.cr(w)
		}
	case Table:
		if entering {
			r.cr(w)
			r.out(w, tableTag)
		} else {
			r.out(w, tableCloseTag)
			r.cr(w)
		}
	case TableCell:
		openTag := tdTag
		closeTag := tdCloseTag
		if node.IsHeader {
			openTag = thTag
			closeTag = thCloseTag
		}
		if entering {
			align := cellAlignment(node.Align)
			if align != "" {
				attrs = append(attrs, fmt.Sprintf(`align="%s"`, align))
			}
			if node.Prev == nil {
				r.cr(w)
			}
			r.tag(w, openTag, attrs)
		} else {
			r.out(w, closeTag)
			r.cr(w)
		}
	case TableHead:
		if entering {
			r.cr(w)
			r.out(w, theadTag)
		} else {
			r.out(w, theadCloseTag)
			r.cr(w)
		}
	case TableBody:
		if entering {
			r.cr(w)

XXX: this is to adhere to a rather silly test. Should fix test.

			if node.FirstChild == nil {
				r.cr(w)
			}
		} else {
			r.out(w, tbodyCloseTag)
			r.cr(w)
		}
	case TableRow:
		if entering {
			r.cr(w)
			r.out(w, trTag)
		} else {
			r.out(w, trCloseTag)
			r.cr(w)
		}
	default:
		panic("Unknown node type " + node.Type.String())
	}
	return GoToNext
}

RenderHeader writes HTML document preamble and TOC if requested.

func (r *HTMLRenderer) RenderHeader(w io.Writer, ast *Node) {
	r.writeDocumentHeader(w)
	if r.Flags&TOC != 0 {
		r.writeTOC(w, ast)
	}
}

RenderFooter writes HTML document footer.

func (r *HTMLRenderer) RenderFooter(w io.Writer, ast *Node) {
	if r.Flags&CompletePage == 0 {
		return
	}
	io.WriteString(w, "\n</body>\n</html>\n")
}

func (r *HTMLRenderer) writeDocumentHeader(w io.Writer) {
	if r.Flags&CompletePage == 0 {
		return
	}
	ending := ""
	if r.Flags&UseXHTML != 0 {
		io.WriteString(w, "<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Transitional//EN\" ")
		io.WriteString(w, "\"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd\">\n")
		io.WriteString(w, "<html xmlns=\"http://www.w3.org/1999/xhtml\">\n")
		ending = " /"
	} else {
		io.WriteString(w, "<!DOCTYPE html>\n")
		io.WriteString(w, "<html>\n")
	}
	io.WriteString(w, "<head>\n")
	io.WriteString(w, "  <title>")
	if r.Flags&Smartypants != 0 {
		r.sr.Process(w, []byte(r.Title))
	} else {
		escapeHTML(w, []byte(r.Title))
	}
	io.WriteString(w, "</title>\n")
	io.WriteString(w, "  <meta name=\"GENERATOR\" content=\"Blackfriday Markdown Processor v")
	io.WriteString(w, Version)
	io.WriteString(w, "\"")
	io.WriteString(w, ending)
	io.WriteString(w, ">\n")
	io.WriteString(w, "  <meta charset=\"utf-8\"")
	io.WriteString(w, ending)
	io.WriteString(w, ">\n")
	if r.CSS != "" {
		io.WriteString(w, "  <link rel=\"stylesheet\" type=\"text/css\" href=\"")
		escapeHTML(w, []byte(r.CSS))
		io.WriteString(w, "\"")
		io.WriteString(w, ending)
		io.WriteString(w, ">\n")
	}
	if r.Icon != "" {
		io.WriteString(w, "  <link rel=\"icon\" type=\"image/x-icon\" href=\"")
		escapeHTML(w, []byte(r.Icon))
		io.WriteString(w, "\"")
		io.WriteString(w, ending)
		io.WriteString(w, ">\n")
	}
	io.WriteString(w, "</head>\n")
	io.WriteString(w, "<body>\n\n")
}

func (r *HTMLRenderer) writeTOC(w io.Writer, ast *Node) {
	buf := bytes.Buffer{}

	inHeading := false
	tocLevel := 0
	headingCount := 0

	ast.Walk(func(node *Node, entering bool) WalkStatus {
		if node.Type == Heading && !node.HeadingData.IsTitleblock {
			inHeading = entering
			if entering {
				node.HeadingID = fmt.Sprintf("toc_%d", headingCount)
				if node.Level == tocLevel {
					buf.WriteString("</li>\n\n<li>")
				} else if node.Level < tocLevel {
					for node.Level < tocLevel {
						tocLevel--
						buf.WriteString("</li>\n</ul>")
					}
					buf.WriteString("</li>\n\n<li>")
				} else {
					for node.Level > tocLevel {
						tocLevel++
						buf.WriteString("\n<ul>\n<li>")
					}
				}

				fmt.Fprintf(&buf, `<a href="#toc_%d">`, headingCount)
				headingCount++
			} else {
				buf.WriteString("</a>")
			}
			return GoToNext
		}

		if inHeading {
			return r.RenderNode(&buf, node, entering)
		}

		return GoToNext
	})

	for ; tocLevel > 0; tocLevel-- {
		buf.WriteString("</li>\n</ul>")
	}

	if buf.Len() > 0 {
		io.WriteString(w, "<nav>\n")
		w.Write(buf.Bytes())
		io.WriteString(w, "\n\n</nav>\n")
	}
	r.lastOutputLen = buf.Len()