Remove callback from Header renderer event

Split Header into two events: BeginHeader and EndHeader, removing the
need for callback.

block.go

@@ -245,10 +245,9 @@ func (p *parser) prefixHeader(out *bytes.Buffer, data []byte) int {
 		if id == "" && p.flags&AutoHeaderIDs != 0 {
 			id = sanitized_anchor_name.Create(string(data[i:end]))
 		}
-		work := func() {
+		tocMarker := p.r.BeginHeader(out, level, id)
 		p.inline(out, data[i:end])
-		}
+		p.r.EndHeader(out, level, id, tocMarker)
-		p.r.Header(out, work, level, id)
 	}
 	return skip
 }
@@ -1318,20 +1317,14 @@ func (p *parser) paragraph(out *bytes.Buffer, data []byte) int {
 					eol--
 				}
 
-				// render the header
-				// this ugly double closure avoids forcing variables onto the heap
-				work := func(o *bytes.Buffer, pp *parser, d []byte) func() {
-					return func() {
-						pp.inline(o, d)
-					}
-				}(out, p, data[prev:eol])
-
 				id := ""
 				if p.flags&AutoHeaderIDs != 0 {
 					id = sanitized_anchor_name.Create(string(data[prev:eol]))
 				}
 
-				p.r.Header(out, work, level, id)
+				tocMarker := p.r.BeginHeader(out, level, id)
+				p.inline(out, data[prev:eol])
+				p.r.EndHeader(out, level, id, tocMarker)
 
 				// find the end of the underline
 				for data[i] != '\n' {

html.go

@@ -202,7 +202,7 @@ func (options *Html) TitleBlock(out *bytes.Buffer, text []byte) {
 	out.WriteString("\n</h1>")
 }
 
-func (options *Html) Header(out *bytes.Buffer, text func(), level int, id string) {
+func (options *Html) BeginHeader(out *bytes.Buffer, level int, id string) int {
 	doubleSpace(out)
 
 	if id == "" && options.flags&Toc != 0 {
@@ -225,12 +225,13 @@ func (options *Html) Header(out *bytes.Buffer, text func(), level int, id string
 		out.WriteString(fmt.Sprintf("<h%d>", level))
 	}
 
-	tocMarker := out.Len()
+	return out.Len()
-	text()
+}
 
+func (r *Html) EndHeader(out *bytes.Buffer, level int, id string, tocMarker int) {
 	// are we building a table of contents?
-	if options.flags&Toc != 0 {
+	if r.flags&Toc != 0 {
-		options.TocHeaderWithAnchor(out.Bytes()[tocMarker:], level, id)
+		r.TocHeaderWithAnchor(out.Bytes()[tocMarker:], level, id)
 	}
 
 	out.WriteString(fmt.Sprintf("</h%d>\n", level))

latex.go

@@ -72,7 +72,7 @@ func (options *Latex) BlockHtml(out *bytes.Buffer, text []byte) {
 	out.WriteString("\n\\end{verbatim}\n")
 }
 
-func (options *Latex) Header(out *bytes.Buffer, text func(), level int, id string) {
+func (r *Latex) BeginHeader(out *bytes.Buffer, level int, id string) int {
 	switch level {
 	case 1:
 		out.WriteString("\n\\section{")
@@ -87,7 +87,10 @@ func (options *Latex) Header(out *bytes.Buffer, text func(), level int, id strin
 	case 6:
 		out.WriteString("\n\\textbf{")
 	}
-	text()
+	return out.Len()
+}
+
+func (r *Latex) EndHeader(out *bytes.Buffer, level int, id string, tocMarker int) {
 	out.WriteString("}\n")
 }
 

markdown.go

@@ -163,7 +163,8 @@ type Renderer interface {
 	BlockCode(out *bytes.Buffer, text []byte, lang string)
 	BlockQuote(out *bytes.Buffer, text []byte)
 	BlockHtml(out *bytes.Buffer, text []byte)
-	Header(out *bytes.Buffer, text func(), level int, id string)
+	BeginHeader(out *bytes.Buffer, level int, id string) int
+	EndHeader(out *bytes.Buffer, level int, id string, tocMarker int)
 	HRule(out *bytes.Buffer)
 	List(out *bytes.Buffer, text func(), flags ListType)
 	ListItem(out *bytes.Buffer, text []byte, flags ListType)