From b72024d2b0824a752171261563664f6cbfa2706b Mon Sep 17 00:00:00 2001
From: Tommi Virtanen <tv@eagain.net>
Date: Wed, 15 Jan 2020 18:47:28 -0700
Subject: [PATCH] Use user input as part of footnote identifier

Named footnotes (`[^foo]` instead of `[^42]`) use the name in HTML
node identifiers. This allows for stable references. Cool URLs don't
change.

Fixes #92
---
 extension/_test/footnote.txt | 18 ++++++++++++++++++
 extension/footnote.go        | 36 ++++++++++++++++++++++++++++--------
 2 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/extension/_test/footnote.txt b/extension/_test/footnote.txt
index 9b06721..6c82688 100644
--- a/extension/_test/footnote.txt
+++ b/extension/_test/footnote.txt
@@ -66,3 +66,21 @@ test![^1]
 </ol>
 </section>
 //= = = = = = = = = = = = = = = = = = = = = = = =//
+
+
+6
+//- - - - - - - - -//
+That's some text with a footnote.[^mynameisjeff]
+
+[^mynameisjeff]: And that's the footnote.
+//- - - - - - - - -//
+<p>That's some text with a footnote.<sup id="fnref:mynameisjeff"><a href="#fn:mynameisjeff" class="footnote-ref" role="doc-noteref">1</a></sup></p>
+<section class="footnotes" role="doc-endnotes">
+<hr>
+<ol>
+<li id="fn:mynameisjeff" role="doc-endnote">
+<p>And that's the footnote. <a href="#fnref:mynameisjeff" class="footnote-backref" role="doc-backlink">&#x21a9;&#xfe0e;</a></p>
+</li>
+</ol>
+</section>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
diff --git a/extension/footnote.go b/extension/footnote.go
index 972418e..bc0ec74 100644
--- a/extension/footnote.go
+++ b/extension/footnote.go
@@ -242,16 +242,36 @@ func (r *FootnoteHTMLRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegist
 	reg.Register(ast.KindFootnoteList, r.renderFootnoteList)
 }
 
+func isNumeric(b []byte) bool {
+	for _, c := range b {
+		if c < '0' || c > '9' {
+			return false
+		}
+	}
+	return true
+}
+
+// footnoteRef decides what to use to refer to the footnote. The
+// returned string is safe to use in HTML attributes.
+func footnoteRef(index int, ref []byte) []byte {
+	if !isNumeric(ref) {
+		return util.EscapeHTML(ref)
+	}
+	return strconv.AppendInt(nil, int64(index), 10)
+}
+
 func (r *FootnoteHTMLRenderer) renderFootnoteLink(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
 	if entering {
 		n := node.(*ast.FootnoteLink)
-		is := strconv.Itoa(n.Index)
+		ref := footnoteRef(n.Index, n.Ref)
 		_, _ = w.WriteString(`<sup id="fnref:`)
-		_, _ = w.WriteString(is)
+		_, _ = w.Write(ref)
 		_, _ = w.WriteString(`"><a href="#fn:`)
-		_, _ = w.WriteString(is)
+		_, _ = w.Write(ref)
 		_, _ = w.WriteString(`" class="footnote-ref" role="doc-noteref">`)
-		_, _ = w.WriteString(is)
+		// the content itself refers to footnotes by number, even when
+		// they are named
+		_, _ = w.WriteString(strconv.Itoa(n.Index))
 		_, _ = w.WriteString(`</a></sup>`)
 	}
 	return gast.WalkContinue, nil
@@ -260,9 +280,9 @@ func (r *FootnoteHTMLRenderer) renderFootnoteLink(w util.BufWriter, source []byt
 func (r *FootnoteHTMLRenderer) renderFootnoteBackLink(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
 	if entering {
 		n := node.(*ast.FootnoteBackLink)
-		is := strconv.Itoa(n.Index)
+		ref := footnoteRef(n.Index, n.Ref)
 		_, _ = w.WriteString(` <a href="#fnref:`)
-		_, _ = w.WriteString(is)
+		_, _ = w.Write(ref)
 		_, _ = w.WriteString(`" class="footnote-backref" role="doc-backlink">`)
 		_, _ = w.WriteString("&#x21a9;&#xfe0e;")
 		_, _ = w.WriteString(`</a>`)
@@ -272,10 +292,10 @@ func (r *FootnoteHTMLRenderer) renderFootnoteBackLink(w util.BufWriter, source [
 
 func (r *FootnoteHTMLRenderer) renderFootnote(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
 	n := node.(*ast.Footnote)
-	is := strconv.Itoa(n.Index)
 	if entering {
+		ref := footnoteRef(n.Index, n.Ref)
 		_, _ = w.WriteString(`<li id="fn:`)
-		_, _ = w.WriteString(is)
+		_, _ = w.Write(ref)
 		_, _ = w.WriteString(`" role="doc-endnote"`)
 		if node.Attributes() != nil {
 			html.RenderAttributes(w, node, html.ListItemAttributeFilter)