Closes #161

- Implement footnote configurations defined in original markdown extra.
- Add OwnerDocument() method to ast.Node
- Add Meta() method to *ast.Document

.github/ISSUE_TEMPLATE.md

@@ -1,7 +1,10 @@
+goldmark has [https://github.com/yuin/goldmark/discussions](Discussions) in github.
+You should post only issues here. Feature requests and questions should be posted at discussions.
+
+
 - [ ] goldmark is fully compliant with the CommonMark. Before submitting issue, you **must** read [CommonMark spec](https://spec.commonmark.org/0.29/) and confirm your output is different from [CommonMark online demo](https://spec.commonmark.org/dingus/).
     - [ ] **Extensions(Autolink without `<` `>`, Table, etc) are not part of CommonMark spec.** You should confirm your output is different from other official renderers correspond with an extension.
 - [ ] **goldmark is not dedicated for Hugo**. If you are Hugo user and your issue was raised by your experience in Hugo, **you should consider create issue at Hugo repository at first** .
-- [ ] Before you make a feature request, **you should consider implement the new feature as an extension by yourself** . To keep goldmark itself simple, most new features should be implemented as an extension.
 
 Please answer the following before submitting your issue:
 
@@ -12,6 +15,3 @@ Please answer the following before submitting your issue:
 5. What did you expect to see? :
 6. What did you see instead? :
 7. Did you confirm your output is different from [CommonMark online demo](https://spec.commonmark.org/dingus/) or other official renderer correspond with an extension?:
-8. (Feature request only): Why you can not implement it as an extension?:
-    - You should avoid saying like "I'm not familiar with this project" "I'm not a Go programmer" as far as possible. This is an open source project and a library for Go programmers. I encourage you to strive to read source codes. 
-    - I absolutely welcome questions that are difficult even if you read the source codes.

README.md

@@ -287,6 +287,89 @@ markdown := goldmark.New(
 )
 ```
 
+### Footnotes extension
+
+The Footnote extension implements [PHP Markdown Extra: Footnotes](https://michelf.ca/projects/php-markdown/extra/#footnotes).
+
+This extension has some options:
+
+| Functional option | Type | Description |
+| ----------------- | ---- | ----------- |
+| `extension.WithFootnoteIDPrefix` | `[]byte` |  a prefix for the id attributes.|
+| `extension.WithFootnoteIDPrefixFunction` | `func(gast.Node) []byte` |  a function that determines the id attribute for given Node.|
+| `extension.WithFootnoteLinkTitle` | `[]byte` |  an optional title attribute for footnote links.|
+| `extension.WithFootnoteBacklinkTitle` | `[]byte` |  an optional title attribute for footnote backlinks. |
+| `extension.WithFootnoteLinkClass` | `[]byte` |  a class for footnote links. This defaults to `footnote-ref`. |
+| `extension.WithFootnoteBacklinkClass` | `[]byte` |  a class for footnote backlinks. This defaults to `footnote-backref`. |
+| `extension.WithFootnoteBacklinkHTML` | `[]byte` |  a class for footnote backlinks. This defaults to `↩︎`. |
+
+Some options can have special substitutions. Occurances of “^^” in the string will be replaced by the corresponding footnote number in the HTML output. Occurances of “%%” will be replaced by a number for the reference (footnotes can have multiple references).
+
+`extension.WithFootnoteIDPrefix` and `extension.WithFootnoteIDPrefixFunction` are useful if you have multiple Markdown documents displayed inside one HTML document to avoid footnote ids to clash each other.
+
+`extension.WithFootnoteIDPrefix` sets fixed id prefix, so you may write codes like the following:
+
+```go
+for _, path := range files {
+    source := readAll(path)
+    prefix := getPrefix(path)
+
+    markdown := goldmark.New(
+        goldmark.WithExtensions(
+            NewFootnote(
+                WithFootnoteIDPrefix([]byte(path)),
+            ),
+        ),
+    )
+    var b bytes.Buffer
+    err := markdown.Convert(source, &b)
+    if err != nil {
+        t.Error(err.Error())
+    }
+}
+```
+
+`extension.WithFootnoteIDPrefixFunction` determines an id prefix by calling given function, so you may write codes like the following:
+
+```go
+markdown := goldmark.New(
+    goldmark.WithExtensions(
+        NewFootnote(
+                WithFootnoteIDPrefixFunction(func(n gast.Node) []byte {
+                    v, ok := n.OwnerDocument().Meta()["footnote-prefix"]
+                    if ok {
+                        return util.StringToReadOnlyBytes(v.(string))
+                    }
+                    return nil
+                }),
+        ),
+    ),
+)
+
+for _, path := range files {
+    source := readAll(path)
+    var b bytes.Buffer
+
+    doc := markdown.Parser().Parse(text.NewReader(source))
+    doc.Meta()["footnote-prefix"] = getPrefix(path)
+    err := markdown.Renderer().Render(&b, source, doc)
+}
+```
+
+You can use [goldmark-meta](https://github.com/yuin/goldmark-meta) to define a id prefix in the markdown document:
+
+
+```markdown
+---
+title: document title
+slug: article1
+footnote-prefix: article1
+---
+
+# My article
+
+```
+ 
 Security
 --------------------
 By default, goldmark does not render raw HTML or potentially-dangerous URLs.

ast/ast.go

@@ -116,6 +116,11 @@ type Node interface {
 	// tail of the children.
 	InsertAfter(self, v1, insertee Node)
 
+	// OwnerDocument returns this node's owner document.
+	// If this node is not a child of the Document node, OwnerDocument
+	// returns nil.
+	OwnerDocument() *Document
+
 	// Dump dumps an AST tree structure to stdout.
 	// This function completely aimed for debugging.
 	// level is a indent level. Implementer should indent informations with
@@ -358,6 +363,22 @@ func (n *BaseNode) InsertBefore(self, v1, insertee Node) {
 	}
 }
 
+// OwnerDocument implements Node.OwnerDocument
+func (n *BaseNode) OwnerDocument() *Document {
+	d := n.Parent()
+	for {
+		p := d.Parent()
+		if p == nil {
+			if v, ok := d.(*Document); ok {
+				return v
+			}
+			break
+		}
+		d = p
+	}
+	return nil
+}
+
 // Text implements Node.Text  .
 func (n *BaseNode) Text(source []byte) []byte {
 	var buf bytes.Buffer

ast/block.go

@@ -50,6 +50,8 @@ func (b *BaseBlock) SetLines(v *textm.Segments) {
 // A Document struct is a root node of Markdown text.
 type Document struct {
 	BaseBlock
+
+	meta map[string]interface{}
 }
 
 // KindDocument is a NodeKind of the Document node.
@@ -70,10 +72,29 @@ func (n *Document) Kind() NodeKind {
 	return KindDocument
 }
 
+// OwnerDocument implements Node.OwnerDocument
+func (n *Document) OwnerDocument() *Document {
+	return n
+}
+
+// Meta returns metadata of this document.
+func (n *Document) Meta() map[string]interface{} {
+	if n.meta == nil {
+		n.meta = map[string]interface{}{}
+	}
+	return n.meta
+}
+
+// SetMeta sets given metadata to this document.
+func (n *Document) SetMeta(meta map[string]interface{}) {
+	n.meta = meta
+}
+
 // NewDocument returns a new Document node.
 func NewDocument() *Document {
 	return &Document{
 		BaseBlock: BaseBlock{},
+		meta:      nil,
 	}
 }
 

extension/ast/footnote.go

@@ -2,6 +2,7 @@ package ast
 
 import (
 	"fmt"
+
 	gast "github.com/yuin/goldmark/ast"
 )
 
@@ -9,13 +10,15 @@ import (
 // (PHP Markdown Extra) text.
 type FootnoteLink struct {
 	gast.BaseInline
-	Index int
+	Index    int
+	RefCount int
 }
 
 // Dump implements Node.Dump.
 func (n *FootnoteLink) Dump(source []byte, level int) {
 	m := map[string]string{}
 	m["Index"] = fmt.Sprintf("%v", n.Index)
+	m["RefCount"] = fmt.Sprintf("%v", n.RefCount)
 	gast.DumpHelper(n, source, level, m, nil)
 }
 
@@ -30,36 +33,40 @@ func (n *FootnoteLink) Kind() gast.NodeKind {
 // NewFootnoteLink returns a new FootnoteLink node.
 func NewFootnoteLink(index int) *FootnoteLink {
 	return &FootnoteLink{
-		Index: index,
+		Index:    index,
+		RefCount: 0,
 	}
 }
 
-// A FootnoteBackLink struct represents a link to a footnote of Markdown
+// A FootnoteBacklink struct represents a link to a footnote of Markdown
 // (PHP Markdown Extra) text.
-type FootnoteBackLink struct {
+type FootnoteBacklink struct {
 	gast.BaseInline
-	Index int
+	Index    int
+	RefCount int
 }
 
 // Dump implements Node.Dump.
-func (n *FootnoteBackLink) Dump(source []byte, level int) {
+func (n *FootnoteBacklink) Dump(source []byte, level int) {
 	m := map[string]string{}
 	m["Index"] = fmt.Sprintf("%v", n.Index)
+	m["RefCount"] = fmt.Sprintf("%v", n.RefCount)
 	gast.DumpHelper(n, source, level, m, nil)
 }
 
-// KindFootnoteBackLink is a NodeKind of the FootnoteBackLink node.
-var KindFootnoteBackLink = gast.NewNodeKind("FootnoteBackLink")
+// KindFootnoteBacklink is a NodeKind of the FootnoteBacklink node.
+var KindFootnoteBacklink = gast.NewNodeKind("FootnoteBacklink")
 
 // Kind implements Node.Kind.
-func (n *FootnoteBackLink) Kind() gast.NodeKind {
-	return KindFootnoteBackLink
+func (n *FootnoteBacklink) Kind() gast.NodeKind {
+	return KindFootnoteBacklink
 }
 
-// NewFootnoteBackLink returns a new FootnoteBackLink node.
-func NewFootnoteBackLink(index int) *FootnoteBackLink {
-	return &FootnoteBackLink{
-		Index: index,
+// NewFootnoteBacklink returns a new FootnoteBacklink node.
+func NewFootnoteBacklink(index int) *FootnoteBacklink {
+	return &FootnoteBacklink{
+		Index:    index,
+		RefCount: 0,
 	}
 }
 

extension/footnote.go

@@ -2,6 +2,8 @@ package extension
 
 import (
 	"bytes"
+	"strconv"
+
 	"github.com/yuin/goldmark"
 	gast "github.com/yuin/goldmark/ast"
 	"github.com/yuin/goldmark/extension/ast"
@@ -10,10 +12,10 @@ import (
 	"github.com/yuin/goldmark/renderer/html"
 	"github.com/yuin/goldmark/text"
 	"github.com/yuin/goldmark/util"
-	"strconv"
 )
 
 var footnoteListKey = parser.NewContextKey()
+var footnoteLinkListKey = parser.NewContextKey()
 
 type footnoteBlockParser struct {
 }
@@ -164,7 +166,17 @@ func (s *footnoteParser) Parse(parent gast.Node, block text.Reader, pc parser.Co
 		return nil
 	}
 
-	return ast.NewFootnoteLink(index)
+	fnlink := ast.NewFootnoteLink(index)
+	var fnlist []*ast.FootnoteLink
+	if tmp := pc.Get(footnoteLinkListKey); tmp != nil {
+		fnlist = tmp.([]*ast.FootnoteLink)
+	} else {
+		fnlist = []*ast.FootnoteLink{}
+		pc.Set(footnoteLinkListKey, fnlist)
+	}
+	pc.Set(footnoteLinkListKey, append(fnlist, fnlink))
+
+	return fnlink
 }
 
 type footnoteASTTransformer struct {
@@ -180,23 +192,46 @@ func NewFootnoteASTTransformer() parser.ASTTransformer {
 
 func (a *footnoteASTTransformer) Transform(node *gast.Document, reader text.Reader, pc parser.Context) {
 	var list *ast.FootnoteList
-	if tlist := pc.Get(footnoteListKey); tlist != nil {
-		list = tlist.(*ast.FootnoteList)
-	} else {
+	var fnlist []*ast.FootnoteLink
+	if tmp := pc.Get(footnoteListKey); tmp != nil {
+		list = tmp.(*ast.FootnoteList)
+	}
+	if tmp := pc.Get(footnoteLinkListKey); tmp != nil {
+		fnlist = tmp.([]*ast.FootnoteLink)
+	}
+
+	pc.Set(footnoteListKey, nil)
+	pc.Set(footnoteLinkListKey, nil)
+
+	if list == nil {
 		return
 	}
-	pc.Set(footnoteListKey, nil)
+
+	counter := map[int]int{}
+	if fnlist != nil {
+		for _, fnlink := range fnlist {
+			if fnlink.Index >= 0 {
+				counter[fnlink.Index]++
+			}
+		}
+		for _, fnlink := range fnlist {
+			fnlink.RefCount = counter[fnlink.Index]
+		}
+	}
 	for footnote := list.FirstChild(); footnote != nil; {
 		var container gast.Node = footnote
 		next := footnote.NextSibling()
 		if fc := container.LastChild(); fc != nil && gast.IsParagraph(fc) {
 			container = fc
 		}
-		index := footnote.(*ast.Footnote).Index
+		fn := footnote.(*ast.Footnote)
+		index := fn.Index
 		if index < 0 {
 			list.RemoveChild(list, footnote)
 		} else {
-			container.AppendChild(container, ast.NewFootnoteBackLink(index))
+			backLink := ast.NewFootnoteBacklink(index)
+			backLink.RefCount = counter[index]
+			container.AppendChild(container, backLink)
 		}
 		footnote = next
 	}
@@ -214,19 +249,250 @@ func (a *footnoteASTTransformer) Transform(node *gast.Document, reader text.Read
 	node.AppendChild(node, list)
 }
 
+// FootnoteConfig holds configuration values for the footnote extension.
+//
+// Link* and Backlink* configurations have some variables:
+// Occurances of “^^” in the string will be replaced by the
+// corresponding footnote number in the HTML output.
+// Occurances of “%%” will be replaced by a number for the
+// reference (footnotes can have multiple references).
+type FootnoteConfig struct {
+	html.Config
+
+	// IDPrefix is a prefix for the id attributes generated by footnotes.
+	IDPrefix []byte
+
+	// IDPrefix is a function that determines the id attribute for given Node.
+	IDPrefixFunction func(gast.Node) []byte
+
+	// LinkTitle is an optional title attribute for footnote links.
+	LinkTitle []byte
+
+	// BacklinkTitle is an optional title attribute for footnote backlinks.
+	BacklinkTitle []byte
+
+	// LinkClass is a class for footnote links.
+	LinkClass []byte
+
+	// BacklinkClass is a class for footnote backlinks.
+	BacklinkClass []byte
+
+	// BacklinkHTML is an HTML content for footnote backlinks.
+	BacklinkHTML []byte
+}
+
+// FootnoteOption interface is a functional option interface for the extension.
+type FootnoteOption interface {
+	renderer.Option
+	// SetFootnoteOption sets given option to the extension.
+	SetFootnoteOption(*FootnoteConfig)
+}
+
+// NewFootnoteConfig returns a new Config with defaults.
+func NewFootnoteConfig() FootnoteConfig {
+	return FootnoteConfig{
+		Config:        html.NewConfig(),
+		LinkTitle:     []byte(""),
+		BacklinkTitle: []byte(""),
+		LinkClass:     []byte("footnote-ref"),
+		BacklinkClass: []byte("footnote-backref"),
+		BacklinkHTML:  []byte("&#x21a9;&#xfe0e;"),
+	}
+}
+
+// SetOption implements renderer.SetOptioner.
+func (c *FootnoteConfig) SetOption(name renderer.OptionName, value interface{}) {
+	switch name {
+	case optFootnoteIDPrefixFunction:
+		c.IDPrefixFunction = value.(func(gast.Node) []byte)
+	case optFootnoteIDPrefix:
+		c.IDPrefix = value.([]byte)
+	case optFootnoteLinkTitle:
+		c.LinkTitle = value.([]byte)
+	case optFootnoteBacklinkTitle:
+		c.BacklinkTitle = value.([]byte)
+	case optFootnoteLinkClass:
+		c.LinkClass = value.([]byte)
+	case optFootnoteBacklinkClass:
+		c.BacklinkClass = value.([]byte)
+	case optFootnoteBacklinkHTML:
+		c.BacklinkHTML = value.([]byte)
+	default:
+		c.Config.SetOption(name, value)
+	}
+}
+
+type withFootnoteHTMLOptions struct {
+	value []html.Option
+}
+
+func (o *withFootnoteHTMLOptions) SetConfig(c *renderer.Config) {
+	if o.value != nil {
+		for _, v := range o.value {
+			v.(renderer.Option).SetConfig(c)
+		}
+	}
+}
+
+func (o *withFootnoteHTMLOptions) SetFootnoteOption(c *FootnoteConfig) {
+	if o.value != nil {
+		for _, v := range o.value {
+			v.SetHTMLOption(&c.Config)
+		}
+	}
+}
+
+// WithFootnoteHTMLOptions is functional option that wraps goldmark HTMLRenderer options.
+func WithFootnoteHTMLOptions(opts ...html.Option) FootnoteOption {
+	return &withFootnoteHTMLOptions{opts}
+}
+
+const optFootnoteIDPrefix renderer.OptionName = "FootnoteIDPrefix"
+
+type withFootnoteIDPrefix struct {
+	value []byte
+}
+
+func (o *withFootnoteIDPrefix) SetConfig(c *renderer.Config) {
+	c.Options[optFootnoteIDPrefix] = o.value
+}
+
+func (o *withFootnoteIDPrefix) SetFootnoteOption(c *FootnoteConfig) {
+	c.IDPrefix = o.value
+}
+
+// WithFootnoteIDPrefix is a functional option that is a prefix for the id attributes generated by footnotes.
+func WithFootnoteIDPrefix(a []byte) FootnoteOption {
+	return &withFootnoteIDPrefix{a}
+}
+
+const optFootnoteIDPrefixFunction renderer.OptionName = "FootnoteIDPrefixFunction"
+
+type withFootnoteIDPrefixFunction struct {
+	value func(gast.Node) []byte
+}
+
+func (o *withFootnoteIDPrefixFunction) SetConfig(c *renderer.Config) {
+	c.Options[optFootnoteIDPrefixFunction] = o.value
+}
+
+func (o *withFootnoteIDPrefixFunction) SetFootnoteOption(c *FootnoteConfig) {
+	c.IDPrefixFunction = o.value
+}
+
+// WithFootnoteIDPrefixFunction is a functional option that is a prefix for the id attributes generated by footnotes.
+func WithFootnoteIDPrefixFunction(a func(gast.Node) []byte) FootnoteOption {
+	return &withFootnoteIDPrefixFunction{a}
+}
+
+const optFootnoteLinkTitle renderer.OptionName = "FootnoteLinkTitle"
+
+type withFootnoteLinkTitle struct {
+	value []byte
+}
+
+func (o *withFootnoteLinkTitle) SetConfig(c *renderer.Config) {
+	c.Options[optFootnoteLinkTitle] = o.value
+}
+
+func (o *withFootnoteLinkTitle) SetFootnoteOption(c *FootnoteConfig) {
+	c.LinkTitle = o.value
+}
+
+// WithFootnoteLinkTitle is a functional option that is an optional title attribute for footnote links.
+func WithFootnoteLinkTitle(a []byte) FootnoteOption {
+	return &withFootnoteLinkTitle{a}
+}
+
+const optFootnoteBacklinkTitle renderer.OptionName = "FootnoteBacklinkTitle"
+
+type withFootnoteBacklinkTitle struct {
+	value []byte
+}
+
+func (o *withFootnoteBacklinkTitle) SetConfig(c *renderer.Config) {
+	c.Options[optFootnoteBacklinkTitle] = o.value
+}
+
+func (o *withFootnoteBacklinkTitle) SetFootnoteOption(c *FootnoteConfig) {
+	c.BacklinkTitle = o.value
+}
+
+// WithFootnoteBacklinkTitle is a functional option that is an optional title attribute for footnote backlinks.
+func WithFootnoteBacklinkTitle(a []byte) FootnoteOption {
+	return &withFootnoteBacklinkTitle{a}
+}
+
+const optFootnoteLinkClass renderer.OptionName = "FootnoteLinkClass"
+
+type withFootnoteLinkClass struct {
+	value []byte
+}
+
+func (o *withFootnoteLinkClass) SetConfig(c *renderer.Config) {
+	c.Options[optFootnoteLinkClass] = o.value
+}
+
+func (o *withFootnoteLinkClass) SetFootnoteOption(c *FootnoteConfig) {
+	c.LinkClass = o.value
+}
+
+// WithFootnoteLinkClass is a functional option that is a class for footnote links.
+func WithFootnoteLinkClass(a []byte) FootnoteOption {
+	return &withFootnoteLinkClass{a}
+}
+
+const optFootnoteBacklinkClass renderer.OptionName = "FootnoteBacklinkClass"
+
+type withFootnoteBacklinkClass struct {
+	value []byte
+}
+
+func (o *withFootnoteBacklinkClass) SetConfig(c *renderer.Config) {
+	c.Options[optFootnoteBacklinkClass] = o.value
+}
+
+func (o *withFootnoteBacklinkClass) SetFootnoteOption(c *FootnoteConfig) {
+	c.BacklinkClass = o.value
+}
+
+// WithFootnoteBacklinkClass is a functional option that is a class for footnote backlinks.
+func WithFootnoteBacklinkClass(a []byte) FootnoteOption {
+	return &withFootnoteBacklinkClass{a}
+}
+
+const optFootnoteBacklinkHTML renderer.OptionName = "FootnoteBacklinkHTML"
+
+type withFootnoteBacklinkHTML struct {
+	value []byte
+}
+
+func (o *withFootnoteBacklinkHTML) SetConfig(c *renderer.Config) {
+	c.Options[optFootnoteBacklinkHTML] = o.value
+}
+
+func (o *withFootnoteBacklinkHTML) SetFootnoteOption(c *FootnoteConfig) {
+	c.BacklinkHTML = o.value
+}
+
+// WithFootnoteBacklinkHTML is an HTML content for footnote backlinks.
+func WithFootnoteBacklinkHTML(a []byte) FootnoteOption {
+	return &withFootnoteBacklinkHTML{a}
+}
+
 // FootnoteHTMLRenderer is a renderer.NodeRenderer implementation that
 // renders FootnoteLink nodes.
 type FootnoteHTMLRenderer struct {
-	html.Config
+	FootnoteConfig
 }
 
 // NewFootnoteHTMLRenderer returns a new FootnoteHTMLRenderer.
-func NewFootnoteHTMLRenderer(opts ...html.Option) renderer.NodeRenderer {
+func NewFootnoteHTMLRenderer(opts ...FootnoteOption) renderer.NodeRenderer {
 	r := &FootnoteHTMLRenderer{
-		Config: html.NewConfig(),
+		FootnoteConfig: NewFootnoteConfig(),
 	}
 	for _, opt := range opts {
-		opt.SetHTMLOption(&r.Config)
+		opt.SetFootnoteOption(&r.FootnoteConfig)
 	}
 	return r
 }
@@ -234,7 +500,7 @@ func NewFootnoteHTMLRenderer(opts ...html.Option) renderer.NodeRenderer {
 // RegisterFuncs implements renderer.NodeRenderer.RegisterFuncs.
 func (r *FootnoteHTMLRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
 	reg.Register(ast.KindFootnoteLink, r.renderFootnoteLink)
-	reg.Register(ast.KindFootnoteBackLink, r.renderFootnoteBackLink)
+	reg.Register(ast.KindFootnoteBacklink, r.renderFootnoteBacklink)
 	reg.Register(ast.KindFootnote, r.renderFootnote)
 	reg.Register(ast.KindFootnoteList, r.renderFootnoteList)
 }
@@ -243,25 +509,45 @@ func (r *FootnoteHTMLRenderer) renderFootnoteLink(w util.BufWriter, source []byt
 	if entering {
 		n := node.(*ast.FootnoteLink)
 		is := strconv.Itoa(n.Index)
-		_, _ = w.WriteString(`<sup id="fnref:`)
+		_, _ = w.WriteString(`<sup id="`)
+		_, _ = w.Write(r.idPrefix(node))
+		_, _ = w.WriteString(`fnref:`)
 		_, _ = w.WriteString(is)
-		_, _ = w.WriteString(`"><a href="#fn:`)
+		_, _ = w.WriteString(`"><a href="#`)
+		_, _ = w.Write(r.idPrefix(node))
+		_, _ = w.WriteString(`fn:`)
 		_, _ = w.WriteString(is)
-		_, _ = w.WriteString(`" class="footnote-ref" role="doc-noteref">`)
+		_, _ = w.WriteString(`" class="`)
+		_, _ = w.Write(applyFootnoteTemplate(r.FootnoteConfig.LinkClass,
+			n.Index, n.RefCount))
+		if len(r.FootnoteConfig.LinkTitle) > 0 {
+			_, _ = w.WriteString(`" title="`)
+			_, _ = w.Write(util.EscapeHTML(applyFootnoteTemplate(r.FootnoteConfig.LinkTitle, n.Index, n.RefCount)))
+		}
+		_, _ = w.WriteString(`" role="doc-noteref">`)
+
 		_, _ = w.WriteString(is)
 		_, _ = w.WriteString(`</a></sup>`)
 	}
 	return gast.WalkContinue, nil
 }
 
-func (r *FootnoteHTMLRenderer) renderFootnoteBackLink(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
+func (r *FootnoteHTMLRenderer) renderFootnoteBacklink(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
 	if entering {
-		n := node.(*ast.FootnoteBackLink)
+		n := node.(*ast.FootnoteBacklink)
 		is := strconv.Itoa(n.Index)
-		_, _ = w.WriteString(` <a href="#fnref:`)
+		_, _ = w.WriteString(` <a href="#`)
+		_, _ = w.Write(r.idPrefix(node))
+		_, _ = w.WriteString(`fnref:`)
 		_, _ = w.WriteString(is)
-		_, _ = w.WriteString(`" class="footnote-backref" role="doc-backlink">`)
-		_, _ = w.WriteString("&#x21a9;&#xfe0e;")
+		_, _ = w.WriteString(`" class="`)
+		_, _ = w.Write(applyFootnoteTemplate(r.FootnoteConfig.BacklinkClass, n.Index, n.RefCount))
+		if len(r.FootnoteConfig.BacklinkTitle) > 0 {
+			_, _ = w.WriteString(`" title="`)
+			_, _ = w.Write(util.EscapeHTML(applyFootnoteTemplate(r.FootnoteConfig.BacklinkTitle, n.Index, n.RefCount)))
+		}
+		_, _ = w.WriteString(`" role="doc-backlink">`)
+		_, _ = w.Write(applyFootnoteTemplate(r.FootnoteConfig.BacklinkHTML, n.Index, n.RefCount))
 		_, _ = w.WriteString(`</a>`)
 	}
 	return gast.WalkContinue, nil
@@ -271,7 +557,9 @@ func (r *FootnoteHTMLRenderer) renderFootnote(w util.BufWriter, source []byte, n
 	n := node.(*ast.Footnote)
 	is := strconv.Itoa(n.Index)
 	if entering {
-		_, _ = w.WriteString(`<li id="fn:`)
+		_, _ = w.WriteString(`<li id="`)
+		_, _ = w.Write(r.idPrefix(node))
+		_, _ = w.WriteString(`fn:`)
 		_, _ = w.WriteString(is)
 		_, _ = w.WriteString(`" role="doc-endnote"`)
 		if node.Attributes() != nil {
@@ -312,11 +600,54 @@ func (r *FootnoteHTMLRenderer) renderFootnoteList(w util.BufWriter, source []byt
 	return gast.WalkContinue, nil
 }
 
+func (r *FootnoteHTMLRenderer) idPrefix(node gast.Node) []byte {
+	if r.FootnoteConfig.IDPrefix != nil {
+		return r.FootnoteConfig.IDPrefix
+	}
+	if r.FootnoteConfig.IDPrefixFunction != nil {
+		return r.FootnoteConfig.IDPrefixFunction(node)
+	}
+	return []byte("")
+}
+
+func applyFootnoteTemplate(b []byte, index, refCount int) []byte {
+	fast := true
+	for i, c := range b {
+		if i != 0 {
+			if b[i-1] == '^' && c == '^' {
+				fast = false
+				break
+			}
+			if b[i-1] == '%' && c == '%' {
+				fast = false
+				break
+			}
+		}
+	}
+	if fast {
+		return b
+	}
+	is := []byte(strconv.Itoa(index))
+	rs := []byte(strconv.Itoa(refCount))
+	ret := bytes.Replace(b, []byte("^^"), is, -1)
+	return bytes.Replace(ret, []byte("%%"), rs, -1)
+}
+
 type footnote struct {
+	options []FootnoteOption
 }
 
 // Footnote is an extension that allow you to use PHP Markdown Extra Footnotes.
-var Footnote = &footnote{}
+var Footnote = &footnote{
+	options: []FootnoteOption{},
+}
+
+// NewFootnote returns a new extension with given options.
+func NewFootnote(opts ...FootnoteOption) goldmark.Extender {
+	return &footnote{
+		options: opts,
+	}
+}
 
 func (e *footnote) Extend(m goldmark.Markdown) {
 	m.Parser().AddOptions(
@@ -331,6 +662,6 @@ func (e *footnote) Extend(m goldmark.Markdown) {
 		),
 	)
 	m.Renderer().AddOptions(renderer.WithNodeRenderers(
-		util.Prioritized(NewFootnoteHTMLRenderer(), 500),
+		util.Prioritized(NewFootnoteHTMLRenderer(e.options...), 500),
 	))
 }

extension/footnote_test.go

@@ -4,8 +4,12 @@ import (
 	"testing"
 
 	"github.com/yuin/goldmark"
+	gast "github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
 	"github.com/yuin/goldmark/renderer/html"
 	"github.com/yuin/goldmark/testutil"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
 )
 
 func TestFootnote(t *testing.T) {
@@ -19,3 +23,121 @@ func TestFootnote(t *testing.T) {
 	)
 	testutil.DoTestCaseFile(markdown, "_test/footnote.txt", t, testutil.ParseCliCaseArg()...)
 }
+
+type footnoteID struct {
+}
+
+func (a *footnoteID) Transform(node *gast.Document, reader text.Reader, pc parser.Context) {
+	node.Meta()["footnote-prefix"] = "article12-"
+}
+
+func TestFootnoteOptions(t *testing.T) {
+	markdown := goldmark.New(
+		goldmark.WithRendererOptions(
+			html.WithUnsafe(),
+		),
+		goldmark.WithExtensions(
+			NewFootnote(
+				WithFootnoteIDPrefix([]byte("article12-")),
+				WithFootnoteLinkClass([]byte("link-class")),
+				WithFootnoteBacklinkClass([]byte("backlink-class")),
+				WithFootnoteLinkTitle([]byte("link-title-%%-^^")),
+				WithFootnoteBacklinkTitle([]byte("backlink-title")),
+				WithFootnoteBacklinkHTML([]byte("^")),
+			),
+		),
+	)
+
+	testutil.DoTestCase(
+		markdown,
+		testutil.MarkdownTestCase{
+			No:          1,
+			Description: "Footnote with options",
+			Markdown: `That's some text with a footnote.[^1]
+
+Same footnote.[^1]
+
+Another one.[^2]
+
+[^1]: And that's the footnote.
+[^2]: Another footnote.
+`,
+			Expected: `<p>That's some text with a footnote.<sup id="article12-fnref:1"><a href="#article12-fn:1" class="link-class" title="link-title-2-1" role="doc-noteref">1</a></sup></p>
+<p>Same footnote.<sup id="article12-fnref:1"><a href="#article12-fn:1" class="link-class" title="link-title-2-1" role="doc-noteref">1</a></sup></p>
+<p>Another one.<sup id="article12-fnref:2"><a href="#article12-fn:2" class="link-class" title="link-title-1-2" role="doc-noteref">2</a></sup></p>
+<section class="footnotes" role="doc-endnotes">
+<hr>
+<ol>
+<li id="article12-fn:1" role="doc-endnote">
+<p>And that's the footnote. <a href="#article12-fnref:1" class="backlink-class" title="backlink-title" role="doc-backlink">^</a></p>
+</li>
+<li id="article12-fn:2" role="doc-endnote">
+<p>Another footnote. <a href="#article12-fnref:2" class="backlink-class" title="backlink-title" role="doc-backlink">^</a></p>
+</li>
+</ol>
+</section>
+`,
+		},
+		t,
+	)
+
+	markdown = goldmark.New(
+		goldmark.WithParserOptions(
+			parser.WithASTTransformers(
+				util.Prioritized(&footnoteID{}, 100),
+			),
+		),
+		goldmark.WithRendererOptions(
+			html.WithUnsafe(),
+		),
+		goldmark.WithExtensions(
+			NewFootnote(
+				WithFootnoteIDPrefixFunction(func(n gast.Node) []byte {
+					v, ok := n.OwnerDocument().Meta()["footnote-prefix"]
+					if ok {
+						return util.StringToReadOnlyBytes(v.(string))
+					}
+					return nil
+				}),
+				WithFootnoteLinkClass([]byte("link-class")),
+				WithFootnoteBacklinkClass([]byte("backlink-class")),
+				WithFootnoteLinkTitle([]byte("link-title-%%-^^")),
+				WithFootnoteBacklinkTitle([]byte("backlink-title")),
+				WithFootnoteBacklinkHTML([]byte("^")),
+			),
+		),
+	)
+
+	testutil.DoTestCase(
+		markdown,
+		testutil.MarkdownTestCase{
+			No:          2,
+			Description: "Footnote with an id prefix function",
+			Markdown: `That's some text with a footnote.[^1]
+
+Same footnote.[^1]
+
+Another one.[^2]
+
+[^1]: And that's the footnote.
+[^2]: Another footnote.
+`,
+			Expected: `<p>That's some text with a footnote.<sup id="article12-fnref:1"><a href="#article12-fn:1" class="link-class" title="link-title-2-1" role="doc-noteref">1</a></sup></p>
+<p>Same footnote.<sup id="article12-fnref:1"><a href="#article12-fn:1" class="link-class" title="link-title-2-1" role="doc-noteref">1</a></sup></p>
+<p>Another one.<sup id="article12-fnref:2"><a href="#article12-fn:2" class="link-class" title="link-title-1-2" role="doc-noteref">2</a></sup></p>
+<section class="footnotes" role="doc-endnotes">
+<hr>
+<ol>
+<li id="article12-fn:1" role="doc-endnote">
+<p>And that's the footnote. <a href="#article12-fnref:1" class="backlink-class" title="backlink-title" role="doc-backlink">^</a></p>
+</li>
+<li id="article12-fn:2" role="doc-endnote">
+<p>Another footnote. <a href="#article12-fnref:2" class="backlink-class" title="backlink-title" role="doc-backlink">^</a></p>
+</li>
+</ol>
+</section>
+`,
+		},
+		t,
+	)
+}

testutil/testutil.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/parser"
 	"github.com/yuin/goldmark/util"
 )
 
@@ -127,14 +128,14 @@ func DoTestCaseFile(m goldmark.Markdown, filename string, t TestingT, no ...int)
 }
 
 // DoTestCases runs a set of test cases.
-func DoTestCases(m goldmark.Markdown, cases []MarkdownTestCase, t TestingT) {
+func DoTestCases(m goldmark.Markdown, cases []MarkdownTestCase, t TestingT, opts ...parser.ParseOption) {
 	for _, testCase := range cases {
-		DoTestCase(m, testCase, t)
+		DoTestCase(m, testCase, t, opts...)
 	}
 }
 
 // DoTestCase runs a test case.
-func DoTestCase(m goldmark.Markdown, testCase MarkdownTestCase, t TestingT) {
+func DoTestCase(m goldmark.Markdown, testCase MarkdownTestCase, t TestingT, opts ...parser.ParseOption) {
 	var ok bool
 	var out bytes.Buffer
 	defer func() {
@@ -176,7 +177,7 @@ Actual
 		}
 	}()
 
-	if err := m.Convert([]byte(testCase.Markdown), &out); err != nil {
+	if err := m.Convert([]byte(testCase.Markdown), &out, opts...); err != nil {
 		panic(err)
 	}
 	ok = bytes.Equal(bytes.TrimSpace(out.Bytes()), bytes.TrimSpace([]byte(testCase.Expected)))