all: implement macro "footnote:"

Macro footnote grammar,

----
"footnote:" [ REF_ID ] "[" STRING "]"
----

In asciidoctor, footnote can be placed anywhere, even after WORD without
space in between.

The REF_ID, define the unique ID for footnote and can be used to reference
the previous footnote.
The first footnote with REF_ID, should have the STRING defined.
The next footnote with the same REF_ID, should not have the STRING
defined; if its defined, the STRING is ignored.

12 files changed, 430 insertions, 52 deletions

diff --git a/README b/README
index 52f975d..0d16fd0 100644
--- a/README
+++ b/README
@@ -90,6 +90,7 @@ The numbered one is based on the old documentation.
 ** 27.2. Defining an Anchor
 ** 27.3. Internal Cross References
 ** 27.5. Customizing the Cross Reference Text
+* https://docs.asciidoctor.org/asciidoc/latest/macros/footnote/[Footnotes]
 * 28. Include Directive
 ** 28.1. Anatomy
 ** 28.2. Processing
diff --git a/_doc/SPECS.adoc b/_doc/SPECS.adoc
index c3f974c..4eae7a5 100644
--- a/_doc/SPECS.adoc
+++ b/_doc/SPECS.adoc
@@ -559,8 +559,25 @@ CELL_ALIGN_VER = "." ("<" / "^" / ">")
 CELL_STYLE     = "a" / "d" / "e" / "h" / "l" / "m" / "s" / "v"
 ----
 
+== Footnote
 
-== Inconsistencies and bugs on asciidoctor
+Syntax,
+
+----
+"footnote:" [ REF_ID ] "[" STRING "]"
+----
+
+In asciidoctor, footnote can be placed anywhere, even after WORD without space
+in between.
+
+The REF_ID, define the unique ID for footnote and can be used to reference the
+previous footnote.
+The first footnote with REF_ID, should have the STRING defined.
+The next footnote with the same REF_ID, should not have the STRING defined;
+if its defined, the STRING is ignored.
+
+
+== Inconsistencies and bugs in asciidoctor
 
 Listing style "[listing]" followed by "...." is become listing block.
 Example,
diff --git a/document.go b/document.go
index 8b9d42e..2b34300 100644
--- a/document.go
+++ b/document.go
@@ -38,6 +38,9 @@ type Document struct {
 	// its ID.
 	titleID map[string]string
 
+	// List of footnote ID and its text.
+	footnotes []*macro
+
 	Revision Revision
 
 	LastUpdated string
@@ -273,6 +276,8 @@ func (doc *Document) toHTMLBody(buf *bytes.Buffer, withHeaderFooter bool) {
 
 	htmlWriteBody(doc, buf)
 
+	htmlWriteFootnoteDefs(doc, buf)
+
 	if withHeaderFooter {
 		_, ok = doc.Attributes[metaNameNoFooter]
 		if !ok {
@@ -313,6 +318,29 @@ func (doc *Document) registerAnchor(id, label string) string {
 	return id
 }
 
+// registerFootnote add footnote with id and text, where id is optional.
+// If the id already exist it will return true.
+func (doc *Document) registerFootnote(id string, rawContent []byte) (mcr *macro, exist bool) {
+	if len(id) != 0 {
+		// Find existing footnote with the same ID.
+		for _, mcr = range doc.footnotes {
+			if mcr.key == id {
+				return mcr, true
+			}
+		}
+	}
+
+	mcr = &macro{
+		key:        id,
+		rawContent: rawContent,
+	}
+	doc.footnotes = append(doc.footnotes, mcr)
+
+	mcr.level = len(doc.footnotes)
+
+	return mcr, false
+}
+
 // tocHTML write table of contents with HTML template into out.
 func (doc *Document) tocHTML(out io.Writer) {
 	var (
diff --git a/element.go b/element.go
index a558055..1abba9e 100644
--- a/element.go
+++ b/element.go
@@ -831,6 +831,9 @@ func (el *element) toHTML(doc *Document, w io.Writer) {
 		}
 		fmt.Fprintf(w, `<a href="#%s">%s</a>`, href, label)
 
+	case elKindFootnote:
+		htmlWriteFootnote(el, w)
+
 	case elKindMacroTOC:
 		if doc.tocIsEnabled && doc.tocPosition == metaValueMacro {
 			doc.tocHTML(w)
diff --git a/html_backend.go b/html_backend.go
index e631448..4c2e467 100644
--- a/html_backend.go
+++ b/html_backend.go
@@ -393,6 +393,54 @@ func htmlWriteFooter(doc *Document, out io.Writer) {
 	fmt.Fprint(out, "\n</div>\n</div>")
 }
 
+// htmlWriteFootnote generate HTML content for footnote.
+// Each unique footnote will have its id, so it can be referenced at footer.
+func htmlWriteFootnote(el *element, out io.Writer) {
+	if len(el.ID) != 0 {
+		// The first footnote with explicit ID.
+		fmt.Fprintf(out, `<sup class="footnote" id="_footnote_%s">[<a id="_footnoteref_%d" class="footnote" href="#_footnotedef_%d" title="View footnote.">%d</a>]</sup>`,
+			el.ID, el.level, el.level, el.level)
+
+	} else if len(el.key) != 0 {
+		// The first footnote without ID.
+		fmt.Fprintf(out, `<sup class="footnote">[<a id="_footnoteref_%d" class="footnote" href="#_footnotedef_%d" title="View footnote.">%d</a>]</sup>`,
+			el.level, el.level, el.level)
+	} else {
+		// The next footnote with same ID.
+		fmt.Fprintf(out, `<sup class="footnoteref">[<a class="footnote" href="#_footnotedef_%d" title="View footnote.">%d</a>]</sup>`,
+			el.level, el.level)
+	}
+}
+
+func htmlWriteFootnoteDefs(doc *Document, out io.Writer) {
+	if len(doc.footnotes) == 0 {
+		return
+	}
+
+	fmt.Fprint(out, "\n")
+	fmt.Fprint(out, `<div id="footnotes">`)
+	fmt.Fprint(out, "\n")
+	fmt.Fprint(out, `<hr>`)
+	fmt.Fprint(out, "\n")
+
+	var (
+		mcr *macro
+	)
+	for _, mcr = range doc.footnotes {
+		fmt.Fprintf(out, `<div class="footnote" id="_footnotedef_%d">`, mcr.level)
+		fmt.Fprint(out, "\n")
+		fmt.Fprintf(out, `<a href="#_footnoteref_%d">%d</a>. `, mcr.level, mcr.level)
+		if mcr.content != nil {
+			mcr.content.toHTML(doc, out)
+		}
+		fmt.Fprint(out, "\n")
+		fmt.Fprint(out, `</div>`)
+		fmt.Fprint(out, "\n")
+	}
+	fmt.Fprint(out, `</div>`)
+	fmt.Fprint(out, "\n")
+}
+
 func htmlWriteHeader(doc *Document, out io.Writer) {
 	fmt.Fprint(out, "\n<div id=\"header\">")
 
diff --git a/inline_parser.go b/inline_parser.go
index 881374e..eea1e39 100644
--- a/inline_parser.go
+++ b/inline_parser.go
@@ -397,20 +397,6 @@ func (pi *inlineParser) escape() {
 	pi.prev = pi.c
 }
 
-func (pi *inlineParser) getBackMacroName() (macroName string, lastc byte) {
-	var (
-		raw   []byte = pi.current.raw
-		start int    = len(raw) - 1
-	)
-	for start >= 0 {
-		if !ascii.IsAlpha(raw[start]) {
-			return string(raw[start+1:]), raw[start]
-		}
-		start--
-	}
-	return string(raw), 0
-}
-
 func (pi *inlineParser) parseCrossRef() bool {
 	var (
 		raw []byte = pi.content[pi.x+2:]
@@ -729,51 +715,48 @@ func (pi *inlineParser) parseInlineImage() *element {
 
 func (pi *inlineParser) parseMacro() bool {
 	var (
-		el    *element
-		name  string
-		lastc byte
+		el   *element
+		name string
+		n    int
 	)
 
-	name, lastc = pi.getBackMacroName()
-	if lastc == '\\' || len(name) == 0 {
+	name = pi.parseMacroName(pi.current.raw)
+	if len(name) == 0 {
 		return false
 	}
 
 	switch name {
-	case ``:
-		return false
-	case macroFTP, macroHTTPS, macroHTTP, macroIRC, macroLink, macroMailto:
-		el = pi.parseURL(name)
+	case macroFootnote:
+		el, n = pi.parseMacroFootnote(pi.content[pi.x+1:])
 		if el == nil {
 			return false
 		}
 
-		pi.current.raw = pi.current.raw[:len(pi.current.raw)-len(name)]
+		pi.x += n
+		pi.prev = 0
 
-		pi.current.addChild(el)
-		el = &element{
-			kind: elKindText,
+	case macroFTP, macroHTTPS, macroHTTP, macroIRC, macroLink, macroMailto:
+		el = pi.parseURL(name)
+		if el == nil {
+			return false
 		}
-		pi.current.addChild(el)
-		pi.current = el
-		return true
+
 	case macroImage:
 		el = pi.parseInlineImage()
 		if el == nil {
 			return false
 		}
+	}
 
-		pi.current.raw = pi.current.raw[:len(pi.current.raw)-len(name)]
+	pi.current.raw = pi.current.raw[:len(pi.current.raw)-len(name)]
 
-		pi.current.addChild(el)
-		el = &element{
-			kind: elKindText,
-		}
-		pi.current.addChild(el)
-		pi.current = el
-		return true
+	pi.current.addChild(el)
+	el = &element{
+		kind: elKindText,
 	}
-	return false
+	pi.current.addChild(el)
+	pi.current = el
+	return true
 }
 
 func (pi *inlineParser) parsePassthrough() bool {
diff --git a/inline_parser_test.go b/inline_parser_test.go
index 62c4f74..0fd62bf 100644
--- a/inline_parser_test.go
+++ b/inline_parser_test.go
@@ -96,3 +96,39 @@ func TestInlineParser_parseInlineID_isForToC(t *testing.T) {
 		test.Assert(t, c.content, c.exp, got)
 	}
 }
+
+func TestInlineParser_macro_footnote(t *testing.T) {
+	var (
+		testFiles = []string{
+			`testdata/inline_parser/macro_footnote_test.txt`,
+			`testdata/inline_parser/macro_footnote_externalized_test.txt`,
+		}
+
+		testFile string
+		got      bytes.Buffer
+		tdata    *test.Data
+		doc      *Document
+		exp      []byte
+		err      error
+	)
+
+	for _, testFile = range testFiles {
+		tdata, err = test.LoadData(testFile)
+		if err != nil {
+			t.Fatalf(`%s: %s`, testFile, err)
+		}
+
+		doc = Parse(tdata.Input[`input.adoc`])
+
+		err = doc.ToHTMLEmbedded(&got)
+		if err != nil {
+			t.Fatalf(`%s: %s`, testFile, err)
+		}
+
+		exp = tdata.Output[`output.html`]
+
+		test.Assert(t, testFile, string(exp), got.String())
+
+		got.Reset()
+	}
+}
diff --git a/macro.go b/macro.go
new file mode 100644
index 0000000..4485660
--- /dev/null
+++ b/macro.go
@@ -0,0 +1,160 @@
+// SPDX-FileCopyrightText: 2022 M. Shulhan <ms@kilabit.info>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package asciidoctor
+
+import (
+	"strconv"
+
+	"github.com/shuLhan/share/lib/ascii"
+)
+
+const (
+	macroFTP      = `ftp`
+	macroFootnote = `footnote`
+	macroHTTP     = `http`
+	macroHTTPS    = `https`
+	macroIRC      = `irc`
+	macroImage    = `image`
+	macroLink     = `link`
+	macroMailto   = `mailto`
+)
+
+var (
+	_macroKind = map[string]int{
+		macroFTP:      elKindURL,
+		macroFootnote: elKindFootnote,
+		macroHTTP:     elKindURL,
+		macroHTTPS:    elKindURL,
+		macroIRC:      elKindURL,
+		macroImage:    elKindInlineImage,
+		macroLink:     elKindURL,
+		macroMailto:   elKindURL,
+	}
+)
+
+type macro struct {
+	content *element
+
+	// key represent the URL in elKindURL or elKindImage;
+	// or ID in elKindFootnote.
+	// For ID, it could be empty.
+	key string
+
+	// val represent the text for URL or image and footnote.
+	rawContent []byte
+
+	// level represent footnoted index number.
+	level int
+}
+
+// parseMacroName parse inline macro.
+//
+// The parser read the content textBefore in backward order until it found one
+// of macro name.
+// Macro can be escaped using the backslash, for example "\link:", and it will
+// be ignored.
+//
+// If macro name and value valid it will return the element for that macro.
+func (pi *inlineParser) parseMacroName(textBefore []byte) (macroName string) {
+	var (
+		x int = len(textBefore) - 1
+
+		ok bool
+	)
+
+	for x >= 0 {
+		if !ascii.IsAlpha(textBefore[x]) {
+			return ``
+		}
+
+		macroName = string(textBefore[x:])
+		_, ok = _macroKind[macroName]
+		if ok {
+			break
+		}
+
+		x--
+	}
+	if !ok {
+		return ``
+	}
+
+	x--
+	if x >= 0 {
+		if textBefore[x] == '\\' {
+			// Macro is escaped.
+			return ``
+		}
+	}
+
+	return macroName
+}
+
+// parseMacroFootnote parse the footnote macro,
+//
+//	"footnote:" [ REF_ID ] "[" STRING "]"
+//
+// The text content does not have "footnote:".
+//
+// The REF_ID, reference the previous footnote defined with the same.
+// The first footnote with REF_ID, should have the STRING defined.
+// The next footnote with the same REF_ID, should not have the STRING
+// defined; if its already defined, the STRING is ignored.
+//
+// It will return an element if footnote is valid.
+func (pi *inlineParser) parseMacroFootnote(text []byte) (el *element, n int) {
+	var (
+		mcr    *macro
+		id     string
+		key    string
+		vbytes []byte
+		x      int
+		exist  bool
+	)
+
+	vbytes, x = indexByteUnescape(text, '[')
+	if x > 0 {
+		if !isValidID(vbytes) {
+			return nil, 0
+		}
+		id = string(vbytes)
+	}
+
+	n = x + 1
+	text = text[n:]
+
+	vbytes, x = indexByteUnescape(text, ']')
+	if x < 0 {
+		return nil, 0
+	}
+
+	n += x + 2
+
+	mcr, exist = pi.doc.registerFootnote(id, vbytes)
+	if exist {
+		id = ``
+		vbytes = nil
+	} else {
+		// Footnote without explicit ID will be set the key with its
+		// level.
+		key = strconv.FormatInt(int64(mcr.level), 10)
+	}
+
+	el = &element{
+		key:   key,
+		raw:   vbytes,
+		kind:  elKindFootnote,
+		level: mcr.level,
+
+		elementAttribute: elementAttribute{
+			ID: id,
+		},
+	}
+
+	if vbytes != nil {
+		mcr.content = parseInlineMarkup(pi.doc, vbytes)
+	}
+
+	return el, n
+}
diff --git a/parser.go b/parser.go
index a210069..033604c 100644
--- a/parser.go
+++ b/parser.go
@@ -11,16 +11,6 @@ import (
 )
 
 const (
-	macroFTP    = `ftp`
-	macroHTTP   = `http`
-	macroHTTPS  = `https`
-	macroIRC    = `irc`
-	macroImage  = `image`
-	macroLink   = `link`
-	macroMailto = `mailto`
-)
-
-const (
 	elKindUnknown                int = iota
 	elKindDocHeader                  // Wrapper.
 	elKindPreamble                   // Wrapper.
@@ -46,6 +36,7 @@ const (
 	elKindBlockSidebar               // "****"
 	elKindBlockVideo                 // "video::"
 	elKindCrossReference             // "<<" REF ("," LABEL) ">>"
+	elKindFootnote                   // footnote:id[]
 	elKindInlineID                   // "[[" REF_ID "]]" TEXT
 	elKindInlineIDShort              // "[#" REF_ID "]#" TEXT "#"
 	elKindInlineImage                // Inline macro for "image:"
diff --git a/testdata/inline_parser/macro_footnote_externalized_test.txt b/testdata/inline_parser/macro_footnote_externalized_test.txt
new file mode 100644
index 0000000..bd8736c
--- /dev/null
+++ b/testdata/inline_parser/macro_footnote_externalized_test.txt
@@ -0,0 +1,56 @@
+>>> input.adoc
+:fn-hail-and-rainbow: footnote:[The _double_ hail-and-rainbow level makes my toes tingle.]
+:fn-disclaimer: footnote:disclaimer[Opinions are my own.]
+
+The hail-and-rainbow protocol can be initiated at five levels:
+
+. double{fn-hail-and-rainbow}
+. tertiary
+. supernumerary
+. supermassive
+. apocalyptic
+
+A bold statement!{fn-disclaimer}
+
+Another outrageous statement.{fn-disclaimer}
+
+<<< output.html
+
+<div class="paragraph">
+<p>The hail-and-rainbow protocol can be initiated at five levels:</p>
+</div>
+<div class="olist arabic">
+<ol class="arabic">
+<li>
+<p>double<sup class="footnote">[<a id="_footnoteref_1" class="footnote" href="#_footnotedef_1" title="View footnote.">1</a>]</sup>
+</p>
+</li>
+<li>
+<p>tertiary</p>
+</li>
+<li>
+<p>supernumerary</p>
+</li>
+<li>
+<p>supermassive</p>
+</li>
+<li>
+<p>apocalyptic</p>
+</li>
+</ol>
+</div>
+<div class="paragraph">
+<p>A bold statement!<sup class="footnote" id="_footnote_disclaimer">[<a id="_footnoteref_2" class="footnote" href="#_footnotedef_2" title="View footnote.">2</a>]</sup></p>
+</div>
+<div class="paragraph">
+<p>Another outrageous statement.<sup class="footnoteref">[<a class="footnote" href="#_footnotedef_2" title="View footnote.">2</a>]</sup></p>
+</div>
+<div id="footnotes">
+<hr>
+<div class="footnote" id="_footnotedef_1">
+<a href="#_footnoteref_1">1</a>. The <em>double</em> hail-and-rainbow level makes my toes tingle.
+</div>
+<div class="footnote" id="_footnotedef_2">
+<a href="#_footnoteref_2">2</a>. Opinions are my own.
+</div>
+</div>
diff --git a/testdata/inline_parser/macro_footnote_test.txt b/testdata/inline_parser/macro_footnote_test.txt
new file mode 100644
index 0000000..f89c0c1
--- /dev/null
+++ b/testdata/inline_parser/macro_footnote_test.txt
@@ -0,0 +1,55 @@
+
+>>> input.adoc
+
+The hail-and-rainbow protocol can be initiated at five levels:
+
+. doublefootnote:[The _double_ hail-and-rainbow level makes my toes tingle.]
+. tertiary
+. supernumerary
+. supermassive
+. apocalyptic
+
+A bold statement!footnote:disclaimer[Opinions are my own.]
+
+Another outrageous statement.footnote:disclaimer[]
+
+<<< output.html
+
+<div class="paragraph">
+<p>The hail-and-rainbow protocol can be initiated at five levels:</p>
+</div>
+<div class="olist arabic">
+<ol class="arabic">
+<li>
+<p>double<sup class="footnote">[<a id="_footnoteref_1" class="footnote" href="#_footnotedef_1" title="View footnote.">1</a>]</sup>
+</p>
+</li>
+<li>
+<p>tertiary</p>
+</li>
+<li>
+<p>supernumerary</p>
+</li>
+<li>
+<p>supermassive</p>
+</li>
+<li>
+<p>apocalyptic</p>
+</li>
+</ol>
+</div>
+<div class="paragraph">
+<p>A bold statement!<sup class="footnote" id="_footnote_disclaimer">[<a id="_footnoteref_2" class="footnote" href="#_footnotedef_2" title="View footnote.">2</a>]</sup></p>
+</div>
+<div class="paragraph">
+<p>Another outrageous statement.<sup class="footnoteref">[<a class="footnote" href="#_footnotedef_2" title="View footnote.">2</a>]</sup></p>
+</div>
+<div id="footnotes">
+<hr>
+<div class="footnote" id="_footnotedef_1">
+<a href="#_footnoteref_1">1</a>. The <em>double</em> hail-and-rainbow level makes my toes tingle.
+</div>
+<div class="footnote" id="_footnotedef_2">
+<a href="#_footnoteref_2">2</a>. Opinions are my own.
+</div>
+</div>
diff --git a/testdata/test.got.html b/testdata/test.got.html
index 33b917d..d21e7a4 100644
--- a/testdata/test.got.html
+++ b/testdata/test.got.html
@@ -6,7 +6,7 @@
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <meta name="author" content="Author A, Author mid dle B">
 <meta name="description" content="meta description">
-<meta name="generator" content="asciidoctor-go 0.3.1">
+<meta name="generator" content="asciidoctor-go 0.3.2">
 <meta name="keywords" content="key, words">
 <title>Example Document title</title>
 <style>
@@ -3000,7 +3000,7 @@ this sidebar.</p>
 <div id="footer">
 <div id="footer-text">
  1.1.1<br>
-Last updated 2022-09-04 23:41:43 +0700
+Last updated 2022-10-18 00:21:58 +0700
 </div>
 </div>
 </body>