Add support for custom numbering of ordered list using [@\d+]

org mode supports [1] setting the value attribute [2] of ordered list items to
change the numbering for the current and following items. Let's do the same. As
the attribute has no meaning for other types of lists [2] we'll just not
support it for those cases [3].

[1] https://orgmode.org/manual/Plain-Lists.html#Plain-Lists
[2] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/li#attributes
[3]
Org mode seems to instead set the id attribute for e.g. unordered lists
starting with `[@\d+]\s` - but I don't really see the value in that and will
skip that for now.

org/html_writer.go

@@ -412,11 +412,14 @@ func (w *HTMLWriter) WriteList(l List) {
 }
 
 func (w *HTMLWriter) WriteListItem(li ListItem) {
-	if li.Status != "" {
-		w.WriteString(fmt.Sprintf("<li class=\"%s\">\n", listItemStatuses[li.Status]))
-	} else {
-		w.WriteString("<li>\n")
+	attributes := ""
+	if li.Value != "" {
+		attributes += fmt.Sprintf(` value="%s"`, li.Value)
 	}
+	if li.Status != "" {
+		attributes += fmt.Sprintf(` class="%s"`, listItemStatuses[li.Status])
+	}
+	w.WriteString(fmt.Sprintf("<li%s>\n", attributes))
 	WriteNodes(w, li.Children...)
 	w.WriteString("</li>\n")
 }

org/list.go

@@ -15,6 +15,7 @@ type List struct {
 type ListItem struct {
 	Bullet   string
 	Status   string
+	Value    string
 	Children []Node
 }
 
@@ -28,6 +29,7 @@ type DescriptiveListItem struct {
 var unorderedListRegexp = regexp.MustCompile(`^(\s*)([+*-])(\s+(.*)|$)`)
 var orderedListRegexp = regexp.MustCompile(`^(\s*)(([0-9]+|[a-zA-Z])[.)])(\s+(.*)|$)`)
 var descriptiveListItemRegexp = regexp.MustCompile(`\s::(\s|$)`)
+var listItemValueRegexp = regexp.MustCompile(`\[@(\d+)\]\s`)
 var listItemStatusRegexp = regexp.MustCompile(`\[( |X|-)\]\s`)
 
 func lexList(line string) (token, bool) {
@@ -80,9 +82,12 @@ func (d *Document) parseList(i int, parentStop stopFn) (int, Node) {
 
 func (d *Document) parseListItem(l List, i int, parentStop stopFn) (int, Node) {
 	start, nodes, bullet := i, []Node{}, d.tokens[i].matches[2]
-	minIndent, dterm, content, status := d.tokens[i].lvl+len(bullet), "", d.tokens[i].content, ""
+	minIndent, dterm, content, status, value := d.tokens[i].lvl+len(bullet), "", d.tokens[i].content, "", ""
 	originalBaseLvl := d.baseLvl
 	d.baseLvl = minIndent + 1
+	if m := listItemValueRegexp.FindStringSubmatch(content); m != nil && l.Kind == "ordered" {
+		value, content = m[1], content[len("[@] ")+len(m[1]):]
+	}
 	if m := listItemStatusRegexp.FindStringSubmatch(content); m != nil {
 		status, content = m[1], content[len("[ ] "):]
 	}
@@ -110,7 +115,7 @@ func (d *Document) parseListItem(l List, i int, parentStop stopFn) (int, Node) {
 	if l.Kind == "descriptive" {
 		return i - start, DescriptiveListItem{bullet, status, d.parseInline(dterm), nodes}
 	}
-	return i - start, ListItem{bullet, status, nodes}
+	return i - start, ListItem{bullet, status, value, nodes}
 }
 
 func (n List) String() string                { return orgWriter.WriteNodesAsString(n) }

org/org_writer.go

@@ -219,6 +219,9 @@ func (w *OrgWriter) WriteListItem(li ListItem) {
 	content := strings.TrimPrefix(w.String(), w.indent)
 	w.Builder, w.indent = originalBuilder, originalIndent
 	w.WriteString(w.indent + li.Bullet)
+	if li.Value != "" {
+		w.WriteString(fmt.Sprintf(" [@%s]", li.Value))
+	}
 	if li.Status != "" {
 		w.WriteString(fmt.Sprintf(" [%s]", li.Status))
 	}

org/testdata/lists.html

@@ -195,3 +195,14 @@ ordered descriptive
 <p>unordered 2</p>
 </li>
 </ul>
+<ol>
+<li value="2">
+<p>use `[@n]` to change the value of list items</p>
+</li>
+<li class="unchecked">
+<p>foobar</p>
+</li>
+<li value="10" class="checked">
+<p>that even works in combination with list statuses (`[ ]`)</p>
+</li>
+</ol>

org/testdata/lists.org

@@ -68,3 +68,8 @@ some list termination tests
 2. ordered descriptive :: 2
 - unordered 1
 - unordered 2
+
+
+1. [@2] use `[@n]` to change the value of list items
+2. [ ] foobar
+3. [@10] [X] that even works in combination with list statuses (`[ ]`)

org/testdata/lists.pretty_org

@@ -68,3 +68,8 @@ some list termination tests
 2. ordered descriptive :: 2
 - unordered 1
 - unordered 2
+
+
+1. [@2] use `[@n]` to change the value of list items
+2. [ ] foobar
+3. [@10] [X] that even works in combination with list statuses (`[ ]`)