html: Improve link abbreviation (#+LINK) resolution

Handle `%s`/`%h` interpolation and add support for shorthand of prefix only
links / links with empty tag (i.e. `[example]` instead of [example:]). See
[1] and [2]. As so often we don't match org mode output completely but our
output looks sane enough to me so let's move along for now.

[1] https://orgmode.org/manual/Link-Abbreviations.html
[2] https://github.com/bzg/org-mode/blob/main/lisp/ol.el#L1011

org/html_writer.go

@@ -10,6 +10,8 @@ import (
 	"strings"
 	"unicode"
 
+	u "net/url"
+
 	h "golang.org/x/net/html"
 	"golang.org/x/net/html/atom"
 )
@@ -361,7 +363,13 @@ func (w *HTMLWriter) WriteRegularLink(l RegularLink) {
 		url = strings.TrimSuffix(url, ".org") + ".html"
 	}
 	if prefix := w.document.Links[l.Protocol]; prefix != "" {
-		url = html.EscapeString(prefix) + strings.TrimPrefix(url, l.Protocol+":")
+		if tag := strings.TrimPrefix(l.URL, l.Protocol+":"); strings.Contains(prefix, "%s") || strings.Contains(prefix, "%h") {
+			url = html.EscapeString(strings.ReplaceAll(strings.ReplaceAll(prefix, "%s", tag), "%h", u.QueryEscape(tag)))
+		} else {
+			url = html.EscapeString(prefix) + tag
+		}
+	} else if prefix := w.document.Links[l.URL]; prefix != "" {
+		url = html.EscapeString(strings.ReplaceAll(strings.ReplaceAll(prefix, "%s", ""), "%h", ""))
 	}
 	switch l.Kind() {
 	case "image":

org/keyword.go

@@ -63,7 +63,7 @@ func (d *Document) parseKeyword(i int, stop stopFn) (int, Node) {
 	case "INCLUDE":
 		return d.parseInclude(k)
 	case "LINK":
-		if parts := strings.Split(k.Value, " "); len(parts) >= 2 {
+		if parts := strings.SplitN(k.Value, " ", 2); len(parts) == 2 {
 			d.Links[parts[0]] = parts[1]
 		}
 		return 1, k

org/testdata/inline.html

@@ -55,7 +55,15 @@ not emphasized/</li>
 </ul>
 </li>
 <li>
-<p><code class="verbatim">#+LINK</code> based links: <a href="https://www.example.com/foobar">https://www.example.com/foobar</a></p>
+<p><code class="verbatim">#+LINK</code> based links:</p>
+<ul>
+<li><a href="https://www.example.com/foobar">https://www.example.com/foobar</a></li>
+<li><a href="https://www.example.com/">https://www.example.com/</a></li>
+<li><a href="https://www.example.com/">https://www.example.com/</a></li>
+<li><a href="https://www.example.com/">description</a></li>
+<li><a href="https://www.example.com?raw_tag=tag value with specical chars % : &amp;">https://www.example.com?raw_tag=tag value with specical chars % : &amp;</a> (w/o tag <a href="https://www.example.com?raw_tag=">https://www.example.com?raw_tag=</a>)</li>
+<li><a href="https://www.example.com?encoded_tag=tag+value+with+specical+chars+%25+%3A+%26">https://www.example.com?encoded_tag=tag+value+with+specical+chars+%25+%3A+%26</a> (w/o tag <a href="https://www.example.com?encoded_tag=">https://www.example.com?encoded_tag=</a>)</li>
+</ul>
 </li>
 <li>
 <p><code class="verbatim">#+MACROs</code>: <p><h1>yolo</h1></p>

org/testdata/inline.org

@@ -39,7 +39,15 @@
   - <2019-01-06 Sun 18:00 +1w>
   - <2019-01-06 18:00>
   - <2019-01-06 18:00 +1w>
-- =#+LINK= based links: [[example:foobar]]
+- =#+LINK= based links:
   #+LINK: example https://www.example.com/
+  #+LINK: example_interpolate_s https://www.example.com?raw_tag=%s
+  #+LINK: example_interpolate_h https://www.example.com?encoded_tag=%h
+  - [[example:foobar]]
+  - [[example:]]
+  - [[example]]
+  - [[example][description]]
+  - [[example_interpolate_s:tag value with specical chars % : &]] (w/o tag [[example_interpolate_s]])
+  - [[example_interpolate_h:tag value with specical chars % : &]] (w/o tag [[example_interpolate_h]])
 - =#+MACROs=: {{{headline(yolo)}}}
   #+MACRO: headline @@html:<h1>$1</h1>@@

org/testdata/inline.pretty_org

@@ -39,7 +39,15 @@
   - <2019-01-06 Sun 18:00 +1w>
   - <2019-01-06 Sun 18:00>
   - <2019-01-06 Sun 18:00 +1w>
-- =#+LINK= based links: [[example:foobar]]
+- =#+LINK= based links:
   #+LINK: example https://www.example.com/
+  #+LINK: example_interpolate_s https://www.example.com?raw_tag=%s
+  #+LINK: example_interpolate_h https://www.example.com?encoded_tag=%h
+  - [[example:foobar]]
+  - [[example:]]
+  - [[example]]
+  - [[example][description]]
+  - [[example_interpolate_s:tag value with specical chars % : &]] (w/o tag [[example_interpolate_s]])
+  - [[example_interpolate_h:tag value with specical chars % : &]] (w/o tag [[example_interpolate_h]])
 - =#+MACROs=: {{{headline(yolo)}}}
   #+MACRO: headline @@html:<h1>$1</h1>@@