Make linkeable lines a link to themselves

Currently its already possible to make line lumbers linkeable. Getting
such a link however requires the end user to look at the pages source
and then to manually edit the URL, which is not a great UX.

This PR changes that to make the line numbers a link to themselves, so
clicking on them gives a link to that line that can then be passed
around, similiar to e.G. GitHub.

README.md

@@ -183,7 +183,7 @@ following constructor options:
 - `ClassPrefix(prefix)` - prefix each generated CSS class.
 - `TabWidth(width)` - Set the rendered tab width, in characters.
 - `WithLineNumbers()` - Render line numbers (style with `LineNumbers`).
-- `LinkableLineNumbers()` - Make the line numbers linkable.
+- `LinkableLineNumbers()` - Make the line numbers linkable and be a link to themselves.
 - `HighlightLines(ranges)` - Highlight lines in these ranges (style with `LineHighlight`).
 - `LineNumbersInTable()` - Use a table for formatting line numbers and code, rather than spans.
 

formatters/html/html.go

@@ -211,7 +211,7 @@ func (f *Formatter) writeHTML(w io.Writer, style *chroma.Style, tokens []chroma.
 				fmt.Fprintf(w, "<span%s>", f.styleAttr(css, chroma.LineHighlight))
 			}
 
-			fmt.Fprintf(w, "<span%s%s>%*d\n</span>", f.styleAttr(css, chroma.LineNumbersTable), f.lineIDAttribute(line), lineDigits, line)
+			fmt.Fprintf(w, "<span%s%s>%s\n</span>", f.styleAttr(css, chroma.LineNumbersTable), f.lineIDAttribute(line), f.lineTitleWithLinkIfNeeded(lineDigits, line))
 
 			if highlight {
 				fmt.Fprintf(w, "</span>")
@@ -237,7 +237,7 @@ func (f *Formatter) writeHTML(w io.Writer, style *chroma.Style, tokens []chroma.
 		}
 
 		if f.lineNumbers && !wrapInTable {
-			fmt.Fprintf(w, "<span%s%s>%*d</span>", f.styleAttr(css, chroma.LineNumbers), f.lineIDAttribute(line), lineDigits, line)
+			fmt.Fprintf(w, "<span%s%s>%s</span>", f.styleAttr(css, chroma.LineNumbers), f.lineIDAttribute(line), f.lineTitleWithLinkIfNeeded(lineDigits, line))
 		}
 
 		for _, token := range tokens {
@@ -272,7 +272,19 @@ func (f *Formatter) lineIDAttribute(line int) string {
 	if !f.linkableLineNumbers {
 		return ""
 	}
-	return fmt.Sprintf(" id=\"%s%d\"", f.lineNumbersIDPrefix, line)
+	return fmt.Sprintf(" id=\"%s\"", f.lineID(line))
+}
+
+func (f *Formatter) lineTitleWithLinkIfNeeded(lineDigits, line int) string {
+	title := fmt.Sprintf("%*d", lineDigits, line)
+	if !f.linkableLineNumbers {
+		return title
+	}
+	return fmt.Sprintf("<a style=\"outline: none; text-decoration:none; color:inherit\" href=\"#%s\">%s</a>", f.lineID(line), title)
+}
+
+func (f *Formatter) lineID(line int) string {
+	return fmt.Sprintf("%s%d", f.lineNumbersIDPrefix, line)
 }
 
 func (f *Formatter) shouldHighlight(highlightIndex, line int) (bool, bool) {

formatters/html/html_test.go

@@ -108,6 +108,32 @@ func TestTableLineNumberNewlines(t *testing.T) {
 </span>`)
 }
 
+func TestLinkeableLineNumbers(t *testing.T) {
+	f := New(WithClasses(true), WithLineNumbers(true), LinkableLineNumbers(true, "line"))
+	it, err := lexers.Get("go").Tokenise(nil, "package main\nfunc main()\n{\nprintln(\"hello world\")\n}\n")
+	assert.NoError(t, err)
+
+	var buf bytes.Buffer
+	err = f.Format(&buf, styles.Fallback, it)
+	assert.NoError(t, err)
+
+	assert.Contains(t, buf.String(), `id="line1"><a style="outline: none; text-decoration:none; color:inherit" href="#line1">1</a>`)
+	assert.Contains(t, buf.String(), `id="line5"><a style="outline: none; text-decoration:none; color:inherit" href="#line5">5</a>`)
+}
+
+func TestTableLinkeableLineNumbers(t *testing.T) {
+	f := New(WithClasses(true), WithLineNumbers(true), LineNumbersInTable(true), LinkableLineNumbers(true, "line"))
+	it, err := lexers.Get("go").Tokenise(nil, "package main\nfunc main()\n{\nprintln(`hello world`)\n}\n")
+	assert.NoError(t, err)
+
+	var buf bytes.Buffer
+	err = f.Format(&buf, styles.Fallback, it)
+	assert.NoError(t, err)
+
+	assert.Contains(t, buf.String(), `id="line1"><a style="outline: none; text-decoration:none; color:inherit" href="#line1">1</a>`)
+	assert.Contains(t, buf.String(), `id="line5"><a style="outline: none; text-decoration:none; color:inherit" href="#line5">5</a>`)
+}
+
 func TestTableLineNumberSpacing(t *testing.T) {
 	testCases := []struct {
 		baseLineNumber int