Fix highlighting code destroying backlinks on unnamed <ref>s

There are multiple formats for these IDs:

cite_ref-1, cite_ref-2, and so on for anonymous <ref>s without a name.

cite_ref-name_1-0 for named references, where "name" is the custom name,
and "_1" is the sequential number for the reference (same number as above).
The final "-0" is counting the usages. If a named <ref> is only used once,
there is no cite_ref-name_1-1 anywhere on the page.

The later was already checked by the code. But we forgot about unnamed
references! As a consequence IDs like the cite_ref-1 above got misdetected
as reused references.

This patch tries hard to extract code into named functions, so it becomes
much more clear what they do, and why.

Bug: T215317
Change-Id: Iedb5b0c3dffae19bad7df9a43ed2d4512b3921ec

extension.json

@@ -87,6 +87,7 @@
 				"ext.cite.highlighting.css"
 			],
 			"messages": [
+				"cite_reference_link_prefix",
 				"cite_references_link_accessibility_label",
 				"cite_references_link_many_accessibility_label",
 				"cite_references_link_accessibility_back_label"

modules/ext.cite.highlighting.js

@@ -4,6 +4,35 @@
 ( function () {
 	'use strict';
 
+	/**
+	 * Checks if the ID uses a composite format that does not only consist of a sequential number,
+	 * as specified in "cite_reference_link_key_with_num".
+	 *
+	 * @param {string} id
+	 * @return {boolean}
+	 */
+	function isNamedReference( id ) {
+		var prefix = mw.msg( 'cite_reference_link_prefix' );
+
+		// Note: This assumes IDs start with the prefix; this is guaranteed by the parser function
+		return /\D/.test( id.slice( prefix.length ) );
+	}
+
+	/**
+	 * @param {string} id
+	 * @param {jQuery} $content
+	 * @return {boolean}
+	 */
+	function isReusedNamedReference( id, $content ) {
+		if ( !isNamedReference( id ) ) {
+			return false;
+		}
+
+		// Either the ID is already a reuse, or at least one reuse exists somewhere else on the page
+		return id.slice( -2 ) !== '-0' ||
+			$content.find( '.references a[href="#' + $.escapeSelector( id.slice( 0, -1 ) ) + '1"]' ).length;
+	}
+
 	/**
 	 * @param {jQuery} $backlinkWrapper
 	 * @return {jQuery}
@@ -72,9 +101,7 @@
 			$content.find( '.' + className ).removeClass( className );
 
 			// Bail out if there is not at least a second backlink ("cite_references_link_many").
-			if ( id.slice( -2 ) === '-0' &&
-				!$content.find( '.references a[href="#' + $.escapeSelector( id.slice( 0, -1 ) ) + '1"]' ).length
-			) {
+			if ( !isReusedNamedReference( id, $content ) ) {
 				return;
 			}