Add backlinks indexer (#4421)

* Add tests for backlinks

* Add backlinks indexer

* Use backlinks indexer in getTiddlerBacklinks if available

* Extract link extraction into its own method

This way we can provide an arbitrary parse tree, rather than just a
title, which will allow us to compare lists of outgoing links between
versions of a single tiddler

* Use new extractLinks method in backlinks indexer

...rather than copy-pasting the implementation

* Remove ES6-isms

TiddlyWiki needs to work with browsers that only support ES5

core/modules/indexers/backlinks-index.js

@@ -0,0 +1,86 @@
+/*\
+title: $:/core/modules/indexers/backlinks-indexer.js
+type: application/javascript
+module-type: indexer
+
+Indexes the tiddlers' backlinks
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global modules: false */
+"use strict";
+
+
+function BacklinksIndexer(wiki) {
+	this.wiki = wiki;
+}
+
+BacklinksIndexer.prototype.init = function() {
+	this.index = null;
+}
+
+BacklinksIndexer.prototype.rebuild = function() {
+	this.index = null;
+}
+
+BacklinksIndexer.prototype._getLinks = function(tiddler) {
+	var parser =  this.wiki.parseText(tiddler.fields.type, tiddler.fields.text, {});
+	if(parser) {
+		return this.wiki.extractLinks(parser.tree);
+	}
+	return [];
+}
+
+BacklinksIndexer.prototype.update = function(updateDescriptor) {
+	if(!this.index) {
+		return;
+	}
+	var newLinks = [],
+	    oldLinks = [],
+	    self = this;
+	if(updateDescriptor.old.exists) {
+		oldLinks = this._getLinks(updateDescriptor.old.tiddler);
+	}
+	if(updateDescriptor.new.exists) {
+		newLinks = this._getLinks(updateDescriptor.new.tiddler);
+	}
+
+	$tw.utils.each(oldLinks,function(link) {
+		if(self.index[link]) {
+			delete self.index[link][updateDescriptor.old.tiddler.fields.title];
+		}
+	});
+	$tw.utils.each(newLinks,function(link) {
+		if(!self.index[link]) {
+			self.index[link] = Object.create(null);
+		}
+		self.index[link][updateDescriptor.new.tiddler.fields.title] = true;
+	});
+}
+
+BacklinksIndexer.prototype.lookup = function(title) {
+	if(!this.index) {
+		this.index = Object.create(null);
+		var self = this;
+		this.wiki.forEachTiddler(function(title,tiddler) {
+			var links = self._getLinks(tiddler);
+			$tw.utils.each(links, function(link) {
+				if(!self.index[link]) {
+					self.index[link] = Object.create(null);
+				}
+				self.index[link][title] = true;
+			});
+		});
+	}
+	if(this.index[title]) {
+		return Object.keys(this.index[title]);
+	} else {
+		return [];
+	}
+}
+
+exports.BacklinksIndexer = BacklinksIndexer;
+
+})();