From ae04a425c06f9b6f262b4e6a11aa71cde70f389d Mon Sep 17 00:00:00 2001
From: Rob Hoelz <rob@hoelz.ro>
Date: Thu, 26 Mar 2020 08:15:02 -0500
Subject: [PATCH] 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      |  86 ++++++++++++
 core/modules/wiki.js                          |  61 ++++----
 .../test/tiddlers/tests/test-backlinks.js     | 132 ++++++++++++++++++
 3 files changed, 255 insertions(+), 24 deletions(-)
 create mode 100644 core/modules/indexers/backlinks-index.js
 create mode 100644 editions/test/tiddlers/tests/test-backlinks.js

diff --git a/core/modules/indexers/backlinks-index.js b/core/modules/indexers/backlinks-index.js
new file mode 100644
index 000000000..5902e2829
--- /dev/null
+++ b/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;
+
+})();
diff --git a/core/modules/wiki.js b/core/modules/wiki.js
index fc8b42eda..d44940cb6 100755
--- a/core/modules/wiki.js
+++ b/core/modules/wiki.js
@@ -414,6 +414,30 @@ exports.forEachTiddler = function(/* [options,]callback */) {
 	}
 };
 
+/*
+Return an array of tiddler titles that are directly linked within the given parse tree
+ */
+exports.extractLinks = function(parseTreeRoot) {
+	// Count up the links
+	var links = [],
+		checkParseTree = function(parseTree) {
+			for(var t=0; t<parseTree.length; t++) {
+				var parseTreeNode = parseTree[t];
+				if(parseTreeNode.type === "link" && parseTreeNode.attributes.to && parseTreeNode.attributes.to.type === "string") {
+					var value = parseTreeNode.attributes.to.value;
+					if(links.indexOf(value) === -1) {
+						links.push(value);
+					}
+				}
+				if(parseTreeNode.children) {
+					checkParseTree(parseTreeNode.children);
+				}
+			}
+		};
+	checkParseTree(parseTreeRoot);
+	return links;
+};
+
 /*
 Return an array of tiddler titles that are directly linked from the specified tiddler
 */
@@ -423,26 +447,10 @@ exports.getTiddlerLinks = function(title) {
 	return this.getCacheForTiddler(title,"links",function() {
 		// Parse the tiddler
 		var parser = self.parseTiddler(title);
-		// Count up the links
-		var links = [],
-			checkParseTree = function(parseTree) {
-				for(var t=0; t<parseTree.length; t++) {
-					var parseTreeNode = parseTree[t];
-					if(parseTreeNode.type === "link" && parseTreeNode.attributes.to && parseTreeNode.attributes.to.type === "string") {
-						var value = parseTreeNode.attributes.to.value;
-						if(links.indexOf(value) === -1) {
-							links.push(value);
-						}
-					}
-					if(parseTreeNode.children) {
-						checkParseTree(parseTreeNode.children);
-					}
-				}
-			};
 		if(parser) {
-			checkParseTree(parser.tree);
+			return self.extractLinks(parser.tree);
 		}
-		return links;
+		return [];
 	});
 };
 
@@ -451,13 +459,18 @@ Return an array of tiddler titles that link to the specified tiddler
 */
 exports.getTiddlerBacklinks = function(targetTitle) {
 	var self = this,
+		backlinksIndexer = this.getIndexer("BacklinksIndexer"),
+		backlinks = backlinksIndexer && backlinksIndexer.lookup(targetTitle);
+
+	if(!backlinks) {
 		backlinks = [];
-	this.forEachTiddler(function(title,tiddler) {
-		var links = self.getTiddlerLinks(title);
-		if(links.indexOf(targetTitle) !== -1) {
-			backlinks.push(title);
-		}
-	});
+		this.forEachTiddler(function(title,tiddler) {
+			var links = self.getTiddlerLinks(title);
+			if(links.indexOf(targetTitle) !== -1) {
+				backlinks.push(title);
+			}
+		});
+	}
 	return backlinks;
 };
 
diff --git a/editions/test/tiddlers/tests/test-backlinks.js b/editions/test/tiddlers/tests/test-backlinks.js
new file mode 100644
index 000000000..68ad18bbd
--- /dev/null
+++ b/editions/test/tiddlers/tests/test-backlinks.js
@@ -0,0 +1,132 @@
+/*\
+title: test-backlinks.js
+type: application/javascript
+tags: [[$:/tags/test-spec]]
+
+Tests the backlinks mechanism.
+
+\*/
+(function(){
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+describe('Backlinks tests', function() {
+	describe('a tiddler with no links to it', function() {
+		var wiki = new $tw.Wiki();
+
+		wiki.addTiddler({
+			title: 'TestIncoming',
+			text: ''});
+
+		it('should have no backlinks', function() {
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('');
+		});
+	});
+
+	describe('A tiddler added to the wiki with a link to it', function() {
+		var wiki = new $tw.Wiki();
+
+		wiki.addTiddler({
+			title: 'TestIncoming',
+			text: ''});
+
+		wiki.addTiddler({
+			title: 'TestOutgoing',
+			text: 'A link to [[TestIncoming]]'});
+
+		it('should have a backlink', function() {
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('TestOutgoing');
+		});
+	});
+
+	describe('A tiddler that has a link added to it later', function() {
+		it('should have an additional backlink', function() {
+			var wiki = new $tw.Wiki();
+
+			wiki.addTiddler({
+				title: 'TestIncoming',
+				text: ''});
+
+			wiki.addTiddler({
+				title: 'TestOutgoing',
+				text: 'A link to [[TestIncoming]]'});
+
+			wiki.addTiddler({
+				title: 'TestOutgoing2',
+				text: 'Nothing yet!'});
+
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('TestOutgoing');
+
+			wiki.addTiddler({
+				title: 'TestOutgoing2',
+				text: 'Updated with link to [[TestIncoming]]'});
+
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('TestOutgoing,TestOutgoing2');
+		});
+	});
+
+	describe('A tiddler that has a link remove from it later', function() {
+		var wiki = new $tw.Wiki();
+
+		wiki.addTiddler({
+			title: 'TestIncoming',
+			text: ''});
+
+		wiki.addTiddler({
+			title: 'TestOutgoing',
+			text: 'A link to [[TestIncoming]]'});
+
+		it('should have one fewer backlink', function() {
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('TestOutgoing');
+
+			wiki.addTiddler({
+				title: 'TestOutgoing',
+				text: 'No link to ~TestIncoming'});
+
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('');
+		});
+	});
+
+	describe('A tiddler linking to another that gets renamed', function() {
+		var wiki = new $tw.Wiki();
+
+		wiki.addTiddler({
+			title: 'TestIncoming',
+			text: ''});
+
+		wiki.addTiddler({
+			title: 'TestOutgoing',
+			text: 'A link to [[TestIncoming]]'});
+
+		it('should have its name changed in the backlinks', function() {
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('TestOutgoing');
+
+			wiki.renameTiddler('TestOutgoing', 'TestExtroverted');
+
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('TestExtroverted');
+		});
+	});
+
+	describe('A tiddler linking to another that gets deleted', function() {
+		var wiki = new $tw.Wiki();
+
+		wiki.addTiddler({
+			title: 'TestIncoming',
+			text: ''});
+
+		wiki.addTiddler({
+			title: 'TestOutgoing',
+			text: 'A link to [[TestIncoming]]'});
+
+		it('should be removed from backlinks', function() {
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('TestOutgoing');
+
+			wiki.deleteTiddler('TestOutgoing');
+
+			expect(wiki.filterTiddlers('TestIncoming +[backlinks[]]').join(',')).toBe('');
+		});
+	});
+});
+
+})();