generic "related topics" functionality

src/topics.js

@@ -192,12 +192,19 @@ var async = require('async'),
 				}, next);
 			},
 			function(results, next) {
+				if (plugins.hasListeners('filter:topic.getRelatedTopics')) {
+					plugins.fireHook('filter:topic.getRelatedTopics', results, next);
+				} else {
+					Topics.getRelatedTopics(results, next);
+				}
+			}, function(results, next) {
 				topicData.posts = results.posts;
 				topicData.category = results.category;
 				topicData.thread_tools = results.threadTools.tools;
 				topicData.tags = results.tags;
 				topicData.isFollowing = results.isFollowing[0];
 				topicData.bookmark = results.bookmark;
+				topicData.related = results.related || [];
 
 				topicData.unreplied = parseInt(topicData.postcount, 10) === 1;
 				topicData.deleted = parseInt(topicData.deleted, 10) === 1;

src/topics/tags.js

@@ -5,6 +5,7 @@ var async = require('async'),
 
 	db = require('../database'),
 	meta = require('../meta'),
+	user = require('../user'),
 	_ = require('underscore'),
 	plugins = require('../plugins');
 
@@ -322,4 +323,41 @@ module.exports = function(Topics) {
 		});
 	};
 
+	Topics.getRelatedTopics = function(topicData, callback) {
+		if (!topicData.tags.length) {
+			return callback(null, topicData);
+		}
+
+		var related = [];
+
+		user.isAdministrator(topicData.threadTools.uid, function(err, isAdministrator) {
+			async.each(topicData.tags, function(tag, next) {
+				tag = tag.value;
+
+				Topics.getTagTids(tag, 0, 5, function(err, tids) {
+					Topics.getTopics(tids, topicData.threadTools.uid, function(err, topics) {
+						related = related.concat(topics.filter(function(topic) {
+							var doesntOwnTopic = parseInt(topic.uid, 10) !== parseInt(topicData.threadTools.uid, 10);
+							var isntSameTopic = parseInt(topic.tid, 10) !== parseInt(topicData.threadTools.topic.tid, 10);
+
+							return doesntOwnTopic && isntSameTopic;
+						}));
+
+						next(err);
+					});
+				});
+			}, function(err) {
+				if (!isAdministrator) {
+					related = related.filter(function(topic) {
+						return topic && !topic.deleted;
+					});
+				}
+
+				related = _.shuffle(related).slice(0, 5);
+
+				topicData.related = related;
+				callback(err, topicData);
+			});
+		});
+	};
 };