crossbar: check common reasons to not match first

Also, put off the use of EJSON.equals until necessary.

Inspired by @hypno2000.  See #3697.

History.md

@@ -76,6 +76,9 @@
   (`application-configuration`, `ctl`, `ctl-helper`, `follower-livedata`,
   `dev-bundle-fetcher`, and `star-translate`).
 
+* Optimize common cases faced by the "crossbar" data structure (used by oplog
+  tailing and DDP method write tracking).  #3697
+
 * Upgraded dependencies:
 
   - node: 0.10.36 (from 0.10.33)

packages/ddp/crossbar.js

@@ -87,6 +87,26 @@ _.extend(DDPServer._Crossbar.prototype, {
   //    (a targeted write to a collection does not match a targeted query
   //     targeted at a different document)
   _matches: function (notification, trigger) {
+    // Most notifications that use the crossbar have a string `collection` and
+    // maybe an `id` that is a string or ObjectID. So let's fast-track "nope,
+    // different collection" and "nope, different ID". This makes a noticeable
+    // performance difference; see https://github.com/meteor/meteor/pull/3697
+    if (typeof(notification.collection) === 'string' &&
+        typeof(trigger.collection) === 'string' &&
+        notification.collection !== trigger.collection) {
+      return false;
+    }
+    if (typeof(notification.id) === 'string' &&
+        typeof(trigger.id) === 'string' &&
+        notification.id !== trigger.id) {
+      return false;
+    }
+    if (notification.id instanceof LocalCollection._ObjectID &&
+        trigger.id instanceof LocalCollection._ObjectID &&
+        ! notification.id.equals(trigger.id)) {
+      return false;
+    }
+
     return _.all(trigger, function (triggerValue, key) {
       return !_.has(notification, key) ||
         EJSON.equals(triggerValue, notification[key]);