Issue 6916 - Avoid Set object for filters with only one subscription

lib/filterClasses.js

Index: lib/filterClasses.js
===================================================================
--- a/lib/filterClasses.js
+++ b/lib/filterClasses.js
@@ -38,20 +38,20 @@
  * @constructor
  */
 function Filter(text)
 {
   this.text = text;
 
   /**
    * Subscriptions to which this filter belongs.
-   * @type {Set.<Subscription>}
+   * @type {(Subscription|Set.<Subscription>)?}
    * @private
    */
-  this._subscriptions = new Set();
+  this._subscriptions = null;
 }
 exports.Filter = Filter;
 
 Filter.prototype =
 {
   /**
    * String representation of the filter
    * @type {string}
@@ -68,46 +68,78 @@
   },
 
   /**
    * Yields subscriptions to which the filter belongs.
    * @yields {Subscription}
    */
   *subscriptions()
   {
-    yield* this._subscriptions;
+    if (this._subscriptions)
+    {
+      if (this._subscriptions instanceof Set)
+        yield* this._subscriptions;
+      else
+        yield this._subscriptions;
+    }
   },
 
   /**
    * The number of subscriptions to which the filter belongs.
    * @type {number}
    */
   get subscriptionCount()
   {
-    return this._subscriptions.size;
+    if (this._subscriptions instanceof Set)
+      return this._subscriptions.size;
+
+    return this._subscriptions ? 1 : 0;
   },
 
   /**
    * Adds a subscription to the set of subscriptions to which the filter
    * belongs.
    * @param {Subscription} subscription
    */
   addSubscription(subscription)
   {
-    this._subscriptions.add(subscription);
+    if (this._subscriptions)
+    {
+      if (this._subscriptions instanceof Set)
+        this._subscriptions.add(subscription);
+      else if (subscription != this._subscriptions)
+        this._subscriptions = new Set([this._subscriptions, subscription]);
+    }
+    else
+    {
+      this._subscriptions = subscription;
+    }
   },
 
   /**
    * Removes a subscription from the set of subscriptions to which the filter
    * belongs.
    * @param {Subscription} subscription
    */
   removeSubscription(subscription)
   {
-    this._subscriptions.delete(subscription);
+    if (this._subscriptions)
+    {
+      if (this._subscriptions instanceof Set)
+      {
+        this._subscriptions.delete(subscription);
+
+        if (this._subscriptions.size == 1)
+          this._subscriptions = [...this._subscriptions][0];
+      }
+      else if (subscription == this._subscriptions)
+      {
+        this._subscriptions = null;
+      }
+    }
   },
 
   /**
    * Serializes the filter to an array of strings for writing out on the disk.
    * @param {string[]} buffer  buffer to push the serialization results into
    */
   serialize(buffer)
   {