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

lib/filterClasses.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 23 matching lines @@
 /**
  * Abstract base class for filters
  *
  * @param {string} text   string representation of the filter
  * @constructor
  */
 function Filter(text)
 {
   this.text = text;
 
-  this._subscriptions = new Set();
+  this._subscriptions = null;
 }
 exports.Filter = Filter;
 
 Filter.prototype =
 {
   /**
    * String representation of the filter
    * @type {string}
    */
   text: null,
 
   /**
    * Filter type as a string, e.g. "blocking".
    * @type {string}
    */
   get type()
   {
     throw new Error("Please define filter type in the subclass");
   },
 
   /**
    * Yields filter subscriptions the filter belongs to
    * @yields {Subscription}
    */
   *subscriptions()
   {
-    yield* this._subscriptions;
+    if (this._subscriptions)
+    {
+      if (this._subscriptions instanceof Set)
+        yield* this._subscriptions;
+      else
+        yield this._subscriptions;
+    }
   },
 
   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;
+    }
   },
 
   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)
   {
     buffer.push("[Filter]");
     buffer.push("text=" + this.text);
@@ skipping 1099 matching lines @@
 
   /**
    * Script that should be executed
    * @type {string}
    */
   get script()
   {
     return this.body;
   }
 });