Index: ext/background.js
===================================================================
new file mode 100644
--- /dev/null
+++ b/ext/background.js
@@ -0,0 +1,65 @@
+/*
+ * This file is part of Adblock Plus <http://adblockplus.org/>,
+ * Copyright (C) 2006-2014 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
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+(function()
+{
+  var nonEmptyPageMaps = {__proto__: null};
+  var pageMapCounter = 0;
+
+  var PageMap = ext.PageMap = function()
+  {
+    this._map = {__proto__: null};
+    this._id = ++pageMapCounter;
+  };
+  PageMap.prototype = {
+    _delete: function(id)
+    {
+      delete this._map[id];
+
+      if (Object.keys(this._map).length == 0)
+        delete nonEmptyPageMaps[this._id];
+    },
+    get: function(page)
+    {
+      return this._map[page._id];
+    },
+    set: function(page, value)
+    {
+      this._map[page._id] = value;
+      nonEmptyPageMaps[this._id] = this;
+    },
+    has: function(page)
+    {
+      return page._id in this._map;
+    },
+    clear: function()
+    {
+      for (var id in this._map)
+        this._delete(id);
+    },
+    delete: function(page)
+    {
+      this._delete(page._id);
+    }
+  };
+
+  ext._removeFromAllPageMaps = function(pageId)
+  {
+    for (var pageMapId in nonEmptyPageMaps)
+      nonEmptyPageMaps[pageMapId]._delete(pageId);
+  };
+})();