Issue 6957 - Return common style sheet for unknown domains

lib/elemHideExceptions.js

Index: lib/elemHideExceptions.js
===================================================================
--- a/lib/elemHideExceptions.js
+++ b/lib/elemHideExceptions.js
@@ -26,16 +26,22 @@
 
 /**
  * Lookup table, lists of element hiding exceptions by selector
  * @type {Map.<string,ElemHideException[]>}
  */
 let exceptions = new Map();
 
 /**
+ * Set containing all known domains
+ * @type {Set.<string>}
+ */
+let knownDomains = new Set();
+
+/**
  * Set containing known element exceptions
  * @type {Set.<ElemHideException>}
  */
 let knownExceptions = new Set();
 
 /**
  * Container for element hiding exceptions
  * @class
@@ -56,17 +62,30 @@
    * Add a new element hiding exception
    * @param {ElemHideException} exception
    */
   add(exception)
   {
     if (knownExceptions.has(exception))
       return;
 
-    let {selector} = exception;
+    let {domains, selector} = exception;
+
+    if (domains)
+    {
+      for (let domain of domains.keys())
+      {
+        // Note: Once a domain is known it never becomes unknown, even when all
+        // the filters containing that domain are removed. This is a best-case
+        // optimization.
+        if (domain != "")
+          knownDomains.add(domain);
+      }
+    }
+
     let list = exceptions.get(selector);
     if (list)
       list.push(exception);
     else
       exceptions.set(selector, [exception]);
 
     knownExceptions.add(exception);
 
@@ -92,16 +111,26 @@
     knownExceptions.delete(exception);
 
     this.emit("removed", exception);
 
     filterNotifier.emit("elemhideupdate");
   },
 
   /**
+   * Checks whether a given domain is known.
+   * @param {string} domain
+   * @returns {boolean}
+   */
+  isKnownDomain(domain)
+  {
+    return knownDomains.has(domain);
+  },
+
+  /**
    * Checks whether any exception rules are registered for a selector
    * @param {string} selector
    * @returns {boolean}
    */
   hasExceptions(selector)
   {
     return exceptions.has(selector);
   },