Issue 6955 - Avoid making copies of common selector list

lib/elemHideExceptions.js

Index: lib/elemHideExceptions.js
===================================================================
--- a/lib/elemHideExceptions.js
+++ b/lib/elemHideExceptions.js
@@ -26,32 +26,39 @@
 
 /**
  * Lookup table, lists of element hiding exceptions by selector
  * @type {Map.<string,ElemHideException[]>}
  */
 let exceptions = new Map();
 
 /**
+ * Number of element hiding exceptions per domain.
+ * @type {Map.<string,number>}
+ */
+let exceptionCountByDomain = new Map();
+
+/**
  * Set containing known element exceptions
  * @type {Set.<ElemHideException>}
  */
 let knownExceptions = new Set();
 
 /**
  * Container for element hiding exceptions
  * @class
  */
 exports.ElemHideExceptions = Object.assign(Object.create(new EventEmitter()), {
   /**
    * Removes all known exceptions
    */
   clear()
   {
     exceptions.clear();
+    exceptionCountByDomain.clear();
     knownExceptions.clear();
 
     filterNotifier.emit("elemhideupdate");
   },
 
   /**
    * Add a new element hiding exception
    * @param {ElemHideException} exception
@@ -63,16 +70,25 @@
 
     let {selector} = exception;
     let list = exceptions.get(selector);
     if (list)
       list.push(exception);
     else
       exceptions.set(selector, [exception]);
 
+    for (let [domain] of exception.domains || [])
+    {
+      if (domain == "")
+        continue;
+
+      let count = exceptionCountByDomain.get(domain) || 0;
+      exceptionCountByDomain.set(domain, count + 1);
+    }
+
     knownExceptions.add(exception);
 
     this.emit("added", exception);
 
     filterNotifier.emit("elemhideupdate");
   },
 
   /**
@@ -84,16 +100,28 @@
     if (!knownExceptions.has(exception))
       return;
 
     let list = exceptions.get(exception.selector);
     let index = list.indexOf(exception);
     if (index >= 0)
       list.splice(index, 1);
 
+    for (let [domain] of exception.domains || [])
+    {
+      if (domain == "")
+        continue;
+
+      let count = exceptionCountByDomain.get(domain) || 0;
+      if (count == 1)
+        exceptionCountByDomain.delete(domain);
+      else if (count > 1)
+        exceptionCountByDomain.set(domain, count - 1);
+    }
+
     knownExceptions.delete(exception);
 
     this.emit("removed", exception);
 
     filterNotifier.emit("elemhideupdate");
   },
 
   /**
@@ -102,16 +130,26 @@
    * @returns {boolean}
    */
   hasExceptions(selector)
   {
     return exceptions.has(selector);
   },
 
   /**
+   * Checks whether any exception rules are registered for a domain
+   * @param {string} domain
+   * @returns {boolean}
+   */
+  domainHasExceptions(domain)
+  {
+    return exceptionCountByDomain.has(domain);
+  },
+
+  /**
    * Checks whether an exception rule is registered for a selector on a
    * particular domain.
    * @param {string} selector
    * @param {?string} docDomain
    * @return {?ElemHideException}
    */
   getException(selector, docDomain)
   {