Issue 6955 - Avoid making copies of common selector list

lib/elemHide.js

Index: lib/elemHide.js
===================================================================
--- a/lib/elemHide.js
+++ b/lib/elemHide.js
@@ -42,16 +42,24 @@
  * This array caches the keys of filterBySelector table (selectors
  * which unconditionally apply on all domains). It will be null if the
  * cache needs to be rebuilt.
  * @type {?string[]}
  */
 let unconditionalSelectors = null;
 
 /**
+ * Cached list of selectors that apply on any domain that neither has any
+ * domain-specific filters or exceptions nor is specifically excluded from any
+ * generic filters or exceptions.
+ * @type {?string[]}
+ */
+let commonSelectors = null;
+
+/**
  * Map to be used instead when a filter has a blank domains property.
  * @type {Map.<string,boolean>}
  * @const
  */
 let defaultDomains = new Map([["", true]]);
 
 /**
  * Set containing known element hiding filters
@@ -88,16 +96,18 @@
   if (!unconditionalSelectors)
     unconditionalSelectors = [...filterBySelector.keys()];
 
   return unconditionalSelectors;
 }
 
 ElemHideExceptions.on("added", ({selector}) =>
 {
+  commonSelectors = null;
+
   // If this is the first exception for a previously unconditionally applied
   // element hiding selector we need to take care to update the lookups.
   let unconditionalFilterForSelector = filterBySelector.get(selector);
   if (unconditionalFilterForSelector)
   {
     addToFiltersByDomain(unconditionalFilterForSelector);
     filterBySelector.delete(selector);
     unconditionalSelectors = null;
@@ -112,29 +122,33 @@
   /**
    * Removes all known filters
    */
   clear()
   {
     for (let collection of [filtersByDomain, filterBySelector, knownFilters])
       collection.clear();
 
+    commonSelectors = null;
     unconditionalSelectors = null;
+
     filterNotifier.emit("elemhideupdate");
   },
 
   /**
    * Add a new element hiding filter
    * @param {ElemHideFilter} filter
    */
   add(filter)
   {
     if (knownFilters.has(filter))
       return;
 
+    commonSelectors = null;
+
     let {selector} = filter;
 
     if (!(filter.domains || ElemHideExceptions.hasExceptions(selector)))
     {
       // The new filter's selector is unconditionally applied to all domains
       filterBySelector.set(selector, filter);
       unconditionalSelectors = null;
     }
@@ -152,16 +166,18 @@
    * Removes an element hiding filter
    * @param {ElemHideFilter} filter
    */
   remove(filter)
   {
     if (!knownFilters.has(filter))
       return;
 
+    commonSelectors = null;
+
     let {selector} = filter;
 
     // Unconditially applied element hiding filters
     if (filterBySelector.get(selector) == filter)
     {
       filterBySelector.delete(selector);
       unconditionalSelectors = null;
     }
@@ -195,26 +211,37 @@
    */
   getSelectorsForDomain(domain, specificOnly = false)
   {
     let selectors = [];
 
     let excluded = new Set();
     let currentDomain = domain ? domain.replace(/\.+$/, "").toLowerCase() : "";
 
+    let commonCase = !specificOnly;
+
     // This code is a performance hot-spot, which is why we've made certain
     // micro-optimisations. Please be careful before making changes.
     while (true)
     {
       if (specificOnly && currentDomain == "")
         break;
 
+      if (commonCase && currentDomain != "" &&
+          ElemHideExceptions.domainHasExceptions(currentDomain))
+      {
+        commonCase = false;
+      }
+
       let filters = filtersByDomain.get(currentDomain);
       if (filters)
       {
+        if (currentDomain != "")
+          commonCase = false;
+
         for (let [filter, isIncluded] of filters)
         {
           if (!isIncluded)
           {
             excluded.add(filter);
           }
           else
           {
@@ -231,13 +258,30 @@
       if (currentDomain == "")
         break;
 
       let nextDot = currentDomain.indexOf(".");
       currentDomain = nextDot == -1 ? "" : currentDomain.substr(nextDot + 1);
     }
 
     if (!specificOnly)
-      selectors = getUnconditionalSelectors().concat(selectors);
+    {
+      // Use the cache of common selectors if there are no filters or
+      // exceptions that apply or don't apply specifically to this domain.
+      if (commonCase)
+      {
+        if (!commonSelectors)
+        {
+          commonSelectors =
+            Object.freeze(getUnconditionalSelectors().concat(selectors));
+        }
+
+        selectors = commonSelectors;
+      }
+      else
+      {
+        selectors = getUnconditionalSelectors().concat(selectors);
+      }
+    }
 
     return selectors;
   }
 };