Issue 4057 - Further speedup ElemHide.getSelectorsforDomain

lib/elemHide.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 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 43 matching lines @@
  */
 var filtersByDomain = Object.create(null);
 
 /**
  * Lookup table, filters by selector. (Only contains filters that have a
  * selector that is unconditionally matched for all domains.)
  */
 var filtersBySelector = Object.create(null);
 
 /**
- * Array of selectors which unconditionally apply to all domains, or null if
- * the array needs to be regenerated.

[Wladimir Palant, 2016/05/25 10:43:03]

Maybe change the description to better indicate what function this array has
now, something like:

  This array caches the keys of filtersBySelector table (selectors which
unconditionally apply on all domains). It will be null if the cache needs to be
rebuilt.

[kzar, 2016/05/25 10:51:28]

Done.

- */
-var unconditionalSelectors = [];

[Wladimir Palant, 2016/05/25 10:43:03]

Nit: Please initialize with null here and in clear(), this is now purely a cache
which is built on demand. While technically initializing with an empty array is
correct, in 99.9999% of the cases we will immediately throw away that array
again.

[kzar, 2016/05/25 10:51:28]

Done.

+ * This array caches the keys of filtersBySelector table (selectors which
+ * unconditionally apply on all domains). It will be null if the cache needs to
+ * be rebuilt.
+ */
+var unconditionalSelectors = null;
 
 /**
  * Object to be used instead when a filter has a blank domains property.
  */
 var defaultDomains = Object.create(null);
 defaultDomains[""] = true;
 
 /**
  * Lookup table, keys are known element hiding exceptions
  * @type Object
@@ skipping 49 matching lines @@
 
   /**
    * Removes all known filters
    */
   clear: function()
   {
     filterByKey = [];
     keyByFilter = Object.create(null);
     filtersByDomain = Object.create(null);
     filtersBySelector = Object.create(null);
-    unconditionalSelectors = [];
+    unconditionalSelectors = null;
     knownExceptions = Object.create(null);
     exceptions = Object.create(null);
     ElemHide.isDirty = false;
     ElemHide.unapply();
   },
 
   _addToFiltersByDomain: function(filter)
   {
     let key = keyByFilter[filter.text];
     let domains = filter.domains || defaultDomains;
@@ skipping 377 matching lines @@
       if (currentDomain == "")
         break;
 
       let nextDot = currentDomain.indexOf(".");
       currentDomain = nextDot == -1 ? "" : currentDomain.substr(nextDot + 1);
     }
 
     return selectors;
   }
 };