Index: lib/matcher.js
===================================================================
--- a/lib/matcher.js
+++ b/lib/matcher.js
@@ -243,17 +243,17 @@
  * rules into two Matcher instances.
  * @constructor
  * @augments Matcher
  */
 function CombinedMatcher()
 {
   this.blacklist = new Matcher();
   this.whitelist = new Matcher();
-  this.resultCache = Object.create(null);
+  this.resultCache = new Map();
 }
 exports.CombinedMatcher = CombinedMatcher;
 
 /**
  * Maximal number of matching cache entries to be kept
  * @type {number}
  */
 CombinedMatcher.maxCacheEntries = 1000;
@@ -269,71 +269,56 @@
   /**
    * Matcher for exception rules.
    * @type {Matcher}
    */
   whitelist: null,
 
   /**
    * Lookup table of previous matchesAny results
-   * @type {Object}
+   * @type {Map.<string,Filter>}
    */
   resultCache: null,
 
   /**
-   * Number of entries in resultCache
-   * @type {number}
-   */
-  cacheEntries: 0,
-
-  /**
    * @see Matcher#clear
    */
   clear()
   {
     this.blacklist.clear();
     this.whitelist.clear();
-    this.resultCache = Object.create(null);
-    this.cacheEntries = 0;
+    this.resultCache.clear();
   },
 
   /**
    * @see Matcher#add
    * @param {Filter} filter
    */
   add(filter)
   {
     if (filter instanceof WhitelistFilter)
       this.whitelist.add(filter);
     else
       this.blacklist.add(filter);
 
-    if (this.cacheEntries > 0)
-    {
-      this.resultCache = Object.create(null);
-      this.cacheEntries = 0;
-    }
+    this.resultCache.clear();
   },
 
   /**
    * @see Matcher#remove
    * @param {Filter} filter
    */
   remove(filter)
   {
     if (filter instanceof WhitelistFilter)
       this.whitelist.remove(filter);
     else
       this.blacklist.remove(filter);
 
-    if (this.cacheEntries > 0)
-    {
-      this.resultCache = Object.create(null);
-      this.cacheEntries = 0;
-    }
+    this.resultCache.clear();
   },
 
   /**
    * @see Matcher#findKeyword
    * @param {Filter} filter
    * @return {string} keyword
    */
   findKeyword(filter)
@@ -419,30 +404,28 @@
   /**
    * @see Matcher#matchesAny
    * @inheritdoc
    */
   matchesAny(location, typeMask, docDomain, thirdParty, sitekey, specificOnly)
   {
     let key = location + " " + typeMask + " " + docDomain + " " + thirdParty +
       " " + sitekey + " " + specificOnly;
-    if (key in this.resultCache)
-      return this.resultCache[key];
 
-    let result = this.matchesAnyInternal(location, typeMask, docDomain,
-                                         thirdParty, sitekey, specificOnly);
+    let result = this.resultCache.get(key);
+    if (typeof result != "undefined")
+      return result;
 
-    if (this.cacheEntries >= CombinedMatcher.maxCacheEntries)
-    {
-      this.resultCache = Object.create(null);
-      this.cacheEntries = 0;
-    }
+    result = this.matchesAnyInternal(location, typeMask, docDomain,
+                                     thirdParty, sitekey, specificOnly);
 
-    this.resultCache[key] = result;
-    this.cacheEntries++;
+    if (this.resultCache.size >= CombinedMatcher.maxCacheEntries)
+      this.resultCache.clear();
+
+    this.resultCache.set(key, result);
 
     return result;
   }
 };
 
 /**
  * Shared CombinedMatcher instance that should usually be used.
  * @type {CombinedMatcher}