Issue 4067 - Make filter "keys" generated for element hiding filters numeric

lib/filterListener.js

Index: lib/filterListener.js
===================================================================
--- a/lib/filterListener.js
+++ b/lib/filterListener.js
@@ -209,23 +209,48 @@ function removeFilter(filter)
   {
     if (filter instanceof CSSPropertyFilter)
       CSSRules.remove(filter);
     else
       ElemHide.remove(filter);
   }
 }
 
+const primes = [101, 109, 131, 149, 163, 179, 193, 211, 229, 241];
+
+function addFilters(filters)
+{
+  // We add filters using pseudo-random ordering. Reason is that ElemHide will
+  // assign consecutive filter IDs that might be visible to the website. The
+  // randomization makes sure that no conclusion can be made about the actual
+  // filters applying there. We have ten prime numbers to use as iteration step,
+  // any of those can be chosen as long as the array length isn't divisible by
+  // it.
+  let len = filters.length;
+  if (!len)
+    return;
+
+  let current = (Math.random() * len) | 0;
+  let step;
+  do
+  {
+    step = primes[(Math.random() * primes.length) | 0];
+  } while (len % step == 0);
+
+  for (let i = 0; i < len; i++, current = (current + step) % len)
+    addFilter(filters[current]);
+}
+
 function onSubscriptionAdded(subscription)
 {
   FilterListener.setDirty(1);
 
   if (!subscription.disabled)
   {
-    subscription.filters.forEach(addFilter);
+    addFilters(subscription.filters);
     flushElemHide();
   }
 }
 
 function onSubscriptionRemoved(subscription)
 {
   FilterListener.setDirty(1);
 
@@ -238,32 +263,32 @@ function onSubscriptionRemoved(subscript
 
 function onSubscriptionDisabled(subscription, newValue)
 {
   FilterListener.setDirty(1);
 
   if (subscription.url in FilterStorage.knownSubscriptions)
   {
     if (newValue == false)
-      subscription.filters.forEach(addFilter);
+      addFilters(subscription.filters);
     else
       subscription.filters.forEach(removeFilter);
     flushElemHide();
   }
 }
 
 function onSubscriptionUpdated(subscription)
 {
   FilterListener.setDirty(1);
 
   if (subscription.url in FilterStorage.knownSubscriptions &&
       !subscription.disabled)
   {
     subscription.oldFilters.forEach(removeFilter);
-    subscription.filters.forEach(addFilter);
+    addFilters(subscription.filters);
     flushElemHide();
   }
 }
 
 function onFilterHitCount(filter, newValue)
 {
   if (newValue == 0)
     FilterListener.setDirty(0);
@@ -318,16 +343,16 @@ function onLoad()
 {
   isDirty = 0;
 
   defaultMatcher.clear();
   ElemHide.clear();
   CSSRules.clear();
   for (let subscription of FilterStorage.subscriptions)
     if (!subscription.disabled)
-      subscription.filters.forEach(addFilter);
+      addFilters(subscription.filters);
   flushElemHide();
 }
 
 function onSave()
 {
   isDirty = 0;
 }