Issue 7015 - Serialize subscriptions for *exportData in one loop

lib/filterStorage.js

Index: lib/filterStorage.js
===================================================================
--- a/lib/filterStorage.js
+++ b/lib/filterStorage.js
@@ -438,49 +438,44 @@
     });
   },
 
   /**
    * Generator serializing filter data and yielding it line by line.
    */
   *exportData()
   {
-    // Do not persist external subscriptions
-    let subscriptions = [];
-    for (let subscription of this.subscriptions())
-    {
-      if (!(subscription instanceof ExternalSubscription))
-        subscriptions.push(subscription);
-    }
-
     yield "# Adblock Plus preferences";
     yield "version=" + formatVersion;
 
     let saved = new Set();
     let buf = [];
 
     // Save subscriptions
-    for (let subscription of subscriptions)
+    for (let subscription of this.subscriptions())
     {
+      // Do not persist external subscriptions
+      if (subscription instanceof ExternalSubscription)
+        continue;
+
       yield "";
+      subscription.serialize(buf);
 
-      subscription.serialize(buf);
       if (subscription.filters.length)
       {
         buf.push("", "[Subscription filters]");
         subscription.serializeFilters(buf);
       }
+
       for (let line of buf)
         yield line;
-      buf.splice(0);
-    }
 
-    // Save filter data
-    for (let subscription of subscriptions)
-    {
+      buf.splice(0);

[Sebastian Noack, 2018/10/03 16:33:24]

While optimizing this code, you might want consider using `buf = []` which is
used to be much faster than splice(). I didn't benchmark it recently though, so
you might want to double check on recent browser versions.

+
+      // Save filter data
       for (let filter of subscription.filters)
       {
         if (!saved.has(filter.text))
         {
           filter.serialize(buf);
           saved.add(filter.text);
           for (let line of buf)
             yield line;