Issue 2824 - Only consider ads subscriptions in chooseFilterSubscription

lib/subscriptionInit.js

Index: lib/subscriptionInit.js
diff --git a/lib/subscriptionInit.js b/lib/subscriptionInit.js
index 9cd17b48988b41eb21d071d7efcd7506557f0375..c14c4b3c971763c8c2ccd78d20576fd9091d5f14 100644
--- a/lib/subscriptionInit.js
+++ b/lib/subscriptionInit.js
@@ -81,6 +81,57 @@ function shouldAddDefaultSubscription()
 }
 
 /**
+ * Finds the element for the default ad blocking filter subscription based
+ * on the user's locale.
+ *
+ * @param {HTMLCollection} subscriptions
+ * @return {Element}
+ */
+function chooseFilterSubscription(subscriptions)
+{
+  let selectedItem = null;
+  let selectedPrefix = null;
+  let matchCount = 0;
+  for (let subscription of subscriptions)
+  {
+    if (!selectedItem)
+      selectedItem = subscription;
+
+    let prefixes = subscription.getAttribute("prefixes");
+    let prefix = prefixes && prefixes.split(",").find(
+      lang => new RegExp("^" + lang + "\\b").test(Utils.appLocale)
+    );
+
+    let subscriptionType = subscription.getAttribute("type");
+
+    if (prefix && subscriptionType == "ads")
+    {
+      if (!selectedPrefix || selectedPrefix.length < prefix.length)
+      {
+        selectedItem = subscription;
+        selectedPrefix = prefix;
+        matchCount = 1;
+      }
+      else if (selectedPrefix && selectedPrefix.length == prefix.length)
+      {
+        matchCount++;
+
+        // If multiple items have a matching prefix of the same length:
+        // Select one of the items randomly, probability should be the same
+        // for all items. So we replace the previous match here with
+        // probability 1/N (N being the number of matches).
+        if (Math.random() * matchCount < 1)
+        {
+          selectedItem = subscription;
+          selectedPrefix = prefix;
+        }
+      }
+    }
+  }
+  return selectedItem;
+}
+
+/**
  * Gets the filter subscriptions to be added when the extnesion is loaded.
  *
  * @return {Promise|Subscription[]}
@@ -119,7 +170,7 @@ function getSubscriptions()
         let doc = new DOMParser().parseFromString(text, "application/xml");
         let nodes = doc.getElementsByTagName("subscription");
 
-        let node = Utils.chooseFilterSubscription(nodes);
+        let node = chooseFilterSubscription(nodes);
         if (node)
         {
           let url = node.getAttribute("url");