muting: Fix calling update_unread_counts in a loop.

Previously, set_muted_topics was calling update_unread_counts once for each
topic in the input; this results in poor performance when there is a large
number of muted topics.

Fixes: #3605

frontend_tests/node_tests/muting.js

@@ -55,6 +55,36 @@ var muting = require('js/muting.js');
     ]);
 }());
 
+(function test_muting_performance() {
+    // This test ensures that each call to mute_topic and set_muted_topics only
+    // results in one call to unread.update_unread_counts.
+
+    // We replace (for the duration of this test) the real update_unread_counts
+    // with a test version that just counts the number of calls.
+    var saved = unread.update_unread_counts;
+    var num_calls = 0;
+    unread.update_unread_counts = function () {
+        num_calls += 1;
+    };
+
+    muting.mute_topic('office', 'gossip');
+    assert.equal(num_calls, 1);
+
+    muting.set_muted_topics([
+        ['social', 'breakfast'],
+    ]);
+    assert.equal(num_calls, 2);
+
+    muting.set_muted_topics([
+        ['social', 'breakfast'],
+        ['design', 'typography'],
+        ['devel', 'java'],
+    ]);
+    assert.equal(num_calls, 3);
+
+    unread.update_unread_counts = saved;
+}());
+
 (function test_case_insensitivity() {
     muting.set_muted_topics([]);
     assert(!muting.is_topic_muted('SOCial', 'breakfast'));

static/js/muting.js

@@ -4,13 +4,17 @@ var exports = {};
 
 var muted_topics = new Dict({fold_case: true});
 
-exports.mute_topic = function (stream, topic) {
+function set_muted_topic(stream, topic) {
     var sub_dict = muted_topics.get(stream);
     if (!sub_dict) {
         sub_dict = new Dict({fold_case: true});
         muted_topics.set(stream, sub_dict);
     }
     sub_dict.set(topic, true);
+}
+
+exports.mute_topic = function (stream, topic) {
+    set_muted_topic(stream, topic);
     unread.update_unread_counts();
 };
 
@@ -47,8 +51,9 @@ exports.set_muted_topics = function (tuples) {
         var stream = tuple[0];
         var topic = tuple[1];
 
-        exports.mute_topic(stream, topic);
+        set_muted_topic(stream, topic);
     });
+    unread.update_unread_counts();
 };
 
 return exports;