buddy list: Shrink overly large lists.

If we would have more than 600 people in a buddy list, it's kind of
cumbersome to scroll through it, and it's also expensive to render
it (short of doing progressive rendering, which adds a lot of
complexity).

So, as a short term measure, we filter out offline users whenever the
list would exceed 600 users.  Note that if you are doing a search that
narrows to fewer 600 users, the offline users will appear again.

frontend_tests/node_tests/buddy_data.js

@@ -0,0 +1,50 @@
+zrequire('people');
+zrequire('presence');
+zrequire('util');
+zrequire('buddy_data');
+
+// The buddy_data module is mostly tested indirectly through
+// activity.js, but we should feel free to add direct tests
+// here.
+
+
+set_global('page_params', {});
+
+(function make_people() {
+    _.each(_.range(1000, 2000), (i) => {
+        const person = {
+            user_id: i,
+            full_name: `Person ${i}`,
+            email: `person${i}@example.com`,
+        };
+        people.add_in_realm(person);
+    });
+}());
+
+(function activate_people() {
+    const server_time = 9999;
+    const info = {
+        website: {
+            status: "active",
+            timestamp: server_time,
+        },
+    };
+
+    // Make 400 of the users active
+    _.each(_.range(1000, 1400), (user_id) => {
+        presence.set_user_status(user_id, info, server_time);
+    });
+
+    // And then 300 not active
+    _.each(_.range(1400, 1700), (user_id) => {
+        presence.set_user_status(user_id, {}, server_time);
+    });
+}());
+
+(function test_user_ids() {
+    const user_ids = buddy_data.get_filtered_and_sorted_user_ids();
+
+    // Even though we have 900 users, we only get the 400 active
+    // users.  This is a consequence of buddy_data.maybe_shrink_list.
+    assert.equal(user_ids.length, 400);
+}());

static/js/buddy_data.js

@@ -11,24 +11,25 @@ var exports = {};
 
 */
 
+var max_size_before_shrinking = 600;
+
 var presence_descriptions = {
     active: 'is active',
     idle:   'is not active',
 };
 
-exports.compare_function = function (a, b) {
-
-    function level(status) {
-        switch (status) {
-            case 'active':
-                return 1;
-            case 'idle':
-                return 2;
-            default:
-                return 3;
-        }
+function level(status) {
+    switch (status) {
+        case 'active':
+            return 1;
+        case 'idle':
+            return 2;
+        default:
+            return 3;
     }
+}
 
+exports.compare_function = function (a, b) {
     var level_a = level(presence.get_status(a));
     var level_b = level(presence.get_status(b));
     var diff = level_a - level_b;
@@ -102,6 +103,21 @@ exports.info_for = function (user_id) {
     };
 };
 
+function user_is_recently_active(user_id) {
+    // return true if the user has a green/orange cirle
+    return level(presence.get_status(user_id)) <= 2;
+}
+
+function maybe_shrink_list(user_ids) {
+    if (user_ids.length <= max_size_before_shrinking) {
+        return user_ids;
+    }
+
+    user_ids = _.filter(user_ids, user_is_recently_active);
+
+    return user_ids;
+}
+
 exports.get_filtered_and_sorted_user_ids = function (filter_text) {
     var user_ids;
 
@@ -116,6 +132,8 @@ exports.get_filtered_and_sorted_user_ids = function (filter_text) {
         user_ids = presence.get_user_ids();
     }
 
+    user_ids = maybe_shrink_list(user_ids);
+
     return exports.sort_users(user_ids);
 };