Prioritize stream subscribers in typeahead list.

When autocompleting names using @mention, subscribers of the stream
would appear at the top of the list. Added new test module to reflect
this change.

frontend_tests/node_tests/typeahead_helper.js

@@ -0,0 +1,103 @@
+var th = require('js/typeahead_helper.js');
+
+global.stub_out_jquery();
+
+set_global('compose', {});
+set_global('page_params', {is_zephyr_mirror_realm: false});
+
+add_dependencies({
+    stream_data: 'js/stream_data.js',
+    people: 'js/people.js',
+});
+
+stream_data.create_streams([
+    {name: 'Dev', subscribed: true, color: 'blue', stream_id: 1},
+    {name: 'Linux', subscribed: true, color: 'red', stream_id: 2}
+]);
+
+var matches = [
+    {
+        email: "[email protected]",
+        full_name: "A zulip test bot",
+        is_admin: false,
+        is_bot: true,
+        user_id: 1,
+    }, {
+        email: "[email protected]",
+        full_name: "A zulip user",
+        is_admin: false,
+        is_bot: false,
+        user_id: 2,
+    }, {
+        email: "[email protected]",
+        full_name: "Bob 1",
+        is_admin: false,
+        is_bot: false,
+        user_id: 3,
+    }, {
+        email: "[email protected]",
+        full_name: "Bob 2",
+        is_admin: true,
+        is_bot: false,
+        user_id: 4,
+    }, {
+        email: "[email protected]",
+        full_name: "B bot",
+        is_admin: false,
+        is_bot: true,
+        user_id: 5,
+    }, {
+        email: "[email protected]",
+        full_name: "Zman",
+        is_admin: false,
+        is_bot: false,
+        user_id: 6,
+    }
+];
+
+_.each(matches, function (person) {
+    global.people.add_in_realm(person);
+});
+
+(function test_sort_recipients() {
+    function get_typeahead_result(query) {
+        var result = th.sort_recipients(global.people.get_realm_persons(), query);
+        return _.map(result, function (person) {
+            return person.email;
+        });
+    }
+
+    global.compose.stream_name = function () { return ""; };
+    assert.deepEqual(get_typeahead_result("b"), [
+        '[email protected]',
+        '[email protected]',
+        '[email protected]',
+        '[email protected]',
+        '[email protected]',
+        '[email protected]'
+     ]);
+
+    global.compose.stream_name = function () { return "Dev"; };
+    var subscriber_email = "[email protected]";
+    stream_data.add_subscriber("Dev", people.get_user_id(subscriber_email));
+    assert.deepEqual(get_typeahead_result("b"), [
+        subscriber_email,
+        '[email protected]',
+        '[email protected]',
+        '[email protected]',
+        '[email protected]',
+        '[email protected]'
+    ]);
+
+    // No match
+    global.compose.stream_name = function () { return "Linux"; };
+    assert.deepEqual(get_typeahead_result("h"), [
+        '[email protected]',
+        '[email protected]',
+        '[email protected]',
+        '[email protected]',
+        '[email protected]',
+        '[email protected]'
+    ]);
+
+}());

static/js/typeahead_helper.js

@@ -95,6 +95,29 @@ function prefix_sort(query, objs, get_item) {
 
 }
 
+function split_by_subscribers(people) {
+    var subscribers = [];
+    var non_subscribers = [];
+    var current_stream = compose.stream_name();
+
+    if (current_stream === "") {
+        // If there is no stream specified, everyone is considered as a subscriber.
+        return {subs: people, non_subs: []};
+    }
+
+    _.each(people, function (person) {
+        if (person.email === "all" || person.email === "everyone") {
+            subscribers.push(person);
+        } else if (stream_data.user_is_subscribed(current_stream, person.email)) {
+            subscribers.push(person);
+        } else {
+            non_subscribers.push(person);
+        }
+    });
+
+    return {subs: subscribers, non_subs: non_subscribers};
+}
+
 exports.sorter = function (query, objs, get_item) {
    var results = prefix_sort(query, objs, get_item);
    return results.matches.concat(results.rest);
@@ -126,18 +149,22 @@ exports.compare_by_pms = function (user_a, user_b) {
     return 1;
 };
 
-exports.sort_by_pms = function (objs) {
-    objs.sort(exports.compare_by_pms);
-    return objs;
+exports.sort_for_at_mentioning = function (objs) {
+    var objs_split = split_by_subscribers(objs);
+
+    var subs_sorted = objs_split.subs.sort(exports.compare_by_pms);
+    var non_subs_sorted = objs_split.non_subs.sort(exports.compare_by_pms);
+    return subs_sorted.concat(non_subs_sorted);
 };
 
 exports.sort_recipients = function (matches, query) {
     var name_results =  prefix_sort(query, matches, function (x) { return x.full_name; });
     var email_results = prefix_sort(query, name_results.rest, function (x) { return x.email; });
-    var matches_sorted_by_pms =
-        exports.sort_by_pms(name_results.matches.concat(email_results.matches));
-    var rest_sorted_by_pms = exports.sort_by_pms(email_results.rest);
-    return matches_sorted_by_pms.concat(rest_sorted_by_pms);
+
+    var matches_sorted =
+        exports.sort_for_at_mentioning(name_results.matches.concat(email_results.matches));
+    var rest_sorted = exports.sort_for_at_mentioning(email_results.rest);
+    return matches_sorted.concat(rest_sorted);
 };
 
 exports.sort_emojis = function (matches, query) {