refactor: Extract is_subscriber_subset().

Extracting the function makes it a bit easier to
test and use in a generic way.

Also, I wanted this to live in stream_data, so that
it's easier to find if we change how we model
subscriber data.

Finally, I use _.every to do the subset check
instead of `_.difference`, since _.difference
is actually N-squared:

  _.difference = restArguments(function(array, rest) {
    rest = flatten(rest, true, true);
    return _.filter(array, function(value){
      return !_.contains(rest, value);
    });
  });

And we don't actually want to build a list only
to check that it's zero vs. nonzero length.

We now do this, which short circuits as soon
as it finds any key that is only in sub1:

    return _.every(sub1.subscribers.keys(), (key) => {
        return sub2_set.has(key);
    });

frontend_tests/node_tests/stream_data.js

@@ -850,6 +850,45 @@ run_test('filter inactives', () => {
     assert(stream_data.is_filtering_inactives());
 });
 
+run_test('is_subscriber_subset', () => {
+    function make_sub(user_ids) {
+        const sub = {};
+        stream_data.set_subscribers(sub, user_ids);
+        return sub;
+    }
+
+    const sub_a = make_sub([1, 2]);
+    const sub_b = make_sub([2, 3]);
+    const sub_c = make_sub([1, 2, 3]);
+
+    // The bogus case should not come up in normal
+    // use.
+    // We simply punt on any calculation if
+    // a stream has no subscriber info (like
+    // maybe Zephyr?).
+    const bogus = {}; // no subscribers
+
+    const matrix = [
+        [sub_a, sub_a, true],
+        [sub_a, sub_b, false],
+        [sub_a, sub_c, true],
+        [sub_b, sub_a, false],
+        [sub_b, sub_b, true],
+        [sub_b, sub_c, true],
+        [sub_c, sub_a, false],
+        [sub_c, sub_b, false],
+        [sub_c, sub_c, true],
+        [bogus, bogus, false],
+    ];
+
+    _.each(matrix, (row) => {
+        assert.equal(
+            stream_data.is_subscriber_subset(row[0], row[1]),
+            row[2]
+        );
+    });
+});
+
 run_test('invite_streams', () => {
     // add default stream
     const orie = {

static/js/compose.js

@@ -817,14 +817,12 @@ exports.warn_if_private_stream_is_linked = function (linked_stream) {
         return;
     }
 
-    if (compose_stream.subscribers && linked_stream.subscribers) {
-        const compose_stream_sub = compose_stream.subscribers.keys();
-        const mentioned_stream_sub = linked_stream.subscribers.keys();
-        // Don't warn if subscribers list of current compose_stream is a subset of
-        // mentioned_stream subscribers list.
-        if (_.difference(compose_stream_sub, mentioned_stream_sub).length === 0) {
-            return;
-        }
+    if (stream_data.is_subscriber_subset(compose_stream, linked_stream)) {
+        // Don't warn if subscribers list of current
+        // compose_stream is a subset of linked_stream's
+        // subscribers list, because everyone will be
+        // subscribed to the linked stream.
+        return;
     }
 
     const stream_name = linked_stream.name;

static/js/stream_data.js

@@ -145,6 +145,18 @@ exports.subscribe_myself = function (sub) {
     stream_info.set_true(sub.name, sub);
 };
 
+exports.is_subscriber_subset = function (sub1, sub2) {
+    if (sub1.subscribers && sub2.subscribers) {
+        const sub2_set = sub2.subscribers;
+
+        return _.every(sub1.subscribers.keys(), (key) => {
+            return sub2_set.has(key);
+        });
+    }
+
+    return false;
+};
+
 exports.unsubscribe_myself = function (sub) {
     // Remove user from subscriber's list
     const user_id = people.my_current_user_id();