Add filter.is_valid_id_from().

We will use this to find the first id from a list of
message ids that matches a filter.  (This will help us
during narrowing to determine whether we have at least
one good message locally, so that we can render something
useful before waiting for the server.)

frontend_tests/node_tests/filter.js

@@ -5,6 +5,7 @@ zrequire('people');
 zrequire('Handlebars', 'handlebars');
 zrequire('Filter', 'js/filter');
 
+set_global('message_store', {});
 set_global('page_params', {});
 set_global('feature_flags', {});
 
@@ -800,6 +801,32 @@ function make_sub(name, stream_id) {
     assert.deepEqual(term_types, ['stream', 'topic', 'sender']);
 }());
 
+(function test_first_valid_id_from() {
+    const terms = [
+        {operator: 'is', operand: 'alerted'},
+    ];
+
+    const filter = new Filter(terms);
+
+    const messages = {
+        5: { id: 5, alerted: true },
+        10: { id: 10 },
+        20: { id: 20, alerted: true },
+        30: { id: 30, type: 'stream' },
+        40: { id: 40, alerted: false },
+    };
+
+    const msg_ids = [10, 20, 30, 40];
+
+    message_store.get = () => {};
+
+    assert.equal(filter.first_valid_id_from(msg_ids), undefined);
+
+    message_store.get = (msg_id) => messages[msg_id];
+
+    assert.equal(filter.first_valid_id_from(msg_ids), 20);
+}());
+
 (function test_update_email() {
     var terms = [
         {operator: 'pm-with', operand: '[email protected]'},

static/js/filter.js

@@ -415,6 +415,22 @@ Filter.prototype = {
         return _.isEqual(term_types, wanted_term_types);
     },
 
+    first_valid_id_from: function (msg_ids) {
+        var predicate = this.predicate();
+
+        var first_id = _.find(msg_ids, function (msg_id) {
+            var message = message_store.get(msg_id);
+
+            if (message === undefined) {
+                return false;
+            }
+
+            return predicate(message);
+        });
+
+        return first_id;
+    },
+
     update_email: function (user_id, new_email) {
         _.each(this._operators, function (term) {
             switch (term.operator) {