user_topics: Add set_user_topic helper function.

As we plan to move towards using `user_topics` instead of
`muted_topics`, this helper method will be used to set an
individual `user_topic` event in the corresponding data
structure in the web app.

frontend_tests/node_tests/muting.js

@@ -2,6 +2,7 @@
 
 const {strict: assert} = require("assert");
 
+const {visibility_policy} = require("../../static/js/user_topics");
 const {zrequire} = require("../zjsunit/namespace");
 const {run_test} = require("../zjsunit/test");
 const blueslip = require("../zjsunit/zblueslip");
@@ -209,6 +210,64 @@ test("unknown streams", () => {
     ]);
 });
 
+test("set_user_topics", () => {
+    blueslip.expect("warn", "Unknown stream ID in set_user_topic: 999");
+
+    user_topics.set_muted_topics([]);
+    assert.ok(!user_topics.is_topic_muted(social.stream_id, "breakfast"));
+    assert.ok(!user_topics.is_topic_muted(design.stream_id, "typography"));
+
+    const user_topic_events = [
+        {
+            stream_id: social.stream_id,
+            topic_name: "breakfast",
+            last_updated: "1577836800",
+            visibility_policy: visibility_policy.MUTED,
+        },
+        {
+            stream_id: design.stream_id,
+            topic_name: "typography",
+            last_updated: "1577836800",
+            visibility_policy: visibility_policy.MUTED,
+        },
+        {
+            stream_id: 999, // BOGUS STREAM ID
+            topic_name: "random",
+            last_updated: "1577836800",
+            visibility_policy: visibility_policy.MUTED,
+        },
+    ];
+
+    for (const user_topic of user_topic_events) {
+        user_topics.set_user_topic(user_topic);
+    }
+
+    assert.deepEqual(user_topics.get_muted_topics().sort(), [
+        {
+            date_muted: 1577836800000,
+            date_muted_str: "Jan\u00A001,\u00A02020",
+            stream: social.name,
+            stream_id: social.stream_id,
+            topic: "breakfast",
+        },
+        {
+            date_muted: 1577836800000,
+            date_muted_str: "Jan\u00A001,\u00A02020",
+            stream: design.name,
+            stream_id: design.stream_id,
+            topic: "typography",
+        },
+    ]);
+
+    user_topics.set_user_topic({
+        stream_id: design.stream_id,
+        topic_name: "typography",
+        last_updated: "1577836800",
+        visibility_policy: visibility_policy.VISIBILITY_POLICY_INHERIT,
+    });
+    assert.ok(!user_topics.is_topic_muted(design.stream_id, "typography"));
+});
+
 test("case_insensitivity", () => {
     user_topics.set_muted_topics([]);
     assert.ok(!user_topics.is_topic_muted(social.stream_id, "breakfast"));

static/js/user_topics.js

@@ -7,6 +7,13 @@ import {get_time_from_date_muted} from "./util";
 
 const muted_topics = new Map();
 
+export const visibility_policy = {
+    VISIBILITY_POLICY_INHERIT: 0,
+    MUTED: 1,
+    UNMUTED: 2,
+    FOLLOWED: 3,
+};
+
 export function add_muted_topic(stream_id, topic, date_muted) {
     let sub_dict = muted_topics.get(stream_id);
     if (!sub_dict) {
@@ -51,6 +58,28 @@ export function get_muted_topics() {
     return topics;
 }
 
+export function set_user_topic(user_topic) {
+    const stream_id = user_topic.stream_id;
+    const topic = user_topic.topic_name;
+    const date_muted = user_topic.last_updated;
+
+    const stream_name = stream_data.maybe_get_stream_name(stream_id);
+
+    if (!stream_name) {
+        blueslip.warn("Unknown stream ID in set_user_topic: " + stream_id);
+        return;
+    }
+
+    switch (user_topic.visibility_policy) {
+        case visibility_policy.MUTED:
+            add_muted_topic(stream_id, topic, date_muted);
+            break;
+        case visibility_policy.VISIBILITY_POLICY_INHERIT:
+            remove_muted_topic(stream_id, topic);
+            break;
+    }
+}
+
 export function set_muted_topics(tuples) {
     muted_topics.clear();