edit_history: Check edit history for stream and topic match.

Creates a helper function in `message_edit.js` that loops over
a message's edit history to see if a stream and topic pair
existed at some point in the message history.

Exports `util.lower_same` function to use for comparing
edit history topics as lowercase.

Also adds test for new function in `mesage_edit` node tests.

frontend_tests/node_tests/message_edit.js

@@ -158,3 +158,114 @@ run_test("get_deletability", ({override}) => {
     message.timestamp = current_timestamp - 60;
     assert.equal(message_edit.get_deletability(message), false);
 });
+
+run_test("stream_and_topic_exist_in_edit_history", () => {
+    // A message with no edit history should always return false;
+    // the message's current stream_id and topic are not compared
+    // to the stream_id and topic parameters.
+    const message_no_edits = {
+        stream_id: 1,
+        topic: "topic match",
+    };
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_no_edits, 2, "no match"),
+        false,
+    );
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_no_edits, 1, "topic match"),
+        false,
+    );
+
+    // A non-stream message (object has no stream_id or topic)
+    // with content edit history, should return false.
+    const private_message = {
+        edit_history: [{prev_content: "content edit to PM"}],
+    };
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(private_message, 1, "topic match"),
+        false,
+    );
+
+    // A stream message with only content edits should return false,
+    // even if the message's current stream_id and topic are a match.
+    const message_content_edit = {
+        stream_id: 1,
+        topic: "topic match",
+        edit_history: [{prev_content: "content edit"}],
+    };
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_content_edit, 1, "topic match"),
+        false,
+    );
+
+    const message_stream_edit = {
+        stream_id: 6,
+        topic: "topic match",
+        edit_history: [{stream: 6, prev_stream: 1}],
+    };
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_stream_edit, 2, "topic match"),
+        false,
+    );
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_stream_edit, 1, "topic match"),
+        true,
+    );
+
+    const message_topic_edit = {
+        stream_id: 1,
+        topic: "final topic",
+        edit_history: [{topic: "final topic", prev_topic: "topic match"}],
+    };
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_topic_edit, 1, "no match"),
+        false,
+    );
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_topic_edit, 1, "topic match"),
+        true,
+    );
+
+    const message_many_edits = {
+        stream_id: 6,
+        topic: "final topic",
+        edit_history: [
+            {stream: 6, prev_stream: 5},
+            {prev_content: "content only edit"},
+            {topic: "final topic", prev_topic: "topic match"},
+            {stream: 5, prev_stream: 1},
+        ],
+    };
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_many_edits, 1, "no match"),
+        false,
+    );
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_many_edits, 2, "topic match"),
+        false,
+    );
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(message_many_edits, 1, "topic match"),
+        true,
+    );
+
+    // When the topic and stream_id exist in the message's edit history
+    // individually, but not together in a historical state, it should return false.
+    const message_no_historical_match = {
+        stream_id: 6,
+        topic: "final topic",
+        edit_history: [
+            {stream: 6, prev_stream: 1}, // stream matches, topic does not
+            {stream: 1, prev_stream: 5}, // neither match
+            {topic: "final topic", prev_topic: "topic match"}, // topic matches, stream does not
+        ],
+    };
+    assert.equal(
+        message_edit.stream_and_topic_exist_in_edit_history(
+            message_no_historical_match,
+            1,
+            "topic match",
+        ),
+        false,
+    );
+});

static/js/message_edit.js

@@ -33,6 +33,7 @@ import * as stream_data from "./stream_data";
 import * as sub_store from "./sub_store";
 import * as ui_report from "./ui_report";
 import * as upload from "./upload";
+import * as util from "./util";
 
 const currently_editing_messages = new Map();
 let currently_deleting_messages = [];
@@ -185,6 +186,53 @@ export function get_deletability(message) {
     return false;
 }
 
+export function stream_and_topic_exist_in_edit_history(message, stream_id, topic) {
+    /*  Checks to see if a stream_id and a topic match any historical
+        stream_id and topic state in the message's edit history.
+
+        Does not check the message's current stream_id and topic for
+        a match to the stream_id and topic parameters.
+     */
+    const narrow_dict = {stream_id, topic};
+    const message_dict = {stream_id: message.stream_id, topic: message.topic};
+
+    if (!message.edit_history) {
+        // If message edit history is disabled in the organization,
+        // the client does not have the information locally to answer
+        // this question correctly.
+        return false;
+    }
+
+    for (const edit_history_event of message.edit_history) {
+        if (!edit_history_event.prev_stream && !edit_history_event.prev_topic) {
+            // Message was not moved in this edit event.
+            continue;
+        }
+
+        if (edit_history_event.prev_stream) {
+            // This edit event changed the stream.  We expect the
+            // following to be true due to the invariants of the edit
+            // history data structure:
+            // edit_history_event.stream === message_dict.stream_id
+            message_dict.stream_id = edit_history_event.prev_stream;
+        }
+
+        if (edit_history_event.prev_topic) {
+            // This edit event changed the topic.  We expect the
+            // following to be true due to the invariants of the edit
+            // history data structure:
+            // util.lower_same(edit_history_event.topic, message_dict.topic)
+            message_dict.topic = edit_history_event.prev_topic;
+        }
+
+        if (util.same_stream_and_topic(narrow_dict, message_dict)) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 export function update_message_topic_editing_pencil() {
     if (page_params.realm_allow_message_editing) {
         $(".on_hover_topic_edit, .always_visible_topic_edit").show();

static/js/util.js

@@ -36,9 +36,9 @@ export function lower_bound(array, value, less) {
     return first;
 }
 
-function lower_same(a, b) {
+export const lower_same = function lower_same(a, b) {
     return a.toLowerCase() === b.toLowerCase();
-}
+};
 
 export const same_stream_and_topic = function util_same_stream_and_topic(a, b) {
     // Streams and topics are case-insensitive.