people: Add dm_matches_search_string for DM filter.

This function does not respect `,` (commas) in the search term and will
treat a comma as any other character. We can decide how to treat comma
separated terms in future iterations. That is also the reason that we
introduce this 2 line function instead of just using the person matcher
directly in future commits. This function still supports search terms
with diacritics because of using person matcher.

web/src/people.ts

@@ -1209,6 +1209,12 @@ export function filter_people_by_search_terms(users: User[], search_string: stri
     return filtered_users;
 }
 
+export function dm_matches_search_string(users: User[], search_string: string): boolean {
+    const matcher = build_person_matcher(search_string);
+
+    return users.some((user) => matcher(user));
+}
+
 export const is_valid_full_name_and_user_id = (full_name: string, user_id: number): boolean => {
     /*
         This function is currently only used for checking

web/tests/people.test.js

@@ -757,6 +757,47 @@ test_people("filtered_users", () => {
     assert.ok(filtered_people.has(noah.user_id));
 });
 
+test_people("dm_matches_search_string", () => {
+    people.add_active_user(charles);
+    people.add_active_user(maria);
+    people.add_active_user(ashton);
+    people.add_active_user(linus);
+    people.add_active_user(noah);
+    people.add_active_user(plain_noah);
+
+    let result = people.dm_matches_search_string([ashton], "a");
+    assert.ok(result);
+    // Maria's email starts with `a`.
+    result = people.dm_matches_search_string([maria], "a");
+    assert.ok(result);
+    // Neither Charles' full name or email start with `a`.
+    result = people.dm_matches_search_string([charles], "a");
+    assert.ok(!result);
+
+    // Empty search terms should always return true
+    result = people.dm_matches_search_string([charles], "");
+    assert.ok(result);
+    result = people.dm_matches_search_string([charles, maria, noah], "");
+    assert.ok(result);
+
+    // Match with email.
+    result = people.dm_matches_search_string([linus], "ltorv");
+    assert.ok(result);
+
+    // Test filtering of names with diacritics. This should match
+    // Nöôáàh by ignoring diacritics, and also match Nooaah.
+    result = people.dm_matches_search_string([noah], "noOa");
+    assert.ok(result);
+    result = people.dm_matches_search_string([plain_noah], "noOa");
+    assert.ok(result);
+
+    // This should match ëmerson, but not emerson.
+    result = people.dm_matches_search_string([noah], "ëm");
+    assert.ok(result);
+    result = people.dm_matches_search_string([plain_noah], "ëm");
+    assert.ok(!result);
+});
+
 test_people("multi_user_methods", () => {
     people.add_active_user(emp401);
     people.add_active_user(emp402);