Map<String, File> scanExistingUsers() throws IOException {
        Map<String, File> users = new HashMap<>();
        File[] userDirectories = listUserDirectories();
        if (userDirectories != null) {
            for (File directory : userDirectories) {
                String userId = idStrategy.idFromFilename(directory.getName());
                users.put(userId, directory);
            }
        }
        addEmptyUsernameIfExists(users);
        return users;
    }
@Test
    public void scanExistingUsersOldLegacy() throws IOException {
        UserIdMigrator migrator = createUserIdMigrator();
        Map<String, File> userMappings = migrator.scanExistingUsers();
        assertThat(userMappings.keySet(), hasSize(4));
        assertThat(userMappings.keySet(), hasItems("make\u1000000", "\u306f\u56fd\u5185\u3067\u6700\u5927", "\u1000yyy", "zzz\u1000"));
    }