(NSComparator)^(id a, id b) {
    // check if user a or user b has unread message
    if( [(UserInfo*)a unreadMsg] > 0 && [(UserInfo*)b unreadMsg] <= 0 ) {   // a has but b
        return a;
    }
    else if [(UserInfo*)a unreadMsg] <= 0 && [(UserInfo*)b unreadMsg] > 0 ) {   // b has but a
        return b;
    }
    else {  // both have unread msg or neither has unread msg
        // then check distance
        if ([(UserInfo*)a distance] < [(UserInfo*)b distance]) {
            return a;
        }
        else {
            return b;
        }
    }
}