enum TokenType {
    Plus,
    Minus,
    Multiply,
    Divide,
    Number,
    OpenBracket,
    CloseBracket
};
struct Token {
    TokenType type;
    std::string data;
    // other useful things
};
std::vector<Token> lex(std::string input) {
    std::unordered_map<char, TokenType> singles;
    singles['+'] = TokenType::Plus;
    singles['-'] = TokenType::Minus;
    singles['('] = TokenType::OpenBracket;
    singles[')'] = TokenType::CloseBracket;
    singles['*'] = TokenType::Multiply;
    singles['/'] = TokenType::Divide;
    std::unordered_set<char> whitespace;
    whitespace.insert(' ');
    whitespace.insert('\t');
    whitespace.insert('\r');
    whitespace.insert('\n');
    std::unordered_set<char> nums;
    for(int i = 0; i < 10; i++)
        nums.insert('0' + i);
    std::vector<Token> out;
    for(auto it = input.begin(); it != input.end(); ++it) {
        if (singles.find(*it) != singles.end()) {
            Token t;
            t.type = singles[*it];
            out.push_back(t);
            continue;
        }
        if (whitespace.find(*it) != whitespace.end()) {
            continue;
        }
        if (nums.find(*it) != nums.end()) {
            Token t;
            t.type = TokenType::Number;
            while(nums.find(*it) != nums.end()) {
                t.data.push_back(*it);
                ++it;
                if (it == input.end())
                    break;
            }
            out.push_back(t);
            continue;
        }
        // error
    }
    return out;
};