Rabin-Karp algorithm

RabinKarp.cpp

int main()
{
    std::string str;
    std::string substr;
    
    cin >> str;
    cin >> substr;
    
    if (str.length() < substr.length()) {
        return 0;
    }
    
    int p_base = 53;
    std::vector<long long> powers(str.length());
    powers[0] = 1;
    for (size_t i = 1; i < powers.size(); i++)
        powers[i] = powers[i - 1] * p_base;
    
    std::vector<long long> str_hashes(powers.size());
    for (int i = 0; i < str_hashes.size(); i++) {
        str_hashes[i] = (str[i] - 'a' + 1) * powers[i];
        if (i) str_hashes[i] += str_hashes[i - 1];
    }
    
    long long substr_hash = 0;
    for (size_t i = 0; i < substr.size(); i++) {
        substr_hash += (substr[i] - 'a' + 1) * powers[i];
    }
    
    for (size_t i = 0; i + substr.length() - 1 < str.length(); ++i) {
        long long hash_t = str_hashes[i + substr.length() - 1];
        
        if (i) hash_t -= str_hashes[i-1];
        
        if (hash_t == substr_hash * powers[i]) {
            std::cout << i << " ";
        }
    }
    
    return 0;
}