#include <cstdio>
#include <cstring>
#include <regex>
#include <chrono>
#include <iostream>

char mail[] = "pippo.paperino234@some.contoso.com";

bool isValidEmail(char* mail) {
    int len = strlen(mail);
    char c;
    int position = 0;
    // check first character
    c = mail[position++];
    if(!((c >= 'A' && c <= 'z') || (c >= '0' && c <= '9'))) return false;

    // check first group before '@'
    c = mail[position++];
    do {
        if(!((c >= 'A' && c <= 'z') || (c >= '0' && c <= '9') || c == '.'))  return false;
        c = mail[position++];
    } while (c != '@' && position < len);

    if(position >= len)  return false;

    // do the second group
    c = mail[position++];
    if(!((c >= 'A' && c <= 'z') || (c >= '0' && c <= '9'))) return false;
    c = mail[position++];
    do {
        if(!((c >= 'A' && c <= 'z') || (c >= '0' && c <= '9'))) return false;
        c = mail[position++];
    } while (c != '.' && position < len);
    if(position >= len)  return false;


    // now we could have something that ends in only letters
    do {
    c = mail[position++];
    if(!((c >= 'A' && c <= 'z') || (c >= '0' && c <= '9')))  return false;
    do {
        if(!((c >= 'A' && c <= 'z') || (c >= '0' && c <= '9')))  return false;
        c = mail[position++];
    } while (c != '.' && position < len);
    if(position >= len && c == '.')  return false;
    } while(position < len);

    return true;

}
int main(int argc, char** argv) {
    auto start = std::chrono::system_clock::now();

{
    bool valid;
    for(int i = 0; i < 1000000; i++) valid = isValidEmail(mail);

    auto end = std::chrono::system_clock::now();
    auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);
    std::cout << "Time for sequential version: " << elapsed.count() << " ms, value: " << valid << std::endl;
}


    const std::regex mail_regex("^[A-z0-9][A-z0-9\\.]+@([A-z0-9]+\\.)+[A-z]+$");

{
    bool valid;
    for(int i = 0; i < 1000000; i++) valid = std::regex_match(mail, mail_regex);

    auto end = std::chrono::system_clock::now();
    auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);
    std::cout << "Time for regex version: " << elapsed.count() << " ms, value: " << valid << std::endl;
}
}