main.cpp

#include <iostream>
#include <string>
#include <cmath>
#include <chrono>

bool isPrime(long long n) {
    if (n <= 1) return false;
    if (n <= 3) return true;
    if (n % 2 == 0 || n % 3 == 0) return false;  

    // Check divisors up to sqrt(n)
    for (long long i = 5; i * i <= n; i += 6) {
        if (n % i == 0 || n % (i + 2) == 0)
            return false;
    }

    return true;
}

int main() {
    std::string num;  // Stored as a string to make truncating easier

    std::cout << "Enter your number: ";
    std::cin >> num;

    // Time, for benchmarking
    auto timeStart = std::chrono::system_clock::now();
    auto millisStart = std::chrono::duration_cast<std::chrono::milliseconds>(
                      timeStart.time_since_epoch()
                  ).count();

    try {
        long long initialCheck = std::stoll(num);
    } catch (...) {
        std::cerr << "Invalid input or number too large." << std::endl;
        return -1;
    }

    if (isPrime(std::stoll(num))) {
        std::cout << num << " is a prime number" << std::endl;

        // Make sure it can be truncated
        if (num.find('0') != std::string::npos) {
            std::cout << "Truncatable primes cannot contain the digit 0." << std::endl;
            std::cout << num << " is not a left truncatable prime number" << std::endl;
            std::cout << num << " is not a right truncatable prime number" << std::endl;
            std::cout << num << " is not a left-and-right truncatable prime number" << std::endl;
            return 0;
        }
        // Check whether it's left truncatable
        bool isLeftTruncatable = true;
        for (int i = 0; i < num.length(); i++) {
            std::string current = num.substr(i);
            long long value = std::stoll(current);

            if (!isPrime(value)) {
                isLeftTruncatable = false;
                break;
            }
        }

        if (isLeftTruncatable)
            std::cout << num << " is a left truncatable prime number" << std::endl;
        else
            std::cout << num << " is not a left truncatable prime number" << std::endl;

        // Check whether it's right truncatable
        bool isRightTruncatable = true;
        for (int i = num.length(); i > 0; i--) {
            std::string current = num.substr(0, i);
            long long value = std::stoll(current);

            if (!isPrime(value)) {
                isRightTruncatable = false;
                break;
            }
        }

        if (isRightTruncatable)
            std::cout << num << " is a right truncatable prime number" << std::endl;
        else
            std::cout << num << " is not a right truncatable prime number" << std::endl;

        // Check whether it's left-and-right truncatable
        bool isBothTruncatable = true;
        std::string current = num;

        if (current.length() > 3) {
            while (!current.empty()) {
                long long value = std::stoll(current);

                if (!isPrime(value)) {
                    isBothTruncatable = false;
                    break;
                }

                // Remove one digit from both ends
                if (current.length() <= 3)  // If it has 2 or fewer digits, we’re done next
                    break;

                current = current.substr(1, current.length() - 2);
            }
        } else {
            isBothTruncatable = false;
        }

        if (isBothTruncatable)
            std::cout << num << " is a left-and-right truncatable prime number" << std::endl;
        else
            std::cout << num << " is not a left-and-right truncatable prime number" << std::endl;
    } else {
        std::cout << num << " is not a prime number" << std::endl;
    }

    // Benchmark
    auto timeEnd = std::chrono::system_clock::now();
    auto millisEnd = std::chrono::duration_cast<std::chrono::milliseconds>(
                      timeEnd.time_since_epoch()
                  ).count();

    std::cout << "Evaluation completed in " << millisEnd - millisStart << " milliseconds" << std::endl;
    
    return 0;
}