The GMP library

#include <gmp.h>
#include <stdio.h>
#include <string.h>

/**
 * Check if a GMP integer is prime
 * @param n: the number to test
 * @return: true if prime, false if composite
 */
bool isPrime(const mpz_t n)
{
    int result = mpz_probab_prime_p(n, 25);
    return result > 0; // Returns true for both "probably prime" and "definitely prime"
}

/**
 * Check if a string representation of a number is prime
 * @param str: string representation of the number
 * @return: true if prime, false if composite or invalid
 */
bool isPrimeStr(const char *str)
{
    mpz_t n;
    mpz_init(n);

    // Try to set the number from string
    if (mpz_set_str(n, str, 10) != 0)
    {
        mpz_clear(n);
        return false; // Invalid number format
    }

    bool result = isPrime(n);
    mpz_clear(n);
    return result;
}

/**
 * Check if a regular integer is prime (for convenience)
 * @param num: the number to test
 * @return: true if prime, false if composite
 */
bool isPrimeInt(unsigned long num)
{
    mpz_t n;
    mpz_init_set_ui(n, num);
    bool result = isPrime(n);
    mpz_clear(n);
    return result;
}

int main()
{
    mpz_t big_number;
    char input[1000];

    mpz_init(big_number);

    // Test with small numbers
    printf("Testing small numbers:\n");
    for (int i = 2; i <= 20; i++)
    {
        printf("%d is %s\n", i, isPrimeInt(i) ? "prime" : "composite");
    }

    // Test with large numbers
    printf("\nTesting large numbers:\n");

    // Test Mersenne prime 2^127 - 1
    mpz_ui_pow_ui(big_number, 2, 127);
    mpz_sub_ui(big_number, big_number, 1);
    printf("2^127 - 1 is %s\n", isPrime(big_number) ? "prime" : "composite");

    // Test from string input
    printf("\nEnter a large number to test: ");
    if (fgets(input, sizeof(input), stdin) != NULL)
    {
        input[strcspn(input, "\n")] = 0; // Remove newline
        printf("%s is %s\n", input, isPrimeStr(input) ? "prime" : "composite");
    }

    mpz_clear(big_number);
    return 0;
}