strong_prime.cpp

#include <cstdio>
#include <cassert>
#include <stddef.h>

#include <gmp.h>



constexpr size_t PRIME_BITS = 1024;
constexpr size_t PRIME_BYTES = PRIME_BITS / 8;
constexpr int PRIME_REPS = 15;

int main(int argc, char** argv)
{
    const char* filename = "/dev/urandom";

    FILE* fp = std::fopen(filename, "rb");
    if (!fp) {
        std::printf("Failed to open input file");
        return 1;
    }

    char buffer[PRIME_BYTES];
    mpz_t r, p, q;
    mpz_inits(r, p, q, NULL);

    for (int c = 0; c < 10000; c++) {
        size_t bytes_read = std::fread(buffer, 1, PRIME_BYTES, fp);
        if (bytes_read < PRIME_BYTES) {
            std::printf("EOF\n");
            break;
        }
        mpz_import(p, PRIME_BYTES, 1, 1, 0, 0, buffer);
        while (mpz_probab_prime_p(p, PRIME_REPS) == 0) {
            mpz_add_ui(p, p, 1);
        }
        mpz_sub_ui(q, p, 1);
        mpz_fdiv_q_ui(q, q, 2);
        int q_prime = mpz_probab_prime_p(q, PRIME_REPS);
        if (q_prime) {
            std::putchar('+');
        } else {
            std::putchar('.');
        }
        std::fflush(stdout);
    }

    mpz_clears(r, p, q, NULL);
    fclose(fp);
    std::putchar('\n');

    return 0;
}