Check for stringy primes

stringy_check.c

/* SPDX-License-Identifier: GPL-2.0-or-later */
/**
 * @file stringy_check.c
 * @author Juuso Alasuutari
 *
 * @brief Parse the raw content bytes of text strings as if the strings were
 *        arbitrarily large integers, and check if the result is a different
 *        prime number in both the big- and little-endian byte orders.
 *
 * I call the group of primes generated by this method "stringy primes"; run
 * `./stringy_check stringy` to see why.
 *
 * Link with "-lgmp".
 */
#include <gmp.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static void
usage (char const *argv0,
       int         ret)
{
	fprintf(stderr, "Usage: %s [-r REPS] STRING...\n"
	                "       %s -h\n", argv0, argv0);
	exit(ret);
}

int
main (int    argc,
      char **argv)
{
	int options_end = argc;
	int input_count = 0;
	int reps        = 100;

	for (int i = 0; ++i < argc; ) {
		char *s = argv[i];

		if (i >= options_end || s[0] != '-') {
			++input_count;
			continue;
		}

		switch (s[1]) {
		case '-':
			if (!s[2]) {
				options_end = i + 1;
				continue;
			}
			break;

		case 'h':
			if (s[2])
				break;
			usage(argv[0], EXIT_SUCCESS);

		case 'r':
			if (s[2])
				s += 2;
			else if (++i >= argc || !*(s = argv[i]))
				break;

			char *end = s;
			long n = strtol(s, &end, 0);
			if (*end || n < 0 || n > INT_MAX)
				break;

			reps = (int)n;
			continue;
		}

		input_count = -input_count;
		break;
	}

	if (input_count < 1)
		usage(argv[0], EXIT_FAILURE);

	mpz_t le, be;
	mpz_init(le);
	mpz_init(be);

	for (int i = 0; ++i < argc; ) {
		char const *s = argv[i];
		if (i < options_end && s[0] == '-')
			// option
			continue;

		if (!*s)
			// empty string
			continue;

		size_t n = strlen(&s[1]), k = n, j = 0;
		for (; j < k && s[j] == s[k]; ++j, --k) {}
		if (j >= k)
			// byte-level palindrome
			continue;
		n++;

		mpz_import(le, 1U, -1, n, -1, 0, argv[i]);
		int lp = mpz_probab_prime_p(le, reps) & 3;
		if (!lp)
			// definitely not prime in little-endian
			continue;

		mpz_import(be, 1U, -1, n,  1, 0, argv[i]);
		int bp = mpz_probab_prime_p(be, reps) & 3;
		if (!bp)
			// definitely not prime in big-endian
			continue;

		printf("%s\t", argv[i]);
		mpz_out_str(stdout, 10, le);
		putchar('\t');
		mpz_out_str(stdout, 10, be);
		printf("\t%c%c\n", "npyy"[lp], "npyy"[bp]);
	}

	mpz_clear(le);
	mpz_clear(be);
}