Prime number generator

main.c

#include <stdio.h>
#include <getopt.h>
#include <string.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdbool.h>

static struct option prime_opts[] =
        {
                {"count", required_argument, NULL, 'c'},
                {"number", required_argument, NULL, 'n'},
                {"range", required_argument, NULL, 'r'},
                {"test", required_argument, NULL, 't'},
                {"help", no_argument, NULL, 'h'},
                {NULL, 0, NULL, 0}
        };


static bool is_prime(int64_t n, int64_t *factor) {
    // Only positive numbers greater than or equal to 2 can be prime
    if (n < 2) {
        return false;
    }

    // 2 is the only even number that is prime, so check if number is even for an early out
    if (n % 2 == 0)
    {
        if (factor) *factor = 2;
        return n == 2;
    }

    // Search for a factor, returning first one found
    int64_t i;
    for (i = 3; i * i <= n; i += 2) {
        if (n % i == 0){
            if (factor)
                *factor = i;
            return false;
        }
    }

    // Return 0 if none could be found, number is prime
    return true;
}

static int primes_in_range(int64_t start, int64_t end) {

    int64_t n = start < end ? start : end;
    int64_t e = end > start ? end : start;

    while (n <= e) {
        if (is_prime(n, NULL)) {
            printf("%ld\n", n);
        }
        n++;
    }
    return EXIT_SUCCESS;
}

int main(int argc, char **argv) {

    int64_t c = 0, n = 2;
    bool process = false;

    int opt;
    while ((opt = getopt_long(argc, argv, "c:n:r:t:h", prime_opts, NULL)) != -1)
    {
        switch (opt)
        {
            case 'c':
            {
                c = atoll(optarg);
                if (c <= 0)
                {
                    printf("invalid argument (use -h for help)\n");
                    return EXIT_FAILURE;
                }
                process = true;
                break;
            }
            case 'n':
            {
                n = atoll(optarg);
                if (n < 0)
                {
                    printf("invalid argument (use -h for help)\n");
                    return EXIT_FAILURE;
                }
                break;
            }
            case 'r':
            {
                char *token;
                int64_t start, finish;
                token = strtok(optarg, ":");
                if (token) {
                    start = atoll(token);
                    token = strtok(NULL, ":");
                    if (token) {
                        finish = atoll(token);

                        if (start < 0 || finish < 0)
                        {
                            printf("invalid argument (use -h for help)\n");
                            return EXIT_FAILURE;
                        }
                        return primes_in_range(start, finish);
                    }
                }
                printf("invalid argument (use -h for help)\n");
                return EXIT_FAILURE;
            }
            case 't':
            {
                int64_t f;
                printf("%ld\n", is_prime(atoll(optarg), &f) ? 0L : f);
                return EXIT_SUCCESS;
            }
            case 'h':
                printf("-c, [--count] <number>        # Sets number of primes to generate\n");
                printf("-n, [--number] <number>       # Sets the initial value to begin generating from\n");
                printf("-r, [--range] <start>:<end>   # Sets start and end points to generate all primes between\n");
                printf("-t, [--test] <number>         # Test if a number if prime. Returns 0 if prime, otherwise the first factor found\n");
                printf("-h, [--help]                  # Displays this help message\n");
                printf("\n");
                printf("All arguments must be positive numbers, as a all primes are positive by definition.\n");
                return EXIT_SUCCESS;
            default: return EXIT_FAILURE;
        }
    }

    if (!process) {
        printf("no operation specified (use -h for help)\n");
        return EXIT_FAILURE;
    }

    int64_t count = 0;
    do {
        if (is_prime(n, NULL)) {
            printf("%ld\n", n);
            count++;
        }
        n++;
    } while (count < c);

    return EXIT_SUCCESS;
}