check whether a number is a prime or not

prime.c

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

static void error(const char *message, ...) {
    va_list args;

    fprintf(stderr, "\e[31mError:\e[0m ");

    va_start(args, message);
    vfprintf(stderr, message, args);
    va_end(args);

    fprintf(stderr, "\n");

    exit(1);
}

// string must be null-terminated
static uint64_t string_to_uint64(const char *string) {
    uint64_t result = 0;
    int index = 0;

    const int MAX_DIGITS = 20; // 18,446,744,073,709,551,615

    while (1) {
        char c = string[index++];

        switch (c) {
            case '\0': {
                if (index == 1) {
                    error("string is empty");
                } else {
                    return result;
                }
            }
            case '0' ... '9': {
                if (index > MAX_DIGITS) {
                    error("too many digits, uint64_t overflow");
                }

                // TODO check overflow for max value

                result = result * 10 + c - '0';
                break;
            }
            default: {
                error("unexpected 'x%02X' at index %d", c, index - 1);
            }
        }
    }
}

int is_prime(uint64_t number) {
    switch (number) {
        case 0: case 1: case 4: case 6: case 8: case 9:
            return 0; // not
        case 2: case 3: case 5: case 7:
            return 1; // prime
    }

    if (number % 2 == 0) {
        return 0; // even, not a prime
    }

    // check other divisors
    // TODO requires heavy optimization
    // += 2 because all other divisors are even

    for (uint64_t divisor = 3; divisor < number; divisor += 2) {
        if (number % divisor == 0) {
            printf("divisor: %llu\n", divisor);
            return 0; // not
        }
    }

    return 1; // prime
}

int main(int argc, char **argv) {
    // argv[0] is program name
    if (argc != 2) {
        error("Usage: prime 123");
    }

    uint64_t number = string_to_uint64(argv[1]);

    if (is_prime(number)) {
        printf("number %llu is prime\n", number);
    } else {
        printf("number %llu is NOT a prime\n", number);
    }
}