prime-generator/Main.c

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

static char* data = NULL;

static int hasBit(size_t index) {
    return (data[index >> 3] & (1 << (index & 7))) == 0;
}

static void setBit(size_t index) {
    data[index >> 3] |= (char)(1 << (index & 7));
}

static size_t divideRound8(size_t i) {
    return (i >> 3) + ((i & 7) != 0);
}

int main(int argAmount, const char** args) {
    if(argAmount < 2) {
        if(argAmount > 0) {
            printf("%s <max_prime>\n", args[0]);
        } else {
            puts("... <max_prime>");
        }
        return 0;
    }
    char* endP = NULL;
    long maxRaw = strtol(args[1], &endP, 10);
    if(maxRaw <= 0 || *args[1] == '\0' || *endP != '\0') {
        puts("max prime should be a positive number");
        return 0;
    }
    size_t max = (size_t)maxRaw;

    size_t end = (max - 1) >> 1;
    data = calloc(divideRound8(end), 1);
    if(data == NULL) {
        puts("failed allocation");
        return 0;
    }

    int found = 0;
    if(max >= 2) {
        puts("2");
        found++;
    }
    for(size_t i = 0; i < end; i++) {
        if(hasBit(i)) {
            size_t prime = (i << 1) + 3;
            for(size_t k = i + prime; k < end; k += prime) {
                setBit(k);
            }
            printf("%zu\n", prime);
            found++;
        }
    }
    printf("found: %d\n", found);

    free(data);
    return 0;
}