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

/*
 * There is different solutions but this one is simple and easy to remember
 * (it's very similar to !(a & (a - 1)) used to check if a is a power of two).
 * If you want to look smart, say that this solution is perfect for embedded
 * systems.
 *
 * Another simple one, more optimized, is to have a lookup table that gives you
 * the number of 1 bit in each integer for integers from 0 to 255 (or 0 to
 * 2**16 - 1).
 */

unsigned int
bit_count(const unsigned char *a, size_t n)
{
    unsigned int c = 0;

    while (n--)
        for (unsigned char v = a[n]; v; v &= v - 1)
            c++;

    return c;
}

int
main(void)
{
    int a[] = { 1, 2, 3, 4 };
    int b = 42;

    printf("%d\n", bit_count((unsigned char *)a, sizeof(a)));
    printf("%d\n", bit_count((unsigned char *)&b, sizeof(b)));

    return 0;
}