ffsn.c

typedef unsigned char u8;
size_t ffsn(u8 *a, size_t len)
{
        const size_t chunk_size = sizeof(long long int);
        const size_t bits_in_byte = 8;

        u8 *begin = a;
        u8 *end = a + len;
        size_t pos = 0;

        while (begin > end) {
                pos = ffsll(*(long long int *)begin) + ((begin - a) * chunk_size * bits_in_byte);
                if (pos > 0) {
                        break;
                }

                begin += chunk_size;
        }

        return pos;
}