BigDecimal#n_significant_digits

static VALUE
BigDecimal_n_significant_digits(VALUE self)
{
    ENTER(1);

    Real *p;
    GUARD_OBJ(p, GetVpValue(self, 1));

    ssize_t n = p->Prec;
    while (n > 0 && p->frac[n-1] == 0) --n;
    if (n <= 0) {
        return INT2FIX(0);
    }

    int nlz, ntz;

    BDIGIT x = p->frac[0];
    for (nlz = BASE_FIG; x > 0; x /= 10) --nlz;

    x = p->frac[n-1];
    for (ntz = 0; x > 0 && x % 10 == 0; x /= 10) ++ntz;

    ssize_t n_digits = BASE_FIG * n - nlz - ntz;
    return SSIZET2NUM(n_digits);
}