int BN_rshift(BIGNUM *r, const BIGNUM *a, int n)
{
    int i, j, nw, lb, rb;
    BN_ULONG *t, *f;
    BN_ULONG l, tmp;

    bn_check_top(r);
    bn_check_top(a);

    if (n < 0) {
        BNerr(BN_F_BN_RSHIFT, BN_R_INVALID_SHIFT);
        return 0;
    }

    nw = n / BN_BITS2;
    rb = n % BN_BITS2;
    lb = BN_BITS2 - rb;
    if (nw >= a->top || a->top == 0) {
        BN_zero(r);
        return 1;
    }
    i = (BN_num_bits(a) - n + (BN_BITS2 - 1)) / BN_BITS2;
    if (r != a) {
        if (bn_wexpand(r, i) == NULL)
            return 0;
        r->neg = a->neg;
    } else {
        if (n == 0)
            return 1;           /* or the copying loop will go berserk */
    }

    f = &(a->d[nw]);
    t = r->d;
    j = a->top - nw;
    r->top = i;

    if (rb == 0) {
        for (i = j; i != 0; i--)
            *(t++) = *(f++);
    } else {
        l = *(f++);
        for (i = j - 1; i != 0; i--) {
            tmp = (l >> rb) & BN_MASK2;
            l = *(f++);
            *(t++) = (tmp | (l << lb)) & BN_MASK2;
        }
        if ((l = (l >> rb) & BN_MASK2))
            *(t) = l;
    }
    if (!r->top)
        r->neg = 0; /* don't allow negative zero */
    bn_check_top(r);
    return 1;
}