BIGNUM *BN_copy(BIGNUM *a, const BIGNUM *b)
{
    int i;
    BN_ULONG *A;
    const BN_ULONG *B;

    bn_check_top(b);

    if (a == b)
        return (a);
    if (bn_wexpand(a, b->top) == NULL)
        return (NULL);

#if 1
    A = a->d;
    B = b->d;
    for (i = b->top >> 2; i > 0; i--, A += 4, B += 4) {
        BN_ULONG a0, a1, a2, a3;
        a0 = B[0];
        a1 = B[1];
        a2 = B[2];
        a3 = B[3];
        A[0] = a0;
        A[1] = a1;
        A[2] = a2;
        A[3] = a3;
    }
    /* ultrix cc workaround, see comments in bn_expand_internal */
    switch (b->top & 3) {
    case 3:
        A[2] = B[2];
    case 2:
        A[1] = B[1];
    case 1:
        A[0] = B[0];
    case 0:;
    }
#else
    memcpy(a->d, b->d, sizeof(b->d[0]) * b->top);
#endif

    a->top = b->top;
    a->neg = b->neg;
    bn_check_top(a);
    return (a);
}