int BN_sqr(BIGNUM *r, const BIGNUM *a, BN_CTX *ctx)
{
    int max, al;
    int ret = 0;
    BIGNUM *tmp, *rr;

    bn_check_top(a);

    al = a->top;
    if (al <= 0) {
        r->top = 0;
        r->neg = 0;
        return 1;
    }

    BN_CTX_start(ctx);
    rr = (a != r) ? r : BN_CTX_get(ctx);
    tmp = BN_CTX_get(ctx);
    if (rr == NULL || tmp == NULL)
        goto err;

    max = 2 * al;               /* Non-zero (from above) */
    if (bn_wexpand(rr, max) == NULL)
        goto err;

    if (al == 4) {
#ifndef BN_SQR_COMBA
        BN_ULONG t[8];
        bn_sqr_normal(rr->d, a->d, 4, t);
#else
        bn_sqr_comba4(rr->d, a->d);
#endif
    } else if (al == 8) {
#ifndef BN_SQR_COMBA
        BN_ULONG t[16];
        bn_sqr_normal(rr->d, a->d, 8, t);
#else
        bn_sqr_comba8(rr->d, a->d);
#endif
    } else {
#if defined(BN_RECURSION)
        if (al < BN_SQR_RECURSIVE_SIZE_NORMAL) {
            BN_ULONG t[BN_SQR_RECURSIVE_SIZE_NORMAL * 2];
            bn_sqr_normal(rr->d, a->d, al, t);
        } else {
            int j, k;

            j = BN_num_bits_word((BN_ULONG)al);
            j = 1 << (j - 1);
            k = j + j;
            if (al == j) {
                if (bn_wexpand(tmp, k * 2) == NULL)
                    goto err;
                bn_sqr_recursive(rr->d, a->d, al, tmp->d);
            } else {
                if (bn_wexpand(tmp, max) == NULL)
                    goto err;
                bn_sqr_normal(rr->d, a->d, al, tmp->d);
            }
        }
#else
        if (bn_wexpand(tmp, max) == NULL)
            goto err;
        bn_sqr_normal(rr->d, a->d, al, tmp->d);
#endif
    }

    rr->neg = 0;
    rr->top = max;
    bn_correct_top(rr);
    if (r != rr && BN_copy(r, rr) == NULL)
        goto err;

    ret = 1;
 err:
    bn_check_top(rr);
    bn_check_top(tmp);
    BN_CTX_end(ctx);
    return ret;
}