/*

 * Copyright 1995-2026 The OpenSSL Project Authors. All Rights Reserved.

 *

 * Licensed under the Apache License 2.0 (the "License").  You may not use

 * this file except in compliance with the License.  You can obtain a copy

 * in the file LICENSE in the source distribution or at

 * https://www.openssl.org/source/license.html

 */

#include "internal/cryptlib.h"

#include "bn_local.h"

/* signed add of b to a. */

int BN_add(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)

{

    int ret, r_neg, cmp_res;

    bn_check_top(a);

    bn_check_top(b);

    if (a->neg == b->neg) {

        r_neg = a->neg;

        ret = BN_uadd(r, a, b);

    } else {

        cmp_res = BN_ucmp(a, b);

        if (cmp_res > 0) {

            r_neg = a->neg;

            ret = BN_usub(r, a, b);

        } else if (cmp_res < 0) {

            r_neg = b->neg;

            ret = BN_usub(r, b, a);

        } else {

            r_neg = 0;

            BN_zero(r);

            ret = 1;

        }

    }

    r->neg = r_neg;

    bn_check_top(r);

    return ret;

}

/* signed sub of b from a. */

int BN_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)

{

    int ret, r_neg, cmp_res;

    bn_check_top(a);

    bn_check_top(b);

    if (a->neg != b->neg) {

        r_neg = a->neg;

        ret = BN_uadd(r, a, b);

    } else {

        cmp_res = BN_ucmp(a, b);

        if (cmp_res > 0) {

            r_neg = a->neg;

            ret = BN_usub(r, a, b);

        } else if (cmp_res < 0) {

            r_neg = !b->neg;

            ret = BN_usub(r, b, a);

        } else {

            r_neg = 0;

            BN_zero(r);

            ret = 1;

        }

    }

    r->neg = r_neg;

    bn_check_top(r);

    return ret;

}

/* unsigned add of b to a, r can be equal to a or b. */

int BN_uadd(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)

{

    int max, min, dif;

    const BN_ULONG *ap, *bp;

    BN_ULONG *rp, carry, t1, t2;

    bn_check_top(a);

    bn_check_top(b);

    if (a->top < b->top) {

        const BIGNUM *tmp;

        tmp = a;

        a = b;

        b = tmp;

    }

    max = a->top;

    min = b->top;

    dif = max - min;

    if (bn_wexpand(r, max + 1) == NULL)

        return 0;

    r->top = max;

    if (max == 0)

        goto end;

    ap = a->d;

    bp = b->d;

    rp = r->d;

    carry = bn_add_words(rp, ap, bp, min);

    rp += min;

    ap += min;

    while (dif) {

        dif--;

        t1 = *(ap++);

        t2 = (t1 + carry) & BN_MASK2;

        *(rp++) = t2;

        carry &= (t2 == 0);

    }

    *rp = carry;

    r->top += carry;

end:

    r->neg = 0;

    bn_check_top(r);

    return 1;

}

/* unsigned subtraction of b from a, a must be larger than b. */

int BN_usub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)

{

    int max, min, dif;

    BN_ULONG t1, t2, borrow, *rp;

    const BN_ULONG *ap, *bp;

    bn_check_top(a);

    bn_check_top(b);

    max = a->top;

    min = b->top;

    dif = max - min;

    if (dif < 0) { /* hmm... should not be happening */

        ERR_raise(ERR_LIB_BN, BN_R_ARG2_LT_ARG3);

        return 0;

    }

    if (bn_wexpand(r, max) == NULL)

        return 0;

    if (max == 0)

        goto end;

    ap = a->d;

    bp = b->d;

    rp = r->d;

    borrow = bn_sub_words(rp, ap, bp, min);

    ap += min;

    rp += min;

    while (dif) {

        dif--;

        t1 = *(ap++);

        t2 = (t1 - borrow) & BN_MASK2;

        *(rp++) = t2;

        borrow &= (t1 == 0);

    }

    while (max && *--rp == 0)

        max--;

end:

    r->top = max;

    r->neg = 0;

    bn_pollute(r);

    return 1;

}