iw7-mod/deps/libtommath/s_mp_add.c

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2024-08-13 05:15:34 -04:00
#include "tommath_private.h"
#ifdef S_MP_ADD_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* low level addition, based on HAC pp.594, Algorithm 14.7 */
mp_err s_mp_add(const mp_int *a, const mp_int *b, mp_int *c)
{
int oldused, min, max, i;
mp_digit u;
mp_err err;
/* find sizes, we let |a| <= |b| which means we have to sort
* them. "x" will point to the input with the most digits
*/
if (a->used < b->used) {
MP_EXCH(const mp_int *, a, b);
}
min = b->used;
max = a->used;
/* init result */
if ((err = mp_grow(c, max + 1)) != MP_OKAY) {
return err;
}
/* get old used digit count and set new one */
oldused = c->used;
c->used = max + 1;
/* zero the carry */
u = 0;
for (i = 0; i < min; i++) {
/* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
c->dp[i] = a->dp[i] + b->dp[i] + u;
/* U = carry bit of T[i] */
u = c->dp[i] >> (mp_digit)MP_DIGIT_BIT;
/* take away carry bit from T[i] */
c->dp[i] &= MP_MASK;
}
/* now copy higher words if any, that is in A+B
* if A or B has more digits add those in
*/
if (min != max) {
for (; i < max; i++) {
/* T[i] = A[i] + U */
c->dp[i] = a->dp[i] + u;
/* U = carry bit of T[i] */
u = c->dp[i] >> (mp_digit)MP_DIGIT_BIT;
/* take away carry bit from T[i] */
c->dp[i] &= MP_MASK;
}
}
/* add carry */
c->dp[i] = u;
/* clear digits above oldused */
s_mp_zero_digs(c->dp + c->used, oldused - c->used);
mp_clamp(c);
return MP_OKAY;
}
#endif