### view libtommath/bn_s_mp_sub.c @ 1734:73646de50f13DROPBEAR_2020.80

version 2020.80
author Matt Johnston Fri, 26 Jun 2020 21:45:59 +0800 1051e4eea25a
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```#include "tommath_private.h"
#ifdef BN_S_MP_SUB_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
{
int    olduse, min, max;
mp_err err;

/* find sizes */
min = b->used;
max = a->used;

/* init result */
if (c->alloc < max) {
if ((err = mp_grow(c, max)) != MP_OKAY) {
return err;
}
}
olduse = c->used;
c->used = max;

{
mp_digit u, *tmpa, *tmpb, *tmpc;
int i;

/* alias for digit pointers */
tmpa = a->dp;
tmpb = b->dp;
tmpc = c->dp;

/* set carry to zero */
u = 0;
for (i = 0; i < min; i++) {
/* T[i] = A[i] - B[i] - U */
*tmpc = (*tmpa++ - *tmpb++) - u;

/* U = carry bit of T[i]
* Note this saves performing an AND operation since
* if a carry does occur it will propagate all the way to the
* MSB.  As a result a single shift is enough to get the carry
*/
u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);

/* Clear carry from T[i] */
}

/* now copy higher words if any, e.g. if A has more digits than B  */
for (; i < max; i++) {
/* T[i] = A[i] - U */
*tmpc = *tmpa++ - u;

/* U = carry bit of T[i] */
u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);

/* Clear carry from T[i] */