### view libtommath/bn_s_mp_sub.c @ 350:01e4180895ba

merge of '9a4e042fd565f46141e81e0c1ab90260303348fe' and 'bea3887a5875cf3ab8a1331e15e698b37b61fe37'
author Matt Johnston Mon, 07 Aug 2006 13:41:23 +0000 eed26cff980b 5ff8218bcee9
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line source
#include <tommath.h>
#ifdef BN_S_MP_SUB_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is a library that provides multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library was designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://math.libtomcrypt.org
*/

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

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

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

{
register mp_digit u, *tmpa, *tmpb, *tmpc;
register 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_digit)(CHAR_BIT * sizeof (mp_digit) - 1));

/* 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_digit)(CHAR_BIT * sizeof (mp_digit) - 1));

/* Clear carry from T[i] */