90 lines
2.2 KiB
C
90 lines
2.2 KiB
C
#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
|
|
* additional optimizations in place.
|
|
*
|
|
* The library is free for all purposes without any express
|
|
* guarantee it works.
|
|
*
|
|
* Tom St Denis, tomstdenis@gmail.com, http://libtom.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] */
|
|
*tmpc++ &= MP_MASK;
|
|
}
|
|
|
|
/* 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] */
|
|
*tmpc++ &= MP_MASK;
|
|
}
|
|
|
|
/* clear digits above used (since we may not have grown result above) */
|
|
for (i = c->used; i < olduse; i++) {
|
|
*tmpc++ = 0;
|
|
}
|
|
}
|
|
|
|
mp_clamp (c);
|
|
return MP_OKAY;
|
|
}
|
|
|
|
#endif
|
|
|
|
/* $Source: /cvs/libtom/libtommath/bn_s_mp_sub.c,v $ */
|
|
/* $Revision: 1.4 $ */
|
|
/* $Date: 2006/12/28 01:25:13 $ */
|