tommath/bn_s_mp_sub.c

90 lines
2.1 KiB
C
Raw Normal View History

#include <tommath_private.h>
2004-10-29 18:07:18 -04:00
#ifdef BN_S_MP_SUB_C
2003-02-28 11:08:34 -05:00
/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
2003-08-04 21:24:44 -04:00
* LibTomMath is a library that provides multiple-precision
2003-02-28 11:08:34 -05:00
* integer arithmetic as well as number theoretic functionality.
*
2003-08-04 21:24:44 -04:00
* The library was designed directly after the MPI library by
2003-02-28 11:08:34 -05:00
* 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, tstdenis82@gmail.com, http://libtom.org
2003-02-28 11:08:34 -05:00
*/
2003-05-17 08:33:54 -04:00
/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
2003-02-28 11:08:34 -05:00
int
s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
{
2003-02-28 11:09:08 -05:00
int olduse, res, min, max;
2003-02-28 11:08:34 -05:00
/* 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;
2003-02-28 11:09:08 -05:00
{
register mp_digit u, *tmpa, *tmpb, *tmpc;
register int i;
2003-02-28 11:08:34 -05:00
2003-02-28 11:09:08 -05:00
/* alias for digit pointers */
tmpa = a->dp;
tmpb = b->dp;
tmpc = c->dp;
2003-02-28 11:08:34 -05:00
2003-02-28 11:09:08 -05:00
/* set carry to zero */
u = 0;
for (i = 0; i < min; i++) {
/* T[i] = A[i] - B[i] - U */
*tmpc = *tmpa++ - *tmpb++ - u;
2003-05-17 08:33:54 -04:00
/* U = carry bit of T[i]
* Note this saves performing an AND operation since
2003-02-28 11:09:08 -05:00
* if a carry does occur it will propagate all the way to the
2003-05-29 09:35:26 -04:00
* MSB. As a result a single shift is enough to get the carry
2003-02-28 11:09:08 -05:00
*/
2003-05-17 08:33:54 -04:00
u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
2003-02-28 11:09:08 -05:00
/* Clear carry from T[i] */
*tmpc++ &= MP_MASK;
}
2003-02-28 11:08:34 -05:00
2003-02-28 11:09:08 -05:00
/* now copy higher words if any, e.g. if A has more digits than B */
2003-02-28 11:08:34 -05:00
for (; i < max; i++) {
/* T[i] = A[i] - U */
2003-02-28 11:09:08 -05:00
*tmpc = *tmpa++ - u;
2003-02-28 11:08:34 -05:00
/* U = carry bit of T[i] */
2003-05-17 08:33:54 -04:00
u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
2003-02-28 11:08:34 -05:00
/* Clear carry from T[i] */
2003-02-28 11:09:08 -05:00
*tmpc++ &= MP_MASK;
2003-02-28 11:08:34 -05:00
}
2003-02-28 11:09:08 -05:00
/* clear digits above used (since we may not have grown result above) */
for (i = c->used; i < olduse; i++) {
*tmpc++ = 0;
}
2003-02-28 11:08:34 -05:00
}
mp_clamp (c);
return MP_OKAY;
}
2003-05-29 09:35:26 -04:00
2004-10-29 18:07:18 -04:00
#endif
2005-08-01 12:37:28 -04:00
/* $Source$ */
/* $Revision$ */
/* $Date$ */