tommath/bn_mp_rshd.c

73 lines
1.6 KiB
C
Raw Normal View History

2004-10-29 18:07:18 -04:00
#include <tommath.h>
#ifdef BN_MP_RSHD_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.
*
2003-03-12 21:11:11 -05:00
* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
2003-02-28 11:08:34 -05:00
*/
/* shift right a certain amount of digits */
2003-12-24 13:59:22 -05:00
void mp_rshd (mp_int * a, int b)
2003-02-28 11:08:34 -05:00
{
2003-02-28 11:09:08 -05:00
int x;
2003-02-28 11:08:34 -05:00
/* if b <= 0 then ignore it */
if (b <= 0) {
return;
}
/* if b > used then simply zero it and return */
2003-05-17 08:33:54 -04:00
if (a->used <= b) {
2003-02-28 11:08:34 -05:00
mp_zero (a);
return;
}
2003-03-12 21:11:11 -05:00
{
2003-05-29 09:35:26 -04:00
register mp_digit *bottom, *top;
2003-03-12 21:11:11 -05:00
/* shift the digits down */
2003-05-29 09:35:26 -04:00
/* bottom */
bottom = a->dp;
2003-03-22 10:10:20 -05:00
2003-05-29 09:35:26 -04:00
/* top [offset into digits] */
top = a->dp + b;
2003-03-22 10:10:20 -05:00
2003-05-17 08:33:54 -04:00
/* this is implemented as a sliding window where
* the window is b-digits long and digits from
* the top of the window are copied to the bottom
2003-03-12 21:11:11 -05:00
*
* e.g.
2003-03-22 10:10:20 -05:00
2003-03-12 21:11:11 -05:00
b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
/\ | ---->
\-------------------/ ---->
2003-03-22 10:10:20 -05:00
*/
2003-03-12 21:11:11 -05:00
for (x = 0; x < (a->used - b); x++) {
2003-05-29 09:35:26 -04:00
*bottom++ = *top++;
2003-03-12 21:11:11 -05:00
}
2003-02-28 11:08:34 -05:00
2003-03-12 21:11:11 -05:00
/* zero the top digits */
for (; x < a->used; x++) {
2003-05-29 09:35:26 -04:00
*bottom++ = 0;
2003-03-12 21:11:11 -05:00
}
2003-02-28 11:08:34 -05:00
}
2003-05-29 09:35:26 -04:00
/* remove excess digits */
a->used -= b;
2003-02-28 11:08:34 -05:00
}
2004-10-29 18:07:18 -04:00
#endif
2005-08-01 12:37:28 -04:00
/* $Source$ */
/* $Revision$ */
/* $Date$ */