80 lines
1.7 KiB
C
80 lines
1.7 KiB
C
#include <tommath.h>
|
|
#ifdef BN_MP_DIV_3_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
|
|
*/
|
|
|
|
/* divide by three (based on routine from MPI and the GMP manual) */
|
|
int
|
|
mp_div_3 (mp_int * a, mp_int *c, mp_digit * d)
|
|
{
|
|
mp_int q;
|
|
mp_word w, t;
|
|
mp_digit b;
|
|
int res, ix;
|
|
|
|
/* b = 2**DIGIT_BIT / 3 */
|
|
b = (((mp_word)1) << ((mp_word)DIGIT_BIT)) / ((mp_word)3);
|
|
|
|
if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
|
|
return res;
|
|
}
|
|
|
|
q.used = a->used;
|
|
q.sign = a->sign;
|
|
w = 0;
|
|
for (ix = a->used - 1; ix >= 0; ix--) {
|
|
w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]);
|
|
|
|
if (w >= 3) {
|
|
/* multiply w by [1/3] */
|
|
t = (w * ((mp_word)b)) >> ((mp_word)DIGIT_BIT);
|
|
|
|
/* now subtract 3 * [w/3] from w, to get the remainder */
|
|
w -= t+t+t;
|
|
|
|
/* fixup the remainder as required since
|
|
* the optimization is not exact.
|
|
*/
|
|
while (w >= 3) {
|
|
t += 1;
|
|
w -= 3;
|
|
}
|
|
} else {
|
|
t = 0;
|
|
}
|
|
q.dp[ix] = (mp_digit)t;
|
|
}
|
|
|
|
/* [optional] store the remainder */
|
|
if (d != NULL) {
|
|
*d = (mp_digit)w;
|
|
}
|
|
|
|
/* [optional] store the quotient */
|
|
if (c != NULL) {
|
|
mp_clamp(&q);
|
|
mp_exch(&q, c);
|
|
}
|
|
mp_clear(&q);
|
|
|
|
return res;
|
|
}
|
|
|
|
#endif
|
|
|
|
/* $Source$ */
|
|
/* $Revision$ */
|
|
/* $Date$ */
|