### view libtommath/bn_mp_div_3.c @ 709:abd99ecd7ec2

use an empty writebuf rather than a NULL one
author Matt Johnston Wed, 20 Mar 2013 22:31:07 +0800 5ff8218bcee9 60fc6476e044
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```#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, [email protected], http://math.libtomcrypt.com
*/

/* 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: /cvs/libtom/libtommath/bn_mp_div_3.c,v \$ */
/* \$Revision: 1.3 \$ */
/* \$Date: 2006/03/31 14:18:44 \$ */
```