### view libtommath/bn_mp_reduce_2k.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_REDUCE_2K_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
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://math.libtomcrypt.com
*/

/* reduces a modulo n where n is of the form 2**p - d */
int mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d)
{
mp_int q;
int    p, res;

if ((res = mp_init(&q)) != MP_OKAY) {
return res;
}

p = mp_count_bits(n);
top:
/* q = a/2**p, a = a mod 2**p */
if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
goto ERR;
}

if (d != 1) {
/* q = q * d */
if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) {
goto ERR;
}
}

/* a = a + q */
if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
goto ERR;
}

if (mp_cmp_mag(a, n) != MP_LT) {
s_mp_sub(a, n, a);
goto top;
}

ERR:
mp_clear(&q);
return res;
}

#endif

/* \$Source: /cvs/libtom/libtommath/bn_mp_reduce_2k.c,v \$ */
/* \$Revision: 1.3 \$ */
/* \$Date: 2006/03/31 14:18:44 \$ */
```