tommath/bn_mp_div_3.c

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#include <tommath.h>
#ifdef BN_MP_DIV_3_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* 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
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*/
/* 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]);
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if (w >= 3) {
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/* multiply w by [1/3] */
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t = (w * ((mp_word)b)) >> ((mp_word)DIGIT_BIT);
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/* now subtract 3 * [w/3] from w, to get the remainder */
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w -= t+t+t;
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/* fixup the remainder as required since
* the optimization is not exact.
*/
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while (w >= 3) {
t += 1;
w -= 3;
}
} else {
t = 0;
}
q.dp[ix] = (mp_digit)t;
}
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/* [optional] store the remainder */
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if (d != NULL) {
*d = (mp_digit)w;
}
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/* [optional] store the quotient */
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if (c != NULL) {
mp_clamp(&q);
mp_exch(&q, c);
}
mp_clear(&q);
return res;
}
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#endif
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/* $Source$ */
/* $Revision$ */
/* $Date$ */