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80 lines
1.8 KiB
C
80 lines
1.8 KiB
C
#include <tommath.h>
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#ifdef BN_MP_DIV_3_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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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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*
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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
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
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*/
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/* divide by three (based on routine from MPI and the GMP manual) */
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int
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mp_div_3 (mp_int * a, mp_int *c, mp_digit * d)
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{
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mp_int q;
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mp_word w, t;
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mp_digit b;
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int res, ix;
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/* b = 2**DIGIT_BIT / 3 */
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b = (((mp_word)1) << ((mp_word)DIGIT_BIT)) / ((mp_word)3);
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if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
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return res;
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}
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q.used = a->used;
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q.sign = a->sign;
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w = 0;
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for (ix = a->used - 1; ix >= 0; ix--) {
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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
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* the optimization is not exact.
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*/
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while (w >= 3) {
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t += 1;
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w -= 3;
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}
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} else {
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t = 0;
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}
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q.dp[ix] = (mp_digit)t;
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}
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/* [optional] store the remainder */
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if (d != NULL) {
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*d = (mp_digit)w;
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}
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/* [optional] store the quotient */
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if (c != NULL) {
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mp_clamp(&q);
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mp_exch(&q, c);
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}
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mp_clear(&q);
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return res;
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}
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#endif
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/* $Source: /cvs/libtom/libtommath/bn_mp_div_3.c,v $ */
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/* $Revision: 1.3 $ */
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/* $Date: 2006/03/31 14:18:44 $ */
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