Mercurial > dropbear

annotate libtommath/bn_mp_div_2.c @ 1734:73646de50f13 DROPBEAR_2020.80

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version 2020.80
author Matt Johnston <matt@ucc.asn.au>
date Fri, 26 Jun 2020 21:45:59 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_DIV_2_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
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4 /* SPDX-License-Identifier: Unlicense */
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6 /* b = a/2 */
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7 mp_err mp_div_2(const mp_int *a, mp_int *b)
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8 {
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9 int x, oldused;
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10 mp_digit r, rr, *tmpa, *tmpb;
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11 mp_err err;
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13 /* copy */
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14 if (b->alloc < a->used) {
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15 if ((err = mp_grow(b, a->used)) != MP_OKAY) {
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16 return err;
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17 }
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18 }
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20 oldused = b->used;
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21 b->used = a->used;
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22
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23 /* source alias */
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24 tmpa = a->dp + b->used - 1;
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26 /* dest alias */
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27 tmpb = b->dp + b->used - 1;
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29 /* carry */
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30 r = 0;
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31 for (x = b->used - 1; x >= 0; x--) {
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32 /* get the carry for the next iteration */
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33 rr = *tmpa & 1u;
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35 /* shift the current digit, add in carry and store */
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36 *tmpb-- = (*tmpa-- >> 1) | (r << (MP_DIGIT_BIT - 1));
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37
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38 /* forward carry to next iteration */
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39 r = rr;
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40 }
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41
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42 /* zero excess digits */
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43 MP_ZERO_DIGITS(b->dp + b->used, oldused - b->used);
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44
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45 b->sign = a->sign;
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46 mp_clamp(b);
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47 return MP_OKAY;
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48 }
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49 #endif