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annotate libtommath/bn_mp_div_3.c @ 1707:41a0ff8d5a89

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void return types for curve25519
author Matt Johnston <matt@ucc.asn.au>
date Sat, 30 May 2020 00:05:49 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_DIV_3_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 /* divide by three (based on routine from MPI and the GMP manual) */
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7 mp_err mp_div_3(const mp_int *a, mp_int *c, mp_digit *d)
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8 {
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9 mp_int q;
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10 mp_word w, t;
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11 mp_digit b;
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12 mp_err err;
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13 int ix;
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14
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15 /* b = 2**MP_DIGIT_BIT / 3 */
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16 b = ((mp_word)1 << (mp_word)MP_DIGIT_BIT) / (mp_word)3;
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17
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18 if ((err = mp_init_size(&q, a->used)) != MP_OKAY) {
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19 return err;
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20 }
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21
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22 q.used = a->used;
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23 q.sign = a->sign;
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24 w = 0;
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25 for (ix = a->used - 1; ix >= 0; ix--) {
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26 w = (w << (mp_word)MP_DIGIT_BIT) | (mp_word)a->dp[ix];
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27
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28 if (w >= 3u) {
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29 /* multiply w by [1/3] */
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30 t = (w * (mp_word)b) >> (mp_word)MP_DIGIT_BIT;
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31
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32 /* now subtract 3 * [w/3] from w, to get the remainder */
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33 w -= t+t+t;
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34
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35 /* fixup the remainder as required since
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36 * the optimization is not exact.
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37 */
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38 while (w >= 3u) {
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39 t += 1u;
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40 w -= 3u;
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41 }
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42 } else {
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43 t = 0;
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44 }
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45 q.dp[ix] = (mp_digit)t;
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46 }
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47
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48 /* [optional] store the remainder */
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49 if (d != NULL) {
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50 *d = (mp_digit)w;
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51 }
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53 /* [optional] store the quotient */
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54 if (c != NULL) {
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55 mp_clamp(&q);
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56 mp_exch(&q, c);
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57 }
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58 mp_clear(&q);
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59
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60 return err;
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61 }
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63 #endif