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annotate libtommath/bn_mp_div_3.c @ 1661:6a6a0bac52f4

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author Matt Johnston <matt@ucc.asn.au>
date Sat, 14 Mar 2020 23:26:37 +0800
parents f52919ffd3b1
children 1051e4eea25a
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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 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * SPDX-License-Identifier: Unlicense
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13 */
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14
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15 /* divide by three (based on routine from MPI and the GMP manual) */
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16 int mp_div_3(const mp_int *a, mp_int *c, mp_digit *d)
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17 {
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18 mp_int q;
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19 mp_word w, t;
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20 mp_digit b;
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21 int res, ix;
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22
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23 /* b = 2**DIGIT_BIT / 3 */
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24 b = ((mp_word)1 << (mp_word)DIGIT_BIT) / (mp_word)3;
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25
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26 if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
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27 return res;
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28 }
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29
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30 q.used = a->used;
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31 q.sign = a->sign;
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32 w = 0;
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33 for (ix = a->used - 1; ix >= 0; ix--) {
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34 w = (w << (mp_word)DIGIT_BIT) | (mp_word)a->dp[ix];
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35
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36 if (w >= 3u) {
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37 /* multiply w by [1/3] */
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38 t = (w * (mp_word)b) >> (mp_word)DIGIT_BIT;
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39
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40 /* now subtract 3 * [w/3] from w, to get the remainder */
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41 w -= t+t+t;
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42
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43 /* fixup the remainder as required since
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44 * the optimization is not exact.
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45 */
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46 while (w >= 3u) {
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47 t += 1u;
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48 w -= 3u;
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49 }
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50 } else {
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51 t = 0;
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52 }
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53 q.dp[ix] = (mp_digit)t;
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54 }
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55
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56 /* [optional] store the remainder */
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57 if (d != NULL) {
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58 *d = (mp_digit)w;
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59 }
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60
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61 /* [optional] store the quotient */
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62 if (c != NULL) {
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63 mp_clamp(&q);
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64 mp_exch(&q, c);
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65 }
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66 mp_clear(&q);
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67
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68 return res;
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69 }
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71 #endif
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73 /* ref: HEAD -> master, tag: v1.1.0 */
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74 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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75 /* commit time: 2019-01-28 20:32:32 +0100 */