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annotate libtommath/bn_mp_div_2d.c @ 1720:e2e4929d057b DROPBEAR_2020.79

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can't call "make lint" without configure
author Matt Johnston <matt@ucc.asn.au>
date Mon, 15 Jun 2020 23:38:08 +0800
parents 1051e4eea25a
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_DIV_2D_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 /* shift right by a certain bit count (store quotient in c, optional remainder in d) */
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7 mp_err mp_div_2d(const mp_int *a, int b, mp_int *c, mp_int *d)
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8 {
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9 mp_digit D, r, rr;
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10 int x;
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11 mp_err err;
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12
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13 /* if the shift count is <= 0 then we do no work */
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14 if (b <= 0) {
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15 err = mp_copy(a, c);
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16 if (d != NULL) {
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17 mp_zero(d);
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18 }
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19 return err;
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20 }
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21
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22 /* copy */
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23 if ((err = mp_copy(a, c)) != MP_OKAY) {
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24 return err;
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25 }
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26 /* 'a' should not be used after here - it might be the same as d */
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27
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28 /* get the remainder */
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29 if (d != NULL) {
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30 if ((err = mp_mod_2d(a, b, d)) != MP_OKAY) {
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31 return err;
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32 }
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33 }
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34
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35 /* shift by as many digits in the bit count */
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36 if (b >= MP_DIGIT_BIT) {
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37 mp_rshd(c, b / MP_DIGIT_BIT);
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38 }
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39
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40 /* shift any bit count < MP_DIGIT_BIT */
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41 D = (mp_digit)(b % MP_DIGIT_BIT);
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42 if (D != 0u) {
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43 mp_digit *tmpc, mask, shift;
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45 /* mask */
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46 mask = ((mp_digit)1 << D) - 1uL;
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48 /* shift for lsb */
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49 shift = (mp_digit)MP_DIGIT_BIT - D;
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51 /* alias */
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52 tmpc = c->dp + (c->used - 1);
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54 /* carry */
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55 r = 0;
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56 for (x = c->used - 1; x >= 0; x--) {
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57 /* get the lower bits of this word in a temp */
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58 rr = *tmpc & mask;
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59
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60 /* shift the current word and mix in the carry bits from the previous word */
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61 *tmpc = (*tmpc >> D) | (r << shift);
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62 --tmpc;
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63
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64 /* set the carry to the carry bits of the current word found above */
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65 r = rr;
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66 }
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67 }
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68 mp_clamp(c);
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69 return MP_OKAY;
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70 }
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71 #endif