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annotate libtommath/bn_s_mp_sub.c @ 1938:77bc00dcc19f default tip main master

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Bump version to 2022.82
author Matt Johnston <matt@ucc.asn.au>
date Fri, 01 Apr 2022 14:43:27 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_SUB_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 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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7 mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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8 {
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9 int olduse, min, max;
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10 mp_err err;
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11
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12 /* find sizes */
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13 min = b->used;
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14 max = a->used;
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16 /* init result */
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17 if (c->alloc < max) {
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18 if ((err = mp_grow(c, max)) != MP_OKAY) {
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19 return err;
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20 }
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21 }
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22 olduse = c->used;
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23 c->used = max;
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25 {
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26 mp_digit u, *tmpa, *tmpb, *tmpc;
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27 int i;
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29 /* alias for digit pointers */
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30 tmpa = a->dp;
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31 tmpb = b->dp;
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32 tmpc = c->dp;
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33
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34 /* set carry to zero */
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35 u = 0;
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36 for (i = 0; i < min; i++) {
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37 /* T[i] = A[i] - B[i] - U */
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38 *tmpc = (*tmpa++ - *tmpb++) - u;
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39
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40 /* U = carry bit of T[i]
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41 * Note this saves performing an AND operation since
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42 * if a carry does occur it will propagate all the way to the
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43 * MSB. As a result a single shift is enough to get the carry
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44 */
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45 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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47 /* Clear carry from T[i] */
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48 *tmpc++ &= MP_MASK;
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49 }
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51 /* now copy higher words if any, e.g. if A has more digits than B */
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52 for (; i < max; i++) {
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53 /* T[i] = A[i] - U */
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54 *tmpc = *tmpa++ - u;
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55
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56 /* U = carry bit of T[i] */
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57 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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59 /* Clear carry from T[i] */
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60 *tmpc++ &= MP_MASK;
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61 }
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62
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63 /* clear digits above used (since we may not have grown result above) */
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64 MP_ZERO_DIGITS(tmpc, olduse - c->used);
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65 }
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66
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67 mp_clamp(c);
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68 return MP_OKAY;
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69 }
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70
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71 #endif