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annotate libtommath/bn_mp_sub_d.c @ 1792:2bf1e97ba3cd

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Update INSTALL (#113) Make Git/Mercurial instructions easier to understand
author Xenhat <commits@xenh.at>
date Thu, 17 Dec 2020 04:35:48 -0500
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_SUB_D_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 /* single digit subtraction */
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7 mp_err mp_sub_d(const mp_int *a, mp_digit b, mp_int *c)
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8 {
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9 mp_digit *tmpa, *tmpc;
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10 mp_err err;
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11 int ix, oldused;
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12
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13 /* grow c as required */
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14 if (c->alloc < (a->used + 1)) {
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15 if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
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16 return err;
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17 }
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18 }
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19
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20 /* if a is negative just do an unsigned
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21 * addition [with fudged signs]
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22 */
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23 if (a->sign == MP_NEG) {
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24 mp_int a_ = *a;
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25 a_.sign = MP_ZPOS;
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26 err = mp_add_d(&a_, b, c);
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27 c->sign = MP_NEG;
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28
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29 /* clamp */
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30 mp_clamp(c);
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31
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32 return err;
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33 }
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34
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35 /* setup regs */
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36 oldused = c->used;
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37 tmpa = a->dp;
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38 tmpc = c->dp;
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40 /* if a <= b simply fix the single digit */
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41 if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {
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42 if (a->used == 1) {
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43 *tmpc++ = b - *tmpa;
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44 } else {
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45 *tmpc++ = b;
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46 }
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47 ix = 1;
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49 /* negative/1digit */
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50 c->sign = MP_NEG;
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51 c->used = 1;
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52 } else {
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53 mp_digit mu = b;
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54
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55 /* positive/size */
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56 c->sign = MP_ZPOS;
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57 c->used = a->used;
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58
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59 /* subtract digits, mu is carry */
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60 for (ix = 0; ix < a->used; ix++) {
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61 *tmpc = *tmpa++ - mu;
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62 mu = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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63 *tmpc++ &= MP_MASK;
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64 }
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65 }
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66
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67 /* zero excess digits */
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68 MP_ZERO_DIGITS(tmpc, oldused - ix);
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69
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70 mp_clamp(c);
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71 return MP_OKAY;
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72 }
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74 #endif