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annotate libtommath/bn_s_mp_sub.c @ 1691:2d3745d58843

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try rearrange travis build matrix
author Matt Johnston <matt@ucc.asn.au>
date Tue, 26 May 2020 23:27:26 +0800
parents f52919ffd3b1
children 1051e4eea25a
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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 *
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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 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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16 int s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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17 {
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18 int olduse, res, min, max;
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20 /* find sizes */
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21 min = b->used;
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22 max = a->used;
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24 /* init result */
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25 if (c->alloc < max) {
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26 if ((res = mp_grow(c, max)) != MP_OKAY) {
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27 return res;
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28 }
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29 }
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30 olduse = c->used;
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31 c->used = max;
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32
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33 {
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34 mp_digit u, *tmpa, *tmpb, *tmpc;
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35 int i;
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37 /* alias for digit pointers */
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38 tmpa = a->dp;
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39 tmpb = b->dp;
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40 tmpc = c->dp;
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41
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42 /* set carry to zero */
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43 u = 0;
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44 for (i = 0; i < min; i++) {
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45 /* T[i] = A[i] - B[i] - U */
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46 *tmpc = (*tmpa++ - *tmpb++) - u;
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48 /* U = carry bit of T[i]
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49 * Note this saves performing an AND operation since
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50 * if a carry does occur it will propagate all the way to the
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51 * MSB. As a result a single shift is enough to get the carry
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52 */
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53 u = *tmpc >> (((size_t)CHAR_BIT * sizeof(mp_digit)) - 1u);
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54
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55 /* Clear carry from T[i] */
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56 *tmpc++ &= MP_MASK;
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57 }
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58
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59 /* now copy higher words if any, e.g. if A has more digits than B */
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60 for (; i < max; i++) {
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61 /* T[i] = A[i] - U */
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62 *tmpc = *tmpa++ - u;
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63
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64 /* U = carry bit of T[i] */
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65 u = *tmpc >> (((size_t)CHAR_BIT * sizeof(mp_digit)) - 1u);
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66
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67 /* Clear carry from T[i] */
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68 *tmpc++ &= MP_MASK;
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69 }
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71 /* clear digits above used (since we may not have grown result above) */
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72 for (i = c->used; i < olduse; i++) {
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73 *tmpc++ = 0;
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74 }
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75 }
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76
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77 mp_clamp(c);
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78 return MP_OKAY;
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79 }
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80
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81 #endif
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83 /* ref: HEAD -> master, tag: v1.1.0 */
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84 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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85 /* commit time: 2019-01-28 20:32:32 +0100 */