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annotate libtommath/bn_mp_sub_d.c @ 1680:5e763ad6e2e0

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author Matt Johnston <matt@ucc.asn.au>
date Sun, 24 May 2020 13:34:19 +0800
parents f52919ffd3b1
children 1051e4eea25a
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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 *
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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 /* single digit subtraction */
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16 int mp_sub_d(const mp_int *a, mp_digit b, mp_int *c)
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17 {
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18 mp_digit *tmpa, *tmpc, mu;
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19 int res, ix, oldused;
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20
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21 /* grow c as required */
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22 if (c->alloc < (a->used + 1)) {
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23 if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
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24 return res;
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25 }
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26 }
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27
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28 /* if a is negative just do an unsigned
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29 * addition [with fudged signs]
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30 */
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31 if (a->sign == MP_NEG) {
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32 mp_int a_ = *a;
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33 a_.sign = MP_ZPOS;
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34 res = mp_add_d(&a_, b, c);
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35 c->sign = MP_NEG;
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36
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37 /* clamp */
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38 mp_clamp(c);
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39
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40 return res;
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41 }
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42
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43 /* setup regs */
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44 oldused = c->used;
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45 tmpa = a->dp;
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46 tmpc = c->dp;
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47
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48 /* if a <= b simply fix the single digit */
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49 if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {
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50 if (a->used == 1) {
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51 *tmpc++ = b - *tmpa;
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52 } else {
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53 *tmpc++ = b;
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54 }
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55 ix = 1;
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56
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57 /* negative/1digit */
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58 c->sign = MP_NEG;
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59 c->used = 1;
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60 } else {
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61 /* positive/size */
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62 c->sign = MP_ZPOS;
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63 c->used = a->used;
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64
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65 /* subtract first digit */
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66 *tmpc = *tmpa++ - b;
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67 mu = *tmpc >> ((sizeof(mp_digit) * (size_t)CHAR_BIT) - 1u);
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68 *tmpc++ &= MP_MASK;
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69
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70 /* handle rest of the digits */
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71 for (ix = 1; ix < a->used; ix++) {
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72 *tmpc = *tmpa++ - mu;
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73 mu = *tmpc >> ((sizeof(mp_digit) * (size_t)CHAR_BIT) - 1u);
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74 *tmpc++ &= MP_MASK;
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75 }
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76 }
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77
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78 /* zero excess digits */
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79 while (ix++ < oldused) {
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80 *tmpc++ = 0;
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81 }
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82 mp_clamp(c);
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83 return MP_OKAY;
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84 }
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86 #endif
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87
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88 /* ref: HEAD -> master, tag: v1.1.0 */
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89 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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90 /* commit time: 2019-01-28 20:32:32 +0100 */