Mercurial > dropbear

annotate libtommath/bn_mp_sub_d.c @ 1552:e46f7f1da56a

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CHANGES for 2018.76
author Matt Johnston <matt@ucc.asn.au>
date Tue, 27 Feb 2018 22:14:04 +0800
parents 8bba51a55704
children f52919ffd3b1
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
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diff changeset
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 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://libtom.org
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16 */
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17
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18 /* single digit subtraction */
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19 int
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20 mp_sub_d (mp_int * a, mp_digit b, mp_int * c)
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21 {
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22 mp_digit *tmpa, *tmpc, mu;
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23 int res, ix, oldused;
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24
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25 /* grow c as required */
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26 if (c->alloc < (a->used + 1)) {
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27 if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
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28 return res;
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29 }
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30 }
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31
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32 /* if a is negative just do an unsigned
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33 * addition [with fudged signs]
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34 */
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35 if (a->sign == MP_NEG) {
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36 a->sign = MP_ZPOS;
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37 res = mp_add_d(a, b, c);
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38 a->sign = c->sign = MP_NEG;
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39
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40 /* clamp */
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41 mp_clamp(c);
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42
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43 return res;
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44 }
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45
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46 /* setup regs */
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47 oldused = c->used;
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48 tmpa = a->dp;
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49 tmpc = c->dp;
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50
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51 /* if a <= b simply fix the single digit */
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52 if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {
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53 if (a->used == 1) {
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54 *tmpc++ = b - *tmpa;
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55 } else {
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56 *tmpc++ = b;
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57 }
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58 ix = 1;
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59
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60 /* negative/1digit */
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61 c->sign = MP_NEG;
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62 c->used = 1;
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63 } else {
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64 /* positive/size */
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65 c->sign = MP_ZPOS;
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66 c->used = a->used;
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67
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68 /* subtract first digit */
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69 *tmpc = *tmpa++ - b;
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70 mu = *tmpc >> ((sizeof(mp_digit) * CHAR_BIT) - 1);
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71 *tmpc++ &= MP_MASK;
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72
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73 /* handle rest of the digits */
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74 for (ix = 1; ix < a->used; ix++) {
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75 *tmpc = *tmpa++ - mu;
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76 mu = *tmpc >> ((sizeof(mp_digit) * CHAR_BIT) - 1);
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77 *tmpc++ &= MP_MASK;
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78 }
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79 }
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80
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81 /* zero excess digits */
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82 while (ix++ < oldused) {
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83 *tmpc++ = 0;
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84 }
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85 mp_clamp(c);
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86 return MP_OKAY;
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87 }
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88
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89 #endif
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90
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91 /* ref: $Format:%D$ */
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92 /* git commit: $Format:%H$ */
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93 /* commit time: $Format:%ai$ */