annotate bn_mp_add_d.c @ 145:a96ff234ff19 libtommath

propagate of fc94c38452d9fd684a8e1eb9e3a73120aac0d38f and 3e4de4cbef3d9035a7b2f0c25e9f86e297f9f6d1 from branch 'au.asn.ucc.matt.ltm-orig' to 'au.asn.ucc.matt.ltm-db'
author Matt Johnston <matt@ucc.asn.au>
date Sun, 19 Dec 2004 15:57:19 +0000
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children
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d29b64170cf0 import of libtommath 0.32
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1 #include <tommath.h>
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2 #ifdef BN_MP_ADD_D_C
2
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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://math.libtomcrypt.org
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16 */
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17
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18 /* single digit addition */
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19 int
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20 mp_add_d (mp_int * a, mp_digit b, mp_int * c)
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21 {
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22 int res, ix, oldused;
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23 mp_digit *tmpa, *tmpc, mu;
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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 and |a| >= b, call c = |a| - b */
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33 if (a->sign == MP_NEG && (a->used > 1 || a->dp[0] >= b)) {
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34 /* temporarily fix sign of a */
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35 a->sign = MP_ZPOS;
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36
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37 /* c = |a| - b */
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38 res = mp_sub_d(a, b, c);
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39
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40 /* fix sign */
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41 a->sign = c->sign = MP_NEG;
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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 /* old number of used digits in c */
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47 oldused = c->used;
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48
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49 /* sign always positive */
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50 c->sign = MP_ZPOS;
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51
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52 /* source alias */
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53 tmpa = a->dp;
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54
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55 /* destination alias */
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56 tmpc = c->dp;
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57
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58 /* if a is positive */
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59 if (a->sign == MP_ZPOS) {
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60 /* add digit, after this we're propagating
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61 * the carry.
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62 */
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63 *tmpc = *tmpa++ + b;
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64 mu = *tmpc >> DIGIT_BIT;
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65 *tmpc++ &= MP_MASK;
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66
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67 /* now handle rest of the digits */
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68 for (ix = 1; ix < a->used; ix++) {
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69 *tmpc = *tmpa++ + mu;
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70 mu = *tmpc >> DIGIT_BIT;
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71 *tmpc++ &= MP_MASK;
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72 }
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73 /* set final carry */
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74 ix++;
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75 *tmpc++ = mu;
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76
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77 /* setup size */
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78 c->used = a->used + 1;
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79 } else {
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80 /* a was negative and |a| < b */
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81 c->used = 1;
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82
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83 /* the result is a single digit */
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84 if (a->used == 1) {
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85 *tmpc++ = b - a->dp[0];
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86 } else {
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87 *tmpc++ = b;
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88 }
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89
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90 /* setup count so the clearing of oldused
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91 * can fall through correctly
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92 */
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93 ix = 1;
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94 }
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95
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96 /* now zero to oldused */
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97 while (ix++ < oldused) {
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98 *tmpc++ = 0;
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99 }
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100 mp_clamp(c);
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101
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102 return MP_OKAY;
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103 }
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104
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105 #endif