annotate bn_s_mp_add.c @ 282:91fbc376f010 libtommath-orig libtommath-0.35

Import of libtommath 0.35 From ltm-0.35.tar.bz2 SHA1 of 3f193dbae9351e92d02530994fa18236f7fde01c
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:16:18 +0000
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1 #include <tommath.h>
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2 #ifdef BN_S_MP_ADD_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://math.libtomcrypt.org
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16 */
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17
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18 /* low level addition, based on HAC pp.594, Algorithm 14.7 */
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19 int
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20 s_mp_add (mp_int * a, mp_int * b, mp_int * c)
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21 {
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22 mp_int *x;
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23 int olduse, res, min, max;
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24
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25 /* find sizes, we let |a| <= |b| which means we have to sort
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26 * them. "x" will point to the input with the most digits
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27 */
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28 if (a->used > b->used) {
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29 min = b->used;
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30 max = a->used;
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31 x = a;
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32 } else {
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33 min = a->used;
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34 max = b->used;
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35 x = b;
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36 }
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37
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38 /* init result */
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39 if (c->alloc < max + 1) {
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40 if ((res = mp_grow (c, max + 1)) != MP_OKAY) {
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41 return res;
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42 }
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43 }
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44
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45 /* get old used digit count and set new one */
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46 olduse = c->used;
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47 c->used = max + 1;
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48
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49 {
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50 register mp_digit u, *tmpa, *tmpb, *tmpc;
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51 register int i;
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52
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53 /* alias for digit pointers */
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54
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55 /* first input */
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56 tmpa = a->dp;
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57
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58 /* second input */
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59 tmpb = b->dp;
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60
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61 /* destination */
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62 tmpc = c->dp;
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63
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64 /* zero the carry */
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65 u = 0;
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66 for (i = 0; i < min; i++) {
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67 /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
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68 *tmpc = *tmpa++ + *tmpb++ + u;
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69
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70 /* U = carry bit of T[i] */
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71 u = *tmpc >> ((mp_digit)DIGIT_BIT);
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72
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73 /* take away carry bit from T[i] */
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74 *tmpc++ &= MP_MASK;
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75 }
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76
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77 /* now copy higher words if any, that is in A+B
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78 * if A or B has more digits add those in
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79 */
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80 if (min != max) {
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81 for (; i < max; i++) {
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82 /* T[i] = X[i] + U */
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83 *tmpc = x->dp[i] + u;
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84
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85 /* U = carry bit of T[i] */
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86 u = *tmpc >> ((mp_digit)DIGIT_BIT);
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87
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88 /* take away carry bit from T[i] */
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89 *tmpc++ &= MP_MASK;
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90 }
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91 }
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92
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93 /* add carry */
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94 *tmpc++ = u;
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95
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96 /* clear digits above oldused */
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97 for (i = c->used; i < olduse; i++) {
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98 *tmpc++ = 0;
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99 }
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100 }
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101
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102 mp_clamp (c);
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103 return MP_OKAY;
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104 }
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105 #endif