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1 /* LibTomMath, multiple-precision integer library -- Tom St Denis |
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2 * |
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3 * LibTomMath is a library that provides multiple-precision |
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4 * integer arithmetic as well as number theoretic functionality. |
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5 * |
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6 * The library was designed directly after the MPI library by |
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7 * Michael Fromberger but has been written from scratch with |
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8 * additional optimizations in place. |
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9 * |
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10 * The library is free for all purposes without any express |
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11 * guarantee it works. |
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12 * |
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13 * Tom St Denis, [email protected], http://math.libtomcrypt.org |
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14 */ |
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15 #include <tommath.h> |
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16 |
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17 /* low level addition, based on HAC pp.594, Algorithm 14.7 */ |
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18 int |
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19 s_mp_add (mp_int * a, mp_int * b, mp_int * c) |
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20 { |
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21 mp_int *x; |
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22 int olduse, res, min, max; |
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23 |
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24 /* find sizes, we let |a| <= |b| which means we have to sort |
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25 * them. "x" will point to the input with the most digits |
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26 */ |
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27 if (a->used > b->used) { |
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28 min = b->used; |
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29 max = a->used; |
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30 x = a; |
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31 } else { |
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32 min = a->used; |
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33 max = b->used; |
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34 x = b; |
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35 } |
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36 |
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37 /* init result */ |
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38 if (c->alloc < max + 1) { |
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39 if ((res = mp_grow (c, max + 1)) != MP_OKAY) { |
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40 return res; |
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41 } |
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42 } |
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43 |
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44 /* get old used digit count and set new one */ |
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45 olduse = c->used; |
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46 c->used = max + 1; |
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47 |
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48 { |
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49 register mp_digit u, *tmpa, *tmpb, *tmpc; |
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50 register int i; |
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51 |
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52 /* alias for digit pointers */ |
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53 |
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54 /* first input */ |
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55 tmpa = a->dp; |
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56 |
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57 /* second input */ |
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58 tmpb = b->dp; |
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59 |
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60 /* destination */ |
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61 tmpc = c->dp; |
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62 |
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63 /* zero the carry */ |
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64 u = 0; |
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65 for (i = 0; i < min; i++) { |
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66 /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */ |
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67 *tmpc = *tmpa++ + *tmpb++ + u; |
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68 |
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69 /* U = carry bit of T[i] */ |
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70 u = *tmpc >> ((mp_digit)DIGIT_BIT); |
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71 |
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72 /* take away carry bit from T[i] */ |
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73 *tmpc++ &= MP_MASK; |
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74 } |
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75 |
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76 /* now copy higher words if any, that is in A+B |
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77 * if A or B has more digits add those in |
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78 */ |
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79 if (min != max) { |
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80 for (; i < max; i++) { |
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81 /* T[i] = X[i] + U */ |
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82 *tmpc = x->dp[i] + u; |
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83 |
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84 /* U = carry bit of T[i] */ |
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85 u = *tmpc >> ((mp_digit)DIGIT_BIT); |
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86 |
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87 /* take away carry bit from T[i] */ |
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88 *tmpc++ &= MP_MASK; |
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89 } |
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90 } |
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91 |
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92 /* add carry */ |
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93 *tmpc++ = u; |
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94 |
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95 /* clear digits above oldused */ |
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96 for (i = c->used; i < olduse; i++) { |
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97 *tmpc++ = 0; |
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98 } |
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99 } |
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100 |
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101 mp_clamp (c); |
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102 return MP_OKAY; |
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103 } |
