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1 #include <tommath.h> |
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2 #ifdef BN_MP_ADD_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://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 |
