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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 /* single digit addition */ |
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18 int |
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19 mp_add_d (mp_int * a, mp_digit b, mp_int * c) |
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20 { |
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21 int res, ix, oldused; |
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22 mp_digit *tmpa, *tmpc, mu; |
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23 |
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24 /* grow c as required */ |
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25 if (c->alloc < a->used + 1) { |
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26 if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) { |
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27 return res; |
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28 } |
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29 } |
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30 |
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31 /* if a is negative and |a| >= b, call c = |a| - b */ |
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32 if (a->sign == MP_NEG && (a->used > 1 || a->dp[0] >= b)) { |
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33 /* temporarily fix sign of a */ |
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34 a->sign = MP_ZPOS; |
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35 |
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36 /* c = |a| - b */ |
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37 res = mp_sub_d(a, b, c); |
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38 |
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39 /* fix sign */ |
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40 a->sign = c->sign = MP_NEG; |
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41 |
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42 return res; |
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43 } |
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44 |
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45 /* old number of used digits in c */ |
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46 oldused = c->used; |
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47 |
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48 /* sign always positive */ |
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49 c->sign = MP_ZPOS; |
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50 |
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51 /* source alias */ |
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52 tmpa = a->dp; |
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53 |
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54 /* destination alias */ |
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55 tmpc = c->dp; |
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56 |
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57 /* if a is positive */ |
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58 if (a->sign == MP_ZPOS) { |
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59 /* add digit, after this we're propagating |
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60 * the carry. |
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61 */ |
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62 *tmpc = *tmpa++ + b; |
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63 mu = *tmpc >> DIGIT_BIT; |
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64 *tmpc++ &= MP_MASK; |
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65 |
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66 /* now handle rest of the digits */ |
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67 for (ix = 1; ix < a->used; ix++) { |
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68 *tmpc = *tmpa++ + mu; |
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69 mu = *tmpc >> DIGIT_BIT; |
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70 *tmpc++ &= MP_MASK; |
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71 } |
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72 /* set final carry */ |
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73 ix++; |
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74 *tmpc++ = mu; |
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75 |
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76 /* setup size */ |
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77 c->used = a->used + 1; |
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78 } else { |
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79 /* a was negative and |a| < b */ |
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80 c->used = 1; |
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81 |
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82 /* the result is a single digit */ |
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83 if (a->used == 1) { |
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84 *tmpc++ = b - a->dp[0]; |
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85 } else { |
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86 *tmpc++ = b; |
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87 } |
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88 |
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89 /* setup count so the clearing of oldused |
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90 * can fall through correctly |
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91 */ |
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92 ix = 1; |
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93 } |
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94 |
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95 /* now zero to oldused */ |
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96 while (ix++ < oldused) { |
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97 *tmpc++ = 0; |
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98 } |
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99 mp_clamp(c); |
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100 |
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101 return MP_OKAY; |
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102 } |
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103 |
