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1 #include <tommath.h> |
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2 #ifdef BN_MP_SUB_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.com |
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16 */ |
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17 |
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18 /* single digit subtraction */ |
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19 int |
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20 mp_sub_d (mp_int * a, mp_digit b, mp_int * c) |
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21 { |
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22 mp_digit *tmpa, *tmpc, mu; |
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23 int res, ix, oldused; |
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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 just do an unsigned |
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33 * addition [with fudged signs] |
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34 */ |
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35 if (a->sign == MP_NEG) { |
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36 a->sign = MP_ZPOS; |
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37 res = mp_add_d(a, b, c); |
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38 a->sign = c->sign = MP_NEG; |
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39 |
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40 /* clamp */ |
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41 mp_clamp(c); |
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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 /* setup regs */ |
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47 oldused = c->used; |
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48 tmpa = a->dp; |
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49 tmpc = c->dp; |
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50 |
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51 /* if a <= b simply fix the single digit */ |
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52 if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) { |
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53 if (a->used == 1) { |
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54 *tmpc++ = b - *tmpa; |
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55 } else { |
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56 *tmpc++ = b; |
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57 } |
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58 ix = 1; |
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59 |
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60 /* negative/1digit */ |
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61 c->sign = MP_NEG; |
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62 c->used = 1; |
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63 } else { |
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64 /* positive/size */ |
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65 c->sign = MP_ZPOS; |
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66 c->used = a->used; |
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67 |
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68 /* subtract first digit */ |
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69 *tmpc = *tmpa++ - b; |
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70 mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1); |
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71 *tmpc++ &= MP_MASK; |
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72 |
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73 /* handle rest of the digits */ |
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74 for (ix = 1; ix < a->used; ix++) { |
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75 *tmpc = *tmpa++ - mu; |
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76 mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1); |
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77 *tmpc++ &= MP_MASK; |
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78 } |
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79 } |
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80 |
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81 /* zero excess digits */ |
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82 while (ix++ < oldused) { |
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83 *tmpc++ = 0; |
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84 } |
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85 mp_clamp(c); |
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86 return MP_OKAY; |
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87 } |
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88 |
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89 #endif |
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90 |
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91 /* $Source: /cvs/libtom/libtommath/bn_mp_sub_d.c,v $ */ |
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92 /* $Revision: 1.5 $ */ |
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93 /* $Date: 2006/03/31 14:18:44 $ */ |
