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, tomstdenis@gmail.com, 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: /cvsroot/tcl/libtommath/bn_mp_sub_d.c,v $ */ |
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92 | /* $Revision: 1.1.1.4 $ */ |
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93 | /* $Date: 2006/12/01 00:08:11 $ */ |
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