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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 /* shift right by a certain bit count (store quotient in c, optional remainder in d) */ |
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18 int mp_div_2d (mp_int * a, int b, mp_int * c, mp_int * d) |
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19 { |
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20 mp_digit D, r, rr; |
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21 int x, res; |
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22 mp_int t; |
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23 |
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24 |
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25 /* if the shift count is <= 0 then we do no work */ |
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26 if (b <= 0) { |
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27 res = mp_copy (a, c); |
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28 if (d != NULL) { |
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29 mp_zero (d); |
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30 } |
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31 return res; |
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32 } |
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33 |
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34 if ((res = mp_init (&t)) != MP_OKAY) { |
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35 return res; |
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36 } |
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37 |
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38 /* get the remainder */ |
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39 if (d != NULL) { |
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40 if ((res = mp_mod_2d (a, b, &t)) != MP_OKAY) { |
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41 mp_clear (&t); |
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42 return res; |
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43 } |
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44 } |
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45 |
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46 /* copy */ |
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47 if ((res = mp_copy (a, c)) != MP_OKAY) { |
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48 mp_clear (&t); |
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49 return res; |
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50 } |
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51 |
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52 /* shift by as many digits in the bit count */ |
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53 if (b >= (int)DIGIT_BIT) { |
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54 mp_rshd (c, b / DIGIT_BIT); |
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55 } |
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56 |
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57 /* shift any bit count < DIGIT_BIT */ |
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58 D = (mp_digit) (b % DIGIT_BIT); |
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59 if (D != 0) { |
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60 register mp_digit *tmpc, mask, shift; |
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61 |
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62 /* mask */ |
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63 mask = (((mp_digit)1) << D) - 1; |
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64 |
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65 /* shift for lsb */ |
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66 shift = DIGIT_BIT - D; |
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67 |
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68 /* alias */ |
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69 tmpc = c->dp + (c->used - 1); |
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70 |
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71 /* carry */ |
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72 r = 0; |
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73 for (x = c->used - 1; x >= 0; x--) { |
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74 /* get the lower bits of this word in a temp */ |
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75 rr = *tmpc & mask; |
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76 |
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77 /* shift the current word and mix in the carry bits from the previous word */ |
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78 *tmpc = (*tmpc >> D) | (r << shift); |
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79 --tmpc; |
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80 |
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81 /* set the carry to the carry bits of the current word found above */ |
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82 r = rr; |
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83 } |
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84 } |
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85 mp_clamp (c); |
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86 if (d != NULL) { |
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87 mp_exch (&t, d); |
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88 } |
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89 mp_clear (&t); |
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90 return MP_OKAY; |
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91 } |
