1 | #include <tommath.h> |
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2 | #ifdef BN_MP_DIV_3_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 | /* divide by three (based on routine from MPI and the GMP manual) */ |
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19 | int |
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20 | mp_div_3 (mp_int * a, mp_int *c, mp_digit * d) |
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21 | { |
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22 | mp_int q; |
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23 | mp_word w, t; |
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24 | mp_digit b; |
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25 | int res, ix; |
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26 | |
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27 | /* b = 2**DIGIT_BIT / 3 */ |
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28 | b = (((mp_word)1) << ((mp_word)DIGIT_BIT)) / ((mp_word)3); |
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29 | |
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30 | if ((res = mp_init_size(&q, a->used)) != MP_OKAY) { |
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31 | return res; |
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32 | } |
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33 | |
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34 | q.used = a->used; |
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35 | q.sign = a->sign; |
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36 | w = 0; |
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37 | for (ix = a->used - 1; ix >= 0; ix--) { |
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38 | w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]); |
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39 | |
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40 | if (w >= 3) { |
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41 | /* multiply w by [1/3] */ |
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42 | t = (w * ((mp_word)b)) >> ((mp_word)DIGIT_BIT); |
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43 | |
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44 | /* now subtract 3 * [w/3] from w, to get the remainder */ |
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45 | w -= t+t+t; |
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46 | |
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47 | /* fixup the remainder as required since |
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48 | * the optimization is not exact. |
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49 | */ |
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50 | while (w >= 3) { |
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51 | t += 1; |
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52 | w -= 3; |
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53 | } |
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54 | } else { |
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55 | t = 0; |
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56 | } |
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57 | q.dp[ix] = (mp_digit)t; |
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58 | } |
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59 | |
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60 | /* [optional] store the remainder */ |
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61 | if (d != NULL) { |
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62 | *d = (mp_digit)w; |
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63 | } |
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64 | |
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65 | /* [optional] store the quotient */ |
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66 | if (c != NULL) { |
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67 | mp_clamp(&q); |
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68 | mp_exch(&q, c); |
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69 | } |
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70 | mp_clear(&q); |
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71 | |
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72 | return res; |
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73 | } |
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74 | |
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75 | #endif |
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76 | |
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77 | /* $Source: /cvsroot/tcl/libtommath/bn_mp_div_3.c,v $ */ |
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78 | /* $Revision: 1.1.1.3 $ */ |
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79 | /* $Date: 2006/12/01 00:08:11 $ */ |
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