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