Mercurial > dropbear

annotate libtommath/bn_mp_div_2d.c @ 1486:cf43bbb6b8ff

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author Matt Johnston <matt@ucc.asn.au>
date Sat, 10 Feb 2018 19:06:30 +0800
parents 8bba51a55704
children f52919ffd3b1
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rev   line source
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diff changeset
1 #include <tommath_private.h>
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2 #ifdef BN_MP_DIV_2D_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://libtom.org
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16 */
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17
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18 /* shift right by a certain bit count (store quotient in c, optional remainder in d) */
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19 int mp_div_2d (mp_int * a, int b, mp_int * c, mp_int * d)
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20 {
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21 mp_digit D, r, rr;
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22 int x, res;
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23
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24 /* if the shift count is <= 0 then we do no work */
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25 if (b <= 0) {
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26 res = mp_copy (a, c);
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27 if (d != NULL) {
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28 mp_zero (d);
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29 }
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30 return res;
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31 }
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32
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33 /* copy */
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34 if ((res = mp_copy (a, c)) != MP_OKAY) {
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35 return res;
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36 }
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37 /* 'a' should not be used after here - it might be the same as d */
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38
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39 /* get the remainder */
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40 if (d != NULL) {
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41 if ((res = mp_mod_2d (a, b, d)) != MP_OKAY) {
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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 /* shift by as many digits in the bit count */
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47 if (b >= (int)DIGIT_BIT) {
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48 mp_rshd (c, b / DIGIT_BIT);
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49 }
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50
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51 /* shift any bit count < DIGIT_BIT */
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52 D = (mp_digit) (b % DIGIT_BIT);
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53 if (D != 0) {
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54 mp_digit *tmpc, mask, shift;
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55
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56 /* mask */
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57 mask = (((mp_digit)1) << D) - 1;
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58
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59 /* shift for lsb */
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60 shift = DIGIT_BIT - D;
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61
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62 /* alias */
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63 tmpc = c->dp + (c->used - 1);
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64
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65 /* carry */
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66 r = 0;
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67 for (x = c->used - 1; x >= 0; x--) {
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68 /* get the lower bits of this word in a temp */
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69 rr = *tmpc & mask;
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70
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71 /* shift the current word and mix in the carry bits from the previous word */
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72 *tmpc = (*tmpc >> D) | (r << shift);
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73 --tmpc;
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74
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75 /* set the carry to the carry bits of the current word found above */
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76 r = rr;
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77 }
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78 }
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79 mp_clamp (c);
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80 return MP_OKAY;
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81 }
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82 #endif
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83
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84 /* ref: $Format:%D$ */
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85 /* git commit: $Format:%H$ */
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86 /* commit time: $Format:%ai$ */