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annotate libtommath/bn_mp_mul_2.c @ 789:ea04e3eb03e2

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author Matt Johnston <matt@ucc.asn.au>
date Thu, 18 Apr 2013 23:15:17 +0800
parents 5ff8218bcee9
children 60fc6476e044
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1 #include <tommath.h>
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2 #ifdef BN_MP_MUL_2_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 *
389
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.com
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16 */
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17
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18 /* b = a*2 */
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19 int mp_mul_2(mp_int * a, mp_int * b)
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20 {
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21 int x, res, oldused;
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22
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23 /* grow to accomodate result */
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24 if (b->alloc < a->used + 1) {
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25 if ((res = mp_grow (b, a->used + 1)) != MP_OKAY) {
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26 return res;
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27 }
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28 }
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29
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30 oldused = b->used;
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31 b->used = a->used;
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32
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33 {
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34 register mp_digit r, rr, *tmpa, *tmpb;
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35
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36 /* alias for source */
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37 tmpa = a->dp;
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38
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39 /* alias for dest */
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40 tmpb = b->dp;
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41
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42 /* carry */
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43 r = 0;
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44 for (x = 0; x < a->used; x++) {
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45
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46 /* get what will be the *next* carry bit from the
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47 * MSB of the current digit
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48 */
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49 rr = *tmpa >> ((mp_digit)(DIGIT_BIT - 1));
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50
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51 /* now shift up this digit, add in the carry [from the previous] */
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52 *tmpb++ = ((*tmpa++ << ((mp_digit)1)) | r) & MP_MASK;
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53
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54 /* copy the carry that would be from the source
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55 * digit into the next iteration
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56 */
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57 r = rr;
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58 }
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59
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60 /* new leading digit? */
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61 if (r != 0) {
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62 /* add a MSB which is always 1 at this point */
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63 *tmpb = 1;
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64 ++(b->used);
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65 }
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66
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67 /* now zero any excess digits on the destination
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68 * that we didn't write to
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69 */
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70 tmpb = b->dp + b->used;
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71 for (x = b->used; x < oldused; x++) {
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72 *tmpb++ = 0;
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73 }
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74 }
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75 b->sign = a->sign;
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76 return MP_OKAY;
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77 }
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78 #endif
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79
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80 /* $Source: /cvs/libtom/libtommath/bn_mp_mul_2.c,v $ */
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81 /* $Revision: 1.3 $ */
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82 /* $Date: 2006/03/31 14:18:44 $ */