Mercurial > dropbear

annotate libtommath/bn_mp_div_2.c @ 322:84aa4e60bd3c

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author Matt Johnston <matt@ucc.asn.au>
date Wed, 07 Jun 2006 13:41:19 +0000
parents eed26cff980b
children 5ff8218bcee9
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284
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1 #include <tommath.h>
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2 #ifdef BN_MP_DIV_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 *
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.org
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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_div_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 /* copy */
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24 if (b->alloc < a->used) {
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25 if ((res = mp_grow (b, a->used)) != 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 register mp_digit r, rr, *tmpa, *tmpb;
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34
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35 /* source alias */
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36 tmpa = a->dp + b->used - 1;
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37
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38 /* dest alias */
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39 tmpb = b->dp + b->used - 1;
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40
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41 /* carry */
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42 r = 0;
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43 for (x = b->used - 1; x >= 0; x--) {
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44 /* get the carry for the next iteration */
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45 rr = *tmpa & 1;
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46
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47 /* shift the current digit, add in carry and store */
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48 *tmpb-- = (*tmpa-- >> 1) | (r << (DIGIT_BIT - 1));
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49
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50 /* forward carry to next iteration */
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51 r = rr;
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52 }
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53
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54 /* zero excess digits */
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55 tmpb = b->dp + b->used;
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56 for (x = b->used; x < oldused; x++) {
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57 *tmpb++ = 0;
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58 }
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59 }
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60 b->sign = a->sign;
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61 mp_clamp (b);
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62 return MP_OKAY;
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63 }
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64 #endif