Mercurial > dropbear

annotate libtommath/bn_mp_rshd.c @ 689:c3de235d9506

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author Matt Johnston <matt@ucc.asn.au>
date Thu, 19 Jul 2012 21:34:27 +0800
parents 5ff8218bcee9
children 60fc6476e044
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1 #include <tommath.h>
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2 #ifdef BN_MP_RSHD_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.com
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16 */
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17
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18 /* shift right a certain amount of digits */
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19 void mp_rshd (mp_int * a, int b)
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20 {
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21 int x;
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22
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23 /* if b <= 0 then ignore it */
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24 if (b <= 0) {
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25 return;
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26 }
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27
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28 /* if b > used then simply zero it and return */
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29 if (a->used <= b) {
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30 mp_zero (a);
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31 return;
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32 }
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33
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34 {
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35 register mp_digit *bottom, *top;
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36
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37 /* shift the digits down */
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38
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39 /* bottom */
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40 bottom = a->dp;
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41
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42 /* top [offset into digits] */
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43 top = a->dp + b;
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44
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45 /* this is implemented as a sliding window where
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46 * the window is b-digits long and digits from
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47 * the top of the window are copied to the bottom
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48 *
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49 * e.g.
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50
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51 b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
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52 /\ | ---->
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53 \-------------------/ ---->
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54 */
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55 for (x = 0; x < (a->used - b); x++) {
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56 *bottom++ = *top++;
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57 }
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58
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59 /* zero the top digits */
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60 for (; x < a->used; x++) {
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61 *bottom++ = 0;
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62 }
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63 }
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64
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65 /* remove excess digits */
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66 a->used -= b;
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67 }
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68 #endif
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69
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70 /* $Source: /cvs/libtom/libtommath/bn_mp_rshd.c,v $ */
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71 /* $Revision: 1.3 $ */
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72 /* $Date: 2006/03/31 14:18:44 $ */