annotate bn_mp_rshd.c @ 139:cc04b085e7dd libtommath

Pristine compilation works.
author Matt Johnston <matt@ucc.asn.au>
date Fri, 17 Dec 2004 06:27:22 +0000
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children d29b64170cf0
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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 /* shift right a certain amount of digits */
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18 void mp_rshd (mp_int * a, int b)
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19 {
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20 int x;
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21
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22 /* if b <= 0 then ignore it */
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23 if (b <= 0) {
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24 return;
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25 }
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26
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27 /* if b > used then simply zero it and return */
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28 if (a->used <= b) {
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29 mp_zero (a);
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30 return;
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31 }
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32
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33 {
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34 register mp_digit *bottom, *top;
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35
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36 /* shift the digits down */
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37
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38 /* bottom */
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39 bottom = a->dp;
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40
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41 /* top [offset into digits] */
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42 top = a->dp + b;
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43
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44 /* this is implemented as a sliding window where
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45 * the window is b-digits long and digits from
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46 * the top of the window are copied to the bottom
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47 *
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48 * e.g.
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49
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50 b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
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51 /\ | ---->
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52 \-------------------/ ---->
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53 */
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54 for (x = 0; x < (a->used - b); x++) {
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55 *bottom++ = *top++;
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56 }
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57
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58 /* zero the top digits */
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59 for (; x < a->used; x++) {
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60 *bottom++ = 0;
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61 }
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62 }
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63
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64 /* remove excess digits */
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65 a->used -= b;
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66 }