annotate libtommath/bn_mp_rshd.c @ 1853:2be3115a8762 default tip main master

debugging test runner authorized_keys perms
author Matt Johnston <matt@ucc.asn.au>
date Tue, 19 Oct 2021 13:45:59 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.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 /* SPDX-License-Identifier: Unlicense */
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6 /* shift right a certain amount of digits */
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7 void mp_rshd(mp_int *a, int b)
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8 {
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9 int x;
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10 mp_digit *bottom, *top;
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11
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12 /* if b <= 0 then ignore it */
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13 if (b <= 0) {
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14 return;
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15 }
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16
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17 /* if b > used then simply zero it and return */
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18 if (a->used <= b) {
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19 mp_zero(a);
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20 return;
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21 }
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22
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23 /* shift the digits down */
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24
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25 /* bottom */
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26 bottom = a->dp;
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27
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28 /* top [offset into digits] */
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29 top = a->dp + b;
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30
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31 /* this is implemented as a sliding window where
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32 * the window is b-digits long and digits from
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33 * the top of the window are copied to the bottom
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34 *
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35 * e.g.
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36
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37 b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
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38 /\ | ---->
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39 \-------------------/ ---->
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40 */
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41 for (x = 0; x < (a->used - b); x++) {
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42 *bottom++ = *top++;
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43 }
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44
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45 /* zero the top digits */
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46 MP_ZERO_DIGITS(bottom, a->used - x);
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47
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48 /* remove excess digits */
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49 a->used -= b;
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50 }
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51 #endif