tommath/bn_mp_rshd.c

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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is library that provides for multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library is designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
#include <tommath.h>
/* shift right a certain amount of digits */
void
mp_rshd (mp_int * a, int b)
{
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int x;
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/* if b <= 0 then ignore it */
if (b <= 0) {
return;
}
/* if b > used then simply zero it and return */
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if (a->used <= b) {
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mp_zero (a);
return;
}
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{
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register mp_digit *bottom, *top;
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/* shift the digits down */
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/* bottom */
bottom = a->dp;
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/* top [offset into digits] */
top = a->dp + b;
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/* this is implemented as a sliding window where
* the window is b-digits long and digits from
* the top of the window are copied to the bottom
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*
* e.g.
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b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
/\ | ---->
\-------------------/ ---->
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*/
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for (x = 0; x < (a->used - b); x++) {
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*bottom++ = *top++;
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}
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/* zero the top digits */
for (; x < a->used; x++) {
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*bottom++ = 0;
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}
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}
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/* remove excess digits */
a->used -= b;
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}