tommath/bn_mp_cnt_lsb.c

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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* 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.
*
* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
*/
#include <tommath.h>
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static const int lnz[16] = {
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
};
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/* Counts the number of lsbs which are zero before the first zero bit */
int mp_cnt_lsb(mp_int *a)
{
int x;
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mp_digit q, qq;
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/* easy out */
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if (mp_iszero(a) == 1) {
return 0;
}
/* scan lower digits until non-zero */
for (x = 0; x < a->used && a->dp[x] == 0; x++);
q = a->dp[x];
x *= DIGIT_BIT;
/* now scan this digit until a 1 is found */
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if ((q & 1) == 0) {
do {
qq = q & 15;
x += lnz[qq];
q >>= 4;
} while (qq == 0);
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}
return x;
}