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comparison 3rdparty/libtommath/bn_mp_cnt_lsb.c @ 0:2cf249471370

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author Takahiro SHIMIZU <anatofuz@cr.ie.u-ryukyu.ac.jp>
date Tue, 08 May 2018 16:09:12 +0900
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1 #include <tommath_private.h>
2 #ifdef BN_MP_CNT_LSB_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
4 *
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, tstdenis82@gmail.com, http://libtom.org
16 */
17
18 static const int lnz[16] = {
19 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
20 };
21
22 /* Counts the number of lsbs which are zero before the first zero bit */
23 int mp_cnt_lsb(mp_int *a)
24 {
25 int x;
26 mp_digit q, qq;
27
28 /* easy out */
29 if (mp_iszero(a) == MP_YES) {
30 return 0;
31 }
32
33 /* scan lower digits until non-zero */
34 for (x = 0; (x < a->used) && (a->dp[x] == 0); x++) {}
35 q = a->dp[x];
36 x *= DIGIT_BIT;
37
38 /* now scan this digit until a 1 is found */
39 if ((q & 1) == 0) {
40 do {
41 qq = q & 15;
42 x += lnz[qq];
43 q >>= 4;
44 } while (qq == 0);
45 }
46 return x;
47 }
48
49 #endif
50
51 /* $Source$ */
52 /* $Revision$ */
53 /* $Date$ */