1 #include <tommath.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, tomstdenis (at) gmail.com, http://math.libtomcrypt.com 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) == 1) { 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: /cvs/libtom/libtommath/bn_mp_cnt_lsb.c,v $ */ 52 /* $Revision: 1.3 $ */ 53 /* $Date: 2006/03/31 14:18:44 $ */ 54
