Mercurial > dropbear

annotate libtommath/bn_mp_cnt_lsb.c @ 350:01e4180895ba

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merge of '9a4e042fd565f46141e81e0c1ab90260303348fe' and 'bea3887a5875cf3ab8a1331e15e698b37b61fe37'
author Matt Johnston <matt@ucc.asn.au>
date Mon, 07 Aug 2006 13:41:23 +0000
parents eed26cff980b
children 5ff8218bcee9
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284
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1 #include <tommath.h>
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2 #ifdef BN_MP_CNT_LSB_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.org
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16 */
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17
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18 static const int lnz[16] = {
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19 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
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20 };
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21
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22 /* Counts the number of lsbs which are zero before the first zero bit */
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23 int mp_cnt_lsb(mp_int *a)
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24 {
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25 int x;
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26 mp_digit q, qq;
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27
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28 /* easy out */
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29 if (mp_iszero(a) == 1) {
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30 return 0;
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31 }
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32
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33 /* scan lower digits until non-zero */
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34 for (x = 0; x < a->used && a->dp[x] == 0; x++);
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35 q = a->dp[x];
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36 x *= DIGIT_BIT;
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37
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38 /* now scan this digit until a 1 is found */
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39 if ((q & 1) == 0) {
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40 do {
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41 qq = q & 15;
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42 x += lnz[qq];
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43 q >>= 4;
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44 } while (qq == 0);
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45 }
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46 return x;
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47 }
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48
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49 #endif