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142
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1 #include <tommath.h> |
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2 #ifdef BN_MP_CNT_LSB_C |
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2
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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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142
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49 #endif |
