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1 /* LibTomMath, multiple-precision integer library -- Tom St Denis |
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2 * |
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3 * LibTomMath is a library that provides multiple-precision |
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4 * integer arithmetic as well as number theoretic functionality. |
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5 * |
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6 * The library was designed directly after the MPI library by |
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7 * Michael Fromberger but has been written from scratch with |
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8 * additional optimizations in place. |
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9 * |
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10 * The library is free for all purposes without any express |
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11 * guarantee it works. |
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12 * |
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13 * Tom St Denis, [email protected], http://math.libtomcrypt.org |
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14 */ |
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15 #include <tommath.h> |
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16 |
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17 /* b = a/2 */ |
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18 int mp_div_2(mp_int * a, mp_int * b) |
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19 { |
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20 int x, res, oldused; |
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21 |
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22 /* copy */ |
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23 if (b->alloc < a->used) { |
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24 if ((res = mp_grow (b, a->used)) != MP_OKAY) { |
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25 return res; |
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26 } |
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27 } |
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28 |
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29 oldused = b->used; |
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30 b->used = a->used; |
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31 { |
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32 register mp_digit r, rr, *tmpa, *tmpb; |
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33 |
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34 /* source alias */ |
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35 tmpa = a->dp + b->used - 1; |
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36 |
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37 /* dest alias */ |
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38 tmpb = b->dp + b->used - 1; |
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39 |
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40 /* carry */ |
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41 r = 0; |
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42 for (x = b->used - 1; x >= 0; x--) { |
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43 /* get the carry for the next iteration */ |
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44 rr = *tmpa & 1; |
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45 |
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46 /* shift the current digit, add in carry and store */ |
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47 *tmpb-- = (*tmpa-- >> 1) | (r << (DIGIT_BIT - 1)); |
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48 |
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49 /* forward carry to next iteration */ |
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50 r = rr; |
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51 } |
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52 |
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53 /* zero excess digits */ |
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54 tmpb = b->dp + b->used; |
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55 for (x = b->used; x < oldused; x++) { |
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56 *tmpb++ = 0; |
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57 } |
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58 } |
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59 b->sign = a->sign; |
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60 mp_clamp (b); |
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61 return MP_OKAY; |
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62 } |
