1 | #include <tommath.h> |
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2 | #ifdef BN_MP_RSHD_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, tomstdenis@gmail.com, http://math.libtomcrypt.com |
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16 | */ |
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17 | |
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18 | /* shift right a certain amount of digits */ |
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19 | void mp_rshd (mp_int * a, int b) |
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20 | { |
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21 | int x; |
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22 | |
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23 | /* if b <= 0 then ignore it */ |
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24 | if (b <= 0) { |
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25 | return; |
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26 | } |
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27 | |
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28 | /* if b > used then simply zero it and return */ |
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29 | if (a->used <= b) { |
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30 | mp_zero (a); |
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31 | return; |
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32 | } |
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33 | |
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34 | { |
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35 | register mp_digit *bottom, *top; |
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36 | |
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37 | /* shift the digits down */ |
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38 | |
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39 | /* bottom */ |
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40 | bottom = a->dp; |
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41 | |
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42 | /* top [offset into digits] */ |
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43 | top = a->dp + b; |
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44 | |
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45 | /* this is implemented as a sliding window where |
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46 | * the window is b-digits long and digits from |
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47 | * the top of the window are copied to the bottom |
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48 | * |
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49 | * e.g. |
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50 | |
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51 | b-2 | b-1 | b0 | b1 | b2 | ... | bb | ----> |
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52 | /\ | ----> |
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53 | \-------------------/ ----> |
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54 | */ |
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55 | for (x = 0; x < (a->used - b); x++) { |
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56 | *bottom++ = *top++; |
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57 | } |
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58 | |
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59 | /* zero the top digits */ |
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60 | for (; x < a->used; x++) { |
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61 | *bottom++ = 0; |
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62 | } |
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63 | } |
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64 | |
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65 | /* remove excess digits */ |
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66 | a->used -= b; |
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67 | } |
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68 | #endif |
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69 | |
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70 | /* $Source: /cvsroot/tcl/libtommath/bn_mp_rshd.c,v $ */ |
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71 | /* $Revision: 1.1.1.3 $ */ |
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72 | /* $Date: 2006/12/01 00:08:11 $ */ |
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