| 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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