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Diffstat (limited to 'libtommath/bn_mp_rshd.c')
-rw-r--r-- | libtommath/bn_mp_rshd.c | 68 |
1 files changed, 68 insertions, 0 deletions
diff --git a/libtommath/bn_mp_rshd.c b/libtommath/bn_mp_rshd.c new file mode 100644 index 0000000..e6095b3 --- /dev/null +++ b/libtommath/bn_mp_rshd.c @@ -0,0 +1,68 @@ +#include <tommath.h> +#ifdef BN_MP_RSHD_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis + * + * LibTomMath is a library that provides multiple-precision + * integer arithmetic as well as number theoretic functionality. + * + * The library was designed directly after the MPI library by + * Michael Fromberger but has been written from scratch with + * additional optimizations in place. + * + * The library is free for all purposes without any express + * guarantee it works. + * + * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com + */ + +/* shift right a certain amount of digits */ +void mp_rshd (mp_int * a, int b) +{ + int x; + + /* if b <= 0 then ignore it */ + if (b <= 0) { + return; + } + + /* if b > used then simply zero it and return */ + if (a->used <= b) { + mp_zero (a); + return; + } + + { + register mp_digit *bottom, *top; + + /* shift the digits down */ + + /* bottom */ + bottom = a->dp; + + /* top [offset into digits] */ + top = a->dp + b; + + /* this is implemented as a sliding window where + * the window is b-digits long and digits from + * the top of the window are copied to the bottom + * + * e.g. + + b-2 | b-1 | b0 | b1 | b2 | ... | bb | ----> + /\ | ----> + \-------------------/ ----> + */ + for (x = 0; x < (a->used - b); x++) { + *bottom++ = *top++; + } + + /* zero the top digits */ + for (; x < a->used; x++) { + *bottom++ = 0; + } + } + + /* remove excess digits */ + a->used -= b; +} +#endif |