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      1 #include <tommath.h>
      2 #ifdef BN_MP_RSHD_C
      3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
      4  *
      5  * LibTomMath is a library that provides multiple-precision
      6  * integer arithmetic as well as number theoretic functionality.
      7  *
      8  * The library was designed directly after the MPI library by
      9  * Michael Fromberger but has been written from scratch with
     10  * additional optimizations in place.
     11  *
     12  * The library is free for all purposes without any express
     13  * guarantee it works.
     14  *
     15  * Tom St Denis, tomstdenis (at) gmail.com, http://math.libtomcrypt.com
     16  */
     17 
     18 /* shift right a certain amount of digits */
     19 void mp_rshd (mp_int * a, int b)
     20 {
     21   int     x;
     22 
     23   /* if b <= 0 then ignore it */
     24   if (b <= 0) {
     25     return;
     26   }
     27 
     28   /* if b > used then simply zero it and return */
     29   if (a->used <= b) {
     30     mp_zero (a);
     31     return;
     32   }
     33 
     34   {
     35     register mp_digit *bottom, *top;
     36 
     37     /* shift the digits down */
     38 
     39     /* bottom */
     40     bottom = a->dp;
     41 
     42     /* top [offset into digits] */
     43     top = a->dp + b;
     44 
     45     /* this is implemented as a sliding window where
     46      * the window is b-digits long and digits from
     47      * the top of the window are copied to the bottom
     48      *
     49      * e.g.
     50 
     51      b-2 | b-1 | b0 | b1 | b2 | ... | bb |   ---->
     52                  /\                   |      ---->
     53                   \-------------------/      ---->
     54      */
     55     for (x = 0; x < (a->used - b); x++) {
     56       *bottom++ = *top++;
     57     }
     58 
     59     /* zero the top digits */
     60     for (; x < a->used; x++) {
     61       *bottom++ = 0;
     62     }
     63   }
     64 
     65   /* remove excess digits */
     66   a->used -= b;
     67 }
     68 #endif
     69 
     70 /* $Source: /cvs/libtom/libtommath/bn_mp_rshd.c,v $ */
     71 /* $Revision: 1.3 $ */
     72 /* $Date: 2006/03/31 14:18:44 $ */
     73