release/src/router/dropbear/libtommath/bn_mp_rshd.c

   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@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 $ */