crypto/external/bsd/heimdal/dist/lib/hcrypto/libtommath/bn_mp_rshd.c

   1 /*      $NetBSD: bn_mp_rshd.c,v 1.1.1.2 2014/04/24 12:45:31 pettai Exp $        */
   2
   3 #include <tommath.h>
   4 #ifdef BN_MP_RSHD_C
   5 /* LibTomMath, multiple-precision integer library -- Tom St Denis
   6  *
   7  * LibTomMath is a library that provides multiple-precision
   8  * integer arithmetic as well as number theoretic functionality.
   9  *
  10  * The library was designed directly after the MPI library by
  11  * Michael Fromberger but has been written from scratch with
  12  * additional optimizations in place.
  13  *
  14  * The library is free for all purposes without any express
  15  * guarantee it works.
  16  *
  17  * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
  18  */
  19
  20 /* shift right a certain amount of digits */
  21 void mp_rshd (mp_int * a, int b)
  22 {
  23   int     x;
  24
  25   /* if b <= 0 then ignore it */
  26   if (b <= 0) {
  27     return;
  28   }
  29
  30   /* if b > used then simply zero it and return */
  31   if (a->used <= b) {
  32     mp_zero (a);
  33     return;
  34   }
  35
  36   {
  37     register mp_digit *bottom, *top;
  38
  39     /* shift the digits down */
  40
  41     /* bottom */
  42     bottom = a->dp;
  43
  44     /* top [offset into digits] */
  45     top = a->dp + b;
  46
  47     /* this is implemented as a sliding window where
  48      * the window is b-digits long and digits from
  49      * the top of the window are copied to the bottom
  50      *
  51      * e.g.
  52
  53      b-2 | b-1 | b0 | b1 | b2 | ... | bb |   ---->
  54                  /\                   |      ---->
  55                   \-------------------/      ---->
  56      */
  57     for (x = 0; x < (a->used - b); x++) {
  58       *bottom++ = *top++;
  59     }
  60
  61     /* zero the top digits */
  62     for (; x < a->used; x++) {
  63       *bottom++ = 0;
  64     }
  65   }
  66
  67   /* remove excess digits */
  68   a->used -= b;
  69 }
  70 #endif
  71
  72 /* Source: /cvs/libtom/libtommath/bn_mp_rshd.c,v  */
  73 /* Revision: 1.4  */
  74 /* Date: 2006/12/28 01:25:13  */