release/src/router/dropbear/libtommath/bn_mp_div_2.c

   1 #include <tommath.h>
   2 #ifdef BN_MP_DIV_2_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 /* b = a/2 */
  19 int mp_div_2(mp_int * a, mp_int * b)
  20 {
  21   int     x, res, oldused;
  22
  23   /* copy */
  24   if (b->alloc < a->used) {
  25     if ((res = mp_grow (b, a->used)) != MP_OKAY) {
  26       return res;
  27     }
  28   }
  29
  30   oldused = b->used;
  31   b->used = a->used;
  32   {
  33     register mp_digit r, rr, *tmpa, *tmpb;
  34
  35     /* source alias */
  36     tmpa = a->dp + b->used - 1;
  37
  38     /* dest alias */
  39     tmpb = b->dp + b->used - 1;
  40
  41     /* carry */
  42     r = 0;
  43     for (x = b->used - 1; x >= 0; x--) {
  44       /* get the carry for the next iteration */
  45       rr = *tmpa & 1;
  46
  47       /* shift the current digit, add in carry and store */
  48       *tmpb-- = (*tmpa-- >> 1) | (r << (DIGIT_BIT - 1));
  49
  50       /* forward carry to next iteration */
  51       r = rr;
  52     }
  53
  54     /* zero excess digits */
  55     tmpb = b->dp + b->used;
  56     for (x = b->used; x < oldused; x++) {
  57       *tmpb++ = 0;
  58     }
  59   }
  60   b->sign = a->sign;
  61   mp_clamp (b);
  62   return MP_OKAY;
  63 }
  64 #endif
  65
  66 /* $Source: /cvs/libtom/libtommath/bn_mp_div_2.c,v $ */
  67 /* $Revision: 1.3 $ */
  68 /* $Date: 2006/03/31 14:18:44 $ */