libtommath/bn_mp_div_2.c

#include "tommath_private.h"
#ifdef BN_MP_DIV_2_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.
 *
 * SPDX-License-Identifier: Unlicense
 */

/* b = a/2 */
int mp_div_2(const mp_int *a, mp_int *b)
{
   int     x, res, oldused;

   /* copy */
   if (b->alloc < a->used) {
      if ((res = mp_grow(b, a->used)) != MP_OKAY) {
         return res;
      }
   }

   oldused = b->used;
   b->used = a->used;
   {
      mp_digit r, rr, *tmpa, *tmpb;

      /* source alias */
      tmpa = a->dp + b->used - 1;

      /* dest alias */
      tmpb = b->dp + b->used - 1;

      /* carry */
      r = 0;
      for (x = b->used - 1; x >= 0; x--) {
         /* get the carry for the next iteration */
         rr = *tmpa & 1u;

         /* shift the current digit, add in carry and store */
         *tmpb-- = (*tmpa-- >> 1) | (r << (DIGIT_BIT - 1));

         /* forward carry to next iteration */
         r = rr;
      }

      /* zero excess digits */
      tmpb = b->dp + b->used;
      for (x = b->used; x < oldused; x++) {
         *tmpb++ = 0;
      }
   }
   b->sign = a->sign;
   mp_clamp(b);
   return MP_OKAY;
}
#endif

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