bn_mp_expt_d.c

#include <tommath.h>
#ifdef BN_MP_EXPT_D_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.
 *
 * The library is free for all purposes without any express
 * guarantee it works.
 *
 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
 */

/* calculate c = a**b  using a square-multiply algorithm */
int mp_expt_d(mp_int *a, mp_digit b, mp_int *c)
{
   int     res;
   mp_int  g;

   if ((res = mp_init_copy(&g, a)) != MP_OKAY) {
      return res;
   }

   /* set initial result */
   mp_set(c, 1);

   while (b > 0) {
      /* if the bit is set multiply */
      if (b & 1) {
         if ((res = mp_mul(c, &g, c)) != MP_OKAY) {
            mp_clear(&g);
            return res;
         }
      }

      /* square */
      if (b > 1 && (res = mp_sqr(&g, &g)) != MP_OKAY) {
         mp_clear(&g);
         return res;
      }

      /* shift to next bit */
      b >>= 1;
   }

   mp_clear(&g);
   return MP_OKAY;
}
#endif

/* $Source$ */
/* $Revision$ */
/* $Date$ */