libtommath/bn_mp_div_2d.c

#include "tommath_private.h"
#ifdef BN_MP_DIV_2D_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
 */

/* shift right by a certain bit count (store quotient in c, optional remainder in d) */
int mp_div_2d(const mp_int *a, int b, mp_int *c, mp_int *d)
{
   mp_digit D, r, rr;
   int     x, res;

   /* if the shift count is <= 0 then we do no work */
   if (b <= 0) {
      res = mp_copy(a, c);
      if (d != NULL) {
         mp_zero(d);
      }
      return res;
   }

   /* copy */
   if ((res = mp_copy(a, c)) != MP_OKAY) {
      return res;
   }
   /* 'a' should not be used after here - it might be the same as d */

   /* get the remainder */
   if (d != NULL) {
      if ((res = mp_mod_2d(a, b, d)) != MP_OKAY) {
         return res;
      }
   }

   /* shift by as many digits in the bit count */
   if (b >= DIGIT_BIT) {
      mp_rshd(c, b / DIGIT_BIT);
   }

   /* shift any bit count < DIGIT_BIT */
   D = (mp_digit)(b % DIGIT_BIT);
   if (D != 0u) {
      mp_digit *tmpc, mask, shift;

      /* mask */
      mask = ((mp_digit)1 << D) - 1uL;

      /* shift for lsb */
      shift = (mp_digit)DIGIT_BIT - D;

      /* alias */
      tmpc = c->dp + (c->used - 1);

      /* carry */
      r = 0;
      for (x = c->used - 1; x >= 0; x--) {
         /* get the lower  bits of this word in a temp */
         rr = *tmpc & mask;

         /* shift the current word and mix in the carry bits from the previous word */
         *tmpc = (*tmpc >> D) | (r << shift);
         --tmpc;

         /* set the carry to the carry bits of the current word found above */
         r = rr;
      }
   }
   mp_clamp(c);
   return MP_OKAY;
}
#endif