libtommath/mp_reduce_2k.c

#include "tommath_private.h"
#ifdef MP_REDUCE_2K_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */

/* reduces a modulo n where n is of the form 2**p - d */
mp_err mp_reduce_2k(mp_int *a, const mp_int *n, mp_digit d)
{
   mp_int q;
   mp_err err;
   int p;

   if ((err = mp_init(&q)) != MP_OKAY) {
      return err;
   }

   p = mp_count_bits(n);
   for (;;) {
      /* q = a/2**p, a = a mod 2**p */
      if ((err = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
         goto LBL_ERR;
      }

      if (d != 1u) {
         /* q = q * d */
         if ((err = mp_mul_d(&q, d, &q)) != MP_OKAY) {
            goto LBL_ERR;
         }
      }

      /* a = a + q */
      if ((err = s_mp_add(a, &q, a)) != MP_OKAY) {
         goto LBL_ERR;
      }

      if (mp_cmp_mag(a, n) == MP_LT) {
         break;
      }
      if ((err = s_mp_sub(a, n, a)) != MP_OKAY) {
         goto LBL_ERR;
      }
   }

LBL_ERR:
   mp_clear(&q);
   return err;
}

#endif