#include "tommath_private.h"

#ifdef BN_S_MP_ADD_C

/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* SPDX-License-Identifier: Unlicense */

/* low level addition, based on HAC pp.594, Algorithm 14.7 */

mp_err s_mp_add(const mp_int *a, const mp_int *b, mp_int *c)

{

   const mp_int *x;

   mp_err err;

   int     olduse, min, max;

   /* find sizes, we let |a| <= |b| which means we have to sort

    * them.  "x" will point to the input with the most digits

    */

   if (a->used > b->used) {

      min = b->used;

      max = a->used;

      x = a;

   } else {

      min = a->used;

      max = b->used;

      x = b;

   }

   /* init result */

   if (c->alloc < (max + 1)) {

      if ((err = mp_grow(c, max + 1)) != MP_OKAY) {

         return err;

      }

   }

   /* get old used digit count and set new one */

   olduse = c->used;

   c->used = max + 1;

   {

      mp_digit u, *tmpa, *tmpb, *tmpc;

      int i;

      /* alias for digit pointers */

      /* first input */

      tmpa = a->dp;

      /* second input */

      tmpb = b->dp;

      /* destination */

      tmpc = c->dp;

      /* zero the carry */

      u = 0;

      for (i = 0; i < min; i++) {

         /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */

         *tmpc = *tmpa++ + *tmpb++ + u;

         /* U = carry bit of T[i] */

         u = *tmpc >> (mp_digit)MP_DIGIT_BIT;

         /* take away carry bit from T[i] */

         *tmpc++ &= MP_MASK;

      }

      /* now copy higher words if any, that is in A+B

       * if A or B has more digits add those in

       */

      if (min != max) {

         for (; i < max; i++) {

            /* T[i] = X[i] + U */

            *tmpc = x->dp[i] + u;

            /* U = carry bit of T[i] */

            u = *tmpc >> (mp_digit)MP_DIGIT_BIT;

            /* take away carry bit from T[i] */

            *tmpc++ &= MP_MASK;

         }

      }

      /* add carry */

      *tmpc++ = u;

      /* clear digits above oldused */

      MP_ZERO_DIGITS(tmpc, olduse - c->used);

   }

   mp_clamp(c);

   return MP_OKAY;

}

#endif