#include "tommath_private.h"

#ifdef BN_S_MP_MUL_DIGS_C

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

/* SPDX-License-Identifier: Unlicense */

/* multiplies |a| * |b| and only computes upto digs digits of result

 * HAC pp. 595, Algorithm 14.12  Modified so you can control how

 * many digits of output are created.

 */

mp_err s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)

{

   mp_int  t;

   mp_err  err;

   int     pa, pb, ix, iy;

   mp_digit u;

   mp_word r;

   mp_digit tmpx, *tmpt, *tmpy;

   if (digs < 0) {

      return MP_VAL;

   }

   /* can we use the fast multiplier? */

   if ((digs < MP_WARRAY) &&

       (MP_MIN(a->used, b->used) < MP_MAXFAST)) {

      return s_mp_mul_digs_fast(a, b, c, digs);

   }

   if ((err = mp_init_size(&t, digs)) != MP_OKAY) {

      return err;

   }

   t.used = digs;

   /* compute the digits of the product directly */

   pa = a->used;

   for (ix = 0; ix < pa; ix++) {

      /* set the carry to zero */

      u = 0;

      /* limit ourselves to making digs digits of output */

      pb = MP_MIN(b->used, digs - ix);

      /* setup some aliases */

      /* copy of the digit from a used within the nested loop */

      tmpx = a->dp[ix];

      /* an alias for the destination shifted ix places */

      tmpt = t.dp + ix;

      /* an alias for the digits of b */

      tmpy = b->dp;

      /* compute the columns of the output and propagate the carry */

      for (iy = 0; iy < pb; iy++) {

         /* compute the column as a mp_word */

         r       = (mp_word)*tmpt +

                   ((mp_word)tmpx * (mp_word)*tmpy++) +

                   (mp_word)u;

         /* the new column is the lower part of the result */

         *tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);

         /* get the carry word from the result */

         u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);

      }

      /* set carry if it is placed below digs */

      if ((ix + iy) < digs) {

         *tmpt = u;

      }

   }

   mp_clamp(&t);

   mp_exch(&t, c);

   mp_clear(&t);

   return MP_OKAY;

}

#endif