bn_s_mp_add.c 2.45 KB
Newer Older
1
#include "tommath_private.h"
2 3 4 5 6 7 8 9 10 11
#ifdef BN_S_MP_ADD_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.
 *
12
 * SPDX-License-Identifier: Unlicense
13 14 15
 */

/* low level addition, based on HAC pp.594, Algorithm 14.7 */
16
int s_mp_add(const mp_int *a, const mp_int *b, mp_int *c)
17
{
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
   const mp_int *x;
   int     olduse, res, 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 ((res = mp_grow(c, max + 1)) != MP_OKAY) {
         return res;
      }
   }

   /* 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)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)DIGIT_BIT;

            /* take away carry bit from T[i] */
            *tmpc++ &= MP_MASK;
         }
87 88
      }

89 90
      /* add carry */
      *tmpc++ = u;
91

92 93 94 95 96
      /* clear digits above oldused */
      for (i = c->used; i < olduse; i++) {
         *tmpc++ = 0;
      }
   }
97

98 99
   mp_clamp(c);
   return MP_OKAY;
100 101 102
}
#endif

103 104 105
/* ref:         HEAD -> master, tag: v1.1.0 */
/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
/* commit time: 2019-01-28 20:32:32 +0100 */