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      1 /*	$NetBSD: bn_s_mp_add.c,v 1.2 2017/01/28 21:31:47 christos Exp $	*/
      2 
      3 #include <tommath.h>
      4 #ifdef BN_S_MP_ADD_C
      5 /* LibTomMath, multiple-precision integer library -- Tom St Denis
      6  *
      7  * LibTomMath is a library that provides multiple-precision
      8  * integer arithmetic as well as number theoretic functionality.
      9  *
     10  * The library was designed directly after the MPI library by
     11  * Michael Fromberger but has been written from scratch with
     12  * additional optimizations in place.
     13  *
     14  * The library is free for all purposes without any express
     15  * guarantee it works.
     16  *
     17  * Tom St Denis, tomstdenis (at) gmail.com, http://libtom.org
     18  */
     19 
     20 /* low level addition, based on HAC pp.594, Algorithm 14.7 */
     21 int
     22 s_mp_add (mp_int * a, mp_int * b, mp_int * c)
     23 {
     24   mp_int *x;
     25   int     olduse, res, min, max;
     26 
     27   /* find sizes, we let |a| <= |b| which means we have to sort
     28    * them.  "x" will point to the input with the most digits
     29    */
     30   if (a->used > b->used) {
     31     min = b->used;
     32     max = a->used;
     33     x = a;
     34   } else {
     35     min = a->used;
     36     max = b->used;
     37     x = b;
     38   }
     39 
     40   /* init result */
     41   if (c->alloc < max + 1) {
     42     if ((res = mp_grow (c, max + 1)) != MP_OKAY) {
     43       return res;
     44     }
     45   }
     46 
     47   /* get old used digit count and set new one */
     48   olduse = c->used;
     49   c->used = max + 1;
     50 
     51   {
     52     register mp_digit u, *tmpa, *tmpb, *tmpc;
     53     register int i;
     54 
     55     /* alias for digit pointers */
     56 
     57     /* first input */
     58     tmpa = a->dp;
     59 
     60     /* second input */
     61     tmpb = b->dp;
     62 
     63     /* destination */
     64     tmpc = c->dp;
     65 
     66     /* zero the carry */
     67     u = 0;
     68     for (i = 0; i < min; i++) {
     69       /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
     70       *tmpc = *tmpa++ + *tmpb++ + u;
     71 
     72       /* U = carry bit of T[i] */
     73       u = *tmpc >> ((mp_digit)DIGIT_BIT);
     74 
     75       /* take away carry bit from T[i] */
     76       *tmpc++ &= MP_MASK;
     77     }
     78 
     79     /* now copy higher words if any, that is in A+B
     80      * if A or B has more digits add those in
     81      */
     82     if (min != max) {
     83       for (; i < max; i++) {
     84         /* T[i] = X[i] + U */
     85         *tmpc = x->dp[i] + u;
     86 
     87         /* U = carry bit of T[i] */
     88         u = *tmpc >> ((mp_digit)DIGIT_BIT);
     89 
     90         /* take away carry bit from T[i] */
     91         *tmpc++ &= MP_MASK;
     92       }
     93     }
     94 
     95     /* add carry */
     96     *tmpc++ = u;
     97 
     98     /* clear digits above oldused */
     99     for (i = c->used; i < olduse; i++) {
    100       *tmpc++ = 0;
    101     }
    102   }
    103 
    104   mp_clamp (c);
    105   return MP_OKAY;
    106 }
    107 #endif
    108 
    109 /* Source: /cvs/libtom/libtommath/bn_s_mp_add.c,v  */
    110 /* Revision: 1.4  */
    111 /* Date: 2006/12/28 01:25:13  */
    112