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      1 #include <tommath.h>
      2 #ifdef BN_S_MP_ADD_C
      3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
      4  *
      5  * LibTomMath is a library that provides multiple-precision
      6  * integer arithmetic as well as number theoretic functionality.
      7  *
      8  * The library was designed directly after the MPI library by
      9  * Michael Fromberger but has been written from scratch with
     10  * additional optimizations in place.
     11  *
     12  * The library is free for all purposes without any express
     13  * guarantee it works.
     14  *
     15  * Tom St Denis, tomstdenis (at) gmail.com, http://math.libtomcrypt.com
     16  */
     17 
     18 /* low level addition, based on HAC pp.594, Algorithm 14.7 */
     19 int
     20 s_mp_add (mp_int * a, mp_int * b, mp_int * c)
     21 {
     22   mp_int *x;
     23   int     olduse, res, min, max;
     24 
     25   /* find sizes, we let |a| <= |b| which means we have to sort
     26    * them.  "x" will point to the input with the most digits
     27    */
     28   if (a->used > b->used) {
     29     min = b->used;
     30     max = a->used;
     31     x = a;
     32   } else {
     33     min = a->used;
     34     max = b->used;
     35     x = b;
     36   }
     37 
     38   /* init result */
     39   if (c->alloc < max + 1) {
     40     if ((res = mp_grow (c, max + 1)) != MP_OKAY) {
     41       return res;
     42     }
     43   }
     44 
     45   /* get old used digit count and set new one */
     46   olduse = c->used;
     47   c->used = max + 1;
     48 
     49   {
     50     register mp_digit u, *tmpa, *tmpb, *tmpc;
     51     register int i;
     52 
     53     /* alias for digit pointers */
     54 
     55     /* first input */
     56     tmpa = a->dp;
     57 
     58     /* second input */
     59     tmpb = b->dp;
     60 
     61     /* destination */
     62     tmpc = c->dp;
     63 
     64     /* zero the carry */
     65     u = 0;
     66     for (i = 0; i < min; i++) {
     67       /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
     68       *tmpc = *tmpa++ + *tmpb++ + u;
     69 
     70       /* U = carry bit of T[i] */
     71       u = *tmpc >> ((mp_digit)DIGIT_BIT);
     72 
     73       /* take away carry bit from T[i] */
     74       *tmpc++ &= MP_MASK;
     75     }
     76 
     77     /* now copy higher words if any, that is in A+B
     78      * if A or B has more digits add those in
     79      */
     80     if (min != max) {
     81       for (; i < max; i++) {
     82         /* T[i] = X[i] + U */
     83         *tmpc = x->dp[i] + u;
     84 
     85         /* U = carry bit of T[i] */
     86         u = *tmpc >> ((mp_digit)DIGIT_BIT);
     87 
     88         /* take away carry bit from T[i] */
     89         *tmpc++ &= MP_MASK;
     90       }
     91     }
     92 
     93     /* add carry */
     94     *tmpc++ = u;
     95 
     96     /* clear digits above oldused */
     97     for (i = c->used; i < olduse; i++) {
     98       *tmpc++ = 0;
     99     }
    100   }
    101 
    102   mp_clamp (c);
    103   return MP_OKAY;
    104 }
    105 #endif
    106 
    107 /* $Source: /cvs/libtom/libtommath/bn_s_mp_add.c,v $ */
    108 /* $Revision: 1.3 $ */
    109 /* $Date: 2006/03/31 14:18:44 $ */
    110