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      1 #include <tommath.h>
      2 #ifdef BN_MP_ADD_D_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 /* single digit addition */
     19 int
     20 mp_add_d (mp_int * a, mp_digit b, mp_int * c)
     21 {
     22   int     res, ix, oldused;
     23   mp_digit *tmpa, *tmpc, mu;
     24 
     25   /* grow c as required */
     26   if (c->alloc < a->used + 1) {
     27      if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
     28         return res;
     29      }
     30   }
     31 
     32   /* if a is negative and |a| >= b, call c = |a| - b */
     33   if (a->sign == MP_NEG && (a->used > 1 || a->dp[0] >= b)) {
     34      /* temporarily fix sign of a */
     35      a->sign = MP_ZPOS;
     36 
     37      /* c = |a| - b */
     38      res = mp_sub_d(a, b, c);
     39 
     40      /* fix sign  */
     41      a->sign = c->sign = MP_NEG;
     42 
     43      /* clamp */
     44      mp_clamp(c);
     45 
     46      return res;
     47   }
     48 
     49   /* old number of used digits in c */
     50   oldused = c->used;
     51 
     52   /* sign always positive */
     53   c->sign = MP_ZPOS;
     54 
     55   /* source alias */
     56   tmpa    = a->dp;
     57 
     58   /* destination alias */
     59   tmpc    = c->dp;
     60 
     61   /* if a is positive */
     62   if (a->sign == MP_ZPOS) {
     63      /* add digit, after this we're propagating
     64       * the carry.
     65       */
     66      *tmpc   = *tmpa++ + b;
     67      mu      = *tmpc >> DIGIT_BIT;
     68      *tmpc++ &= MP_MASK;
     69 
     70      /* now handle rest of the digits */
     71      for (ix = 1; ix < a->used; ix++) {
     72         *tmpc   = *tmpa++ + mu;
     73         mu      = *tmpc >> DIGIT_BIT;
     74         *tmpc++ &= MP_MASK;
     75      }
     76      /* set final carry */
     77      ix++;
     78      *tmpc++  = mu;
     79 
     80      /* setup size */
     81      c->used = a->used + 1;
     82   } else {
     83      /* a was negative and |a| < b */
     84      c->used  = 1;
     85 
     86      /* the result is a single digit */
     87      if (a->used == 1) {
     88         *tmpc++  =  b - a->dp[0];
     89      } else {
     90         *tmpc++  =  b;
     91      }
     92 
     93      /* setup count so the clearing of oldused
     94       * can fall through correctly
     95       */
     96      ix       = 1;
     97   }
     98 
     99   /* now zero to oldused */
    100   while (ix++ < oldused) {
    101      *tmpc++ = 0;
    102   }
    103   mp_clamp(c);
    104 
    105   return MP_OKAY;
    106 }
    107 
    108 #endif
    109 
    110 /* $Source: /cvs/libtom/libtommath/bn_mp_add_d.c,v $ */
    111 /* $Revision: 1.4 $ */
    112 /* $Date: 2006/03/31 14:18:44 $ */
    113