release/src/router/dropbear/libtommath/bn_mp_add_d.c

   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@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 $ */