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

   1 #include <tommath.h>
   2 #ifdef BN_MP_SUB_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 subtraction */
  19 int
  20 mp_sub_d (mp_int * a, mp_digit b, mp_int * c)
  21 {
  22   mp_digit *tmpa, *tmpc, mu;
  23   int       res, ix, oldused;
  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 just do an unsigned
  33    * addition [with fudged signs]
  34    */
  35   if (a->sign == MP_NEG) {
  36      a->sign = MP_ZPOS;
  37      res     = mp_add_d(a, b, c);
  38      a->sign = c->sign = MP_NEG;
  39
  40      /* clamp */
  41      mp_clamp(c);
  42
  43      return res;
  44   }
  45
  46   /* setup regs */
  47   oldused = c->used;
  48   tmpa    = a->dp;
  49   tmpc    = c->dp;
  50
  51   /* if a <= b simply fix the single digit */
  52   if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) {
  53      if (a->used == 1) {
  54         *tmpc++ = b - *tmpa;
  55      } else {
  56         *tmpc++ = b;
  57      }
  58      ix      = 1;
  59
  60      /* negative/1digit */
  61      c->sign = MP_NEG;
  62      c->used = 1;
  63   } else {
  64      /* positive/size */
  65      c->sign = MP_ZPOS;
  66      c->used = a->used;
  67
  68      /* subtract first digit */
  69      *tmpc    = *tmpa++ - b;
  70      mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
  71      *tmpc++ &= MP_MASK;
  72
  73      /* handle rest of the digits */
  74      for (ix = 1; ix < a->used; ix++) {
  75         *tmpc    = *tmpa++ - mu;
  76         mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
  77         *tmpc++ &= MP_MASK;
  78      }
  79   }
  80
  81   /* zero excess digits */
  82   while (ix++ < oldused) {
  83      *tmpc++ = 0;
  84   }
  85   mp_clamp(c);
  86   return MP_OKAY;
  87 }
  88
  89 #endif
  90
  91 /* $Source: /cvs/libtom/libtommath/bn_mp_sub_d.c,v $ */
  92 /* $Revision: 1.5 $ */
  93 /* $Date: 2006/03/31 14:18:44 $ */