Line data    Source code

       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://libtom.org
      16             :  */
      17             : 
      18             : /* single digit subtraction */
      19             : int
      20           0 : 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           0 :   if (c->alloc < a->used + 1) {
      27           0 :      if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
      28           0 :         return res;
      29             :      }
      30             :   }
      31             : 
      32             :   /* if a is negative just do an unsigned
      33             :    * addition [with fudged signs]
      34             :    */
      35           0 :   if (a->sign == MP_NEG) {
      36           0 :      a->sign = MP_ZPOS;
      37           0 :      res     = mp_add_d(a, b, c);
      38           0 :      a->sign = c->sign = MP_NEG;
      39             : 
      40             :      /* clamp */
      41           0 :      mp_clamp(c);
      42             : 
      43           0 :      return res;
      44             :   }
      45             : 
      46             :   /* setup regs */
      47           0 :   oldused = c->used;
      48           0 :   tmpa    = a->dp;
      49           0 :   tmpc    = c->dp;
      50             : 
      51             :   /* if a <= b simply fix the single digit */
      52           0 :   if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) {
      53           0 :      if (a->used == 1) {
      54           0 :         *tmpc++ = b - *tmpa;
      55             :      } else {
      56           0 :         *tmpc++ = b;
      57             :      }
      58           0 :      ix      = 1;
      59             : 
      60             :      /* negative/1digit */
      61           0 :      c->sign = MP_NEG;
      62           0 :      c->used = 1;
      63             :   } else {
      64             :      /* positive/size */
      65           0 :      c->sign = MP_ZPOS;
      66           0 :      c->used = a->used;
      67             : 
      68             :      /* subtract first digit */
      69           0 :      *tmpc    = *tmpa++ - b;
      70           0 :      mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
      71           0 :      *tmpc++ &= MP_MASK;
      72             : 
      73             :      /* handle rest of the digits */
      74           0 :      for (ix = 1; ix < a->used; ix++) {
      75           0 :         *tmpc    = *tmpa++ - mu;
      76           0 :         mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
      77           0 :         *tmpc++ &= MP_MASK;
      78             :      }
      79             :   }
      80             : 
      81             :   /* zero excess digits */
      82           0 :   while (ix++ < oldused) {
      83           0 :      *tmpc++ = 0;
      84             :   }
      85           0 :   mp_clamp(c);
      86           0 :   return MP_OKAY;
      87             : }
      88             : 
      89             : #endif
      90             : 
      91             : /* $Source: /cvs/libtom/libtommath/bn_mp_sub_d.c,v $ */
      92             : /* $Revision: 1.6 $ */
      93             : /* $Date: 2006/12/28 01:25:13 $ */