/home/ntakagi/work/tcl8.5.1/libtommath/bn_mp_sub_d.c Source File

00001 #include <tommath.h>
00002 #ifdef BN_MP_SUB_D_C
00003 /* LibTomMath, multiple-precision integer library -- Tom St Denis
00004  *
00005  * LibTomMath is a library that provides multiple-precision
00006  * integer arithmetic as well as number theoretic functionality.
00007  *
00008  * The library was designed directly after the MPI library by
00009  * Michael Fromberger but has been written from scratch with
00010  * additional optimizations in place.
00011  *
00012  * The library is free for all purposes without any express
00013  * guarantee it works.
00014  *
00015  * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
00016  */
00017 
00018 /* single digit subtraction */
00019 int
00020 mp_sub_d (mp_int * a, mp_digit b, mp_int * c)
00021 {
00022   mp_digit *tmpa, *tmpc, mu;
00023   int       res, ix, oldused;
00024 
00025   /* grow c as required */
00026   if (c->alloc < a->used + 1) {
00027      if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
00028         return res;
00029      }
00030   }
00031 
00032   /* if a is negative just do an unsigned
00033    * addition [with fudged signs]
00034    */
00035   if (a->sign == MP_NEG) {
00036      a->sign = MP_ZPOS;
00037      res     = mp_add_d(a, b, c);
00038      a->sign = c->sign = MP_NEG;
00039 
00040      /* clamp */
00041      mp_clamp(c);
00042 
00043      return res;
00044   }
00045 
00046   /* setup regs */
00047   oldused = c->used;
00048   tmpa    = a->dp;
00049   tmpc    = c->dp;
00050 
00051   /* if a <= b simply fix the single digit */
00052   if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) {
00053      if (a->used == 1) {
00054         *tmpc++ = b - *tmpa;
00055      } else {
00056         *tmpc++ = b;
00057      }
00058      ix      = 1;
00059 
00060      /* negative/1digit */
00061      c->sign = MP_NEG;
00062      c->used = 1;
00063   } else {
00064      /* positive/size */
00065      c->sign = MP_ZPOS;
00066      c->used = a->used;
00067 
00068      /* subtract first digit */
00069      *tmpc    = *tmpa++ - b;
00070      mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
00071      *tmpc++ &= MP_MASK;
00072 
00073      /* handle rest of the digits */
00074      for (ix = 1; ix < a->used; ix++) {
00075         *tmpc    = *tmpa++ - mu;
00076         mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
00077         *tmpc++ &= MP_MASK;
00078      }
00079   }
00080 
00081   /* zero excess digits */
00082   while (ix++ < oldused) {
00083      *tmpc++ = 0;
00084   }
00085   mp_clamp(c);
00086   return MP_OKAY;
00087 }
00088 
00089 #endif
00090 
00091 /* $Source: /cvsroot/tcl/libtommath/bn_mp_sub_d.c,v $ */
00092 /* $Revision: 1.1.1.4 $ */
00093 /* $Date: 2006/12/01 00:08:11 $ */