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

   1 #include <tommath.h>
   2 #ifdef BN_MP_SUB_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 /* high level subtraction (handles signs) */
  19 int
  20 mp_sub (mp_int * a, mp_int * b, mp_int * c)
  21 {
  22   int     sa, sb, res;
  23
  24   sa = a->sign;
  25   sb = b->sign;
  26
  27   if (sa != sb) {
  28     /* subtract a negative from a positive, OR */
  29     /* subtract a positive from a negative. */
  30     /* In either case, ADD their magnitudes, */
  31     /* and use the sign of the first number. */
  32     c->sign = sa;
  33     res = s_mp_add (a, b, c);
  34   } else {
  35     /* subtract a positive from a positive, OR */
  36     /* subtract a negative from a negative. */
  37     /* First, take the difference between their */
  38     /* magnitudes, then... */
  39     if (mp_cmp_mag (a, b) != MP_LT) {
  40       /* Copy the sign from the first */
  41       c->sign = sa;
  42       /* The first has a larger or equal magnitude */
  43       res = s_mp_sub (a, b, c);
  44     } else {
  45       /* The result has the *opposite* sign from */
  46       /* the first number. */
  47       c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
  48       /* The second has a larger magnitude */
  49       res = s_mp_sub (b, a, c);
  50     }
  51   }
  52   return res;
  53 }
  54
  55 #endif
  56
  57 /* $Source: /cvs/libtom/libtommath/bn_mp_sub.c,v $ */
  58 /* $Revision: 1.3 $ */
  59 /* $Date: 2006/03/31 14:18:44 $ */