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) */
19int
20mp_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: /cvsroot/tcl/libtommath/bn_mp_sub.c,v $ */
58/* $Revision: 1.1.1.3 $ */
59/* $Date: 2006/12/01 00:08:11 $ */
60