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comparison bn_mp_sub.c @ 1:22d5cf7d4b1a libtommath

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Renaming branch
author Matt Johnston <matt@ucc.asn.au>
date Mon, 31 May 2004 18:23:46 +0000
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-1:000000000000 1:22d5cf7d4b1a
1 /* LibTomMath, multiple-precision integer library -- Tom St Denis
2 *
3 * LibTomMath is a library that provides multiple-precision
4 * integer arithmetic as well as number theoretic functionality.
5 *
6 * The library was designed directly after the MPI library by
7 * Michael Fromberger but has been written from scratch with
8 * additional optimizations in place.
9 *
10 * The library is free for all purposes without any express
11 * guarantee it works.
12 *
13 * Tom St Denis, [email protected], http://math.libtomcrypt.org
14 */
15 #include <tommath.h>
16
17 /* high level subtraction (handles signs) */
18 int
19 mp_sub (mp_int * a, mp_int * b, mp_int * c)
20 {
21 int sa, sb, res;
22
23 sa = a->sign;
24 sb = b->sign;
25
26 if (sa != sb) {
27 /* subtract a negative from a positive, OR */
28 /* subtract a positive from a negative. */
29 /* In either case, ADD their magnitudes, */
30 /* and use the sign of the first number. */
31 c->sign = sa;
32 res = s_mp_add (a, b, c);
33 } else {
34 /* subtract a positive from a positive, OR */
35 /* subtract a negative from a negative. */
36 /* First, take the difference between their */
37 /* magnitudes, then... */
38 if (mp_cmp_mag (a, b) != MP_LT) {
39 /* Copy the sign from the first */
40 c->sign = sa;
41 /* The first has a larger or equal magnitude */
42 res = s_mp_sub (a, b, c);
43 } else {
44 /* The result has the *opposite* sign from */
45 /* the first number. */
46 c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
47 /* The second has a larger magnitude */
48 res = s_mp_sub (b, a, c);
49 }
50 }
51 return res;
52 }
53