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comparison libtommath/bn_mp_sub.c @ 284:eed26cff980b

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 6c790cad5a7fa866ad062cb3a0c279f7ba788583) to branch 'au.asn.ucc.matt.dropbear' (head fff0894a0399405a9410ea1c6d118f342cf2aa64)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:49 +0000
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children 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
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, [email protected], http://math.libtomcrypt.org
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