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comparison libtommath/bn_mp_sub_d.c @ 389:5ff8218bcee9

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 2af95f00ebd5bb7a28b3817db1218442c935388e) to branch 'au.asn.ucc.matt.dropbear' (head ecd779509ef23a8cdf64888904fc9b31d78aa933)
author Matt Johnston <matt@ucc.asn.au>
date Thu, 11 Jan 2007 03:14:55 +0000
parents eed26cff980b
children 60fc6476e044
comparison
equal deleted inserted replaced
388:fb54020f78e1 389:5ff8218bcee9
1 #include <tommath.h>
2 #ifdef BN_MP_SUB_D_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.com
16 */
17
18 /* single digit subtraction */
19 int
20 mp_sub_d (mp_int * a, mp_digit b, mp_int * c)
21 {
22 mp_digit *tmpa, *tmpc, mu;
23 int res, ix, oldused;
24
25 /* grow c as required */
26 if (c->alloc < a->used + 1) {
27 if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
28 return res;
29 }
30 }
31
32 /* if a is negative just do an unsigned
33 * addition [with fudged signs]
34 */
35 if (a->sign == MP_NEG) {
36 a->sign = MP_ZPOS;
37 res = mp_add_d(a, b, c);
38 a->sign = c->sign = MP_NEG;
39
40 /* clamp */
41 mp_clamp(c);
42
43 return res;
44 }
45
46 /* setup regs */
47 oldused = c->used;
48 tmpa = a->dp;
49 tmpc = c->dp;
50
51 /* if a <= b simply fix the single digit */
52 if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) {
53 if (a->used == 1) {
54 *tmpc++ = b - *tmpa;
55 } else {
56 *tmpc++ = b;
57 }
58 ix = 1;
59
60 /* negative/1digit */
61 c->sign = MP_NEG;
62 c->used = 1;
63 } else {
64 /* positive/size */
65 c->sign = MP_ZPOS;
66 c->used = a->used;
67
68 /* subtract first digit */
69 *tmpc = *tmpa++ - b;
70 mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
71 *tmpc++ &= MP_MASK;
72
73 /* handle rest of the digits */
74 for (ix = 1; ix < a->used; ix++) {
75 *tmpc = *tmpa++ - mu;
76 mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
77 *tmpc++ &= MP_MASK;
78 }
79 }
80
81 /* zero excess digits */
82 while (ix++ < oldused) {
83 *tmpc++ = 0;
84 }
85 mp_clamp(c);
86 return MP_OKAY;
87 }
88
89 #endif
90
91 /* $Source: /cvs/libtom/libtommath/bn_mp_sub_d.c,v $ */
92 /* $Revision: 1.5 $ */
93 /* $Date: 2006/03/31 14:18:44 $ */