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comparison libtommath/bn_mp_sub_d.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_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.org
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 return res;
40 }
41
42 /* setup regs */
43 oldused = c->used;
44 tmpa = a->dp;
45 tmpc = c->dp;
46
47 /* if a <= b simply fix the single digit */
48 if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) {
49 if (a->used == 1) {
50 *tmpc++ = b - *tmpa;
51 } else {
52 *tmpc++ = b;
53 }
54 ix = 1;
55
56 /* negative/1digit */
57 c->sign = MP_NEG;
58 c->used = 1;
59 } else {
60 /* positive/size */
61 c->sign = MP_ZPOS;
62 c->used = a->used;
63
64 /* subtract first digit */
65 *tmpc = *tmpa++ - b;
66 mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
67 *tmpc++ &= MP_MASK;
68
69 /* handle rest of the digits */
70 for (ix = 1; ix < a->used; ix++) {
71 *tmpc = *tmpa++ - mu;
72 mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
73 *tmpc++ &= MP_MASK;
74 }
75 }
76
77 /* zero excess digits */
78 while (ix++ < oldused) {
79 *tmpc++ = 0;
80 }
81 mp_clamp(c);
82 return MP_OKAY;
83 }
84
85 #endif