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comparison libtommath/bn_s_mp_add.c @ 285:1b9e69c058d2

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propagate from branch 'au.asn.ucc.matt.ltc.dropbear' (head 20dccfc09627970a312d77fb41dc2970b62689c3) to branch 'au.asn.ucc.matt.dropbear' (head fdf4a7a3b97ae5046139915de7e40399cceb2c01)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:58 +0000
parents eed26cff980b
children 5ff8218bcee9
comparison
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281:997e6f7dc01e 285:1b9e69c058d2
1 #include <tommath.h>
2 #ifdef BN_S_MP_ADD_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 /* low level addition, based on HAC pp.594, Algorithm 14.7 */
19 int
20 s_mp_add (mp_int * a, mp_int * b, mp_int * c)
21 {
22 mp_int *x;
23 int olduse, res, min, max;
24
25 /* find sizes, we let |a| <= |b| which means we have to sort
26 * them. "x" will point to the input with the most digits
27 */
28 if (a->used > b->used) {
29 min = b->used;
30 max = a->used;
31 x = a;
32 } else {
33 min = a->used;
34 max = b->used;
35 x = b;
36 }
37
38 /* init result */
39 if (c->alloc < max + 1) {
40 if ((res = mp_grow (c, max + 1)) != MP_OKAY) {
41 return res;
42 }
43 }
44
45 /* get old used digit count and set new one */
46 olduse = c->used;
47 c->used = max + 1;
48
49 {
50 register mp_digit u, *tmpa, *tmpb, *tmpc;
51 register int i;
52
53 /* alias for digit pointers */
54
55 /* first input */
56 tmpa = a->dp;
57
58 /* second input */
59 tmpb = b->dp;
60
61 /* destination */
62 tmpc = c->dp;
63
64 /* zero the carry */
65 u = 0;
66 for (i = 0; i < min; i++) {
67 /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
68 *tmpc = *tmpa++ + *tmpb++ + u;
69
70 /* U = carry bit of T[i] */
71 u = *tmpc >> ((mp_digit)DIGIT_BIT);
72
73 /* take away carry bit from T[i] */
74 *tmpc++ &= MP_MASK;
75 }
76
77 /* now copy higher words if any, that is in A+B
78 * if A or B has more digits add those in
79 */
80 if (min != max) {
81 for (; i < max; i++) {
82 /* T[i] = X[i] + U */
83 *tmpc = x->dp[i] + u;
84
85 /* U = carry bit of T[i] */
86 u = *tmpc >> ((mp_digit)DIGIT_BIT);
87
88 /* take away carry bit from T[i] */
89 *tmpc++ &= MP_MASK;
90 }
91 }
92
93 /* add carry */
94 *tmpc++ = u;
95
96 /* clear digits above oldused */
97 for (i = c->used; i < olduse; i++) {
98 *tmpc++ = 0;
99 }
100 }
101
102 mp_clamp (c);
103 return MP_OKAY;
104 }
105 #endif