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comparison bn_s_mp_add.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 /* low level addition, based on HAC pp.594, Algorithm 14.7 */
18 int
19 s_mp_add (mp_int * a, mp_int * b, mp_int * c)
20 {
21 mp_int *x;
22 int olduse, res, min, max;
23
24 /* find sizes, we let |a| <= |b| which means we have to sort
25 * them. "x" will point to the input with the most digits
26 */
27 if (a->used > b->used) {
28 min = b->used;
29 max = a->used;
30 x = a;
31 } else {
32 min = a->used;
33 max = b->used;
34 x = b;
35 }
36
37 /* init result */
38 if (c->alloc < max + 1) {
39 if ((res = mp_grow (c, max + 1)) != MP_OKAY) {
40 return res;
41 }
42 }
43
44 /* get old used digit count and set new one */
45 olduse = c->used;
46 c->used = max + 1;
47
48 {
49 register mp_digit u, *tmpa, *tmpb, *tmpc;
50 register int i;
51
52 /* alias for digit pointers */
53
54 /* first input */
55 tmpa = a->dp;
56
57 /* second input */
58 tmpb = b->dp;
59
60 /* destination */
61 tmpc = c->dp;
62
63 /* zero the carry */
64 u = 0;
65 for (i = 0; i < min; i++) {
66 /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
67 *tmpc = *tmpa++ + *tmpb++ + u;
68
69 /* U = carry bit of T[i] */
70 u = *tmpc >> ((mp_digit)DIGIT_BIT);
71
72 /* take away carry bit from T[i] */
73 *tmpc++ &= MP_MASK;
74 }
75
76 /* now copy higher words if any, that is in A+B
77 * if A or B has more digits add those in
78 */
79 if (min != max) {
80 for (; i < max; i++) {
81 /* T[i] = X[i] + U */
82 *tmpc = x->dp[i] + u;
83
84 /* U = carry bit of T[i] */
85 u = *tmpc >> ((mp_digit)DIGIT_BIT);
86
87 /* take away carry bit from T[i] */
88 *tmpc++ &= MP_MASK;
89 }
90 }
91
92 /* add carry */
93 *tmpc++ = u;
94
95 /* clear digits above oldused */
96 for (i = c->used; i < olduse; i++) {
97 *tmpc++ = 0;
98 }
99 }
100
101 mp_clamp (c);
102 return MP_OKAY;
103 }