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comparison bn_mp_add_d.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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children d29b64170cf0
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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 /* single digit addition */
18 int
19 mp_add_d (mp_int * a, mp_digit b, mp_int * c)
20 {
21 int res, ix, oldused;
22 mp_digit *tmpa, *tmpc, mu;
23
24 /* grow c as required */
25 if (c->alloc < a->used + 1) {
26 if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
27 return res;
28 }
29 }
30
31 /* if a is negative and |a| >= b, call c = |a| - b */
32 if (a->sign == MP_NEG && (a->used > 1 || a->dp[0] >= b)) {
33 /* temporarily fix sign of a */
34 a->sign = MP_ZPOS;
35
36 /* c = |a| - b */
37 res = mp_sub_d(a, b, c);
38
39 /* fix sign */
40 a->sign = c->sign = MP_NEG;
41
42 return res;
43 }
44
45 /* old number of used digits in c */
46 oldused = c->used;
47
48 /* sign always positive */
49 c->sign = MP_ZPOS;
50
51 /* source alias */
52 tmpa = a->dp;
53
54 /* destination alias */
55 tmpc = c->dp;
56
57 /* if a is positive */
58 if (a->sign == MP_ZPOS) {
59 /* add digit, after this we're propagating
60 * the carry.
61 */
62 *tmpc = *tmpa++ + b;
63 mu = *tmpc >> DIGIT_BIT;
64 *tmpc++ &= MP_MASK;
65
66 /* now handle rest of the digits */
67 for (ix = 1; ix < a->used; ix++) {
68 *tmpc = *tmpa++ + mu;
69 mu = *tmpc >> DIGIT_BIT;
70 *tmpc++ &= MP_MASK;
71 }
72 /* set final carry */
73 ix++;
74 *tmpc++ = mu;
75
76 /* setup size */
77 c->used = a->used + 1;
78 } else {
79 /* a was negative and |a| < b */
80 c->used = 1;
81
82 /* the result is a single digit */
83 if (a->used == 1) {
84 *tmpc++ = b - a->dp[0];
85 } else {
86 *tmpc++ = b;
87 }
88
89 /* setup count so the clearing of oldused
90 * can fall through correctly
91 */
92 ix = 1;
93 }
94
95 /* now zero to oldused */
96 while (ix++ < oldused) {
97 *tmpc++ = 0;
98 }
99 mp_clamp(c);
100
101 return MP_OKAY;
102 }
103