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comparison bn_s_mp_sub.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 subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
18 int
19 s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
20 {
21 int olduse, res, min, max;
22
23 /* find sizes */
24 min = b->used;
25 max = a->used;
26
27 /* init result */
28 if (c->alloc < max) {
29 if ((res = mp_grow (c, max)) != MP_OKAY) {
30 return res;
31 }
32 }
33 olduse = c->used;
34 c->used = max;
35
36 {
37 register mp_digit u, *tmpa, *tmpb, *tmpc;
38 register int i;
39
40 /* alias for digit pointers */
41 tmpa = a->dp;
42 tmpb = b->dp;
43 tmpc = c->dp;
44
45 /* set carry to zero */
46 u = 0;
47 for (i = 0; i < min; i++) {
48 /* T[i] = A[i] - B[i] - U */
49 *tmpc = *tmpa++ - *tmpb++ - u;
50
51 /* U = carry bit of T[i]
52 * Note this saves performing an AND operation since
53 * if a carry does occur it will propagate all the way to the
54 * MSB. As a result a single shift is enough to get the carry
55 */
56 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
57
58 /* Clear carry from T[i] */
59 *tmpc++ &= MP_MASK;
60 }
61
62 /* now copy higher words if any, e.g. if A has more digits than B */
63 for (; i < max; i++) {
64 /* T[i] = A[i] - U */
65 *tmpc = *tmpa++ - u;
66
67 /* U = carry bit of T[i] */
68 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
69
70 /* Clear carry from T[i] */
71 *tmpc++ &= MP_MASK;
72 }
73
74 /* clear digits above used (since we may not have grown result above) */
75 for (i = c->used; i < olduse; i++) {
76 *tmpc++ = 0;
77 }
78 }
79
80 mp_clamp (c);
81 return MP_OKAY;
82 }
83