1 /* $NetBSD: bn_s_mp_sub.c,v 1.2 2017/01/28 21:31:47 christos Exp $ */
2
3 #include <tommath.h>
4 #ifdef BN_S_MP_SUB_C
5 /* LibTomMath, multiple-precision integer library -- Tom St Denis
6 *
7 * LibTomMath is a library that provides multiple-precision
8 * integer arithmetic as well as number theoretic functionality.
9 *
10 * The library was designed directly after the MPI library by
11 * Michael Fromberger but has been written from scratch with
12 * additional optimizations in place.
13 *
14 * The library is free for all purposes without any express
15 * guarantee it works.
16 *
17 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
18 */
19
20 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
21 int
s_mp_sub(mp_int * a,mp_int * b,mp_int * c)22 s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
23 {
24 int olduse, res, min, max;
25
26 /* find sizes */
27 min = b->used;
28 max = a->used;
29
30 /* init result */
31 if (c->alloc < max) {
32 if ((res = mp_grow (c, max)) != MP_OKAY) {
33 return res;
34 }
35 }
36 olduse = c->used;
37 c->used = max;
38
39 {
40 register mp_digit u, *tmpa, *tmpb, *tmpc;
41 register int i;
42
43 /* alias for digit pointers */
44 tmpa = a->dp;
45 tmpb = b->dp;
46 tmpc = c->dp;
47
48 /* set carry to zero */
49 u = 0;
50 for (i = 0; i < min; i++) {
51 /* T[i] = A[i] - B[i] - U */
52 *tmpc = *tmpa++ - *tmpb++ - u;
53
54 /* U = carry bit of T[i]
55 * Note this saves performing an AND operation since
56 * if a carry does occur it will propagate all the way to the
57 * MSB. As a result a single shift is enough to get the carry
58 */
59 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
60
61 /* Clear carry from T[i] */
62 *tmpc++ &= MP_MASK;
63 }
64
65 /* now copy higher words if any, e.g. if A has more digits than B */
66 for (; i < max; i++) {
67 /* T[i] = A[i] - U */
68 *tmpc = *tmpa++ - u;
69
70 /* U = carry bit of T[i] */
71 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
72
73 /* Clear carry from T[i] */
74 *tmpc++ &= MP_MASK;
75 }
76
77 /* clear digits above used (since we may not have grown result above) */
78 for (i = c->used; i < olduse; i++) {
79 *tmpc++ = 0;
80 }
81 }
82
83 mp_clamp (c);
84 return MP_OKAY;
85 }
86
87 #endif
88
89 /* Source: /cvs/libtom/libtommath/bn_s_mp_sub.c,v */
90 /* Revision: 1.4 */
91 /* Date: 2006/12/28 01:25:13 */
92