1 /* $NetBSD: bn_mp_rshd.c,v 1.1.1.2 2014/04/24 12:45:31 pettai Exp $ */
2
3 #include <tommath.h>
4 #ifdef BN_MP_RSHD_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 /* shift right a certain amount of digits */
mp_rshd(mp_int * a,int b)21 void mp_rshd (mp_int * a, int b)
22 {
23 int x;
24
25 /* if b <= 0 then ignore it */
26 if (b <= 0) {
27 return;
28 }
29
30 /* if b > used then simply zero it and return */
31 if (a->used <= b) {
32 mp_zero (a);
33 return;
34 }
35
36 {
37 register mp_digit *bottom, *top;
38
39 /* shift the digits down */
40
41 /* bottom */
42 bottom = a->dp;
43
44 /* top [offset into digits] */
45 top = a->dp + b;
46
47 /* this is implemented as a sliding window where
48 * the window is b-digits long and digits from
49 * the top of the window are copied to the bottom
50 *
51 * e.g.
52
53 b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
54 /\ | ---->
55 \-------------------/ ---->
56 */
57 for (x = 0; x < (a->used - b); x++) {
58 *bottom++ = *top++;
59 }
60
61 /* zero the top digits */
62 for (; x < a->used; x++) {
63 *bottom++ = 0;
64 }
65 }
66
67 /* remove excess digits */
68 a->used -= b;
69 }
70 #endif
71
72 /* Source: /cvs/libtom/libtommath/bn_mp_rshd.c,v */
73 /* Revision: 1.4 */
74 /* Date: 2006/12/28 01:25:13 */
75