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comparison bn_mp_rshd.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 /* shift right a certain amount of digits */
18 void mp_rshd (mp_int * a, int b)
19 {
20 int x;
21
22 /* if b <= 0 then ignore it */
23 if (b <= 0) {
24 return;
25 }
26
27 /* if b > used then simply zero it and return */
28 if (a->used <= b) {
29 mp_zero (a);
30 return;
31 }
32
33 {
34 register mp_digit *bottom, *top;
35
36 /* shift the digits down */
37
38 /* bottom */
39 bottom = a->dp;
40
41 /* top [offset into digits] */
42 top = a->dp + b;
43
44 /* this is implemented as a sliding window where
45 * the window is b-digits long and digits from
46 * the top of the window are copied to the bottom
47 *
48 * e.g.
49
50 b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
51 /\ | ---->
52 \-------------------/ ---->
53 */
54 for (x = 0; x < (a->used - b); x++) {
55 *bottom++ = *top++;
56 }
57
58 /* zero the top digits */
59 for (; x < a->used; x++) {
60 *bottom++ = 0;
61 }
62 }
63
64 /* remove excess digits */
65 a->used -= b;
66 }