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comparison libtommath/bn_mp_div_2d.c @ 284:eed26cff980b

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 6c790cad5a7fa866ad062cb3a0c279f7ba788583) to branch 'au.asn.ucc.matt.dropbear' (head fff0894a0399405a9410ea1c6d118f342cf2aa64)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:49 +0000
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children 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
1 #include <tommath.h>
2 #ifdef BN_MP_DIV_2D_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
4 *
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, [email protected], http://math.libtomcrypt.org
16 */
17
18 /* shift right by a certain bit count (store quotient in c, optional remainder in d) */
19 int mp_div_2d (mp_int * a, int b, mp_int * c, mp_int * d)
20 {
21 mp_digit D, r, rr;
22 int x, res;
23 mp_int t;
24
25
26 /* if the shift count is <= 0 then we do no work */
27 if (b <= 0) {
28 res = mp_copy (a, c);
29 if (d != NULL) {
30 mp_zero (d);
31 }
32 return res;
33 }
34
35 if ((res = mp_init (&t)) != MP_OKAY) {
36 return res;
37 }
38
39 /* get the remainder */
40 if (d != NULL) {
41 if ((res = mp_mod_2d (a, b, &t)) != MP_OKAY) {
42 mp_clear (&t);
43 return res;
44 }
45 }
46
47 /* copy */
48 if ((res = mp_copy (a, c)) != MP_OKAY) {
49 mp_clear (&t);
50 return res;
51 }
52
53 /* shift by as many digits in the bit count */
54 if (b >= (int)DIGIT_BIT) {
55 mp_rshd (c, b / DIGIT_BIT);
56 }
57
58 /* shift any bit count < DIGIT_BIT */
59 D = (mp_digit) (b % DIGIT_BIT);
60 if (D != 0) {
61 register mp_digit *tmpc, mask, shift;
62
63 /* mask */
64 mask = (((mp_digit)1) << D) - 1;
65
66 /* shift for lsb */
67 shift = DIGIT_BIT - D;
68
69 /* alias */
70 tmpc = c->dp + (c->used - 1);
71
72 /* carry */
73 r = 0;
74 for (x = c->used - 1; x >= 0; x--) {
75 /* get the lower bits of this word in a temp */
76 rr = *tmpc & mask;
77
78 /* shift the current word and mix in the carry bits from the previous word */
79 *tmpc = (*tmpc >> D) | (r << shift);
80 --tmpc;
81
82 /* set the carry to the carry bits of the current word found above */
83 r = rr;
84 }
85 }
86 mp_clamp (c);
87 if (d != NULL) {
88 mp_exch (&t, d);
89 }
90 mp_clear (&t);
91 return MP_OKAY;
92 }
93 #endif