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comparison libtommath/bn_mp_div_d.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
parents
children 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
1 #include <tommath.h>
2 #ifdef BN_MP_DIV_D_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 static int s_is_power_of_two(mp_digit b, int *p)
19 {
20 int x;
21
22 for (x = 1; x < DIGIT_BIT; x++) {
23 if (b == (((mp_digit)1)<<x)) {
24 *p = x;
25 return 1;
26 }
27 }
28 return 0;
29 }
30
31 /* single digit division (based on routine from MPI) */
32 int mp_div_d (mp_int * a, mp_digit b, mp_int * c, mp_digit * d)
33 {
34 mp_int q;
35 mp_word w;
36 mp_digit t;
37 int res, ix;
38
39 /* cannot divide by zero */
40 if (b == 0) {
41 return MP_VAL;
42 }
43
44 /* quick outs */
45 if (b == 1 || mp_iszero(a) == 1) {
46 if (d != NULL) {
47 *d = 0;
48 }
49 if (c != NULL) {
50 return mp_copy(a, c);
51 }
52 return MP_OKAY;
53 }
54
55 /* power of two ? */
56 if (s_is_power_of_two(b, &ix) == 1) {
57 if (d != NULL) {
58 *d = a->dp[0] & ((((mp_digit)1)<<ix) - 1);
59 }
60 if (c != NULL) {
61 return mp_div_2d(a, ix, c, NULL);
62 }
63 return MP_OKAY;
64 }
65
66 #ifdef BN_MP_DIV_3_C
67 /* three? */
68 if (b == 3) {
69 return mp_div_3(a, c, d);
70 }
71 #endif
72
73 /* no easy answer [c'est la vie]. Just division */
74 if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
75 return res;
76 }
77
78 q.used = a->used;
79 q.sign = a->sign;
80 w = 0;
81 for (ix = a->used - 1; ix >= 0; ix--) {
82 w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]);
83
84 if (w >= b) {
85 t = (mp_digit)(w / b);
86 w -= ((mp_word)t) * ((mp_word)b);
87 } else {
88 t = 0;
89 }
90 q.dp[ix] = (mp_digit)t;
91 }
92
93 if (d != NULL) {
94 *d = (mp_digit)w;
95 }
96
97 if (c != NULL) {
98 mp_clamp(&q);
99 mp_exch(&q, c);
100 }
101 mp_clear(&q);
102
103 return res;
104 }
105
106 #endif