annotate bn_mp_div_d.c @ 282:91fbc376f010 libtommath-orig libtommath-0.35

Import of libtommath 0.35 From ltm-0.35.tar.bz2 SHA1 of 3f193dbae9351e92d02530994fa18236f7fde01c
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:16:18 +0000
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1 #include <tommath.h>
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2 #ifdef BN_MP_DIV_D_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.org
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16 */
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17
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18 static int s_is_power_of_two(mp_digit b, int *p)
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19 {
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20 int x;
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21
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22 for (x = 1; x < DIGIT_BIT; x++) {
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23 if (b == (((mp_digit)1)<<x)) {
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24 *p = x;
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25 return 1;
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26 }
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27 }
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28 return 0;
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29 }
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30
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31 /* single digit division (based on routine from MPI) */
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32 int mp_div_d (mp_int * a, mp_digit b, mp_int * c, mp_digit * d)
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33 {
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34 mp_int q;
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35 mp_word w;
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36 mp_digit t;
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37 int res, ix;
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38
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39 /* cannot divide by zero */
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40 if (b == 0) {
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41 return MP_VAL;
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42 }
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43
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44 /* quick outs */
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45 if (b == 1 || mp_iszero(a) == 1) {
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46 if (d != NULL) {
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47 *d = 0;
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48 }
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49 if (c != NULL) {
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50 return mp_copy(a, c);
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51 }
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52 return MP_OKAY;
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53 }
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54
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55 /* power of two ? */
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56 if (s_is_power_of_two(b, &ix) == 1) {
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57 if (d != NULL) {
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58 *d = a->dp[0] & ((((mp_digit)1)<<ix) - 1);
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59 }
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60 if (c != NULL) {
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61 return mp_div_2d(a, ix, c, NULL);
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62 }
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63 return MP_OKAY;
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64 }
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65
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66 #ifdef BN_MP_DIV_3_C
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67 /* three? */
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68 if (b == 3) {
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69 return mp_div_3(a, c, d);
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70 }
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71 #endif
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72
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73 /* no easy answer [c'est la vie]. Just division */
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74 if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
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75 return res;
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76 }
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77
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78 q.used = a->used;
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79 q.sign = a->sign;
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80 w = 0;
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81 for (ix = a->used - 1; ix >= 0; ix--) {
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82 w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]);
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83
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84 if (w >= b) {
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85 t = (mp_digit)(w / b);
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86 w -= ((mp_word)t) * ((mp_word)b);
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87 } else {
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88 t = 0;
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89 }
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90 q.dp[ix] = (mp_digit)t;
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91 }
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92
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93 if (d != NULL) {
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94 *d = (mp_digit)w;
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95 }
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96
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97 if (c != NULL) {
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98 mp_clamp(&q);
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99 mp_exch(&q, c);
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100 }
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101 mp_clear(&q);
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102
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103 return res;
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104 }
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105
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106 #endif