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annotate tomsfastmath/src/divide/fp_div_d.c @ 646:f10335e5e42f dropbear-tfm

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- More asm constraint fixes. Now seems to build OK on 32-bit OS X.
author Matt Johnston <matt@ucc.asn.au>
date Wed, 30 Nov 2011 23:03:47 +0800
parents a362b62d38b2
children
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1 /* TomsFastMath, a fast ISO C bignum library.
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2 *
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3 * This project is meant to fill in where LibTomMath
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4 * falls short. That is speed ;-)
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5 *
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6 * This project is public domain and free for all purposes.
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7 *
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8 * Tom St Denis, [email protected]
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9 */
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10 #include <tfm.h>
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12 static int s_is_power_of_two(fp_digit b, int *p)
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13 {
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14 int x;
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16 /* fast return if no power of two */
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17 if ((b==0) || (b & (b-1))) {
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18 return 0;
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19 }
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21 for (x = 0; x < DIGIT_BIT; x++) {
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22 if (b == (((fp_digit)1)<<x)) {
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23 *p = x;
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24 return 1;
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25 }
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26 }
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27 return 0;
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28 }
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30 /* a/b => cb + d == a */
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31 int fp_div_d(fp_int *a, fp_digit b, fp_int *c, fp_digit *d)
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32 {
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33 fp_int q;
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34 fp_word w;
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35 fp_digit t;
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36 int ix;
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37
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38 /* cannot divide by zero */
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39 if (b == 0) {
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40 return FP_VAL;
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41 }
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42
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43 /* quick outs */
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44 if (b == 1 || fp_iszero(a) == 1) {
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45 if (d != NULL) {
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46 *d = 0;
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47 }
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48 if (c != NULL) {
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49 fp_copy(a, c);
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50 }
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51 return FP_OKAY;
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52 }
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53
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54 /* power of two ? */
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55 if (s_is_power_of_two(b, &ix) == 1) {
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56 if (d != NULL) {
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57 *d = a->dp[0] & ((((fp_digit)1)<<ix) - 1);
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58 }
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59 if (c != NULL) {
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60 fp_div_2d(a, ix, c, NULL);
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61 }
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62 return FP_OKAY;
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63 }
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64
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65 /* no easy answer [c'est la vie]. Just division */
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66 fp_init(&q);
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67
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68 q.used = a->used;
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69 q.sign = a->sign;
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70 w = 0;
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71 for (ix = a->used - 1; ix >= 0; ix--) {
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72 w = (w << ((fp_word)DIGIT_BIT)) | ((fp_word)a->dp[ix]);
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73
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74 if (w >= b) {
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75 t = (fp_digit)(w / b);
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76 w -= ((fp_word)t) * ((fp_word)b);
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77 } else {
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78 t = 0;
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79 }
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80 q.dp[ix] = (fp_digit)t;
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81 }
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82
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83 if (d != NULL) {
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84 *d = (fp_digit)w;
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85 }
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86
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87 if (c != NULL) {
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88 fp_clamp(&q);
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89 fp_copy(&q, c);
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90 }
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91
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92 return FP_OKAY;
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93 }
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94
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95
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96 /* $Source$ */
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97 /* $Revision$ */
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98 /* $Date$ */