Mercurial > dropbear

annotate libtommath/bn_mp_div_3.c @ 310:4b4478d0d2b9 ucc-axis-hack

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add a comment on usage
author Matt Johnston <matt@ucc.asn.au>
date Sun, 26 Mar 2006 08:28:16 +0000
parents eed26cff980b
children 5ff8218bcee9
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284
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1 #include <tommath.h>
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2 #ifdef BN_MP_DIV_3_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 /* divide by three (based on routine from MPI and the GMP manual) */
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19 int
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20 mp_div_3 (mp_int * a, mp_int *c, mp_digit * d)
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21 {
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22 mp_int q;
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23 mp_word w, t;
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24 mp_digit b;
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25 int res, ix;
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26
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27 /* b = 2**DIGIT_BIT / 3 */
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28 b = (((mp_word)1) << ((mp_word)DIGIT_BIT)) / ((mp_word)3);
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29
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30 if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
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31 return res;
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32 }
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33
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34 q.used = a->used;
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35 q.sign = a->sign;
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36 w = 0;
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37 for (ix = a->used - 1; ix >= 0; ix--) {
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38 w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]);
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39
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40 if (w >= 3) {
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41 /* multiply w by [1/3] */
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42 t = (w * ((mp_word)b)) >> ((mp_word)DIGIT_BIT);
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43
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44 /* now subtract 3 * [w/3] from w, to get the remainder */
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45 w -= t+t+t;
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46
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47 /* fixup the remainder as required since
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48 * the optimization is not exact.
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49 */
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50 while (w >= 3) {
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51 t += 1;
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52 w -= 3;
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53 }
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54 } else {
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55 t = 0;
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56 }
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57 q.dp[ix] = (mp_digit)t;
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58 }
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59
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60 /* [optional] store the remainder */
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61 if (d != NULL) {
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62 *d = (mp_digit)w;
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63 }
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64
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65 /* [optional] store the quotient */
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66 if (c != NULL) {
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67 mp_clamp(&q);
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68 mp_exch(&q, c);
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69 }
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70 mp_clear(&q);
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71
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72 return res;
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73 }
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74
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75 #endif