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annotate libtommath/bn_s_mp_mul_digs.c @ 1680:5e763ad6e2e0

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author Matt Johnston <matt@ucc.asn.au>
date Sun, 24 May 2020 13:34:19 +0800
parents f52919ffd3b1
children 1051e4eea25a
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_MUL_DIGS_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 * SPDX-License-Identifier: Unlicense
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13 */
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14
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15 /* multiplies |a| * |b| and only computes upto digs digits of result
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16 * HAC pp. 595, Algorithm 14.12 Modified so you can control how
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17 * many digits of output are created.
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18 */
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19 int s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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20 {
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21 mp_int t;
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22 int res, pa, pb, ix, iy;
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23 mp_digit u;
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24 mp_word r;
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25 mp_digit tmpx, *tmpt, *tmpy;
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26
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27 /* can we use the fast multiplier? */
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28 if ((digs < (int)MP_WARRAY) &&
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29 (MIN(a->used, b->used) <
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30 (int)(1u << (((size_t)CHAR_BIT * sizeof(mp_word)) - (2u * (size_t)DIGIT_BIT))))) {
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31 return fast_s_mp_mul_digs(a, b, c, digs);
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32 }
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33
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34 if ((res = mp_init_size(&t, digs)) != MP_OKAY) {
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35 return res;
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36 }
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37 t.used = digs;
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38
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39 /* compute the digits of the product directly */
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40 pa = a->used;
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41 for (ix = 0; ix < pa; ix++) {
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42 /* set the carry to zero */
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43 u = 0;
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44
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45 /* limit ourselves to making digs digits of output */
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46 pb = MIN(b->used, digs - ix);
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47
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48 /* setup some aliases */
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49 /* copy of the digit from a used within the nested loop */
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50 tmpx = a->dp[ix];
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51
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52 /* an alias for the destination shifted ix places */
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53 tmpt = t.dp + ix;
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54
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55 /* an alias for the digits of b */
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56 tmpy = b->dp;
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57
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58 /* compute the columns of the output and propagate the carry */
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59 for (iy = 0; iy < pb; iy++) {
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60 /* compute the column as a mp_word */
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61 r = (mp_word)*tmpt +
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62 ((mp_word)tmpx * (mp_word)*tmpy++) +
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63 (mp_word)u;
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64
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65 /* the new column is the lower part of the result */
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66 *tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
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67
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68 /* get the carry word from the result */
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69 u = (mp_digit)(r >> (mp_word)DIGIT_BIT);
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70 }
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71 /* set carry if it is placed below digs */
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72 if ((ix + iy) < digs) {
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73 *tmpt = u;
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74 }
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75 }
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76
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77 mp_clamp(&t);
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78 mp_exch(&t, c);
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79
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80 mp_clear(&t);
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81 return MP_OKAY;
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82 }
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83 #endif
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84
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85 /* ref: HEAD -> master, tag: v1.1.0 */
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86 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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87 /* commit time: 2019-01-28 20:32:32 +0100 */