Mercurial > dropbear

annotate libtommath/bn_s_mp_mul_high_digs.c @ 1926:e356093a6812

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Add c89 build test
author Matt Johnston <matt@ucc.asn.au>
date Wed, 30 Mar 2022 14:29:18 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_MUL_HIGH_DIGS_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
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4 /* SPDX-License-Identifier: Unlicense */
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6 /* multiplies |a| * |b| and does not compute the lower digs digits
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7 * [meant to get the higher part of the product]
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8 */
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9 mp_err s_mp_mul_high_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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10 {
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11 mp_int t;
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12 int pa, pb, ix, iy;
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13 mp_err err;
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14 mp_digit u;
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15 mp_word r;
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16 mp_digit tmpx, *tmpt, *tmpy;
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18 /* can we use the fast multiplier? */
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19 if (MP_HAS(S_MP_MUL_HIGH_DIGS_FAST)
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20 && ((a->used + b->used + 1) < MP_WARRAY)
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21 && (MP_MIN(a->used, b->used) < MP_MAXFAST)) {
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22 return s_mp_mul_high_digs_fast(a, b, c, digs);
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23 }
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24
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25 if ((err = mp_init_size(&t, a->used + b->used + 1)) != MP_OKAY) {
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26 return err;
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27 }
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28 t.used = a->used + b->used + 1;
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30 pa = a->used;
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31 pb = b->used;
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32 for (ix = 0; ix < pa; ix++) {
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33 /* clear the carry */
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34 u = 0;
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36 /* left hand side of A[ix] * B[iy] */
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37 tmpx = a->dp[ix];
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39 /* alias to the address of where the digits will be stored */
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40 tmpt = &(t.dp[digs]);
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42 /* alias for where to read the right hand side from */
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43 tmpy = b->dp + (digs - ix);
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44
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45 for (iy = digs - ix; iy < pb; iy++) {
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46 /* calculate the double precision result */
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47 r = (mp_word)*tmpt +
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48 ((mp_word)tmpx * (mp_word)*tmpy++) +
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49 (mp_word)u;
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51 /* get the lower part */
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52 *tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
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54 /* carry the carry */
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55 u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
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56 }
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57 *tmpt = u;
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58 }
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59 mp_clamp(&t);
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60 mp_exch(&t, c);
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61 mp_clear(&t);
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62 return MP_OKAY;
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63 }
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64 #endif