Mercurial > dropbear

annotate libtommath/bn_s_mp_mul_digs.c @ 1612:470aa609c7e4

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author Matt Johnston <matt@ucc.asn.au>
date Tue, 24 Jul 2018 20:19:05 +0800
parents 8bba51a55704
children f52919ffd3b1
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rev   line source
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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 * 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://libtom.org
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16 */
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17
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18 /* multiplies |a| * |b| and only computes upto digs digits of result
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19 * HAC pp. 595, Algorithm 14.12 Modified so you can control how
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20 * many digits of output are created.
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21 */
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22 int s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
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23 {
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24 mp_int t;
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25 int res, pa, pb, ix, iy;
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26 mp_digit u;
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27 mp_word r;
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28 mp_digit tmpx, *tmpt, *tmpy;
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29
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30 /* can we use the fast multiplier? */
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31 if (((digs) < MP_WARRAY) &&
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32 (MIN (a->used, b->used) <
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33 (1 << ((CHAR_BIT * sizeof(mp_word)) - (2 * DIGIT_BIT))))) {
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34 return fast_s_mp_mul_digs (a, b, c, digs);
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35 }
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36
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37 if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
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38 return res;
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39 }
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40 t.used = digs;
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41
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42 /* compute the digits of the product directly */
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43 pa = a->used;
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44 for (ix = 0; ix < pa; ix++) {
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45 /* set the carry to zero */
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46 u = 0;
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47
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48 /* limit ourselves to making digs digits of output */
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49 pb = MIN (b->used, digs - ix);
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50
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51 /* setup some aliases */
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52 /* copy of the digit from a used within the nested loop */
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53 tmpx = a->dp[ix];
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54
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55 /* an alias for the destination shifted ix places */
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56 tmpt = t.dp + ix;
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57
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58 /* an alias for the digits of b */
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59 tmpy = b->dp;
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60
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61 /* compute the columns of the output and propagate the carry */
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62 for (iy = 0; iy < pb; iy++) {
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63 /* compute the column as a mp_word */
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64 r = (mp_word)*tmpt +
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65 ((mp_word)tmpx * (mp_word)*tmpy++) +
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66 (mp_word)u;
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67
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68 /* the new column is the lower part of the result */
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69 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
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70
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71 /* get the carry word from the result */
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72 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
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73 }
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74 /* set carry if it is placed below digs */
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75 if ((ix + iy) < digs) {
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76 *tmpt = u;
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77 }
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78 }
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79
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80 mp_clamp (&t);
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81 mp_exch (&t, c);
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82
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83 mp_clear (&t);
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84 return MP_OKAY;
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85 }
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86 #endif
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87
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88 /* ref: $Format:%D$ */
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89 /* git commit: $Format:%H$ */
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90 /* commit time: $Format:%ai$ */