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annotate 3rdparty/libtommath/bn_fast_s_mp_mul_digs.c @ 40:9b496a0c430a

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author anatofuz
date Tue, 27 Nov 2018 11:25:43 +0900
parents 2cf249471370
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Takahiro SHIMIZU <anatofuz@cr.ie.u-ryukyu.ac.jp>
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1 #include <tommath_private.h>
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2 #ifdef BN_FAST_S_MP_MUL_DIGS_C
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Takahiro SHIMIZU <anatofuz@cr.ie.u-ryukyu.ac.jp>
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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Takahiro SHIMIZU <anatofuz@cr.ie.u-ryukyu.ac.jp>
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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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Takahiro SHIMIZU <anatofuz@cr.ie.u-ryukyu.ac.jp>
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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, tstdenis82@gmail.com, http://libtom.org
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16 */
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17
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Takahiro SHIMIZU <anatofuz@cr.ie.u-ryukyu.ac.jp>
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18 /* Fast (comba) multiplier
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19 *
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20 * This is the fast column-array [comba] multiplier. It is
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Takahiro SHIMIZU <anatofuz@cr.ie.u-ryukyu.ac.jp>
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21 * designed to compute the columns of the product first
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22 * then handle the carries afterwards. This has the effect
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23 * of making the nested loops that compute the columns very
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24 * simple and schedulable on super-scalar processors.
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25 *
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26 * This has been modified to produce a variable number of
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27 * digits of output so if say only a half-product is required
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28 * you don't have to compute the upper half (a feature
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29 * required for fast Barrett reduction).
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30 *
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31 * Based on Algorithm 14.12 on pp.595 of HAC.
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32 *
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33 */
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34 int fast_s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
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35 {
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36 int olduse, res, pa, ix, iz;
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37 mp_digit W[MP_WARRAY];
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38 mp_word _W;
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39
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40 /* grow the destination as required */
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41 if (c->alloc < digs) {
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42 if ((res = mp_grow (c, digs)) != MP_OKAY) {
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43 return res;
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44 }
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45 }
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46
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47 /* number of output digits to produce */
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48 pa = MIN(digs, a->used + b->used);
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49
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50 /* clear the carry */
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51 _W = 0;
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52 for (ix = 0; ix < pa; ix++) {
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53 int tx, ty;
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54 int iy;
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55 mp_digit *tmpx, *tmpy;
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56
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57 /* get offsets into the two bignums */
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58 ty = MIN(b->used-1, ix);
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59 tx = ix - ty;
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60
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61 /* setup temp aliases */
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62 tmpx = a->dp + tx;
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63 tmpy = b->dp + ty;
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64
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65 /* this is the number of times the loop will iterrate, essentially
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66 while (tx++ < a->used && ty-- >= 0) { ... }
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67 */
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68 iy = MIN(a->used-tx, ty+1);
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69
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70 /* execute loop */
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71 for (iz = 0; iz < iy; ++iz) {
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72 _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
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73
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74 }
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75
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76 /* store term */
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77 W[ix] = ((mp_digit)_W) & MP_MASK;
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78
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79 /* make next carry */
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80 _W = _W >> ((mp_word)DIGIT_BIT);
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81 }
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82
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83 /* setup dest */
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84 olduse = c->used;
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85 c->used = pa;
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86
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87 {
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88 mp_digit *tmpc;
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89 tmpc = c->dp;
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90 for (ix = 0; ix < (pa + 1); ix++) {
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91 /* now extract the previous digit [below the carry] */
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92 *tmpc++ = W[ix];
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93 }
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94
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95 /* clear unused digits [that existed in the old copy of c] */
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96 for (; ix < olduse; ix++) {
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97 *tmpc++ = 0;
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98 }
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99 }
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100 mp_clamp (c);
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101 return MP_OKAY;
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102 }
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103 #endif
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104
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105 /* $Source$ */
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106 /* $Revision$ */
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107 /* $Date$ */