annotate bn_fast_s_mp_sqr.c @ 200:c5c969ed76f3 libtommath

propagate from branch 'au.asn.ucc.matt.ltm-orig' (head 7fa10cba9535de3461cedb14b877c24858826204) to branch 'au.asn.ucc.matt.dropbear.ltm' (head fc26f60de0370ab0a281fa41a2d13fb17c9d90a8)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 11 May 2005 16:15:27 +0000
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1 #include <tommath.h>
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2 #ifdef BN_FAST_S_MP_SQR_C
2
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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 /* the jist of squaring...
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19 * you do like mult except the offset of the tmpx [one that
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20 * starts closer to zero] can't equal the offset of tmpy.
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21 * So basically you set up iy like before then you min it with
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22 * (ty-tx) so that it never happens. You double all those
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23 * you add in the inner loop
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24
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25 After that loop you do the squares and add them in.
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26 */
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27
2
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28 int fast_s_mp_sqr (mp_int * a, mp_int * b)
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29 {
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30 int olduse, res, pa, ix, iz;
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31 mp_digit W[MP_WARRAY], *tmpx;
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32 mp_word W1;
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33
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34 /* grow the destination as required */
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35 pa = a->used + a->used;
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36 if (b->alloc < pa) {
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37 if ((res = mp_grow (b, pa)) != MP_OKAY) {
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38 return res;
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39 }
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40 }
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41
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42 /* number of output digits to produce */
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43 W1 = 0;
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44 for (ix = 0; ix < pa; ix++) {
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45 int tx, ty, iy;
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46 mp_word _W;
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47 mp_digit *tmpy;
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48
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49 /* clear counter */
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50 _W = 0;
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51
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52 /* get offsets into the two bignums */
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53 ty = MIN(a->used-1, ix);
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54 tx = ix - ty;
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55
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56 /* setup temp aliases */
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57 tmpx = a->dp + tx;
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58 tmpy = a->dp + ty;
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59
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60 /* this is the number of times the loop will iterrate, essentially
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61 while (tx++ < a->used && ty-- >= 0) { ... }
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62 */
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63 iy = MIN(a->used-tx, ty+1);
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64
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65 /* now for squaring tx can never equal ty
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66 * we halve the distance since they approach at a rate of 2x
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67 * and we have to round because odd cases need to be executed
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68 */
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69 iy = MIN(iy, (ty-tx+1)>>1);
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70
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71 /* execute loop */
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72 for (iz = 0; iz < iy; iz++) {
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73 _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
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74 }
2
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75
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76 /* double the inner product and add carry */
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77 _W = _W + _W + W1;
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78
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79 /* even columns have the square term in them */
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80 if ((ix&1) == 0) {
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81 _W += ((mp_word)a->dp[ix>>1])*((mp_word)a->dp[ix>>1]);
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82 }
2
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83
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84 /* store it */
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85 W[ix] = (mp_digit)(_W & MP_MASK);
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86
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87 /* make next carry */
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88 W1 = _W >> ((mp_word)DIGIT_BIT);
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89 }
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90
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91 /* setup dest */
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92 olduse = b->used;
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93 b->used = a->used+a->used;
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94
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95 {
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96 mp_digit *tmpb;
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97 tmpb = b->dp;
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98 for (ix = 0; ix < pa; ix++) {
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99 *tmpb++ = W[ix] & MP_MASK;
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100 }
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101
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102 /* clear unused digits [that existed in the old copy of c] */
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103 for (; ix < olduse; ix++) {
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104 *tmpb++ = 0;
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105 }
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106 }
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107 mp_clamp (b);
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108 return MP_OKAY;
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109 }
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110 #endif