annotate bn_s_mp_mul_digs.c @ 142:d29b64170cf0 libtommath-orig

import of libtommath 0.32
author Matt Johnston <matt@ucc.asn.au>
date Sun, 19 Dec 2004 11:33:56 +0000
parents 86e0b50a9b58
children d8254fc979e9
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142
d29b64170cf0 import of libtommath 0.32
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1 #include <tommath.h>
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2 #ifdef BN_S_MP_MUL_DIGS_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 /* 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
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23 s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
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24 {
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25 mp_int t;
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26 int res, pa, pb, ix, iy;
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27 mp_digit u;
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28 mp_word r;
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29 mp_digit tmpx, *tmpt, *tmpy;
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30
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31 /* can we use the fast multiplier? */
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32 if (((digs) < MP_WARRAY) &&
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33 MIN (a->used, b->used) <
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34 (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
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35 return fast_s_mp_mul_digs (a, b, c, digs);
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36 }
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37
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38 if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
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39 return res;
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40 }
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41 t.used = digs;
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42
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43 /* compute the digits of the product directly */
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44 pa = a->used;
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45 for (ix = 0; ix < pa; ix++) {
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46 /* set the carry to zero */
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47 u = 0;
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48
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49 /* limit ourselves to making digs digits of output */
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50 pb = MIN (b->used, digs - ix);
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51
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52 /* setup some aliases */
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53 /* copy of the digit from a used within the nested loop */
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54 tmpx = a->dp[ix];
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55
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56 /* an alias for the destination shifted ix places */
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57 tmpt = t.dp + ix;
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58
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59 /* an alias for the digits of b */
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60 tmpy = b->dp;
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61
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62 /* compute the columns of the output and propagate the carry */
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63 for (iy = 0; iy < pb; iy++) {
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64 /* compute the column as a mp_word */
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65 r = ((mp_word)*tmpt) +
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66 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
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67 ((mp_word) u);
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68
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69 /* the new column is the lower part of the result */
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70 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
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71
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72 /* get the carry word from the result */
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73 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
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74 }
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75 /* set carry if it is placed below digs */
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76 if (ix + iy < digs) {
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77 *tmpt = u;
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78 }
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79 }
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80
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81 mp_clamp (&t);
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82 mp_exch (&t, c);
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83
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84 mp_clear (&t);
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85 return MP_OKAY;
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86 }
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87 #endif