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57354e1 added libtommath-0.12
Tom St Denis authored
1 /* LibTomMath, multiple-precision integer library -- Tom St Denis
2 *
3 * LibTomMath is library that provides for multiple-precision
4 * integer arithmetic as well as number theoretic functionality.
5 *
6 * The library is designed directly after the MPI library by
7 * Michael Fromberger but has been written from scratch with
8 * additional optimizations in place.
9 *
10 * The library is free for all purposes without any express
11 * guarantee it works.
12 *
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13 * Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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14 */
15 #include <tommath.h>
16
17 /* multiplies |a| * |b| and only computes upto digs digits of result
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18 * HAC pp. 595, Algorithm 14.12 Modified so you can control how
19 * many digits of output are created.
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20 */
21 int
22 s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
23 {
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24 mp_int t;
25 int res, pa, pb, ix, iy;
26 mp_digit u;
27 mp_word r;
28 mp_digit tmpx, *tmpt, *tmpy;
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29
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30 /* can we use the fast multiplier? */
31 if (((digs) < MP_WARRAY) &&
32 MIN (a->used, b->used) <
33 (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
34 return fast_s_mp_mul_digs (a, b, c, digs);
35 }
36
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37 if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
38 return res;
39 }
40 t.used = digs;
41
42 /* compute the digits of the product directly */
43 pa = a->used;
44 for (ix = 0; ix < pa; ix++) {
45 /* set the carry to zero */
46 u = 0;
47
48 /* limit ourselves to making digs digits of output */
49 pb = MIN (b->used, digs - ix);
50
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
55 /* an alias for the destination shifted ix places */
56 tmpt = t.dp + ix;
57
58 /* an alias for the digits of b */
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59 tmpy = b->dp;
60
61 /* compute the columns of the output and propagate the carry */
62 for (iy = 0; iy < pb; iy++) {
63 /* compute the column as a mp_word */
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64 r = ((mp_word) *tmpt) +
65 ((mp_word) tmpx) * ((mp_word) * tmpy++) +
66 ((mp_word) u);
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67
68 /* the new column is the lower part of the result */
69 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
70
71 /* get the carry word from the result */
72 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
73 }
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74 /* set carry if it is placed below digs */
75 if (ix + iy < digs) {
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76 *tmpt = u;
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77 }
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78 }
79
80 mp_clamp (&t);
81 mp_exch (&t, c);
82
83 mp_clear (&t);
84 return MP_OKAY;
85 }
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