84 lines
2.2 KiB
C
84 lines
2.2 KiB
C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is library that provides for multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library is designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://libtommath.iahu.ca
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*/
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#include <tommath.h>
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/* multiplies |a| * |b| and only computes upto digs digits of result
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* HAC pp. 595, Algorithm 14.12 Modified so you can control how many digits of
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* output are created.
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*/
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int
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s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
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{
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mp_int t;
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int res, pa, pb, ix, iy;
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mp_digit u;
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mp_word r;
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mp_digit tmpx, *tmpt, *tmpy;
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/* can we use the fast multiplier?
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*
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* The fast multiplier can be used if the output will have less than
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* 512 digits and the number of digits won't affect carry propagation
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*/
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if ((digs < 512)
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&& digs < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
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res = fast_s_mp_mul_digs (a, b, c, digs);
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return res;
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}
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if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
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return res;
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}
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t.used = digs;
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/* compute the digits of the product directly */
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pa = a->used;
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for (ix = 0; ix < pa; ix++) {
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/* set the carry to zero */
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u = 0;
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/* limit ourselves to making digs digits of output */
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pb = MIN (b->used, digs - ix);
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/* setup some aliases */
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tmpx = a->dp[ix];
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tmpt = &(t.dp[ix]);
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tmpy = b->dp;
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/* compute the columns of the output and propagate the carry */
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for (iy = 0; iy < pb; iy++) {
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/* compute the column as a mp_word */
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r =
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((mp_word) * tmpt) + ((mp_word) tmpx) * ((mp_word) * tmpy++) +
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((mp_word) u);
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/* the new column is the lower part of the result */
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*tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
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/* get the carry word from the result */
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u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
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}
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if (ix + iy < digs)
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*tmpt = u;
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}
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mp_clamp (&t);
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mp_exch (&t, c);
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mp_clear (&t);
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return MP_OKAY;
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}
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