tommath/bn_s_mp_mul_digs.c

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#include <tommath_private.h>
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#ifdef BN_S_MP_MUL_DIGS_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, tstdenis82@gmail.com, http://libtom.org
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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 output are created.
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*/
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int 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;
int res, pa, pb, ix, iy;
mp_digit u;
mp_word r;
mp_digit tmpx, *tmpt, *tmpy;
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/* can we use the fast multiplier? */
if (((digs) < MP_WARRAY) &&
(MIN (a->used, b->used) <
(1 << ((CHAR_BIT * sizeof(mp_word)) - (2 * DIGIT_BIT))))) {
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return fast_s_mp_mul_digs (a, b, c, digs);
}
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if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
return res;
}
t.used = digs;
/* compute the digits of the product directly */
pa = a->used;
for (ix = 0; ix < pa; ix++) {
/* set the carry to zero */
u = 0;
/* limit ourselves to making digs digits of output */
pb = MIN (b->used, digs - ix);
/* setup some aliases */
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/* copy of the digit from a used within the nested loop */
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tmpx = a->dp[ix];
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/* an alias for the destination shifted ix places */
tmpt = t.dp + ix;
/* an alias for the digits of b */
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tmpy = b->dp;
/* compute the columns of the output and propagate the carry */
for (iy = 0; iy < pb; iy++) {
/* compute the column as a mp_word */
r = (mp_word)*tmpt +
((mp_word)tmpx * (mp_word)*tmpy++) +
(mp_word)u;
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/* the new column is the lower part of the result */
*tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
/* 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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/* set carry if it is placed below digs */
if ((ix + iy) < digs) {
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*tmpt = u;
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}
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}
mp_clamp (&t);
mp_exch (&t, c);
mp_clear (&t);
return MP_OKAY;
}
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#endif
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/* $Source$ */
/* $Revision$ */
/* $Date$ */