tommath/bn_fast_s_mp_mul_high_digs.c

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#include <tommath.h>
#ifdef BN_FAST_S_MP_MUL_HIGH_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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*/
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/* this is a modified version of fast_s_mul_digs that only produces
* output digits *above* digs. See the comments for fast_s_mul_digs
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* to see how it works.
*
* This is used in the Barrett reduction since for one of the multiplications
* only the higher digits were needed. This essentially halves the work.
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*
* Based on Algorithm 14.12 on pp.595 of HAC.
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*/
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int fast_s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
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{
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int olduse, res, pa, ix, iz;
mp_digit W[MP_WARRAY];
mp_word _W;
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/* grow the destination as required */
pa = a->used + b->used;
if (c->alloc < pa) {
if ((res = mp_grow (c, pa)) != MP_OKAY) {
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return res;
}
}
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/* number of output digits to produce */
pa = a->used + b->used;
_W = 0;
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for (ix = digs; ix < pa; ix++) {
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int tx, ty, iy;
mp_digit *tmpx, *tmpy;
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/* get offsets into the two bignums */
ty = MIN(b->used-1, ix);
tx = ix - ty;
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/* setup temp aliases */
tmpx = a->dp + tx;
tmpy = b->dp + ty;
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/* this is the number of times the loop will iterrate, essentially its
while (tx++ < a->used && ty-- >= 0) { ... }
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*/
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iy = MIN(a->used-tx, ty+1);
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/* execute loop */
for (iz = 0; iz < iy; iz++) {
_W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
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}
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/* store term */
W[ix] = ((mp_digit)_W) & MP_MASK;
/* make next carry */
_W = _W >> ((mp_word)DIGIT_BIT);
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}
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/* setup dest */
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olduse = c->used;
c->used = pa;
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{
register mp_digit *tmpc;
tmpc = c->dp + digs;
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for (ix = digs; ix < pa; ix++) {
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/* now extract the previous digit [below the carry] */
*tmpc++ = W[ix];
}
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/* clear unused digits [that existed in the old copy of c] */
for (; ix < olduse; ix++) {
*tmpc++ = 0;
}
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}
mp_clamp (c);
return MP_OKAY;
}
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#endif
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/* $Source$ */
/* $Revision$ */
/* $Date$ */