/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is a library that provides multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library was designed directly after the MPI library by
* 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, tomstdenis@iahu.ca, http://math.libtomcrypt.org
*/
#include <tommath.h>
/* this is a modified version of fast_s_mp_mul_digs that only produces
* output digits *above* digs. See the comments for fast_s_mp_mul_digs
* 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.
*
* Based on Algorithm 14.12 on pp.595 of HAC.
*/
int
fast_s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
{
int oldused, newused, res, pa, pb, ix;
mp_word W[MP_WARRAY];
/* calculate size of product and allocate more space if required */
newused = a->used + b->used + 1;
if (c->alloc < newused) {
if ((res = mp_grow (c, newused)) != MP_OKAY) {
return res;
}
}
/* like the other comba method we compute the columns first */
pa = a->used;
pb = b->used;
memset (W + digs, 0, (pa + pb + 1 - digs) * sizeof (mp_word));
for (ix = 0; ix < pa; ix++) {
{
register mp_digit tmpx, *tmpy;
register int iy;
register mp_word *_W;
/* work todo, that is we only calculate digits that are at "digs" or above */
iy = digs - ix;
/* copy of word on the left of A[ix] * B[iy] */
tmpx = a->dp[ix];
/* alias for right side */
tmpy = b->dp + iy;
/* alias for the columns of output. Offset to be equal to or above the
* smallest digit place requested
*/
_W = W + digs;
/* skip cases below zero where ix > digs */
if (iy < 0) {
iy = abs(iy);
tmpy += iy;
_W += iy;
iy = 0;
}
/* compute column products for digits above the minimum */
for (; iy < pb; iy++) {
*_W++ += ((mp_word) tmpx) * ((mp_word)*tmpy++);
}
}
}
/* setup dest */
oldused = c->used;
c->used = newused;
/* now convert the array W downto what we need
*
* See comments in bn_fast_s_mp_mul_digs.c
*/
for (ix = digs + 1; ix < newused; ix++) {
W[ix] += (W[ix - 1] >> ((mp_word) DIGIT_BIT));
c->dp[ix - 1] = (mp_digit) (W[ix - 1] & ((mp_word) MP_MASK));
}
c->dp[newused - 1] = (mp_digit) (W[newused - 1] & ((mp_word) MP_MASK));
for (; ix < oldused; ix++) {
c->dp[ix] = 0;
}
mp_clamp (c);
return MP_OKAY;
}
