### view libtommath/bn_s_mp_mul_high_digs.c @ 1470:8bba51a55704

Update to libtommath v1.0.1
author Matt Johnston Thu, 08 Feb 2018 23:11:40 +0800 60fc6476e044 f52919ffd3b1
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```#include <tommath_private.h>
#ifdef BN_S_MP_MUL_HIGH_DIGS_C
/* 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
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://libtom.org
*/

/* multiplies |a| * |b| and does not compute the lower digs digits
* [meant to get the higher part of the product]
*/
int
s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
{
mp_int  t;
int     res, pa, pb, ix, iy;
mp_digit u;
mp_word r;
mp_digit tmpx, *tmpt, *tmpy;

/* can we use the fast multiplier? */
#ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
if (((a->used + b->used + 1) < MP_WARRAY)
&& (MIN (a->used, b->used) < (1 << ((CHAR_BIT * sizeof(mp_word)) - (2 * DIGIT_BIT))))) {
return fast_s_mp_mul_high_digs (a, b, c, digs);
}
#endif

if ((res = mp_init_size (&t, a->used + b->used + 1)) != MP_OKAY) {
return res;
}
t.used = a->used + b->used + 1;

pa = a->used;
pb = b->used;
for (ix = 0; ix < pa; ix++) {
/* clear the carry */
u = 0;

/* left hand side of A[ix] * B[iy] */
tmpx = a->dp[ix];

/* alias to the address of where the digits will be stored */
tmpt = &(t.dp[digs]);

/* alias for where to read the right hand side from */
tmpy = b->dp + (digs - ix);

for (iy = digs - ix; iy < pb; iy++) {
/* calculate the double precision result */
r       = (mp_word)*tmpt +
((mp_word)tmpx * (mp_word)*tmpy++) +
(mp_word)u;

/* get the lower part */
*tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));

/* carry the carry */
u       = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
}
*tmpt = u;
}
mp_clamp (&t);
mp_exch (&t, c);
mp_clear (&t);
return MP_OKAY;
}
#endif

/* ref:         \$Format:%D\$ */
/* git commit:  \$Format:%H\$ */
/* commit time: \$Format:%ai\$ */
```