### view libtommath/bn_s_mp_mul_digs.c @ 1734:73646de50f13DROPBEAR_2020.80

version 2020.80
author Matt Johnston Fri, 26 Jun 2020 21:45:59 +0800 1051e4eea25a
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```#include "tommath_private.h"
#ifdef BN_S_MP_MUL_DIGS_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* multiplies |a| * |b| and only computes upto digs digits of result
* HAC pp. 595, Algorithm 14.12  Modified so you can control how
* many digits of output are created.
*/
mp_err s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
{
mp_int  t;
mp_err  err;
int     pa, pb, ix, iy;
mp_digit u;
mp_word r;
mp_digit tmpx, *tmpt, *tmpy;

/* can we use the fast multiplier? */
if ((digs < MP_WARRAY) &&
(MP_MIN(a->used, b->used) < MP_MAXFAST)) {
return s_mp_mul_digs_fast(a, b, c, digs);
}

if ((err = mp_init_size(&t, digs)) != MP_OKAY) {
return err;
}
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 = MP_MIN(b->used, digs - ix);

/* setup some aliases */
/* copy of the digit from a used within the nested loop */
tmpx = a->dp[ix];

/* an alias for the destination shifted ix places */
tmpt = t.dp + ix;

/* an alias for the digits of b */
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;

/* the new column is the lower part of the result */

/* get the carry word from the result */
u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
}
/* set carry if it is placed below digs */
if ((ix + iy) < digs) {
*tmpt = u;
}
}

mp_clamp(&t);
mp_exch(&t, c);

mp_clear(&t);
return MP_OKAY;
}
#endif
```