### view libtommath/bn_s_mp_sqr_fast.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_SQR_FAST_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* the jist of squaring...
* you do like mult except the offset of the tmpx [one that
* starts closer to zero] can't equal the offset of tmpy.
* So basically you set up iy like before then you min it with
* (ty-tx) so that it never happens.  You double all those
* you add in the inner loop

After that loop you do the squares and add them in.
*/

mp_err s_mp_sqr_fast(const mp_int *a, mp_int *b)
{
int       olduse, pa, ix, iz;
mp_digit  W[MP_WARRAY], *tmpx;
mp_word   W1;
mp_err    err;

/* grow the destination as required */
pa = a->used + a->used;
if (b->alloc < pa) {
if ((err = mp_grow(b, pa)) != MP_OKAY) {
return err;
}
}

/* number of output digits to produce */
W1 = 0;
for (ix = 0; ix < pa; ix++) {
int      tx, ty, iy;
mp_word  _W;
mp_digit *tmpy;

/* clear counter */
_W = 0;

/* get offsets into the two bignums */
ty = MP_MIN(a->used-1, ix);
tx = ix - ty;

/* setup temp aliases */
tmpx = a->dp + tx;
tmpy = a->dp + ty;

/* this is the number of times the loop will iterrate, essentially
while (tx++ < a->used && ty-- >= 0) { ... }
*/
iy = MP_MIN(a->used-tx, ty+1);

/* now for squaring tx can never equal ty
* we halve the distance since they approach at a rate of 2x
* and we have to round because odd cases need to be executed
*/
iy = MP_MIN(iy, ((ty-tx)+1)>>1);

/* execute loop */
for (iz = 0; iz < iy; iz++) {
_W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
}

/* double the inner product and add carry */
_W = _W + _W + W1;

/* even columns have the square term in them */
if (((unsigned)ix & 1u) == 0u) {
_W += (mp_word)a->dp[ix>>1] * (mp_word)a->dp[ix>>1];
}

/* store it */

/* make next carry */
W1 = _W >> (mp_word)MP_DIGIT_BIT;
}

/* setup dest */
olduse  = b->used;
b->used = a->used+a->used;

{
mp_digit *tmpb;
tmpb = b->dp;
for (ix = 0; ix < pa; ix++) {