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#include "tommath_private.h"
#ifdef BN_MP_DIV_3_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* divide by three (based on routine from MPI and the GMP manual) */
mp_err mp_div_3(const mp_int *a, mp_int *c, mp_digit *d)
{
mp_int q;
mp_word w, t;
mp_digit b;
mp_err err;
int ix;
/* b = 2**MP_DIGIT_BIT / 3 */
b = ((mp_word)1 << (mp_word)MP_DIGIT_BIT) / (mp_word)3;
if ((err = mp_init_size(&q, a->used)) != MP_OKAY) {
return err;
}
q.used = a->used;
q.sign = a->sign;
w = 0;
for (ix = a->used - 1; ix >= 0; ix--) {
w = (w << (mp_word)MP_DIGIT_BIT) | (mp_word)a->dp[ix];
if (w >= 3u) {
/* multiply w by [1/3] */
t = (w * (mp_word)b) >> (mp_word)MP_DIGIT_BIT;
/* now subtract 3 * [w/3] from w, to get the remainder */
w -= t+t+t;
/* fixup the remainder as required since
* the optimization is not exact.
*/
while (w >= 3u) {
t += 1u;
w -= 3u;
}
} else {
t = 0;
}
q.dp[ix] = (mp_digit)t;
}
/* [optional] store the remainder */
if (d != NULL) {
*d = (mp_digit)w;
}
/* [optional] store the quotient */
if (c != NULL) {
mp_clamp(&q);
mp_exch(&q, c);
}
mp_clear(&q);
return err;
}
#endif
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