### view libtommath/bn_mp_reduce.c @ 1938:77bc00dcc19fdefaulttipmainmaster

Bump version to 2022.82
author Matt Johnston Fri, 01 Apr 2022 14:43:27 +0800 1051e4eea25a
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```#include "tommath_private.h"
#ifdef BN_MP_REDUCE_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* reduces x mod m, assumes 0 < x < m**2, mu is
* precomputed via mp_reduce_setup.
* From HAC pp.604 Algorithm 14.42
*/
mp_err mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
{
mp_int  q;
mp_err  err;
int     um = m->used;

/* q = x */
if ((err = mp_init_copy(&q, x)) != MP_OKAY) {
return err;
}

/* q1 = x / b**(k-1)  */
mp_rshd(&q, um - 1);

/* according to HAC this optimization is ok */
if ((mp_digit)um > ((mp_digit)1 << (MP_DIGIT_BIT - 1))) {
if ((err = mp_mul(&q, mu, &q)) != MP_OKAY) {
goto CLEANUP;
}
} else if (MP_HAS(S_MP_MUL_HIGH_DIGS)) {
if ((err = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
goto CLEANUP;
}
} else if (MP_HAS(S_MP_MUL_HIGH_DIGS_FAST)) {
if ((err = s_mp_mul_high_digs_fast(&q, mu, &q, um)) != MP_OKAY) {
goto CLEANUP;
}
} else {
err = MP_VAL;
goto CLEANUP;
}

/* q3 = q2 / b**(k+1) */
mp_rshd(&q, um + 1);

/* x = x mod b**(k+1), quick (no division) */
if ((err = mp_mod_2d(x, MP_DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
goto CLEANUP;
}

/* q = q * m mod b**(k+1), quick (no division) */
if ((err = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
goto CLEANUP;
}

/* x = x - q */
if ((err = mp_sub(x, &q, x)) != MP_OKAY) {
goto CLEANUP;
}

/* If x < 0, add b**(k+1) to it */
if (mp_cmp_d(x, 0uL) == MP_LT) {
mp_set(&q, 1uL);
if ((err = mp_lshd(&q, um + 1)) != MP_OKAY) {
goto CLEANUP;
}
if ((err = mp_add(x, &q, x)) != MP_OKAY) {
goto CLEANUP;
}
}

/* Back off if it's too big */
while (mp_cmp(x, m) != MP_LT) {
if ((err = s_mp_sub(x, m, x)) != MP_OKAY) {
goto CLEANUP;
}
}

CLEANUP:
mp_clear(&q);

return err;
}
#endif
```