### view libtommath/bn_mp_root_u32.c @ 1734:73646de50f13DROPBEAR_2020.80

version 2020.80
author Matt Johnston Fri, 26 Jun 2020 21:45:59 +0800 1051e4eea25a
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line source
```#include "tommath_private.h"
#ifdef BN_MP_ROOT_U32_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* find the n'th root of an integer
*
* Result found such that (c)**b <= a and (c+1)**b > a
*
* This algorithm uses Newton's approximation
* x[i+1] = x[i] - f(x[i])/f'(x[i])
* which will find the root in log(N) time where
* each step involves a fair bit.
*/
mp_err mp_root_u32(const mp_int *a, uint32_t b, mp_int *c)
{
mp_int t1, t2, t3, a_;
mp_ord cmp;
int    ilog2;
mp_err err;

/* input must be positive if b is even */
if (((b & 1u) == 0u) && (a->sign == MP_NEG)) {
return MP_VAL;
}

if ((err = mp_init_multi(&t1, &t2, &t3, NULL)) != MP_OKAY) {
return err;
}

/* if a is negative fudge the sign but keep track */
a_ = *a;
a_.sign = MP_ZPOS;

/* Compute seed: 2^(log_2(n)/b + 2)*/
ilog2 = mp_count_bits(a);

/*
If "b" is larger than INT_MAX it is also larger than
log_2(n) because the bit-length of the "n" is measured
with an int and hence the root is always < 2 (two).
*/
if (b > (uint32_t)(INT_MAX/2)) {
mp_set(c, 1uL);
c->sign = a->sign;
err = MP_OKAY;
goto LBL_ERR;
}

/* "b" is smaller than INT_MAX, we can cast safely */
if (ilog2 < (int)b) {
mp_set(c, 1uL);
c->sign = a->sign;
err = MP_OKAY;
goto LBL_ERR;
}
ilog2 =  ilog2 / ((int)b);
if (ilog2 == 0) {
mp_set(c, 1uL);
c->sign = a->sign;
err = MP_OKAY;
goto LBL_ERR;
}
/* Start value must be larger than root */
ilog2 += 2;
if ((err = mp_2expt(&t2,ilog2)) != MP_OKAY)                    goto LBL_ERR;
do {
/* t1 = t2 */
if ((err = mp_copy(&t2, &t1)) != MP_OKAY)                   goto LBL_ERR;

/* t2 = t1 - ((t1**b - a) / (b * t1**(b-1))) */

/* t3 = t1**(b-1) */
if ((err = mp_expt_u32(&t1, b - 1u, &t3)) != MP_OKAY)       goto LBL_ERR;

/* numerator */
/* t2 = t1**b */
if ((err = mp_mul(&t3, &t1, &t2)) != MP_OKAY)               goto LBL_ERR;

/* t2 = t1**b - a */
if ((err = mp_sub(&t2, &a_, &t2)) != MP_OKAY)               goto LBL_ERR;

/* denominator */
/* t3 = t1**(b-1) * b  */
if ((err = mp_mul_d(&t3, b, &t3)) != MP_OKAY)               goto LBL_ERR;

/* t3 = (t1**b - a)/(b * t1**(b-1)) */
if ((err = mp_div(&t2, &t3, &t3, NULL)) != MP_OKAY)         goto LBL_ERR;

if ((err = mp_sub(&t1, &t3, &t2)) != MP_OKAY)               goto LBL_ERR;

/*
Number of rounds is at most log_2(root). If it is more it
got stuck, so break out of the loop and do the rest manually.
*/
if (ilog2-- == 0) {
break;
}
}  while (mp_cmp(&t1, &t2) != MP_EQ);

/* result can be off by a few so check */
/* Loop beneath can overshoot by one if found root is smaller than actual root */
for (;;) {
if ((err = mp_expt_u32(&t1, b, &t2)) != MP_OKAY)            goto LBL_ERR;
cmp = mp_cmp(&t2, &a_);
if (cmp == MP_EQ) {
err = MP_OKAY;
goto LBL_ERR;
}
if (cmp == MP_LT) {
if ((err = mp_add_d(&t1, 1uL, &t1)) != MP_OKAY)          goto LBL_ERR;
} else {
break;
}
}
/* correct overshoot from above or from recurrence */
for (;;) {
if ((err = mp_expt_u32(&t1, b, &t2)) != MP_OKAY)            goto LBL_ERR;
if (mp_cmp(&t2, &a_) == MP_GT) {
if ((err = mp_sub_d(&t1, 1uL, &t1)) != MP_OKAY)          goto LBL_ERR;
} else {
break;
}
}

/* set the result */
mp_exch(&t1, c);

/* set the sign of the result */
c->sign = a->sign;

err = MP_OKAY;

LBL_ERR:
mp_clear_multi(&t1, &t2, &t3, NULL);
return err;
}

#endif
```