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#include "tommath_private.h"
#ifdef BN_S_MP_LOG_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
mp_err s_mp_log(const mp_int *a, mp_digit base, int *c)
{
mp_err err;
int high, low;
mp_int bracket_low, bracket_high, bracket_mid, t, bi_base;
mp_ord cmp = mp_cmp_d(a, base);
if ((cmp == MP_LT) || (cmp == MP_EQ)) {
*c = cmp == MP_EQ;
return MP_OKAY;
}
if ((err =
mp_init_multi(&bracket_low, &bracket_high,
&bracket_mid, &t, &bi_base, (void *)NULL)) != MP_OKAY) {
return err;
}
low = 0;
mp_set(&bracket_low, 1uL);
high = 1;
mp_set(&bracket_high, base);
/*
A kind of Giant-step/baby-step algorithm.
Idea shamelessly stolen from https://programmingpraxis.com/2010/05/07/integer-logarithms/2/
The effect is asymptotic, hence needs benchmarks to test if the Giant-step should be skipped
for small n.
*/
while (mp_cmp(&bracket_high, a) == MP_LT) {
low = high;
if ((err = mp_copy(&bracket_high, &bracket_low)) != MP_OKAY) {
goto LBL_END;
}
high <<= 1;
if ((err = mp_sqr(&bracket_high, &bracket_high)) != MP_OKAY) {
goto LBL_END;
}
}
mp_set(&bi_base, base);
while ((high - low) > 1) {
int mid = (high + low) >> 1;
if ((err = mp_expt_n(&bi_base, mid - low, &t)) != MP_OKAY) {
goto LBL_END;
}
if ((err = mp_mul(&bracket_low, &t, &bracket_mid)) != MP_OKAY) {
goto LBL_END;
}
cmp = mp_cmp(a, &bracket_mid);
if (cmp == MP_LT) {
high = mid;
mp_exch(&bracket_mid, &bracket_high);
}
if (cmp == MP_GT) {
low = mid;
mp_exch(&bracket_mid, &bracket_low);
}
if (cmp == MP_EQ) {
*c = mid;
goto LBL_END;
}
}
*c = (mp_cmp(&bracket_high, a) == MP_EQ) ? high : low;
LBL_END:
mp_clear_multi(&bracket_low, &bracket_high, &bracket_mid,
&t, &bi_base, (void *)NULL);
return err;
}
#endif
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