83 lines
2.1 KiB
C
83 lines
2.1 KiB
C
#include "tommath_private.h"
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#ifdef S_MP_LOG_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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mp_err s_mp_log(const mp_int *a, uint32_t base, uint32_t *c)
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{
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mp_err err;
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mp_ord cmp;
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uint32_t high, low, mid;
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mp_int bracket_low, bracket_high, bracket_mid, t, bi_base;
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cmp = mp_cmp_d(a, base);
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if ((cmp == MP_LT) || (cmp == MP_EQ)) {
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*c = cmp == MP_EQ;
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return MP_OKAY;
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}
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if ((err =
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mp_init_multi(&bracket_low, &bracket_high,
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&bracket_mid, &t, &bi_base, NULL)) != MP_OKAY) {
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return err;
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}
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low = 0u;
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mp_set(&bracket_low, 1uL);
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high = 1u;
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mp_set(&bracket_high, base);
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/*
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A kind of Giant-step/baby-step algorithm.
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Idea shamelessly stolen from https://programmingpraxis.com/2010/05/07/integer-logarithms/2/
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The effect is asymptotic, hence needs benchmarks to test if the Giant-step should be skipped
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for small n.
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*/
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while (mp_cmp(&bracket_high, a) == MP_LT) {
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low = high;
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if ((err = mp_copy(&bracket_high, &bracket_low)) != MP_OKAY) {
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goto LBL_END;
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}
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high <<= 1;
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if ((err = mp_sqr(&bracket_high, &bracket_high)) != MP_OKAY) {
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goto LBL_END;
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}
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}
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mp_set(&bi_base, base);
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while ((high - low) > 1u) {
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mid = (high + low) >> 1;
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if ((err = mp_expt_u32(&bi_base, (uint32_t)(mid - low), &t)) != MP_OKAY) {
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goto LBL_END;
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}
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if ((err = mp_mul(&bracket_low, &t, &bracket_mid)) != MP_OKAY) {
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goto LBL_END;
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}
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cmp = mp_cmp(a, &bracket_mid);
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if (cmp == MP_LT) {
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high = mid;
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mp_exch(&bracket_mid, &bracket_high);
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}
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if (cmp == MP_GT) {
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low = mid;
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mp_exch(&bracket_mid, &bracket_low);
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}
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if (cmp == MP_EQ) {
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*c = mid;
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goto LBL_END;
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}
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}
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*c = (mp_cmp(&bracket_high, a) == MP_EQ) ? high : low;
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LBL_END:
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mp_clear_multi(&bracket_low, &bracket_high, &bracket_mid,
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&t, &bi_base, NULL);
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return err;
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}
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#endif
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