84 lines
3.2 KiB
C
84 lines
3.2 KiB
C
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#include "tommath_private.h"
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#ifdef S_MP_FP_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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/* Fixed point bitwise logarithm base two of "a" with precision "p", Big-integer version. */
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static mp_err s_mp_fp_log_fraction(const mp_int *a, int p, mp_int *c)
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{
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mp_int b, L_out, twoep, a_bar;
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int L, pmL;
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int i;
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mp_err err;
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if ((err = mp_init_multi(&b, &L_out, &twoep, &a_bar, NULL)) != MP_OKAY) {
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return err;
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}
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L = mp_count_bits(a) - 1;
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pmL = (p < L) ? L - p: p - L;
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if ((err = mp_mul_2d(a, pmL, &a_bar)) != MP_OKAY) goto LTM_ERR;
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if ((err = mp_2expt(&b, p - 1)) != MP_OKAY) goto LTM_ERR;
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mp_set_i32(&L_out, L);
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if ((err = mp_mul_2d(&L_out, p, &L_out)) != MP_OKAY) goto LTM_ERR;
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if ((err = mp_2expt(&twoep, p + 1)) != MP_OKAY) goto LTM_ERR;
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for (i = 0; i < p; i++) {
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if ((err = mp_sqr(&a_bar, &a_bar)) != MP_OKAY) goto LTM_ERR;
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if ((err = mp_div_2d(&a_bar, p, &a_bar, NULL)) != MP_OKAY) goto LTM_ERR;
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if (mp_cmp(&a_bar, &twoep) != MP_LT) {
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if ((err = mp_div_2(&a_bar, &a_bar)) != MP_OKAY) goto LTM_ERR;
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if ((err = mp_add(&L_out, &b, &L_out)) != MP_OKAY) goto LTM_ERR;
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}
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if ((err = mp_div_2(&b, &b)) != MP_OKAY) goto LTM_ERR;
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}
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mp_exch(c, &L_out);
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LTM_ERR:
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mp_clear_multi(&b, &L_out, &twoep, &a_bar, NULL);
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return err;
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}
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mp_err s_mp_fp_log(const mp_int *a, mp_int *c)
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{
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mp_int La, t;
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mp_err err;
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int fla;
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/*
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We have arbitrary precision here and could adapt "prec" to actual precision,
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there is no need for lowering precision for small numbers. Some quick runtime
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tests revealed a gain that can only be called marginally at best.
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But: YMMV, as always.
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*/
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int prec = MP_PRECISION_FIXED_LOG;
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fla = mp_count_bits(a) - 1;
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if ((err = mp_init_multi(&La, &t, NULL)) != MP_OKAY) {
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return err;
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}
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if (fla > prec) {
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if ((err = mp_div_2d(a, fla - prec, &t, NULL)) != MP_OKAY) goto LTM_ERR;
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if ((err = s_mp_fp_log_fraction(&t, prec,
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&La)) != MP_OKAY) goto LTM_ERR;
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mp_set_i32(&t,fla - prec);
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if ((err = mp_mul_2d(&t,prec, &t)) != MP_OKAY) goto LTM_ERR;
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if ((err = mp_add(&La, &t, &La)) != MP_OKAY) goto LTM_ERR;
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} else {
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if ((err = s_mp_fp_log_fraction(a, prec,
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&La)) != MP_OKAY) goto LTM_ERR;
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}
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mp_exch(&La, c);
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LTM_ERR:
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mp_clear_multi(&La, &t, NULL);
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return err;
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}
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#endif
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