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- /* Copyright 2012-2019, The Tor Project, Inc
- * See LICENSE for licensing information */
- /** prob_distr_mpfr_ref.c
- *
- * Example reference file for GNU MPFR vectors tested in test_prob_distr.c .
- * Code by Riastradh.
- */
- #include <complex.h>
- #include <float.h>
- #include <math.h>
- #include <stdio.h>
- /* Must come after <stdio.h> so we get mpfr_printf. */
- #include <mpfr.h>
- /* gcc -o mpfr prob_distr_mpfr_ref.c -lmpfr -lm */
- /* Computes logit(p) for p = .49999 */
- int
- main(void)
- {
- mpfr_t p, q, r;
- mpfr_init(p);
- mpfr_set_prec(p, 200);
- mpfr_init(q);
- mpfr_set_prec(q, 200);
- mpfr_init(r);
- mpfr_set_prec(r, 200);
- mpfr_set_d(p, .49999, MPFR_RNDN);
- mpfr_set_d(q, 1, MPFR_RNDN);
- /* r := q - p = 1 - p */
- mpfr_sub(r, q, p, MPFR_RNDN);
- /* q := p/r = p/(1 - p) */
- mpfr_div(q, p, r, MPFR_RNDN);
- /* r := log(q) = log(p/(1 - p)) */
- mpfr_log(r, q, MPFR_RNDN);
- mpfr_printf("mpfr 200-bit\t%.128Rg\n", r);
- /*
- * Print a double approximation to logit three different ways. All
- * three agree bit for bit on the libms I tried, with the nextafter
- * adjustment (which is well within the 10 eps relative error bound
- * advertised). Apparently I must have used the Goldberg expression
- * for what I wrote down in the test case.
- */
- printf("mpfr 53-bit\t%.17g\n", nextafter(mpfr_get_d(r, MPFR_RNDN), 0), 0);
- volatile double p0 = .49999;
- printf("log1p\t\t%.17g\n", nextafter(-log1p((1 - 2*p0)/p0), 0));
- volatile double x = (1 - 2*p0)/p0;
- volatile double xp1 = x + 1;
- printf("Goldberg\t%.17g\n", -x*log(xp1)/(xp1 - 1));
- /*
- * Print a bad approximation, using the naive expression, to see a
- * lot of wrong digits, far beyond the 10 eps relative error attained
- * by -log1p((1 - 2*p)/p).
- */
- printf("naive\t\t%.17g\n", log(p0/(1 - p0)));
- fflush(stdout);
- return ferror(stdout);
- }
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