1/* Test of the double rounding effect.
2 *
3 * This example was presented at the CNC'2 summer school on MPFR and MPC
4 * at LORIA, Nancy, France.
5 *
6 * Arguments: max difference of exponents dmax, significand size n.
7 * Optional argument: extended precision p (with double rounding).
8 *
9 * Return all the couples of positive machine numbers (x,y) such that
10 * 1/2 <= y < 1, 0 <= Ex - Ey <= dmax, x - y is exactly representable
11 * in precision n and the results of floor(x/y) in the rounding modes
12 * toward 0 and to nearest are different.
13 */
14
15/*
16Copyright 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
17Contributed by the AriC and Caramel projects, INRIA.
18
19This file is part of the GNU MPFR Library.
20
21The GNU MPFR Library is free software; you can redistribute it and/or modify
22it under the terms of the GNU Lesser General Public License as published by
23the Free Software Foundation; either version 3 of the License, or (at your
24option) any later version.
25
26The GNU MPFR Library is distributed in the hope that it will be useful, but
27WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
28or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
29License for more details.
30
31You should have received a copy of the GNU Lesser General Public License
32along with the GNU MPFR Library; see the file COPYING.LESSER.  If not, see
33http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
3451 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.
35*/
36
37#include <stdio.h>
38#include <stdlib.h>
39#include <mpfr.h>
40
41#define PRECN x, y, z
42#define VARS PRECN, t
43
44static unsigned long
45eval (mpfr_t x, mpfr_t y, mpfr_t z, mpfr_t t, mpfr_rnd_t rnd)
46{
47  mpfr_div (t, x, y, rnd);  /* the division x/y in precision p */
48  mpfr_set (z, t, rnd);     /* the rounding to the precision n */
49  mpfr_rint_floor (z, z, rnd);
50  return mpfr_get_ui (z, rnd);
51}
52
53int main (int argc, char *argv[])
54{
55  int dmax, n, p;
56  mpfr_t VARS;
57
58  if (argc != 3 && argc != 4)
59    {
60      fprintf (stderr, "Usage: divworst <dmax> <n> [ <p> ]\n");
61      exit (EXIT_FAILURE);
62    }
63
64  dmax = atoi (argv[1]);
65  n = atoi (argv[2]);
66  p = argc == 3 ? n : atoi (argv[3]);
67  if (p < n)
68    {
69      fprintf (stderr, "divworst: p must be greater or equal to n\n");
70      exit (EXIT_FAILURE);
71    }
72
73  mpfr_inits2 (n, PRECN, (mpfr_ptr) 0);
74  mpfr_init2 (t, p);
75
76  for (mpfr_set_ui_2exp (x, 1, -1, MPFR_RNDN);
77       mpfr_get_exp (x) <= dmax;
78       mpfr_nextabove (x))
79    for (mpfr_set_ui_2exp (y, 1, -1, MPFR_RNDN);
80         mpfr_get_exp (y) == 0;
81         mpfr_nextabove (y))
82      {
83        unsigned long rz, rn;
84
85        if (mpfr_sub (z, x, y, MPFR_RNDZ) != 0)
86          continue;  /* x - y is not representable in precision n */
87        rz = eval (x, y, z, t, MPFR_RNDZ);
88        rn = eval (x, y, z, t, MPFR_RNDN);
89        if (rz == rn)
90          continue;
91        mpfr_printf ("x = %.*Rb ; y = %.*Rb ; Z: %lu ; N: %lu\n",
92                     n - 1, x, n - 1, y, rz, rn);
93      }
94
95  mpfr_clears (VARS, (mpfr_ptr) 0);
96  return 0;
97}
98