1/* mpfr_reldiff -- compute relative difference of two floating-point numbers.
2
3Copyright 2000-2001, 2004-2023 Free Software Foundation, Inc.
4Contributed by the AriC and Caramba projects, INRIA.
5
6This file is part of the GNU MPFR Library.
7
8The GNU MPFR Library is free software; you can redistribute it and/or modify
9it under the terms of the GNU Lesser General Public License as published by
10the Free Software Foundation; either version 3 of the License, or (at your
11option) any later version.
12
13The GNU MPFR Library is distributed in the hope that it will be useful, but
14WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
15or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
16License for more details.
17
18You should have received a copy of the GNU Lesser General Public License
19along with the GNU MPFR Library; see the file COPYING.LESSER.  If not, see
20https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
2151 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
22
23#include "mpfr-impl.h"
24
25/* reldiff(b, c) = abs(b-c)/b */
26void
27mpfr_reldiff (mpfr_ptr a, mpfr_srcptr b, mpfr_srcptr c, mpfr_rnd_t rnd_mode)
28{
29  mpfr_t b_copy;
30
31  if (MPFR_ARE_SINGULAR (b, c))
32    {
33      if (MPFR_IS_NAN (b) || MPFR_IS_INF (b) || MPFR_IS_NAN (c) ||
34          (MPFR_IS_ZERO (b) && MPFR_IS_ZERO (c)))
35        {
36          MPFR_SET_NAN (a);
37          return;
38        }
39      if (MPFR_IS_ZERO (b) || MPFR_IS_INF (c))
40        {
41          MPFR_SET_SAME_SIGN (a, b);
42          MPFR_SET_INF (a);
43          return;
44        }
45      /* The case c = 0 with b regular, which should give sign(b) exactly,
46         cannot be optimized here as it is documented in the MPFR manual
47         that this function just computes abs(b-c)/b using the precision
48         of a and the rounding mode rnd_mode for all operations. So let's
49         prefer the potentially "incorrect" result. Note that the correct
50         result is not necessarily better because if could break properties
51         (like monotonicity?) implied by the documentation. */
52    }
53
54  if (a == b)
55    {
56      mpfr_init2 (b_copy, MPFR_PREC(b));
57      mpfr_set (b_copy, b, MPFR_RNDN);
58    }
59
60  mpfr_sub (a, b, c, rnd_mode);
61  MPFR_SET_SIGN (a, 1);
62  mpfr_div (a, a, a == b ? b_copy : b, rnd_mode);
63
64  if (a == b)
65    mpfr_clear (b_copy);
66
67}
68