lib/ftoastr.c

   1 /* floating point to accurate string
   2
   3    Copyright (C) 2010-2017 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 /* This code can misbehave on some buggy or older platforms, when
  21    operating on arguments on floating types other than 'double', or
  22    when given unusual combinations of options.  Gnulib's
  23    snprintf-posix module works around many of these problems.
  24
  25    This code relies on sprintf, strtod, etc. operating accurately;
  26    otherwise, the resulting strings could be inaccurate or too long.  */
  27
  28 #include <config.h>
  29
  30 #include "ftoastr.h"
  31
  32 #include <float.h>
  33 #include <stdio.h>
  34 #include <stdlib.h>
  35
  36 #if LENGTH == 3
  37 # define FLOAT long double
  38 # define FLOAT_DIG LDBL_DIG
  39 # define FLOAT_MIN LDBL_MIN
  40 # define FLOAT_PREC_BOUND _GL_LDBL_PREC_BOUND
  41 # define FTOASTR ldtoastr
  42 # if HAVE_C99_STRTOLD
  43 #  define STRTOF strtold
  44 # endif
  45 #elif LENGTH == 2
  46 # define FLOAT double
  47 # define FLOAT_DIG DBL_DIG
  48 # define FLOAT_MIN DBL_MIN
  49 # define FLOAT_PREC_BOUND _GL_DBL_PREC_BOUND
  50 # define FTOASTR dtoastr
  51 #else
  52 # define LENGTH 1
  53 # define FLOAT float
  54 # define FLOAT_DIG FLT_DIG
  55 # define FLOAT_MIN FLT_MIN
  56 # define FLOAT_PREC_BOUND _GL_FLT_PREC_BOUND
  57 # define FTOASTR ftoastr
  58 # if HAVE_STRTOF
  59 #  define STRTOF strtof
  60 # endif
  61 #endif
  62
  63 /* On pre-C99 hosts, approximate strtof and strtold with strtod.  This
  64    may generate one or two extra digits, but that's better than not
  65    working at all.  */
  66 #ifndef STRTOF
  67 # define STRTOF strtod
  68 #endif
  69
  70 /* On hosts where it's not known that snprintf works, use sprintf to
  71    implement the subset needed here.  Typically BUFSIZE is big enough
  72    and there's little or no performance hit.  */
  73 #if ! GNULIB_SNPRINTF
  74 # undef snprintf
  75 # define snprintf ftoastr_snprintf
  76 static int
  77 ftoastr_snprintf (char *buf, size_t bufsize, char const *format,
  78                   int width, int prec, FLOAT x)
  79 {
  80   char width_0_buffer[LENGTH == 1 ? FLT_BUFSIZE_BOUND
  81                       : LENGTH == 2 ? DBL_BUFSIZE_BOUND
  82                       : LDBL_BUFSIZE_BOUND];
  83   int n = width;
  84   if (bufsize < sizeof width_0_buffer)
  85     {
  86       n = sprintf (width_0_buffer, format, 0, prec, x);
  87       if (n < 0)
  88         return n;
  89       if (n < width)
  90         n = width;
  91     }
  92   if (n < bufsize)
  93     n = sprintf (buf, format, width, prec, x);
  94   return n;
  95 }
  96 #endif
  97
  98 int
  99 FTOASTR (char *buf, size_t bufsize, int flags, int width, FLOAT x)
 100 {
 101   /* The following method is simple but slow.
 102      For ideas about speeding things up, please see:
 103
 104      Florian Loitsch, Printing floating-point numbers quickly and accurately
 105      with integers.  ACM SIGPLAN notices 46, 6 (June 2010), 233-243
 106      <http://dx.doi.org/10.1145/1809028.1806623>; also see the
 107      2010-03-21 draft <http://florian.loitsch.com/tmp/article.pdf>.  */
 108
 109   char format[sizeof "%-+ 0*.*Lg"];
 110   FLOAT abs_x = x < 0 ? -x : x;
 111   int prec;
 112
 113   char *p = format;
 114   *p++ = '%';
 115
 116   /* Support flags that generate output parsable by strtof.  */
 117   *p = '-'; p += (flags & FTOASTR_LEFT_JUSTIFY  ) != 0;
 118   *p = '+'; p += (flags & FTOASTR_ALWAYS_SIGNED ) != 0;
 119   *p = ' '; p += (flags & FTOASTR_SPACE_POSITIVE) != 0;
 120   *p = '0'; p += (flags & FTOASTR_ZERO_PAD      ) != 0;
 121
 122   *p++ = '*';
 123   *p++ = '.';
 124   *p++ = '*';
 125   *p = 'L'; p += 2 < LENGTH;
 126   *p++ = flags & FTOASTR_UPPER_E ? 'G' : 'g';
 127   *p = '\0';
 128
 129   for (prec = abs_x < FLOAT_MIN ? 1 : FLOAT_DIG; ; prec++)
 130     {
 131       int n = snprintf (buf, bufsize, format, width, prec, x);
 132       if (n < 0
 133           || FLOAT_PREC_BOUND <= prec
 134           || (n < bufsize && STRTOF (buf, NULL) == x))
 135         return n;
 136     }
 137 }