lib/ftoastr.c

   1 /* floating point to accurate string
   2
   3    Copyright (C) 2010-2011 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 "intprops.h"
  33 #include <float.h>
  34 #include <stdio.h>
  35 #include <stdlib.h>
  36
  37 #if LENGTH == 3
  38 # define FLOAT long double
  39 # define FLOAT_DIG LDBL_DIG
  40 # define FLOAT_MIN LDBL_MIN
  41 # define FLOAT_PREC_BOUND _GL_LDBL_PREC_BOUND
  42 # define FTOASTR ldtoastr
  43 # if HAVE_C99_STRTOLD
  44 #  define STRTOF strtold
  45 # endif
  46 #elif LENGTH == 2
  47 # define FLOAT double
  48 # define FLOAT_DIG DBL_DIG
  49 # define FLOAT_MIN DBL_MIN
  50 # define FLOAT_PREC_BOUND _GL_DBL_PREC_BOUND
  51 # define FTOASTR dtoastr
  52 #else
  53 # define LENGTH 1
  54 # define FLOAT float
  55 # define FLOAT_DIG FLT_DIG
  56 # define FLOAT_MIN FLT_MIN
  57 # define FLOAT_PREC_BOUND _GL_FLT_PREC_BOUND
  58 # define FTOASTR ftoastr
  59 # if HAVE_STRTOF
  60 #  define STRTOF strtof
  61 # endif
  62 #endif
  63
  64 /* On pre-C99 hosts, approximate strtof and strtold with strtod.  This
  65    may generate one or two extra digits, but that's better than not
  66    working at all.  */
  67 #ifndef STRTOF
  68 # define STRTOF strtod
  69 #endif
  70
  71 /* On hosts where it's not known that snprintf works, use sprintf to
  72    implement the subset needed here.  Typically BUFSIZE is big enough
  73    and there's little or no performance hit.  */
  74 #if ! GNULIB_SNPRINTF
  75 # undef snprintf
  76 # define snprintf ftoastr_snprintf
  77 static int
  78 ftoastr_snprintf (char *buf, size_t bufsize, char const *format,
  79                   int width, int prec, FLOAT x)
  80 {
  81   char width_0_buffer[LENGTH == 1 ? FLT_BUFSIZE_BOUND
  82                       : LENGTH == 2 ? DBL_BUFSIZE_BOUND
  83                       : LDBL_BUFSIZE_BOUND];
  84   int n = width;
  85   if (bufsize < sizeof width_0_buffer)
  86     {
  87       n = sprintf (width_0_buffer, format, 0, prec, x);
  88       if (n < 0)
  89         return n;
  90       if (n < width)
  91         n = width;
  92     }
  93   if (n < bufsize)
  94     n = sprintf (buf, format, width, prec, x);
  95   return n;
  96 }
  97 #endif
  98
  99 int
 100 FTOASTR (char *buf, size_t bufsize, int flags, int width, FLOAT x)
 101 {
 102   /* The following method is simple but slow.
 103      For ideas about speeding things up, please see:
 104
 105      Florian Loitsch, Printing floating-point numbers quickly and accurately
 106      with integers.  ACM SIGPLAN notices 46, 6 (June 2010), 233-243
 107      <http://dx.doi.org/10.1145/1809028.1806623>; also see the
 108      2010-03-21 draft <http://florian.loitsch.com/tmp/article.pdf>.  */
 109
 110   char format[sizeof "%-+ 0*.*Lg"];
 111   FLOAT abs_x = x < 0 ? -x : x;
 112   int prec;
 113
 114   char *p = format;
 115   *p++ = '%';
 116
 117   /* Support flags that generate output parsable by strtof.  */
 118   *p = '-'; p += (flags & FTOASTR_LEFT_JUSTIFY  ) != 0;
 119   *p = '+'; p += (flags & FTOASTR_ALWAYS_SIGNED ) != 0;
 120   *p = ' '; p += (flags & FTOASTR_SPACE_POSITIVE) != 0;
 121   *p = '0'; p += (flags & FTOASTR_ZERO_PAD      ) != 0;
 122
 123   *p++ = '*';
 124   *p++ = '.';
 125   *p++ = '*';
 126   *p = 'L'; p += 2 < LENGTH;
 127   *p++ = flags & FTOASTR_UPPER_E ? 'G' : 'g';
 128   *p = '\0';
 129
 130   for (prec = abs_x < FLOAT_MIN ? 1 : FLOAT_DIG; ; prec++)
 131     {
 132       int n = snprintf (buf, bufsize, format, width, prec, x);
 133       if (n < 0
 134           || FLOAT_PREC_BOUND <= prec
 135           || (n < bufsize && STRTOF (buf, NULL) == x))
 136         return n;
 137     }
 138 }