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