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 <https://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 # define PROMOTED_FLOAT long double
  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 # define PROMOTED_FLOAT double
  53 #else
  54 # define LENGTH 1
  55 # define FLOAT float
  56 # define FLOAT_DIG FLT_DIG
  57 # define FLOAT_MIN FLT_MIN
  58 # define FLOAT_PREC_BOUND _GL_FLT_PREC_BOUND
  59 # define FTOASTR ftoastr
  60 # define PROMOTED_FLOAT double
  61 # if HAVE_STRTOF
  62 #  define STRTOF strtof
  63 # endif
  64 #endif
  65
  66 /* On pre-C99 hosts, approximate strtof and strtold with strtod.  This
  67    may generate one or two extra digits, but that's better than not
  68    working at all.  */
  69 #ifndef STRTOF
  70 # define STRTOF strtod
  71 #endif
  72
  73 /* On hosts where it's not known that snprintf works, use sprintf to
  74    implement the subset needed here.  Typically BUFSIZE is big enough
  75    and there's little or no performance hit.  */
  76 #if ! GNULIB_SNPRINTF
  77 # undef snprintf
  78 # define snprintf ftoastr_snprintf
  79 static int
  80 ftoastr_snprintf (char *buf, size_t bufsize, char const *format,
  81                   int width, int prec, FLOAT x)
  82 {
  83   PROMOTED_FLOAT promoted_x = x;
  84   char width_0_buffer[LENGTH == 1 ? FLT_BUFSIZE_BOUND
  85                       : LENGTH == 2 ? DBL_BUFSIZE_BOUND
  86                       : LDBL_BUFSIZE_BOUND];
  87   int n = width;
  88   if (bufsize < sizeof width_0_buffer)
  89     {
  90       n = sprintf (width_0_buffer, format, 0, prec, promoted_x);
  91       if (n < 0)
  92         return n;
  93       if (n < width)
  94         n = width;
  95     }
  96   if (n < bufsize)
  97     n = sprintf (buf, format, width, prec, promoted_x);
  98   return n;
  99 }
 100 #endif
 101
 102 int
 103 FTOASTR (char *buf, size_t bufsize, int flags, int width, FLOAT x)
 104 {
 105   /* The following method is simple but slow.
 106      For ideas about speeding things up, please see:
 107
 108      Andrysco M, Jhala R, Lerner S. Printing floating-point numbers:
 109      a faster, always correct method. ACM SIGPLAN notices - POPL '16.
 110      2016;51(1):555-67 <http://dx.doi.org/10.1145/2914770.2837654>; draft at
 111      <https://cseweb.ucsd.edu/~lerner/papers/fp-printing-popl16.pdf>.  */
 112
 113   PROMOTED_FLOAT promoted_x = x;
 114   char format[sizeof "%-+ 0*.*Lg"];
 115   FLOAT abs_x = x < 0 ? -x : x;
 116   int prec;
 117
 118   char *p = format;
 119   *p++ = '%';
 120
 121   /* Support flags that generate output parsable by strtof.  */
 122   *p = '-'; p += (flags & FTOASTR_LEFT_JUSTIFY  ) != 0;
 123   *p = '+'; p += (flags & FTOASTR_ALWAYS_SIGNED ) != 0;
 124   *p = ' '; p += (flags & FTOASTR_SPACE_POSITIVE) != 0;
 125   *p = '0'; p += (flags & FTOASTR_ZERO_PAD      ) != 0;
 126
 127   *p++ = '*';
 128   *p++ = '.';
 129   *p++ = '*';
 130   *p = 'L'; p += 2 < LENGTH;
 131   *p++ = flags & FTOASTR_UPPER_E ? 'G' : 'g';
 132   *p = '\0';
 133
 134   for (prec = abs_x < FLOAT_MIN ? 1 : FLOAT_DIG; ; prec++)
 135     {
 136       int n = snprintf (buf, bufsize, format, width, prec, promoted_x);
 137       if (n < 0
 138           || FLOAT_PREC_BOUND <= prec
 139           || (n < bufsize && STRTOF (buf, NULL) == x))
 140         return n;
 141     }
 142 }