lib/ftoastr.c

   1 /* floating point to accurate string
   2
   3    Copyright (C) 2010-2019 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 # 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 PROMOTED_FLOAT double
  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 PROMOTED_FLOAT double
  59 # if HAVE_STRTOF
  60 #  define STRTOF strtof
  61 # endif
  62 #endif
  63
  64 /* On pre-C99 hosts, approximate strtof 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   PROMOTED_FLOAT promoted_x = x;
  82   char width_0_buffer[LENGTH == 1 ? FLT_BUFSIZE_BOUND
  83                       : LENGTH == 2 ? DBL_BUFSIZE_BOUND
  84                       : LDBL_BUFSIZE_BOUND];
  85   int n = width;
  86   if (bufsize < sizeof width_0_buffer)
  87     {
  88       n = sprintf (width_0_buffer, format, 0, prec, promoted_x);
  89       if (n < 0)
  90         return n;
  91       if (n < width)
  92         n = width;
  93     }
  94   if (n < bufsize)
  95     n = sprintf (buf, format, width, prec, promoted_x);
  96   return n;
  97 }
  98 #endif
  99
 100 int
 101 FTOASTR (char *buf, size_t bufsize, int flags, int width, FLOAT x)
 102 {
 103   /* The following method is simple but slow.
 104      For ideas about speeding things up, please see:
 105
 106      Andrysco M, Jhala R, Lerner S. Printing floating-point numbers:
 107      a faster, always correct method. ACM SIGPLAN notices - POPL '16.
 108      2016;51(1):555-67 <https://doi.org/10.1145/2914770.2837654>; draft at
 109      <https://cseweb.ucsd.edu/~lerner/papers/fp-printing-popl16.pdf>.  */
 110
 111   PROMOTED_FLOAT promoted_x = x;
 112   char format[sizeof "%-+ 0*.*Lg"];
 113   FLOAT abs_x = x < 0 ? -x : x;
 114   int prec;
 115
 116   char *p = format;
 117   *p++ = '%';
 118
 119   /* Support flags that generate output parsable by strtof.  */
 120   *p = '-'; p += (flags & FTOASTR_LEFT_JUSTIFY  ) != 0;
 121   *p = '+'; p += (flags & FTOASTR_ALWAYS_SIGNED ) != 0;
 122   *p = ' '; p += (flags & FTOASTR_SPACE_POSITIVE) != 0;
 123   *p = '0'; p += (flags & FTOASTR_ZERO_PAD      ) != 0;
 124
 125   *p++ = '*';
 126   *p++ = '.';
 127   *p++ = '*';
 128   *p = 'L'; p += 2 < LENGTH;
 129   *p++ = flags & FTOASTR_UPPER_E ? 'G' : 'g';
 130   *p = '\0';
 131
 132   for (prec = abs_x < FLOAT_MIN ? 1 : FLOAT_DIG; ; prec++)
 133     {
 134       int n = snprintf (buf, bufsize, format, width, prec, promoted_x);
 135       if (n < 0
 136           || FLOAT_PREC_BOUND <= prec
 137           || (n < bufsize && STRTOF (buf, NULL) == x))
 138         return n;
 139     }
 140 }