strtod.h

#define HUGE_VAL (__builtin_huge_val())

//
// strtod.c
//
// Convert string to double 
//
// Copyright (C) 2002 Michael Ringgaard. All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 
// 1. Redistributions of source code must retain the above copyright 
//    notice, this list of conditions and the following disclaimer.  
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.  
// 3. Neither the name of the project nor the names of its contributors
//    may be used to endorse or promote products derived from this software
//    without specific prior written permission. 
// 
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
// OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 
// SUCH DAMAGE.
// 

static int isspace(char c){ return c==' '; }
static int isdigit(char c){ return (c>='0'&&c<='9'); }

static double strtod(const char *str, char **endptr)
{
  double number;
  int exponent;
  int negative;
  char *p = (char *) str;
  double p10;
  int n;
  int num_digits;
  int num_decimals;

  // Skip leading whitespace
  while (isspace(*p)) p++;

  // Handle optional sign
  negative = 0;
  switch (*p) 
  {             
    case '-': negative = 1; // Fall through to increment position
    case '+': p++;
  }

  number = 0.;
  exponent = 0;
  num_digits = 0;
  num_decimals = 0;

  // Process string of digits
  while (isdigit(*p))
  {
    number = number * 10. + (*p - '0');
    p++;
    num_digits++;
  }

  // Process decimal part
  if (*p == '.') 
  {
    p++;

    while (isdigit(*p))
    {
      number = number * 10. + (*p - '0');
      p++;
      num_digits++;
      num_decimals++;
    }

    exponent -= num_decimals;
  }

  if (num_digits == 0)
  {
    return 0.0;
  }

  // Correct for sign
  if (negative) number = -number;

  // Process an exponent string
  if (*p == 'e' || *p == 'E') 
  {
    // Handle optional sign
    negative = 0;
    switch (*++p) 
    {   
      case '-': negative = 1;   // Fall through to increment pos
      case '+': p++;
    }

    // Process string of digits
    n = 0;
    while (isdigit(*p)) 
    {   
      n = n * 10 + (*p - '0');
      p++;
    }

    if (negative) 
      exponent -= n;
    else
      exponent += n;
  }

  if (exponent < __DBL_MIN_EXP__  || exponent > __DBL_MAX_EXP__)
  {
    return HUGE_VAL;
  }

  // Scale the result
  p10 = 10.;
  n = exponent;
  if (n < 0) n = -n;
  while (n) 
  {
    if (n & 1) 
    {
      if (exponent < 0)
        number /= p10;
      else
        number *= p10;
    }
    n >>= 1;
    p10 *= p10;
  }

  if (endptr) *endptr = p;

  return number;
}