atof-generic.c

早期freebsd实现

      precision = (address_of_generic_floating_point_number -> high
		   - address_of_generic_floating_point_number -> low
		   + 1
		   );		/* Number of destination littlenums. */
				/* Includes guard bits (two littlenums worth) */
      maximum_useful_digits = (  ((double) (precision - 2))
			       * ((double) (LITTLENUM_NUMBER_OF_BITS))
			       / (LOG_TO_BASE_2_OF_10)
			       )
	+ 2;			/* 2 :: guard digits. */
      if (number_of_digits_available > maximum_useful_digits)
	{
	  number_of_digits_to_use = maximum_useful_digits;
	}
      else
	{
	  number_of_digits_to_use = number_of_digits_available;
	}
      decimal_exponent += number_of_digits_before_decimal - number_of_digits_to_use;

      more_than_enough_bits_for_digits
	= ((((double)number_of_digits_to_use) * LOG_TO_BASE_2_OF_10) + 1);
      more_than_enough_littlenums_for_digits
	= (  more_than_enough_bits_for_digits
	   / LITTLENUM_NUMBER_OF_BITS
	   )
	  + 2;
      
      /*
       * Compute (digits) part. In "12.34E56" this is the "1234" part.
       * Arithmetic is exact here. If no digits are supplied then
       * this part is a 0 valued binary integer.
       * Allocate room to build up the binary number as littlenums.
       * We want this memory to disappear when we leave this function.
       * Assume no alignment problems => (room for n objects) ==
       * n * (room for 1 object).
       */
      
      size_of_digits_in_littlenums = more_than_enough_littlenums_for_digits;
      size_of_digits_in_chars = size_of_digits_in_littlenums
	* sizeof( LITTLENUM_TYPE );
      digits_binary_low = (LITTLENUM_TYPE *)
	alloca (size_of_digits_in_chars);
      bzero ((char *)digits_binary_low, size_of_digits_in_chars);

      /* Digits_binary_low[] is allocated and zeroed. */
      
      {
	/*
	 * Parse the decimal digits as if * digits_low was in the units position.
	 * Emit a binary number into digits_binary_low[].
	 *
	 * Use a large-precision version of:
	 * (((1st-digit) * 10 + 2nd-digit) * 10 + 3rd-digit ...) * 10 + last-digit
	 */

	char *		p;
	char		c;
	int		count;	/* Number of useful digits left to scan. */

	for (p = first_digit, count = number_of_digits_to_use;
	     count;
	     p ++,  -- count)
	  {
	    c = * p;
	    if (isdigit(c))
	      {
		/*
		 * Multiply by 10. Assume can never overflow.
		 * Add this digit to digits_binary_low[].
		 */

		long int	carry;
		LITTLENUM_TYPE *	littlenum_pointer;
		LITTLENUM_TYPE *	littlenum_limit;

		littlenum_limit
		  =     digits_binary_low
		    +   more_than_enough_littlenums_for_digits
		      - 1;
		carry = c - '0';	/* char -> binary */
		for (littlenum_pointer = digits_binary_low;
		     littlenum_pointer <= littlenum_limit;
		     littlenum_pointer ++)
		  {
		    long int	work;
		    
		    work = carry + 10 * (long)(*littlenum_pointer);
		    * littlenum_pointer = work & LITTLENUM_MASK;
		    carry = work >> LITTLENUM_NUMBER_OF_BITS;
		  }
		if (carry != 0)
		  {
		    /*
		     * We have a GROSS internal error.
		     * This should never happen.
		     */
		    abort();	/* RMS prefers abort() to any message. */
		  }
	      }
	    else
	      {
		++ count;	/* '.' doesn't alter digits used count. */
	      }		/* if valid digit */
	  }			/* for each digit */
      }

      /*
       * Digits_binary_low[] properly encodes the value of the digits.
       * Forget about any high-order littlenums that are 0.
       */
      while (digits_binary_low [size_of_digits_in_littlenums - 1] == 0
	     && size_of_digits_in_littlenums >= 2)
	  size_of_digits_in_littlenums --;

      digits_flonum . low	= digits_binary_low;
      digits_flonum . high	= digits_binary_low + size_of_digits_in_littlenums - 1;
      digits_flonum . leader	= digits_flonum . high;
      digits_flonum . exponent	= 0;
      /*
       * The value of digits_flonum . sign should not be important.
       * We have already decided the output's sign.
       * We trust that the sign won't influence the other parts of the number!
       * So we give it a value for these reasons:
       * (1) courtesy to humans reading/debugging
       *     these numbers so they don't get excited about strange values
       * (2) in future there may be more meaning attached to sign,
       *     and what was
       *     harmless noise may become disruptive, ill-conditioned (or worse)
       *     input.
       */
      digits_flonum . sign	= '+';

      {
	/*
	 * Compute the mantssa (& exponent) of the power of 10.
	 * If sucessful, then multiply the power of 10 by the digits
	 * giving return_binary_mantissa and return_binary_exponent.
	 */

	LITTLENUM_TYPE *power_binary_low;
	int		decimal_exponent_is_negative;
				/* This refers to the "-56" in "12.34E-56". */
				/* FALSE: decimal_exponent is positive (or 0) */
				/* TRUE:  decimal_exponent is negative */
	FLONUM_TYPE	temporary_flonum;
	LITTLENUM_TYPE *temporary_binary_low;
	int		size_of_power_in_littlenums;
	int		size_of_power_in_chars;

	size_of_power_in_littlenums = precision;
/* Precision has a built-in fudge factor so we get a few guard bits. */


	decimal_exponent_is_negative = decimal_exponent < 0;
	if (decimal_exponent_is_negative)
	  {
	    decimal_exponent = - decimal_exponent;
	  }
	/* From now on: the decimal exponent is > 0. Its sign is seperate. */
	
	size_of_power_in_chars
	  =   size_of_power_in_littlenums
	    * sizeof( LITTLENUM_TYPE ) + 2;
	power_binary_low = (LITTLENUM_TYPE *) alloca ( size_of_power_in_chars );
	temporary_binary_low = (LITTLENUM_TYPE *) alloca ( size_of_power_in_chars );
	bzero ((char *)power_binary_low, size_of_power_in_chars);
	* power_binary_low = 1;
	power_of_10_flonum . exponent	= 0;
	power_of_10_flonum . low	= power_binary_low;
	power_of_10_flonum . leader	= power_binary_low;
	power_of_10_flonum . high	= power_binary_low	+ size_of_power_in_littlenums - 1;
	power_of_10_flonum . sign	= '+';
	temporary_flonum . low	= temporary_binary_low;
	temporary_flonum . high	= temporary_binary_low		+ size_of_power_in_littlenums - 1;
	/*
	 * (power) == 1.
	 * Space for temporary_flonum allocated.
	 */
	
	/*
	 * ...
	 *
	 * WHILE	more bits
	 * DO	find next bit (with place value)
	 *	multiply into power mantissa
	 * OD
	 */
	{
	  int		place_number_limit;
				/* Any 10^(2^n) whose "n" exceeds this */
				/* value will fall off the end of */
				/* flonum_XXXX_powers_of_ten[]. */
	  int		place_number;
	  const FLONUM_TYPE * multiplicand; /* -> 10^(2^n) */

	  place_number_limit = table_size_of_flonum_powers_of_ten;
	  multiplicand
	    = (  decimal_exponent_is_negative
	       ? flonum_negative_powers_of_ten
	       : flonum_positive_powers_of_ten);
	  for (place_number = 1;	/* Place value of this bit of exponent. */
	       decimal_exponent;	/* Quit when no more 1 bits in exponent. */
	       decimal_exponent >>= 1
	       , place_number ++)
	    {
	      if (decimal_exponent & 1)
		{
		  if (place_number > place_number_limit)
		    {
		      /*
		       * The decimal exponent has a magnitude so great that
		       * our tables can't help us fragment it.  Although this
		       * routine is in error because it can't imagine a
		       * number that big, signal an error as if it is the
		       * user's fault for presenting such a big number.
		       */
		      return_value = ERROR_EXPONENT_OVERFLOW;
		      /*
		       * quit out of loop gracefully
		       */
		      decimal_exponent = 0;
		    }
		  else
		    {
#ifdef TRACE
printf("before multiply, place_number = %d., power_of_10_flonum:\n", place_number);
flonum_print( & power_of_10_flonum );
(void)putchar('\n');
#endif
		      flonum_multip (multiplicand + place_number, & power_of_10_flonum, & temporary_flonum);
		      flonum_copy (& temporary_flonum, & power_of_10_flonum);
		    }		/* If this bit of decimal_exponent was computable.*/
		}			/* If this bit of decimal_exponent was set. */
	    }			/* For each bit of binary representation of exponent */
#ifdef TRACE
printf( " after computing power_of_10_flonum: " );
flonum_print( & power_of_10_flonum );
(void)putchar('\n');
#endif
	}

      }

      /*
       * power_of_10_flonum is power of ten in binary (mantissa) , (exponent).
       * It may be the number 1, in which case we don't NEED to multiply.
       *
       * Multiply (decimal digits) by power_of_10_flonum.
       */

      flonum_multip (& power_of_10_flonum, & digits_flonum, address_of_generic_floating_point_number);
      /* Assert sign of the number we made is '+'. */
      address_of_generic_floating_point_number -> sign = digits_sign_char;

    }				/* If we had any significant digits. */
  return (return_value);
}				/* atof_generic () */

/* end: atof_generic.c */