strtod.c

/* 
 * strtod.c --
 *
 *	Source code for the "strtod" library procedure.
 *
 * Copyright (c) 1988-1993 The Regents of the University of California.
 * Copyright (c) 1994 Sun Microsystems, Inc.
 *
 * Permission to use, copy, modify, and distribute this
 * software and its documentation for any purpose and without
 * fee is hereby granted, provided that the above copyright
 * notice appear in all copies.  The University of California
 * makes no representations about the suitability of this
 * software for any purpose.  It is provided "as is" without
 * express or implied warranty.
 *
 * RCS: @(#) $Id$
 */

#include "config.h"
#ifdef HAVE_STDLIB_H
#   include <stdlib.h>
#endif
#include <ctype.h>
#include <errno.h>
extern  int     errno;

#ifndef __STDC__
# ifdef __GNUC__
#  define const __const__
# else
#  define const
# endif
#endif

#ifndef TRUE
#define TRUE 1
#define FALSE 0
#endif
#ifndef NULL
#define NULL 0
#endif

static int maxExponent = 511;	/* Largest possible base 10 exponent.  Any
				 * exponent larger than this will already
				 * produce underflow or overflow, so there's
				 * no need to worry about additional digits.
				 */
static double powersOf10[] = {	/* Table giving binary powers of 10.  Entry */
    10.,			/* is 10^2^i.  Used to convert decimal */
    100.,			/* exponents into floating-point numbers. */
    1.0e4,
    1.0e8,
    1.0e16,
    1.0e32,
    1.0e64,
    1.0e128,
    1.0e256
};

/*
 *----------------------------------------------------------------------
 *
 * strtod --
 *
 *	This procedure converts a floating-point number from an ASCII
 *	decimal representation to internal double-precision format.
 *
 * Results:
 *	The return value is the double-precision floating-point
 *	representation of the characters in string.  If endPtr isn't
 *	NULL, then *endPtr is filled in with the address of the
 *	next character after the last one that was part of the
 *	floating-point number.
 *
 * Side effects:
 *	None.
 *
 *----------------------------------------------------------------------
 */

double
strtod(string, endPtr)
    const char *string;		/* A decimal ASCII floating-point number,
				 * optionally preceded by white space.
				 * Must have form "-I.FE-X", where I is the
				 * integer part of the mantissa, F is the
				 * fractional part of the mantissa, and X
				 * is the exponent.  Either of the signs
				 * may be "+", "-", or omitted.  Either I
				 * or F may be omitted, or both.  The decimal
				 * point isn't necessary unless F is present.
				 * The "E" may actually be an "e".  E and X
				 * may both be omitted (but not just one).
				 */
    char **endPtr;		/* If non-NULL, store terminating character's
				 * address here. */
{
    int sign, expSign = FALSE;
    double fraction, dblExp, *d;
    register const char *p;
    register int c;
    int exp = 0;		/* Exponent read from "EX" field. */
    int fracExp = 0;		/* Exponent that derives from the fractional
				 * part.  Under normal circumstatnces, it is
				 * the negative of the number of digits in F.
				 * However, if I is very long, the last digits
				 * of I get dropped (otherwise a long I with a
				 * large negative exponent could cause an
				 * unnecessary overflow on I alone).  In this
				 * case, fracExp is incremented one for each
				 * dropped digit. */
    int mantSize;		/* Number of digits in mantissa. */
    int decPt;			/* Number of mantissa digits BEFORE decimal
				 * point. */
    const char *pExp;		/* Temporarily holds location of exponent
				 * in string. */

    /*
     * Strip off leading blanks and check for a sign.
     */

    p = string;
    while (isspace(*p)) {
	p += 1;
    }
    if (*p == '-') {
	sign = TRUE;
	p += 1;
    } else {
	if (*p == '+') {
	    p += 1;
	}
	sign = FALSE;
    }

    /*
     * Count the number of digits in the mantissa (including the decimal
     * point), and also locate the decimal point.
     */

    decPt = -1;
    for (mantSize = 0; ; mantSize += 1)
    {
	c = *p;
	if (!isdigit(c)) {
	    if ((c != '.') || (decPt >= 0)) {
		break;
	    }
	    decPt = mantSize;
	}
	p += 1;
    }

    /*
     * Now suck up the digits in the mantissa.  Use two integers to
     * collect 9 digits each (this is faster than using floating-point).
     * If the mantissa has more than 18 digits, ignore the extras, since
     * they can't affect the value anyway.
     */
    
    pExp  = p;
    p -= mantSize;
    if (decPt < 0) {
	decPt = mantSize;
    } else {
	mantSize -= 1;			/* One of the digits was the point. */
    }
    if (mantSize > 18) {
	fracExp = decPt - 18;
	mantSize = 18;
    } else {
	fracExp = decPt - mantSize;
    }
    if (mantSize == 0) {
	fraction = 0.0;
	p = string;
	goto done;
    } else {
	int frac1, frac2;
	frac1 = 0;
	for ( ; mantSize > 9; mantSize -= 1)
	{
	    c = *p;
	    p += 1;
	    if (c == '.') {
		c = *p;
		p += 1;
	    }
	    frac1 = 10*frac1 + (c - '0');
	}
	frac2 = 0;
	for (; mantSize > 0; mantSize -= 1)
	{
	    c = *p;
	    p += 1;
	    if (c == '.') {
		c = *p;
		p += 1;
	    }
	    frac2 = 10*frac2 + (c - '0');
	}
	fraction = (1.0e9 * frac1) + frac2;
    }

    /*
     * Skim off the exponent.
     */

    p = pExp;
    if ((*p == 'E') || (*p == 'e')) {
	p += 1;
	if (*p == '-') {
	    expSign = TRUE;
	    p += 1;
	} else {
	    if (*p == '+') {
		p += 1;
	    }
	    expSign = FALSE;
	}
	while (isdigit(*p)) {
	    exp = exp * 10 + (*p - '0');
	    p += 1;
	}
    }
    if (expSign) {
	exp = fracExp - exp;
    } else {
	exp = fracExp + exp;
    }

    /*
     * Generate a floating-point number that represents the exponent.
     * Do this by processing the exponent one bit at a time to combine
     * many powers of 2 of 10. Then combine the exponent with the
     * fraction.
     */
    
    if (exp < 0) {
	expSign = TRUE;
	exp = -exp;
    } else {
	expSign = FALSE;
    }
    if (exp > maxExponent) {
	exp = maxExponent;
	errno = ERANGE;
    }
    dblExp = 1.0;
    for (d = powersOf10; exp != 0; exp >>= 1, d += 1) {
	if (exp & 01) {
	    dblExp *= *d;
	}
    }
    if (expSign) {
	fraction /= dblExp;
    } else {
	fraction *= dblExp;
    }

done:
    if (endPtr != NULL) {
	*endPtr = (char *) p;
    }

    if (sign) {
	return -fraction;
    }
    return fraction;
}