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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.
 *
 * See the file "license.terms" for information on usage and redistribution
 * of this file, and for a DISCLAIMER OF ALL WARRANTIES.
 *
 * RCS: @(#) $Id: strtod.c,v 1.2 1998/09/14 18:39:45 stanton Exp $
 */

#include "tcl.h"
#ifdef NO_STDLIB_H
#   include "../compat/stdlib.h"
#else
#   include <stdlib.h>
#endif
#include <ctype.h>

#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;
    }
    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;
}

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