将ACII码转为浮点数
2024-04-12 04:45:29
今天看到这样一些代码，情不自禁的把它写入博客，想要好好研究一下这段代码，很优秀的代码，我要研究他们是怎么写出来的。
同时对自己寄予厚望，希望能够有帮助。呵呵
* 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: @(#) $Id$
*/
#include "tclInt.h"
#ifndef TRUE
#define TRUE 1
#define FALSE 0
#endif
#ifndef NULL
#define NULL 0
#endif
static const 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 const 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(
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;
const double *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(UCHAR(*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;
}
if (!isdigit(UCHAR(*p))) {
p = pExp;
goto done;
}
while (isdigit(UCHAR(*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) {
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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