strtod.c

/* vi:set ts=8 sts=4 sw=4: */
/*
 * strtod implementation.
 * author: Yasuhiro Matsumoto
 * license: public domain
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <ctype.h>
#include <errno.h>

    char*
skipwhite(q)
    char	*q;
{
    char	*p = q;
    while (isspace(*p))
	++p;
    return p;
}
#define vim_isdigit(x) isdigit(x)

    double
vim_strtod(str, end)
    const char *str;
    char **end;
{
    double d = 0.0;
    int sign;
    int n = 0;
    const char *p, *a;

    a = p = str;
    p = skipwhite(p);

    /* decimal part */
    sign = 1;
    if (*p == '-')
    {
	sign = -1;
	++p;
    } else if (*p == '+')
	++p;
    if (vim_isdigit(*p))
    {
	d = (double)(*p++ - '0');
	while (*p && vim_isdigit(*p))
	{
	    d = d * 10.0 + (double)(*p - '0');
	    ++p;
	    ++n;
	}
	a = p;
    } else if (*p != '.')
	goto done;
    d *= sign;

    /* fraction part */
    if (*p == '.')
    {
	double f = 0.0;
	double base = 0.1;
	++p;

	if (vim_isdigit(*p))
	{
	    while (*p && vim_isdigit(*p))
	    {
		f += base * (*p - '0') ;
		base /= 10.0;
		++p;
		++n;
	    }
	}
	d += f * sign;
	a = p;
    }

    /* exponential part */
    if ((*p == 'E') || (*p == 'e'))
    {
	int e = 0;
	++p;

	sign = 1;
	if (*p == '-')
	{
	    sign = -1;
	    ++p;
	} else if (*p == '+')
	    ++p;

	if (vim_isdigit(*p))
	{
	    while (*p == '0')
		++p;
            if (*p == '\0') --p;
	    e = (int)(*p++ - '0');
	    while (*p && vim_isdigit(*p))
	    {
		e = e * 10 + (int)(*p - '0');
		++p;
	    }
	    e *= sign;
	}
	else if (!vim_isdigit(*(a-1)))
	{
	    a = str;
	    goto done;
	}
	else if (*p == 0)
	    goto done;

	if (d == 2.2250738585072011 && e == -308)
	{
	    d = 0.0;
	    a = p;
	    errno = ERANGE;
	    goto done;
	}
	if (d == 2.2250738585072012 && e <= -308)
	{
	    d *= 1.0e-308;
	    a = p;
	    goto done;
	}
	d *= pow(10.0, (double) e);
	a = p;
    }
    else if (p > str && !vim_isdigit(*(p-1)))
    {
	a = str;
	goto done;
    }

done:
    if (end)
	*end = (char*)a;
    return d;
}

    void
test(char* str)
{
    double d1, d2;
    char *e1, *e2;
    int x1, x2;

    printf("CASE: %s\n", str);

    errno = 0;
    e1 = NULL;
    d1 = vim_strtod(str, &e1);
    x1 = errno;

    errno = 0;
    e2 = NULL;
    d2 = strtod(str, &e2);
    x2 = errno;

    if (d1 != d2 || e1 != e2 || x1 != x2) {
	printf("  ERR: %s, %s\n", str, strerror(errno));
	printf("    E1 %f, %g, %s, %d\n", d1, d1, e1 ? e1 : "", x1);
	printf("    E2 %f, %g, %s, %d\n", d2, d2, e2 ? e2 : "", x2);
       if (d1 != d2) puts("different value");
       if (e1 != e2) puts("different end position");
       if (x1 != x2) puts("different errno");
    } else {
	printf("  SUCCESS [%f][%s]: %s\n", d1, e1 ? e1 : "", strerror(errno));
    }
    printf("\n");
}

    int
main()
{
    test(".1");
    test("  .");
    test("  1.2e3");
    test(" +1.2e3");
    test("1.2e3");
    test("+1.2e3");
    test("+1.e3");
    test("-1.2e3");
    test("-1.2e3.5");
    test("-1.2e");
    test("--1.2e3.5");
    test("--1-.2e3.5");
    test("-a");
    test("a");
    test(".1e");
    test(".1e3");
    test(".1e-3");
    test(".1e-");
    test(" .e-");
    test(" .e");
    test(" e");
    test(" e0");
    test(" ee");
    test(" -e");
    test(" .9");
    test(" ..9");
    test("009");
    test("0.09e02");
    /* http://thread.gmane.org/gmane.editors.vim.devel/19268/ */
    test("0.9999999999999999999999999999999999");
    test("2.2250738585072010e-308"); // BUG
    /* PHP (slashdot.jp): http://opensource.slashdot.jp/story/11/01/08/0527259/PHP%E3%81%AE%E6%B5%AE%E5%8B%95%E5%B0%8F%E6%95%B0%E7%82%B9%E5%87%A6%E7%90%86%E3%81%AB%E7%84%A1%E9%99%90%E3%83%AB%E3%83%BC%E3%83%97%E3%81%AE%E3%83%90%E3%82%B0 */
    test("2.2250738585072011e-308");
    /* Gauche: http://blog.practical-scheme.net/gauche/20110203-bitten-by-floating-point-numbers-again */
    test("2.2250738585072012e-308");
    test("2.2250738585072013e-308"); // Hmm.
    test("2.2250738585072014e-308"); // Hmm.
}

Thanks for this code. Please note that it fails on exponent values of 0; e.g., 1e0.

I believe it can be fixed by inserting the code if (*p == '\0') --p; after line 97.

@tkmiller3 Thanks. Sorry I noticed your comment just now.