#include <stdlib.h>
#include <string.h>
#include <ctype.h>

double strtod(const char *str, char **ptr)
{
    if (ptr == (char **)0) {
        return atof(str);
    }

    size_t len = strlen(str);
    char rstr[len + 1];
    strcpy(rstr, str);

    char *p = rstr;

    while (isspace(*p)) {
        ++p;
    }

    if (*p == '+' || *p == '-') {
        ++p;
    }

    if ((p[0] == 'i' || p[0] == 'I')
            && (p[1] == 'n' || p[1] == 'N')
            && (p[2] == 'f' || p[2] == 'F')) {
        if ((p[3] == 'i' || p[3] == 'I')
                && (p[4] == 'n' || p[4] == 'N')
                && (p[5] == 'i' || p[5] == 'I')
                && (p[6] == 't' || p[6] == 'T')
                && (p[7] == 'y' || p[7] == 'Y')) {
            *ptr = p + 8;
            return atof(str);
        } else {
            *ptr = p + 3;
            return atof(str);
        }
    }

    if ((p[0] == 'n' || p[0] == 'N')
            && (p[1] == 'a' || p[1] == 'A')
            && (p[2] == 'n' || p[2] == 'N')) {
        p += 3;
        if (*p == '(') {
            ++p;
            while (*p != '\0' && *p != ')') {
                ++p;
            }

            if (*p == ')') {
                ++p;
            }
        }

        *ptr = p;
        return atof(str);
    }

    if (isdigit(*p) || *p == '.') {
        int got_dot = 0;
        while (isdigit(*p) || (!got_dot && *p == '.')) {
            if (*p == '.')
                got_dot = 1;
            ++p;
        }

        if (*p == 'e' || *p == 'E') {
            int i;
            i = 1;
            if (p[i] == '+' || p[i] == '-') {
                ++i;
            }

            if (isdigit (p[i])) {
                while (isdigit (p[i])) {
                    ++i;
                }

                *ptr = p + i;
                return atof(str);
            }
        }

        *ptr = p;
        return atof(str);
    }

    *ptr = (char *)str;
    return 0.0;
}