strtod

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#include <stdio.h>

#include <locale.h>

const double __stdlib_string_to_double_table[17] =
{
    0.0,
    0.1,
    0.01,
    0.001,
    0.0001,
    0.00001,
    0.000001,
    0.0000001,
    0.00000001,
    0.000000001,
    0.0000000001,
    0.00000000001,
    0.000000000001,
    0.0000000000001,
    0.00000000000001,
    0.000000000000001,
    0.0000000000000001,
};

const int __stdlib_string_to_int_table_hex[6] =
{
    10, 11, 12, 13, 14, 15
};

const long long __stdlib_string_to_double_table_hex[17] =
{
    1,
    16,
    256,
    4096,
    65536,
    1048576,
    16777216,
    268435456,
    4294967296,
    68719476736,
    1099511627776,
    17592186044416,
    281474976710656,
    4503599627370496,
    72057594037927936,
    1152921504606846976,
    -1,
};

const char*
__string_skip_spaces(const char* str)
{
    while (1)
    {
        if (*str > 0 && *str < 33)
            ++str;
        else
            break;

        if (!*str)
            break;
    }
    return str;
}

double
strtod(const char* nptr, char** endptr)
{
    double result = 0.0;
    long leftPart = 0;
    long rightPart = 0;
    int e_right = 0;
    int flags = 0; /* 1(-) 2(hex) 3(e) 4(e-)*/

    const char* str_ptr = __string_skip_spaces(nptr);
    const struct lconv* lc = localeconv();
    char decimal_point = *lc->decimal_point;

    /*with - or not*/
    if (*str_ptr == '-')
    {
        ++str_ptr;
        flags |= 0x1;
    }

    if (!*str_ptr)
        goto end;

    /*hex or not, skip 00*/
    if (*str_ptr == '0')
    {
        ++str_ptr;
        if (!*str_ptr)
            goto end;

        if (*str_ptr == 'x' || *str_ptr == 'X')
        {
            flags |= 0x2;
            ++str_ptr;
        }
        else if (*str_ptr == decimal_point)
            --str_ptr;
        else if (*str_ptr < '1' || *str_ptr > '9')
            goto end;
    }

    if (!*str_ptr)
        goto end;

    long part_2_count = 0;

    if (flags & 0x2)/*hex*/
    {
        int charNum = 0;
        const unsigned char* save_str_ptr = str_ptr;
        while (*str_ptr)
        {
            if ((*str_ptr >= 'a' && *str_ptr <= 'f')
                || (*str_ptr >= 'A' && *str_ptr <= 'F'))
                ++charNum;
            else
                break;

            ++str_ptr;

            if (!*str_ptr)
                break;
        }

        if (charNum)
        {
            str_ptr = save_str_ptr;
            int charNum2 = charNum;
            for (int i = 0; i < charNum; ++i)
            {
                int curr_char = str_ptr[i];
                if (curr_char < 'a')
                    curr_char += 32;

                int curr_int = __stdlib_string_to_int_table_hex[curr_char - 97];

                leftPart += curr_int * (long)__stdlib_string_to_double_table_hex[charNum2 - 1];
                --charNum2;
            }
        }
    }
    else
    {
        /* left part 1234. */
        while (*str_ptr >= '0' && *str_ptr <= '9')
        {
            leftPart *= 10;
            leftPart += *str_ptr - '0';
            ++str_ptr;

            if (!*str_ptr)
                break;
        }

        if (*str_ptr == 'e' || *str_ptr == 'E')
        {
            flags |= 0x4;
            ++str_ptr;

            goto do_e;
        }

        /* right part .5678 */
        if (*str_ptr == decimal_point)
        {
            ++str_ptr;

            if (!*str_ptr)
                goto finish;

            while (*str_ptr >= '0' && *str_ptr <= '9')
            {
                rightPart *= 10;
                rightPart += *str_ptr - '0';
                ++str_ptr;
                ++part_2_count;
            }

            if (*str_ptr == 'e' || *str_ptr == 'E')
            {
                flags |= 0x4;
                ++str_ptr;

                goto do_e;
            }
        }
    }

finish:;
    result = leftPart + ((float)rightPart * __stdlib_string_to_double_table[part_2_count]);

    if (flags & 0x4)
    {
        double em = 1.0;

        for (int i = 0; i < e_right; ++i)
        {
            if (flags & 0x8) /* - */
                em *= 0.1;
            else
                em *= 10.0;
        }

        result = result * em;
    }

    result = (flags & 0x1) ? -result : result;

end:;
    if (endptr)
        *endptr = (char*)str_ptr;

    return result;

do_e:;

    if (!*str_ptr)
        goto finish;

    if (*str_ptr == '-')
    {
        flags |= 0x8;

        ++str_ptr;
        if (!*str_ptr)
            goto finish;
    }
    else if (*str_ptr == '+')
    {
        ++str_ptr;
        if (!*str_ptr)
            goto finish;
    }

    while (*str_ptr >= '0' && *str_ptr <= '9')
    {
        e_right *= 10;
        e_right += *str_ptr - '0';
        ++str_ptr;
    }
    goto finish;
}

int main()
{
    printf("Hello World %f\n", strtod("3.14159265359", 0));

    return 0;
}