Colloquium_4_task_2

typedef char string_t[256];

void string_to_double(double *d, const string_t s)
{
    int flag_point = 257;
    int flag_minus = 1;
    int flag_e = 257;
    int i = 0, len = 0;
    long int rank = 1;

    if (s[i] == '-')
    {
        flag_minus = -1;
        i++;
    }

    while(s[len])
    {
        if (s[len] == 'e' && flag_e == 257)
        {
            flag_e = len;
            rank = pow(10, len-1);
        }

        else if (s[len] == 'e' && flag_e != 257)
            return;

        else if (s[len] == '.' && flag_point == 257)
        {
            flag_point = len;
            rank = powf(10, len-1);
        }

        else if (s[len] == '.' && flag_point != 257)
            return;

        else if (s[len] > '9' && s[len < '0'])
            return;

        else if (s[len] == '-' && len != 0 && len != flag_e+1)
            return;
        len++;
    }

    if (flag_point == 257 && flag_e == 257)
        rank = powf(10, len-1);

    if (flag_e != 257)
    {
        int k = 1;
        int first = 0;
        double second = 0;
        int e_minus = 1;

        while (s[i])
        {
            if (i < flag_e)
            {
                first += (s[i] - '0') * rank;
                rank /= 10;
            }

            else if (i == flag_e)
            {
                rank = 1;
            }

            else if (i == flag_e + 1 && s[i] == '-')
            {
                e_minus = -1;
            }

            else if (i > flag_e)
            {
                second += (double)(s[i] - '0') * rank;
                rank *= 10;
            }
            i++;
        }
        *d = flag_minus * first * pow(10, e_minus * second);
    }

    else
    {
        while (s[i])
        {
            if (i < flag_point)
            {
                *d += (s[i] - '0') * rank;
                rank /= 10;
            }

            else if (i == flag_point)
                rank = 10;

            else if (i > flag_point)
            {
                *d += (double)(s[i] - '0') / rank;
                rank *= 10;
            }
            i++;
        }
        *d *= flag_minus;
    }
}