Convert a string to a long using ' _str2num '

unsigned long long _str2num(const char *string, unsigned base, int sign)
{
  unsigned long long result = 0;
  int length = 0;
  int negative = 0;
  int count = 0;

  if (string == NULL)
    {
      errno = ERR_NULLPARAMETER;
      return (0);
    }

  // Here we measure the string length
  length = strlen(string);
  // Here we check the string sign
  if (sign && (string[0] == '-'))
    {
      negative = 1;
      count += 1;
    }


  // Do a loop to add to the value of 'result'.
  for ( ; count < length; count ++)
    {
      switch (base)
    {
  // DEC
    case 10:
      if (!isdigit(string[count]))
        {
          errno = ERR_INVALID;
          goto out;
        }
      result *= base;
      result += (string[count] - '0');
      break;
  // HEX
    case 16:
      if (!isxdigit(string[count]))
        {
          errno = ERR_INVALID;
          goto out;
        }
      result *= base;
      if ((string[count] >= '0') && (string[count] <= '9'))
        result += (string[count] - '0');
      else if ((string[count] >= 'a') && (string[count] <= 'f'))
        result += ((string[count] - 'a') + 10);
      else
        result += ((string[count] - 'A') + 10);
      break;

    default:
      errno = ERR_NOTIMPLEMENTED;
      goto out;
    }
    }

 out:
 
// If negative is 1 then 'result' = 'result' * -1
  if (negative)
    result = ((long long) result * -1);
 
  return (result);
}