"""Helper functions."""
from ctypes import c_ushort
import socket

from .exceptions import exception


def get_local_ip() -> str:
    """Try to determine the local IP address of the machine."""
    # Useful for VPNs.
    try:
        local_ip_address = socket.gethostbyname(socket.gethostname())
        if not local_ip_address.startswith('127.'):
            return local_ip_address
    except socket.gaierror:
        raise exception(-4013)  # DNS Error

    # Connecting to UDP address does not send packets.
    with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
        s.connect(('8.8.8.8', 53))
        return s.getsockname()[0]


def calculate_crc16(input_data) -> int:
    """Calculate CRC-16."""
    crc16_tab = []
    crc16_constant = 0xA001

    for i in range(0, 256):
        crc = c_ushort(i).value
        for j in range(0, 8):
            if crc & 0x0001:
                crc = c_ushort(crc >> 1).value ^ crc16_constant
            else:
                crc = c_ushort(crc >> 1).value
        crc16_tab.append(hex(crc))

    try:
        is_string = isinstance(input_data, str)
        is_bytes = isinstance(input_data, bytes)

        if not is_string and not is_bytes:
            raise Exception(
                "Please provide a string or a byte sequence "
                "as argument for calculation."
            )

        crcValue = 0xFFFF

        for c in input_data:
            d = ord(c) if is_string else c
            tmp = crcValue ^ d
            rotated = c_ushort(crcValue >> 8).value
            crcValue = rotated ^ int(crc16_tab[(tmp & 0x00FF)], 0)

        return crcValue
    except Exception as e:
        print("EXCEPTION(calculate): {}".format(e))