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


def calculate_crc16(input_data: bytes) -> int:
    """Calculate the CRC-16 of a byte string."""
    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))

    crcValue = 0xFFFF

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

    return crcValue