"""
CRC-ITU Implementation for KKS Badge Protocol

Uses CRC-16/X-25 (reflected CRC-CCITT):
  Polynomial: 0x8408 (reflected 0x1021)
  Initial value: 0xFFFF
  Final XOR: 0xFFFF
"""

from typing import List

# ---------------------------------------------------------------------------
# Pre-computed CRC lookup table (256 entries, reflected polynomial 0x8408)
# ---------------------------------------------------------------------------

_CRC_TABLE: List[int] = []


def _generate_crc_table() -> List[int]:
    """Generate the CRC-16/X-25 lookup table for reflected polynomial 0x8408."""
    table: List[int] = []
    for i in range(256):
        crc = i
        for _ in range(8):
            if crc & 1:
                crc = (crc >> 1) ^ 0x8408
            else:
                crc >>= 1
        table.append(crc)
    return table


_CRC_TABLE = _generate_crc_table()


def crc_itu(data: bytes) -> int:
    """
    Compute the CRC-ITU checksum for the given data.

    Uses the CRC-16/X-25 algorithm (reflected CRC-CCITT with final XOR).
    For a KKS protocol packet this should be the bytes from (and including)
    the packet-length field through the serial-number field.

    Parameters
    ----------
    data : bytes
        The data to compute the CRC over.

    Returns
    -------
    int
        16-bit CRC value.
    """
    crc: int = 0xFFFF
    for byte in data:
        crc = (crc >> 8) ^ _CRC_TABLE[(crc ^ byte) & 0xFF]
    return crc ^ 0xFFFF


def verify_crc(data: bytes, expected_crc: int) -> bool:
    """
    Verify that *data* produces the *expected_crc*.

    Parameters
    ----------
    data : bytes
        The data slice to check (same range used when computing the CRC).
    expected_crc : int
        The 16-bit CRC value to compare against.

    Returns
    -------
    bool
        ``True`` if the computed CRC matches *expected_crc*.
    """
    return crc_itu(data) == (expected_crc & 0xFFFF)