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"""Helper functions and classes."""
from typing import Dict, List, Sequence
class CRC16:
"""Helps with CRC-16 calculation.
CRC tables are cached for performance.
"""
_cache: Dict[int, List[int]] = {}
@classmethod
def get_table(cls, polynomial: int) -> List[int]:
"""Return the CRC-16 table for a polynomial."""
try:
crc_table = cls._cache[polynomial]
except KeyError:
crc_table = []
for dividend in range(0, 256):
remainder = dividend
for _ in range(0, 8):
if remainder & 1:
remainder = remainder >> 1 ^ polynomial
else:
remainder = remainder >> 1
crc_table.append(remainder)
cls._cache[polynomial] = crc_table
return crc_table
@classmethod
def calculate(
cls,
sequence: Sequence[int],
polynomial: int = 0xA001, # CRC-16-ANSI.
init_value: int = 0xFFFF,
) -> int:
"""Calculate the CRC-16 of a sequence of integers."""
crc_table = cls.get_table(polynomial)
crc = init_value
for item in sequence:
crc = crc >> 8 ^ crc_table[(crc ^ item) & 0xFF]
return crc
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