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