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https://github.com/mjg59/python-broadlink.git
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Improve CRC-16 function (#565)
* Rename calculate_crc16 to crc16 * Apply PEP-8 naming conventions * Remove unnecessary import * Accept any sequence type * Remove unnecessary conversions * Expose polynomial and initial value as kwargs * Remove unnecessary bitwise operations * Store the CRC-16 table for performance * Add missing type hints * Update docstring * General improvements
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@ -3,7 +3,7 @@ from typing import List
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from . import exceptions as e
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from .device import device
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from .helpers import calculate_crc16
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from .helpers import crc16
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class hysen(device):
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@ -19,7 +19,7 @@ class hysen(device):
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def send_request(self, input_payload: bytes) -> bytes:
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"""Send a request to the device."""
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crc = calculate_crc16(input_payload)
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crc = crc16(input_payload)
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# first byte is length, +2 for CRC16
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request_payload = bytearray([len(input_payload) + 2, 0x00])
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@ -40,7 +40,7 @@ class hysen(device):
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raise ValueError(
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"hysen_response_error", "first byte of response is not length"
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)
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crc = calculate_crc16(response_payload[2:response_payload_len])
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crc = crc16(response_payload[2:response_payload_len])
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if (response_payload[response_payload_len] == crc & 0xFF) and (
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response_payload[response_payload_len + 1] == (crc >> 8) & 0xFF
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):
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@ -1,26 +1,32 @@
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"""Helper functions."""
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from ctypes import c_ushort
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import typing as t
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_crc16_cache = {}
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def calculate_crc16(input_data: bytes) -> int:
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"""Calculate the CRC-16 of a byte string."""
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crc16_tab = []
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crc16_constant = 0xA001
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def crc16(
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sequence: t.Sequence[int],
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polynomial: int = 0xA001, # Default: Modbus CRC-16.
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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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global _crc16_cache
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for i in range(0, 256):
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crc = c_ushort(i).value
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for j in range(0, 8):
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if crc & 0x0001:
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crc = c_ushort(crc >> 1).value ^ crc16_constant
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else:
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crc = c_ushort(crc >> 1).value
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crc16_tab.append(hex(crc))
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try:
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crc_table = _crc16_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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_crc16_cache[polynomial] = crc_table
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crcValue = 0xFFFF
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for c in input_data:
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tmp = crcValue ^ c
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rotated = c_ushort(crcValue >> 8).value
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crcValue = rotated ^ int(crc16_tab[(tmp & 0x00FF)], 0)
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return crcValue
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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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