66 lines
2.9 KiB
Python
66 lines
2.9 KiB
Python
from hamming_8_4_codec import hamming_8_4_encode, hamming_8_4_decode
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from crc import Configuration, Crc16
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PROTOCOL_IDENTIFIER = 0x0
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# CRC_POLY = (0xed2f << 1) + 1 # from https://users.ece.cmu.edu/~koopman/crc/index.html as "best 16-bit CRC" on 2025-10-24
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ONBEAT_CRC = Configuration(Crc16.IBM_3740)
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class Onbeat_Header:
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"""This class represents a header for a single packet of PLSTV."""
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def __init__(self, protocol_id, protocol_configuration: int, callsign: str, pkt_len: int, pkt_sequence_id: int):
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if protocol_id >= 2 << 4:
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raise OverflowError(
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f"protocol identifier should be confined to 4 bits, got {protocol_id}")
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self.protocol_id = protocol_id
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if protocol_configuration >= 2 << 4:
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raise OverflowError(
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f"protocol configuration should be confined to 4 bits, got {protocol_configuration}")
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self.protocol_configuration = protocol_configuration
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call_len = len(callsign.encode())
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if call_len >= 10:
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raise OverflowError(
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f"Callsign must be confined to 10 bytes, got {callsign} with length {call_len} (using UTF-8)")
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self.callsign = callsign
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if pkt_len >= 2 << 16:
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raise OverflowError(
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f"Maximum allowed packet size is {2 << 16 - 1} got {pkt_len}")
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self.pkt_len = pkt_len
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if pkt_sequence_id >= 2 << 8:
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raise OverflowError(
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f"Packet sequence ID must be confined to 8 bits, got {pkt_sequence_id}")
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self.pkt_sequence_id = pkt_sequence_id
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def encode(self) -> list[int]:
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header_asints = []
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header_asints += [(PROTOCOL_IDENTIFIER << 4) +
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self.protocol_configuration]
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header_asints += [(int.from_bytes(self.callsign.encode())
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>> 8*i) % 2 << 8 for i in range(0, 10)]
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header_asints += [(self.pkt_len >> 8), self.pkt_len % 2 << 8]
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header_asints += [self.pkt_sequence_id]
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header_crc = ONBEAT_CRC.checksum(bytes(header_asints))
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header_asints += [header_crc]
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return hamming_8_4_encode(header_asints)
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def decode(self, header_encoded: list[int]):
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header_corrected = hamming_8_4_decode(header_encoded)
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checksum = ONBEAT_CRC.checksum(
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bytes(header_corrected[14] << 8 + header_corrected[15]))
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if checksum != 0:
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raise ValueError(
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"Checksum of header is non-zero, packet is invalid")
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self.protocol_id = header_corrected[0] >> 4
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self.protocol_configuration = header_corrected[0] % 16
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callsign_numeric = 0
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for i in range(1, 11):
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callsign_numeric += header_corrected[i]
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callsign_numeric <<= 8
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self.callsign = int.to_bytes(callsign_numeric, byteorder="big").decode()
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self.pkt_len = header_corrected[11] << 8 + header_corrected[12]
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self.pkt_sequence_id = header_corrected[13]
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