feat(framing): class for header with hamming(7,4)

header.py

@@ -0,0 +1,65 @@
+from hamming_8_4_codec import hamming_8_4_encode, hamming_8_4_decode
+from crc import Configuration, Crc16
+PROTOCOL_IDENTIFIER = 0x0
+# CRC_POLY = (0xed2f << 1) + 1 # from https://users.ece.cmu.edu/~koopman/crc/index.html as "best 16-bit CRC" on 2025-10-24
+ONBEAT_CRC = Configuration(Crc16.IBM_3740)
+
+
+class Onbeat_Header:
+    """This class represents a header for a single packet of PLSTV."""
+
+    def __init__(self, protocol_id, protocol_configuration: int, callsign: str, pkt_len: int, pkt_sequence_id: int):
+        if protocol_id >= 2 << 4:
+            raise OverflowError(
+                f"protocol identifier should be confined to 4 bits, got {protocol_id}")
+        self.protocol_id = protocol_id
+
+        if protocol_configuration >= 2 << 4:
+            raise OverflowError(
+                f"protocol configuration should be confined to 4 bits, got {protocol_configuration}")
+        self.protocol_configuration = protocol_configuration
+
+        call_len = len(callsign.encode())
+        if call_len >= 10:
+            raise OverflowError(
+                f"Callsign must be confined to 10 bytes, got {callsign} with length {call_len} (using UTF-8)")
+        self.callsign = callsign
+
+        if pkt_len >= 2 << 16:
+            raise OverflowError(
+                f"Maximum allowed packet size is {2 << 16 - 1} got {pkt_len}")
+        self.pkt_len = pkt_len
+
+        if pkt_sequence_id >= 2 << 8:
+            raise OverflowError(
+                f"Packet sequence ID must be confined to 8 bits, got {pkt_sequence_id}")
+        self.pkt_sequence_id = pkt_sequence_id
+
+    def encode(self) -> list[int]:
+        header_asints = []
+        header_asints += [(PROTOCOL_IDENTIFIER << 4) +
+                          self.protocol_configuration]
+        header_asints += [(int.from_bytes(self.callsign.encode())
+                           >> 8*i) % 2 << 8 for i in range(0, 10)]
+        header_asints += [(self.pkt_len >> 8), self.pkt_len % 2 << 8]
+        header_asints += [self.pkt_sequence_id]
+        header_crc = ONBEAT_CRC.checksum(bytes(header_asints))
+        header_asints += [header_crc]
+        return hamming_8_4_encode(header_asints)
+
+    def decode(self, header_encoded: list[int]):
+        header_corrected = hamming_8_4_decode(header_encoded)
+        checksum = ONBEAT_CRC.checksum(
+            bytes(header_corrected[14] << 8 + header_corrected[15]))
+        if checksum != 0:
+            raise ValueError(
+                "Checksum of header is non-zero, packet is invalid")
+        self.protocol_id = header_corrected[0] >> 4
+        self.protocol_configuration = header_corrected[0] % 16
+        callsign_numeric = 0
+        for i in range(1, 11):
+            callsign_numeric += header_corrected[i]
+            callsign_numeric <<= 8
+        self.callsign = int.to_bytes(callsign_numeric, byteorder="big").decode()
+        self.pkt_len = header_corrected[11] << 8 + header_corrected[12]
+        self.pkt_sequence_id = header_corrected[13]