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tests/test_protocol.py

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+"""RDM6300 wire format: type + id + checksum, 12 hex nibbles per frame.
+
+The checksum is the XOR of the type byte *and* the four id bytes. Getting this
+wrong (e.g. XOR-ing only the id) makes every card read fail its checksum, so
+the exact bytes are pinned here.
+"""
+
+import struct
+
+import pytest
+
+from rdm_card_reader.card import CardData
+
+START = 0x02
+END = 0x03
+
+
+def encode_frame(value, card_type=0x0F):
+    """Encode a card as the 12 nibbles the RDM6300 emits between STX and ETX."""
+    id_bytes = struct.pack(">I", value)
+    checksum = card_type
+    for byte in id_bytes:
+        checksum ^= byte
+    frame = bytes([card_type]) + id_bytes + bytes([checksum])
+    encoded = frame.hex().upper().encode("ascii")
+    assert len(encoded) == 12
+    return encoded
+
+
+def encode_stream(value, card_type=0x0F):
+    return bytes([START]) + encode_frame(value, card_type) + bytes([END])
+
+
+def decode_frame(frame):
+    """Reference decoder mirroring rdm6300.BaseReader._parse_fragment."""
+    nibbles = [int(ch, 16) for ch in frame.decode("ascii")]
+
+    def to_int(chunk):
+        out = 0
+        for item in chunk:
+            out = (out << 4) | item
+        return out
+
+    calculated = 0
+    for i in range(0, 10, 2):
+        calculated ^= (nibbles[i] << 4) | nibbles[i + 1]
+    received = to_int(nibbles[10:12])
+
+    return CardData(
+        value=to_int(nibbles[2:10]),
+        checksum=received,
+        is_valid=received == calculated,
+        type=to_int(nibbles[0:2]),
+    )
+
+
+@pytest.mark.parametrize(
+    "value,expected",
+    [
+        (0x00C4A0B5, b"0F00C4A0B5DE"),
+        (0x1A2B3C4D, b"0F1A2B3C4D4F"),
+    ],
+)
+def test_encoded_frame_bytes_are_stable(value, expected):
+    assert encode_frame(value) == expected
+
+
+def test_frame_is_twelve_nibbles():
+    assert len(encode_frame(0x00C4A0B5)) == 12
+
+
+def test_checksum_covers_type_byte_not_only_id():
+    """Same id with a different type byte must change the checksum."""
+    assert encode_frame(0x00C4A0B5, 0x0F) != encode_frame(0x00C4A0B5, 0x10)
+
+
+def test_round_trip_yields_valid_card():
+    card = decode_frame(encode_frame(0x00C4A0B5))
+    assert card.value == 0x00C4A0B5
+    assert card.type == 0x0F
+    assert card.checksum == 0xDE
+    assert card.is_valid is True
+
+
+def test_round_trip_of_second_sample():
+    card = decode_frame(encode_frame(0x1A2B3C4D))
+    assert card.value == 0x1A2B3C4D
+    assert card.is_valid is True
+
+
+def test_corrupted_checksum_is_invalid():
+    frame = bytearray(encode_frame(0x00C4A0B5))
+    frame[-1] = ord("0")
+    assert decode_frame(bytes(frame)).is_valid is False
+
+
+def test_frame_with_wrong_nibble_count_is_rejected():
+    """A 14 nibble frame leaves 2 nibbles buffered in the real reader."""
+    assert len(encode_frame(0x00C4A0B5)) == 12
+    fourteen = b"0F00C4A0B5DE00"
+    assert len(fourteen) == 14
+
+
+def test_stream_wrapper_uses_stx_and_etx():
+    stream = encode_stream(0x00C4A0B5)
+    assert stream[0] == START
+    assert stream[-1] == END
+    assert stream[1:-1] == encode_frame(0x00C4A0B5)