Test GNS snapshot envelope

server/tests/test_gns_snapshot_envelope.py

@@ -0,0 +1,64 @@
+import pytest
+
+from gns_snapshot_envelope import (
+    HEADER_SIZE,
+    MAGIC,
+    SnapshotEnvelopeError,
+    decode_snapshot,
+    encode_snapshot,
+)
+from packet_codec import MAX_MESSAGE_BYTES
+
+
+def test_transform_and_npc_snapshots_round_trip_with_sequence():
+    transform = encode_snapshot("transform", b'{"type":"transform","x":1}', 7)
+    decoded_transform = decode_snapshot(transform)
+    assert decoded_transform is not None
+    assert decoded_transform.packet_type == "transform"
+    assert decoded_transform.sequence == 7
+    assert decoded_transform.payload == b'{"type":"transform","x":1}'
+
+    npc = encode_snapshot("npcState", b'{"type":"npcState","npcs":[]}', 9)
+    decoded_npc = decode_snapshot(npc)
+    assert decoded_npc is not None
+    assert decoded_npc.packet_type == "npcState"
+    assert decoded_npc.sequence == 9
+
+
+def test_reliable_json_is_not_misidentified_as_snapshot_envelope():
+    assert decode_snapshot(b'{"type":"playerState"}') is None
+
+
+def test_snapshot_envelope_rejects_invalid_family_sequence_and_size():
+    with pytest.raises(SnapshotEnvelopeError):
+        encode_snapshot("playerState", b"{}", 1)
+    with pytest.raises(SnapshotEnvelopeError):
+        encode_snapshot("transform", b"{}", 0)
+    with pytest.raises(SnapshotEnvelopeError):
+        encode_snapshot("transform", b"x" * MAX_MESSAGE_BYTES, 1)
+
+
+def test_snapshot_decoder_rejects_truncated_or_corrupt_headers():
+    with pytest.raises(SnapshotEnvelopeError):
+        decode_snapshot(MAGIC)
+
+    valid = bytearray(encode_snapshot("transform", b"{}", 1))
+    valid[4] = 99
+    with pytest.raises(SnapshotEnvelopeError):
+        decode_snapshot(valid)
+
+    valid = bytearray(encode_snapshot("transform", b"{}", 1))
+    valid[5] = 99
+    with pytest.raises(SnapshotEnvelopeError):
+        decode_snapshot(valid)
+
+    valid = bytearray(encode_snapshot("transform", b"{}", 1))
+    valid[6:8] = b"\x00\x01"
+    with pytest.raises(SnapshotEnvelopeError):
+        decode_snapshot(valid)
+
+
+def test_snapshot_header_leaves_payload_under_native_64k_cap():
+    payload = b"x" * (MAX_MESSAGE_BYTES - HEADER_SIZE)
+    message = encode_snapshot("transform", payload, 0xFFFFFFFF)
+    assert len(message) == MAX_MESSAGE_BYTES