1 files changed, 34 insertions, 0 deletions
diff --git a/stacks/crdop/lib/test_bell202_line_code.py b/stacks/crdop/lib/test_bell202_line_code.py
new file mode 100644
index 0000000..3b22af6
--- /dev/null
+++ b/stacks/crdop/lib/test_bell202_line_code.py
@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+"""Unit tests for Bell 202 frequency-toggle line code."""
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+LIB = Path(__file__).resolve().parent
+sys.path.insert(0, str(LIB))
+
+from bell202_line_code import decode_tones_to_bits, encode_bits_to_tones  # noqa: E402
+
+
+def test_roundtrip_single_byte() -> None:
+    bits = bytes([0x55])  # 01010101 → many toggles
+    tones = encode_bits_to_tones(bits, start_tone=0)
+    back = decode_tones_to_bits(tones, start_tone=0)
+    assert back == bits
+
+
+def test_all_ones_no_toggle() -> None:
+    tones = encode_bits_to_tones(bytes([0xFF]), start_tone=0)
+    assert len(set(tones)) == 1
+
+
+def main() -> int:
+    test_roundtrip_single_byte()
+    test_all_ones_no_toggle()
+    print("OK: bell202_line_code tests")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())