Define wrap-safe snapshot sequencing

server/tests/test_snapshot_sequence.py

@@ -0,0 +1,43 @@
+from snapshot_sequence import MAX_SEQUENCE, SequenceCounter, SequenceWindow, is_newer, next_sequence
+
+
+def test_next_sequence_skips_zero_after_wrap():
+    assert next_sequence(0) == 1
+    assert next_sequence(1) == 2
+    assert next_sequence(MAX_SEQUENCE) == 1
+
+
+def test_is_newer_is_wrap_safe():
+    assert is_newer(1, 0)
+    assert is_newer(2, 1)
+    assert not is_newer(1, 1)
+    assert not is_newer(1, 2)
+    assert is_newer(1, MAX_SEQUENCE)
+    assert not is_newer(MAX_SEQUENCE, 1)
+    assert not is_newer(0, MAX_SEQUENCE)
+
+
+def test_counter_and_window_enforce_latest_wins():
+    counter = SequenceCounter()
+    assert counter.current == 0
+    assert counter.advance() == 1
+    assert counter.advance() == 2
+    counter.reset()
+    assert counter.advance() == 1
+
+    window = SequenceWindow()
+    assert window.accept(1)
+    assert not window.accept(1)
+    assert window.accept(2)
+    assert not window.accept(1)
+    window.reset()
+    assert window.accept(MAX_SEQUENCE)
+    assert window.accept(1)
+    assert not window.accept(MAX_SEQUENCE)
+
+
+def test_invalid_sequence_inputs_are_rejected():
+    assert not is_newer(-1, 0)
+    assert not is_newer(1, -1)
+    assert not is_newer(True, 0)
+    assert not is_newer(1, MAX_SEQUENCE + 1)