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tests/test_cost_state.py

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+"""Tests for hameter.cost_state persistence."""
+
+import json
+
+from hameter.cost_state import load_cost_state, save_cost_state
+
+
+class TestCostStatePersistence:
+    """Tests for load/save of cost state."""
+
+    def test_save_and_load(self, tmp_path):
+        path = str(tmp_path / "cost_state.json")
+        states = {
+            "123": {
+                "cumulative_cost": 156.78,
+                "last_calibrated_reading": 59830.5,
+                "billing_period_start": "2026-03-01T00:00:00Z",
+                "last_updated": "2026-03-05T14:30:00Z",
+                "fixed_charges_applied": 9.65,
+            }
+        }
+        save_cost_state(states, path)
+        loaded = load_cost_state(path)
+        assert loaded["123"]["cumulative_cost"] == 156.78
+        assert loaded["123"]["last_calibrated_reading"] == 59830.5
+        assert loaded["123"]["fixed_charges_applied"] == 9.65
+
+    def test_load_nonexistent_returns_empty(self, tmp_path):
+        path = str(tmp_path / "nonexistent.json")
+        result = load_cost_state(path)
+        assert result == {}
+
+    def test_load_corrupt_returns_empty(self, tmp_path):
+        path = str(tmp_path / "cost_state.json")
+        with open(path, "w") as f:
+            f.write("not json")
+        result = load_cost_state(path)
+        assert result == {}
+
+    def test_creates_parent_directory(self, tmp_path):
+        path = str(tmp_path / "subdir" / "cost_state.json")
+        save_cost_state({"1": {"cumulative_cost": 0}}, path)
+        loaded = load_cost_state(path)
+        assert "1" in loaded
+
+    def test_no_temp_file_left(self, tmp_path):
+        path = str(tmp_path / "cost_state.json")
+        save_cost_state({"1": {"cumulative_cost": 10.0}}, path)
+        files = list(tmp_path.iterdir())
+        assert len(files) == 1
+        assert files[0].name == "cost_state.json"
+
+    def test_multiple_meters(self, tmp_path):
+        path = str(tmp_path / "cost_state.json")
+        states = {
+            "111": {"cumulative_cost": 50.0, "last_calibrated_reading": 1000.0},
+            "222": {"cumulative_cost": 25.0, "last_calibrated_reading": 500.0},
+        }
+        save_cost_state(states, path)
+        loaded = load_cost_state(path)
+        assert len(loaded) == 2
+        assert loaded["111"]["cumulative_cost"] == 50.0
+        assert loaded["222"]["cumulative_cost"] == 25.0
+
+    def test_atomic_write_format(self, tmp_path):
+        path = str(tmp_path / "cost_state.json")
+        states = {"123": {"cumulative_cost": 42.0}}
+        save_cost_state(states, path)
+        with open(path) as f:
+            raw = f.read()
+        # Should be pretty-printed JSON with trailing newline
+        assert raw.endswith("\n")
+        parsed = json.loads(raw)
+        assert parsed == states