Add hierarchical policy simulation helpers

tests/test_policy_hierarchy.py

@@ -0,0 +1,67 @@
+from __future__ import annotations
+
+import unittest
+
+from govoplan_policy.backend.hierarchy import (
+    PolicyRestrictionRule,
+    simulate_hierarchical_policy_change,
+    validate_hierarchical_policy_patch,
+)
+
+
+class PolicyHierarchyTests(unittest.TestCase):
+    def test_parent_locks_block_lower_level_field_changes_and_limit_reenable(self) -> None:
+        issues = validate_hierarchical_policy_patch(
+            parent_policy={"retention_days": 30},
+            patch={"retention_days": 20, "allow_lower_level_limits": {"retention_days": True}},
+            field_keys=("retention_days",),
+            parent_allow_lower_level_limits={"retention_days": False},
+        )
+
+        self.assertEqual(["field_locked_by_parent", "lower_level_limit_locked_by_parent"], [issue.code for issue in issues])
+        self.assertEqual(["retention_days", "retention_days"], [issue.field for issue in issues])
+
+    def test_restriction_rules_block_less_restrictive_patch(self) -> None:
+        issues = validate_hierarchical_policy_patch(
+            parent_policy={"retention_days": 30},
+            patch={"retention_days": 45},
+            field_keys=("retention_days",),
+            parent_allow_lower_level_limits={"retention_days": True},
+            restriction_rules=(
+                PolicyRestrictionRule(
+                    field="retention_days",
+                    is_more_restrictive_or_equal=lambda parent, requested: int(requested) <= int(parent),
+                    less_restrictive_message="retention_days cannot be widened",
+                ),
+            ),
+        )
+
+        self.assertEqual(1, len(issues))
+        self.assertEqual("less_restrictive_than_parent", issues[0].code)
+        self.assertEqual("retention_days cannot be widened", issues[0].message)
+
+    def test_policy_simulation_returns_decision_payload(self) -> None:
+        simulation = simulate_hierarchical_policy_change(
+            parent_policy={"retention_days": 30},
+            current_policy={"retention_days": 30},
+            patch={"retention_days": 45},
+            field_keys=("retention_days",),
+            parent_allow_lower_level_limits={"retention_days": True},
+            restriction_rules=(
+                PolicyRestrictionRule(
+                    field="retention_days",
+                    is_more_restrictive_or_equal=lambda parent, requested: int(requested) <= int(parent),
+                    less_restrictive_message="retention_days cannot be widened",
+                ),
+            ),
+        )
+
+        self.assertFalse(simulation.allowed)
+        self.assertEqual(("retention_days",), simulation.changed_fields)
+        self.assertIsNotNone(simulation.decision)
+        self.assertEqual(("retention_days",), simulation.decision.requirements)
+        self.assertEqual("Requested policy change is blocked by parent policy constraints.", simulation.decision.reason)
+
+
+if __name__ == "__main__":
+    unittest.main()