Extract compute_income_change and add exact value test

Pulls the 3-line formula into a named function so it can be tested
directly with exact value assertions, not just bucket assignment.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: PavelMakarchuk

policyengine_api/endpoints/economy/compare.py

@@ -310,6 +310,14 @@ def poverty_impact(baseline: dict, reform: dict) -> dict:
     )
 
 
+def compute_income_change(baseline_values, reform_values):
+    """Percentage income change with a floor of 1 on the baseline
+    to avoid division by zero for zero/negative incomes."""
+    absolute_change = reform_values - baseline_values
+    capped_baseline = np.maximum(baseline_values, 1)
+    return absolute_change / capped_baseline
+
+
 def intra_decile_impact(baseline: dict, reform: dict) -> dict:
     baseline_income = MicroSeries(
         baseline["household_net_income"], weights=baseline["household_weight"]
@@ -321,9 +329,9 @@ def intra_decile_impact(baseline: dict, reform: dict) -> dict:
         baseline["household_count_people"], weights=baseline_income.weights
     )
     decile = MicroSeries(baseline["household_income_decile"]).values
-    absolute_change = (reform_income - baseline_income).values
-    capped_baseline_income = np.maximum(baseline_income.values, 1)
-    income_change = absolute_change / capped_baseline_income
+    income_change = compute_income_change(
+        baseline_income.values, reform_income.values
+    )
 
     # Within each decile, calculate the percentage of people who:
     # 1. Gained more than 5% of their income
@@ -378,9 +386,9 @@ def intra_wealth_decile_impact(baseline: dict, reform: dict) -> dict:
         baseline["household_count_people"], weights=baseline_income.weights
     )
     decile = MicroSeries(baseline["household_wealth_decile"]).values
-    absolute_change = (reform_income - baseline_income).values
-    capped_baseline_income = np.maximum(baseline_income.values, 1)
-    income_change = absolute_change / capped_baseline_income
+    income_change = compute_income_change(
+        baseline_income.values, reform_income.values
+    )
 
     # Within each decile, calculate the percentage of people who:
     # 1. Gained more than 5% of their income

tests/unit/endpoints/economy/test_compare.py

@@ -11,6 +11,7 @@
     UKLocalAuthorityBreakdown,
     uk_constituency_breakdown,
     uk_local_authority_breakdown,
+    compute_income_change,
     intra_decile_impact,
     intra_wealth_decile_impact,
 )
@@ -752,6 +753,14 @@ def _make_economy(
     }
 
 
+class TestComputeIncomeChange:
+    """Direct unit tests for the income change formula."""
+
+    def test__income_change_formula_exact(self):
+        result = compute_income_change(np.array([1000.0]), np.array([1040.0]))
+        assert result[0] == pytest.approx(0.04)
+
+
 class TestIntraDecileImpact:
     """Tests for the intra_decile_impact function — verifying correct
     percentage change calculation and bucket assignment."""