src/hooks/useBudget.test.ts

@@ -0,0 +1,50 @@
+import { describe, it, expect } from "vitest";
+import { buildPrevYearTotalMap } from "./useBudget";
+
+/**
+ * Unit tests for the previous-year actuals normalization logic used in useBudget.
+ *
+ * Transaction amounts in the database use signed values (expenses are negative,
+ * income is positive). The buildPrevYearTotalMap function preserves these signs
+ * as-is, because the budget display layer does NOT apply a sign multiplier to
+ * previous year actuals (unlike planned budget amounts).
+ */
+
+describe("buildPrevYearTotalMap", () => {
+  it("should preserve negative sign for expense actuals", () => {
+    const actuals = [{ category_id: 1, actual: -500 }]; // expense: negative in DB
+    const map = buildPrevYearTotalMap(actuals);
+    expect(map.get(1)).toBe(-500);
+  });
+
+  it("should preserve positive sign for income actuals", () => {
+    const actuals = [{ category_id: 2, actual: 3000 }]; // income: positive in DB
+    const map = buildPrevYearTotalMap(actuals);
+    expect(map.get(2)).toBe(3000);
+  });
+
+  it("should skip null category_id entries", () => {
+    const actuals = [{ category_id: null, actual: -100 }];
+    const map = buildPrevYearTotalMap(actuals);
+    expect(map.size).toBe(0);
+  });
+
+  it("should handle zero actuals", () => {
+    const actuals = [{ category_id: 3, actual: 0 }];
+    const map = buildPrevYearTotalMap(actuals);
+    expect(map.get(3)).toBe(0);
+  });
+
+  it("should handle multiple categories", () => {
+    const actuals = [
+      { category_id: 1, actual: -200 },
+      { category_id: 2, actual: 1500 },
+      { category_id: 3, actual: -75.5 },
+    ];
+    const map = buildPrevYearTotalMap(actuals);
+    expect(map.size).toBe(3);
+    expect(map.get(1)).toBe(-200);
+    expect(map.get(2)).toBe(1500);
+    expect(map.get(3)).toBe(-75.5);
+  });
+});

src/hooks/useBudget.ts

@@ -63,6 +63,21 @@ function reducer(state: BudgetState, action: BudgetAction): BudgetState {
 
 const TYPE_ORDER: Record<string, number> = { expense: 0, income: 1, transfer: 2 };
 
+/**
+ * Build a map of category_id -> annual actual total from raw actuals.
+ * Transaction amounts are already signed (expenses negative, income positive),
+ * so they are stored as-is without normalization.
+ */
+export function buildPrevYearTotalMap(
+  actuals: Array<{ category_id: number | null; actual: number }>
+): Map<number, number> {
+  const prevYearTotalMap = new Map<number, number>();
+  for (const a of actuals) {
+    if (a.category_id != null) prevYearTotalMap.set(a.category_id, a.actual);
+  }
+  return prevYearTotalMap;
+}
+
 export function useBudget() {
   const [state, dispatch] = useReducer(reducer, undefined, initialState);
   const fetchIdRef = useRef(0);
@@ -90,11 +105,7 @@ export function useBudget() {
       }
 
       // Build a map for previous year actuals: categoryId -> annual actual total
-      // Amounts are already signed (expenses negative, income positive) — stored as-is.
+      const prevYearTotalMap = buildPrevYearTotalMap(prevYearActuals);
-      const prevYearTotalMap = new Map<number, number>();
-      for (const a of prevYearActuals) {
-        if (a.category_id != null) prevYearTotalMap.set(a.category_id, a.actual);
-      }
 
       // Helper: build months array from entryMap
       const buildMonths = (catId: number) => {