feat: export getAveragePurchaseInterval function and create a unit te…

…st for it

src/utils/dates.js

@@ -142,7 +142,7 @@ function getNextPurchaseEstimate(purchaseIntervals, totalPurchases) {
  * @param {number} estimatedDaysUntilPurchase - The estimated number of days until the next purchase.
  * @returns {number} The average purchase interval calculated from the provided intervals.
  */
-function getAveragePurchaseInterval(
+export function getAveragePurchaseInterval(
 	purchaseIntervals,
 	estimatedDaysUntilPurchase,
 ) {

tests/getAveragePurchaseInterval.test.js

@@ -0,0 +1,47 @@
+import { describe, it, expect } from 'vitest';
+import { getAveragePurchaseInterval } from '../src/utils/dates';
+
+describe('getAveragePurchaseInterval function', () => {
+	it('correctly calculates average purchase intervals', () => {
+		const purchaseIntervals = {
+			lastEstimatedInterval: 4,
+			daysSinceLastPurchase: 6,
+		};
+		const estimatedDaysUntilPurchase = 5;
+		const result = getAveragePurchaseInterval(
+			purchaseIntervals,
+			estimatedDaysUntilPurchase,
+		);
+		expect(result).toBe(5);
+	});
+
+	it('handles zero values in the intervals', () => {
+		const purchaseIntervals = {
+			lastEstimatedInterval: 0,
+			daysSinceLastPurchase: 6,
+		};
+		const estimatedDaysUntilPurchase = 5;
+
+		const result = getAveragePurchaseInterval(
+			purchaseIntervals,
+			estimatedDaysUntilPurchase,
+		);
+
+		expect(result).toBeCloseTo(3.67, 2);
+	});
+
+	it('returns 0 when all intervals are zero', () => {
+		const purchaseIntervals = {
+			lastEstimatedInterval: 0,
+			daysSinceLastPurchase: 0,
+		};
+		const estimatedDaysUntilPurchase = 0;
+
+		const result = getAveragePurchaseInterval(
+			purchaseIntervals,
+			estimatedDaysUntilPurchase,
+		);
+
+		expect(result).toBe(0);
+	});
+});