test(functions): add date and financial tests

test/functions_financial_test.dart

@@ -0,0 +1,60 @@
+import 'package:cellar/cellar.dart';
+import 'package:test/test.dart';
+
+CellValue ev(String formula) {
+  final sheet = Sheet();
+  sheet.setValue('Z1', formula);
+  return sheet.valueOf('Z1');
+}
+
+double num(CellValue value) => (value as NumberValue).value;
+
+void main() {
+  group('financial functions', () {
+    test('PMT for a mortgage', () {
+      expect(num(ev('=PMT(0.05/12, 360, 200000)')), closeTo(-1073.64, 0.01));
+    });
+
+    test('PMT with a zero rate', () {
+      expect(num(ev('=PMT(0, 10, 1000)')), -100);
+    });
+
+    test('FV of a savings plan', () {
+      expect(num(ev('=FV(0.06/12, 120, -100)')), closeTo(16387.93, 0.01));
+      expect(num(ev('=FV(0, 10, -100)')), 1000);
+    });
+
+    test('PV of an annuity', () {
+      expect(num(ev('=PV(0.05, 10, -1000)')), closeTo(7721.73, 0.01));
+      expect(num(ev('=PV(0, 10, -100)')), 1000);
+    });
+
+    test('NPV discounts future flows', () {
+      expect(num(ev('=NPV(0.1, 100, 200, 300)')), closeTo(481.59, 0.01));
+    });
+
+    test('IRR of a project', () {
+      final sheet = Sheet();
+      sheet.setValue('A1', '-1000');
+      sheet.setValue('A2', '400');
+      sheet.setValue('A3', '400');
+      sheet.setValue('A4', '400');
+      sheet.setValue('B1', '=IRR(A1:A4)');
+      expect(num(sheet.valueOf('B1')), closeTo(0.0971, 0.001));
+    });
+
+    test('NPER inverts PMT', () {
+      final sheet = Sheet();
+      sheet.setValue('A1', '=PMT(0.01, 24, 1000)');
+      sheet.setValue('A2', '=NPER(0.01, A1, 1000)');
+      expect(num(sheet.valueOf('A2')), closeTo(24, 0.01));
+    });
+
+    test('RATE inverts PMT', () {
+      final sheet = Sheet();
+      sheet.setValue('A1', '=PMT(0.02, 36, 5000)');
+      sheet.setValue('A2', '=RATE(36, A1, 5000)');
+      expect(num(sheet.valueOf('A2')), closeTo(0.02, 1e-4));
+    });
+  });
+}