A whole lot of stuff

test/estimated_refund_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:fuel_tax_tracker/services/estimated_refund.dart';
+
+void main() {
+  test('the rate itself is 12.5 cents per gallon', () {
+    expect(moHighwayFuelTaxRefundRatePerGallon, 0.125);
+  });
+
+  test('multiplies whole gallons by the rate', () {
+    expect(estimatedFuelRefund(10), 1.25);
+    expect(estimatedFuelRefund(20), 2.50);
+    expect(estimatedFuelRefund(0), 0);
+  });
+
+  test('rounds fractional gallons to the nearest whole gallon before multiplying', () {
+    // 10.523 rounds up to 11 gallons: 11 * $0.125 = $1.375, floored to $1.37.
+    expect(estimatedFuelRefund(10.523), 1.37);
+    // 10.4 rounds down to 10 gallons: 10 * $0.125 = $1.25 exactly.
+    expect(estimatedFuelRefund(10.4), 1.25);
+  });
+
+  test('rounds the resulting cents down, never up', () {
+    // 1 gallon * $0.125 = $0.125 — rounding to the nearest cent would give
+    // $0.13, but this must floor to $0.12 instead.
+    expect(estimatedFuelRefund(1), 0.12);
+    // 3 gallons * $0.125 = $0.375 — floors to $0.37, not $0.38.
+    expect(estimatedFuelRefund(3), 0.37);
+  });
+}