Custom Money Generator

Sometimes you need domain-specific values with tailored shrinking. This example demonstrates how to write a MoneyGenerator that favours round amounts and shrinks toward $0.00.

import 'dart:math';

import 'package:property_testing/property_testing.dart';
import 'package:test/test.dart';

class Money {
  final String currency;
  final int cents;

  const Money(this.currency, this.cents);

  @override
  String toString() => '$currency ${(cents / 100).toStringAsFixed(2)}';
}

class MoneyGenerator extends Generator<Money> {
  final List<String> currencies;

  MoneyGenerator({this.currencies = const ['USD', 'EUR', 'GBP']});

  @override
  ShrinkableValue<Money> generate(Random random) {
    final currency = currencies[random.nextInt(currencies.length)];
    final cents = random.nextInt(20_000); // up to 200.00 in smallest unit
    final money = Money(currency, cents);

    return ShrinkableValue(money, () sync* {
      // Prefer whole dollars first.
      if (money.cents % 100 != 0) {
        yield ShrinkableValue.leaf(Money(currency, money.cents - money.cents % 100));
      }

      // Halve towards zero for finer shrinking.
      var current = money.cents;
      while (current > 0) {
        current ~/= 2;
        yield ShrinkableValue.leaf(Money(currency, current));
      }

      yield ShrinkableValue.leaf(Money(currency, 0));
    });
  }
}

Money applyDiscount(Money money) {
  final discountedCents = max(0, money.cents - 1500); // clamp at zero
  return Money(money.currency, discountedCents);
}

void main() {
  test('discount never produces negative money', () async {
    final moneyGen = MoneyGenerator();

    final runner = PropertyTestRunner(
      moneyGen,
      (money) {
        final discounted = applyDiscount(money);
        expect(discounted.cents, greaterThanOrEqualTo(0),
            reason: 'Negative result for $money');
        expect(discounted.cents, lessThanOrEqualTo(money.cents),
            reason: 'Discount increased value for $money');
      },
      PropertyConfig(numTests: 200),
    );

    final result = await runner.run();
    expect(result.success, isTrue, reason: result.report);
  });
}

The custom shrinker first drops cents to the nearest dollar and then halves the value repeatedly, so failing cases quickly converge on the smallest problematic amount.