nearle_pos/test/unit/promo_engine_test.dart

import 'package:flutter_test/flutter_test.dart';
import 'package:nearle_pos/domain/entities/cart.dart';
import 'package:nearle_pos/domain/entities/product.dart';
import 'package:nearle_pos/domain/entities/promo.dart';
import 'package:nearle_pos/domain/services/promo_engine.dart';

/// Campaigns give money away automatically, so the rules that decide how much
/// are the ones worth pinning down hardest.
void main() {
  Product product({
    String id = 'p1',
    ProductCategory category = ProductCategory.beverages,
    double price = 100,
    double gstRate = 0.18,
  }) =>
      Product(
        id: id,
        name: 'Item $id',
        barcode: 'bc-$id',
        sku: 'sku-$id',
        category: category,
        price: price,
        stock: 100,
        gstRate: gstRate,
      );

  Cart cartOf(List<({Product product, double qty})> items) => Cart(
        lines: [
          for (final i in items)
            CartLine(product: i.product, quantity: i.qty),
        ],
      );

  final monday = DateTime(2026, 8, 3, 11);

  group('amount', () {
    test('a percentage off the bill', () {
      final cart = cartOf([(product: product(), qty: 3)]); // 300
      const promo = Promo(
        id: 'a',
        name: '10% off',
        type: PromoType.percentOffBill,
        value: 10,
      );

      expect(PromoEngine.amountFor(promo: promo, cart: cart), 30);
    });

    test('a flat amount off the bill', () {
      final cart = cartOf([(product: product(), qty: 3)]);
      const promo = Promo(
        id: 'a',
        name: '50 off',
        type: PromoType.flatOffBill,
        value: 50,
      );

      expect(PromoEngine.amountFor(promo: promo, cart: cart), 50);
    });

    test('a category promo touches only that category', () {
      final cart = cartOf([
        (product: product(category: ProductCategory.beverages), qty: 2), // 200
        (product: product(id: 'p2', category: ProductCategory.snacks), qty: 3),
      ]);
      const promo = Promo(
        id: 'a',
        name: '20% off drinks',
        type: PromoType.percentOffCategory,
        value: 20,
        targetId: 'beverages',
      );

      expect(PromoEngine.amountFor(promo: promo, cart: cart), 40);
    });

    test('a product promo touches only that product', () {
      final cart = cartOf([
        (product: product(), qty: 2),
        (product: product(id: 'p2'), qty: 4),
      ]);
      const promo = Promo(
        id: 'a',
        name: '50% off p2',
        type: PromoType.percentOffProduct,
        value: 50,
        targetId: 'p2',
      );

      expect(PromoEngine.amountFor(promo: promo, cart: cart), 200);
    });

    test('a promo whose target is not in the bill is worth nothing', () {
      final cart = cartOf([(product: product(), qty: 2)]);
      const promo = Promo(
        id: 'a',
        name: '20% off Dairy',
        type: PromoType.percentOffCategory,
        value: 20,
        targetId: 'dairy',
      );

      expect(PromoEngine.amountFor(promo: promo, cart: cart), 0);
    });

    test('a cap stops a large trolley giving away more than was costed', () {
      final cart = cartOf([(product: product(), qty: 50)]); // 5000
      const promo = Promo(
        id: 'a',
        name: '20% off, max 200',
        type: PromoType.percentOffBill,
        value: 20,
        maxDiscount: 200,
      );

      expect(PromoEngine.amountFor(promo: promo, cart: cart), 200);
    });
  });

  group('buy X get Y', () {
    const promo = Promo(
      id: 'bxgy',
      name: 'Buy 2 get 1',
      type: PromoType.buyXGetY,
      targetId: 'p1',
      buyQuantity: 2,
      freeQuantity: 1,
    );

    test('gives nothing until a whole group is in the bill', () {
      for (final qty in [1.0, 2.0]) {
        final cart = cartOf([(product: product(), qty: qty)]);
        expect(PromoEngine.amountFor(promo: promo, cart: cart), 0,
            reason: '$qty items should not earn a free one',);
      }
    });

    test('gives one free per completed group, and no more', () {
      // Three earns one. Five still earns one — the fifth has not paid for a
      // second group.
      expect(
        PromoEngine.amountFor(
          promo: promo,
          cart: cartOf([(product: product(), qty: 3)]),
        ),
        100,
      );
      expect(
        PromoEngine.amountFor(
          promo: promo,
          cart: cartOf([(product: product(), qty: 5)]),
        ),
        100,
      );
      expect(
        PromoEngine.amountFor(
          promo: promo,
          cart: cartOf([(product: product(), qty: 6)]),
        ),
        200,
      );
    });

    test('prices the free unit at what is actually being charged', () {
      // A line already carrying a manual discount must not refund more than it
      // took in the first place.
      final cart = Cart(
        lines: [
          CartLine(
            product: product(),
            quantity: 3,
            discount: const Discount(type: DiscountType.percentage, value: 50),
          ),
        ],
      );

      expect(PromoEngine.amountFor(promo: promo, cart: cart), 50);
    });

    test('a malformed buy-X-get-Y is worth nothing rather than everything', () {
      final cart = cartOf([(product: product(), qty: 10)]);
      const broken = Promo(
        id: 'b',
        name: 'Buy 0 get 0',
        type: PromoType.buyXGetY,
        targetId: 'p1',
      );

      expect(PromoEngine.amountFor(promo: broken, cart: cart), 0);
    });
  });

  group('eligibility', () {
    final cart = Cart(
      lines: [CartLine(product: product(), quantity: 3)],
    ); // 300

    List<AppliedPromo> run(List<Promo> promos, {DateTime? at}) =>
        PromoEngine.evaluate(cart: cart, promos: promos, at: at ?? monday);

    test('an inactive campaign never fires', () {
      expect(
        run([
          const Promo(
            id: 'a',
            name: 'Paused',
            type: PromoType.percentOffBill,
            value: 10,
            isActive: false,
          ),
        ]),
        isEmpty,
      );
    });

    test('a campaign outside its dates never fires', () {
      const promo = Promo(
        id: 'a',
        name: 'August',
        type: PromoType.percentOffBill,
        value: 10,
      );

      final august = promo.copyWith(
        validFrom: DateTime(2026, 8),
        validTo: DateTime(2026, 8, 31),
      );

      expect(run([august], at: DateTime(2026, 7, 31, 23)), isEmpty);
      expect(run([august], at: DateTime(2026, 8, 15)), hasLength(1));

      // Inclusive of the closing day: a campaign "to the 31st" runs all of it.
      expect(run([august], at: DateTime(2026, 8, 31, 22)), hasLength(1));
      expect(run([august], at: DateTime(2026, 9, 1)), isEmpty);
    });

    test('a weekday-restricted campaign fires only on those days', () {
      const weekend = Promo(
        id: 'a',
        name: 'Weekends',
        type: PromoType.percentOffBill,
        value: 10,
        daysOfWeek: {6, 7},
      );

      expect(run([weekend], at: DateTime(2026, 8, 3)), isEmpty); // Monday
      expect(run([weekend], at: DateTime(2026, 8, 8)), hasLength(1)); // Sat
      expect(run([weekend], at: DateTime(2026, 8, 9)), hasLength(1)); // Sun
    });

    test('a minimum bill value is enforced', () {
      const promo = Promo(
        id: 'a',
        name: 'Over 500',
        type: PromoType.flatOffBill,
        value: 50,
        minBillValue: 500,
      );

      expect(run([promo]), isEmpty, reason: 'the bill is only 300');
    });
  });

  group('stacking', () {
    final cart = Cart(
      lines: [CartLine(product: product(), quantity: 10)],
    ); // 1000

    List<AppliedPromo> run(List<Promo> promos) =>
        PromoEngine.evaluate(cart: cart, promos: promos, at: monday);

    test('only the best exclusive campaign applies', () {
      // Two stacking percentages compound into a discount nobody costed, and
      // the shop finds out at the end of the month.
      final applied = run(const [
        Promo(id: 'a', name: '5%', type: PromoType.percentOffBill, value: 5),
        Promo(id: 'b', name: '15%', type: PromoType.percentOffBill, value: 15),
        Promo(id: 'c', name: '10%', type: PromoType.percentOffBill, value: 10),
      ]);

      expect(applied, hasLength(1));
      expect(applied.single.promo.id, 'b');
      expect(applied.single.amount, 150);
    });

    test('the best means best for the shopper, not highest percentage', () {
      final applied = run(const [
        Promo(id: 'pct', name: '5%', type: PromoType.percentOffBill, value: 5),
        Promo(
          id: 'flat',
          name: '200 off',
          type: PromoType.flatOffBill,
          value: 200,
        ),
      ]);

      expect(applied.single.promo.id, 'flat');
      expect(applied.single.amount, 200);
    });

    test('stackable campaigns all apply, alongside one exclusive', () {
      final applied = run(const [
        Promo(
          id: 's1',
          name: 'Stack 50',
          type: PromoType.flatOffBill,
          value: 50,
          stackable: true,
          priority: 10,
        ),
        Promo(
          id: 's2',
          name: 'Stack 30',
          type: PromoType.flatOffBill,
          value: 30,
          stackable: true,
          priority: 20,
        ),
        Promo(id: 'e1', name: '10%', type: PromoType.percentOffBill, value: 10),
        Promo(id: 'e2', name: '5%', type: PromoType.percentOffBill, value: 5),
      ]);

      expect(applied.map((a) => a.promo.id), ['s1', 's2', 'e1']);
      expect(applied.fold<double>(0, (s, a) => s + a.amount), 180);
    });

    test('priority breaks a tie between equal exclusive campaigns', () {
      final applied = run(const [
        Promo(
          id: 'low',
          name: 'A',
          type: PromoType.flatOffBill,
          value: 100,
          priority: 50,
        ),
        Promo(
          id: 'high',
          name: 'B',
          type: PromoType.flatOffBill,
          value: 100,
          priority: 10,
        ),
      ]);

      expect(applied.single.promo.id, 'high');
    });

    test('no combination can drive the bill below zero', () {
      // The one outcome that must be impossible: a sale that becomes a payout.
      final applied = run(const [
        Promo(
          id: 'a',
          name: 'Huge',
          type: PromoType.flatOffBill,
          value: 900,
          stackable: true,
        ),
        Promo(
          id: 'b',
          name: 'Also huge',
          type: PromoType.flatOffBill,
          value: 900,
          stackable: true,
        ),
      ]);

      final total = applied.fold<double>(0, (s, a) => s + a.amount);
      expect(total, lessThanOrEqualTo(1000));
      expect(total, 1000);
    });
  });

  group('on the bill', () {
    test('a promo reduces the total and is included in savings', () {
      final base = Cart(lines: [CartLine(product: product(), quantity: 10)]);
      expect(base.grandTotal, 1000);

      const promo = Promo(
        id: 'a',
        name: '10% off',
        type: PromoType.percentOffBill,
        value: 10,
      );

      final withPromo = base.copyWith(
        appliedPromos: [const AppliedPromo(promo: promo, amount: 100)],
      );

      expect(withPromo.promoDiscountAmount, 100);
      expect(withPromo.billDiscountTotal, 100);
      expect(withPromo.grandTotal, 900);
      expect(withPromo.totalSavings, 100);
    });

    test('GST is recomputed against the reduced total, not the original', () {
      // Prices are GST-inclusive, so a discount reduces the tax collected. Left
      // unapportioned the shop would remit tax on money it never took.
      final base = Cart(lines: [CartLine(product: product(), quantity: 10)]);
      const promo = Promo(
        id: 'a',
        name: '10% off',
        type: PromoType.percentOffBill,
        value: 10,
      );

      final withPromo = base.copyWith(
        appliedPromos: [const AppliedPromo(promo: promo, amount: 100)],
      );

      expect(withPromo.taxAmount, lessThan(base.taxAmount));
      expect(withPromo.taxAmount, closeTo(base.taxAmount * 0.9, 0.02));
      expect(
        withPromo.taxableAmount + withPromo.taxAmount,
        closeTo(withPromo.netAmount, 0.02),
      );
    });

    test('a promo and a tier discount both apply, without going negative', () {
      final base = Cart(lines: [CartLine(product: product(), quantity: 10)]);
      const promo = Promo(
        id: 'a',
        name: 'Everything free',
        type: PromoType.flatOffBill,
        value: 5000,
      );

      final withPromo = base.copyWith(
        appliedPromos: [const AppliedPromo(promo: promo, amount: 5000)],
      );

      expect(withPromo.billDiscountTotal, 1000);
      expect(withPromo.grandTotal, 0);
      expect(withPromo.netAmount, greaterThanOrEqualTo(0));
    });

    test('an empty cart earns nothing', () {
      expect(
        PromoEngine.evaluate(
          cart: Cart.empty,
          promos: const [
            Promo(id: 'a', name: 'x', type: PromoType.flatOffBill, value: 50),
          ],
          at: monday,
        ),
        isEmpty,
      );
    });
  });
}