feat: add Cart::fake(), assertion methods, and CartFactory

Adds Cart::fake() which swaps the active driver to NullDriver and returns
a CartFactory fluent test builder. Adds assertion methods (assertContains,
assertCount, assertTotalQuantity, assertSubTotal, assertTotal,
assertConditionApplied, assertEmpty, assertNotEmpty) to Cart for use in
tests. CartFactory provides withItems(), withCondition(), and withCoupon()
as chainable helpers for seeding cart state.

src/Cart.php

@@ -5,10 +5,13 @@
 namespace Wearepixel\Cart;
 
 use Closure;
+use PHPUnit\Framework\Assert;
 use Wearepixel\Cart\Coupons\Coupon;
+use Wearepixel\Cart\Drivers\NullDriver;
 use Wearepixel\Cart\Helpers\Helpers;
 use Wearepixel\Cart\Shipping\ShippingRate;
 use Wearepixel\Cart\Tax\TaxRule;
+use Wearepixel\Cart\Testing\CartFactory;
 use Wearepixel\Cart\Validators\CartItemValidator;
 use Wearepixel\Cart\Exceptions\InvalidItemException;
 use Wearepixel\Cart\Exceptions\UnknownModelException;
@@ -49,6 +52,81 @@ public function getDriver(): CartDriver
         return $this->driver;
     }
 
+    public function fake(): CartFactory
+    {
+        $this->driver = new NullDriver($this->driver->getSessionKey());
+
+        return new CartFactory($this);
+    }
+
+    public function assertContains(int|string $id): void
+    {
+        Assert::assertTrue(
+            $this->has($id),
+            "Failed asserting that cart contains item [{$id}]."
+        );
+    }
+
+    public function assertCount(int $count): void
+    {
+        Assert::assertCount(
+            $count,
+            $this->getContent(),
+            "Failed asserting that cart has [{$count}] items."
+        );
+    }
+
+    public function assertTotalQuantity(int|float $quantity): void
+    {
+        Assert::assertEquals(
+            $quantity,
+            $this->getTotalQuantity(),
+            "Failed asserting that cart total quantity is [{$quantity}]."
+        );
+    }
+
+    public function assertSubTotal(float $amount): void
+    {
+        Assert::assertEquals(
+            $amount,
+            $this->getSubTotal(false),
+            "Failed asserting that cart subtotal is [{$amount}]."
+        );
+    }
+
+    public function assertTotal(float $amount): void
+    {
+        Assert::assertEquals(
+            $amount,
+            $this->getTotal(),
+            "Failed asserting that cart total is [{$amount}]."
+        );
+    }
+
+    public function assertConditionApplied(string $conditionName): void
+    {
+        Assert::assertNotNull(
+            $this->getCondition($conditionName),
+            "Failed asserting that condition [{$conditionName}] is applied to the cart."
+        );
+    }
+
+    public function assertEmpty(): void
+    {
+        Assert::assertTrue(
+            $this->isEmpty(),
+            'Failed asserting that the cart is empty.'
+        );
+    }
+
+    public function assertNotEmpty(): void
+    {
+        Assert::assertFalse(
+            $this->isEmpty(),
+            'Failed asserting that the cart is not empty.'
+        );
+    }
+
     public function getSessionKey(): string
     {
         return $this->driver->getSessionKey();

src/Testing/CartFactory.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Wearepixel\Cart\Testing;
+
+use Wearepixel\Cart\Cart;
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Coupons\Coupon;
+
+class CartFactory
+{
+    public function __construct(private Cart $cart) {}
+
+    public function withItems(int $count): static
+    {
+        for ($i = 1; $i <= $count; $i++) {
+            $this->cart->add($i, "Test Item {$i}", 10.00, 1);
+        }
+
+        return $this;
+    }
+
+    public function withCondition(CartCondition $condition): static
+    {
+        $this->cart->condition($condition);
+
+        return $this;
+    }
+
+    public function withCoupon(Coupon $coupon): static
+    {
+        $this->cart->coupon($coupon);
+
+        return $this;
+    }
+}

tests/Unit/Testing/CartFactoryTest.php

@@ -0,0 +1,60 @@
+<?php
+
+use Wearepixel\Cart\Cart;
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Drivers\SessionDriver;
+use Wearepixel\Cart\Testing\CartFactory;
+use Wearepixel\Cart\Tests\Helpers\SessionMock;
+
+function makeCartForFactory(): Cart
+{
+    $events = Mockery::mock('Illuminate\Contracts\Events\Dispatcher');
+    $events->shouldReceive('dispatch');
+
+    return new Cart(
+        new SessionDriver(new SessionMock, 'TESTKEY'),
+        $events,
+        'cart',
+        require __DIR__ . '/../../Helpers/ConfigMock.php',
+    );
+}
+
+beforeEach(function () {
+    $this->cart = makeCartForFactory();
+    $this->factory = new CartFactory($this->cart);
+});
+
+afterEach(fn() => Mockery::close());
+
+describe('CartFactory', function () {
+    test('withItems adds N generic items to the cart', function () {
+        $this->factory->withItems(3);
+
+        expect($this->cart->getContent()->count())->toBe(3);
+        expect($this->cart->getTotalQuantity())->toBe(3);
+    });
+
+    test('withCondition adds a condition to the cart', function () {
+        $condition = new CartCondition(['name' => 'GST', 'type' => 'tax', 'value' => '10%', 'target' => 'subtotal']);
+        $this->factory->withCondition($condition);
+
+        expect($this->cart->getCondition('GST'))->not->toBeNull();
+    });
+
+    test('methods are chainable', function () {
+        $result = $this->factory->withItems(2)->withCondition(
+            new CartCondition(['name' => 'Discount', 'type' => 'discount', 'value' => '-5%', 'target' => 'subtotal'])
+        );
+
+        expect($result)->toBeInstanceOf(CartFactory::class);
+        expect($this->cart->getContent()->count())->toBe(2);
+        expect($this->cart->getCondition('Discount'))->not->toBeNull();
+    });
+
+    test('withItems generates unique item IDs', function () {
+        $this->factory->withItems(3);
+        $ids = $this->cart->getContent()->keys()->toArray();
+
+        expect(array_unique($ids))->toHaveCount(3);
+    });
+});

tests/Unit/Testing/FakeCartTest.php

@@ -0,0 +1,112 @@
+<?php
+
+use Wearepixel\Cart\Cart;
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Drivers\SessionDriver;
+use Wearepixel\Cart\Tests\Helpers\SessionMock;
+
+function makeFakeableCart(): Cart
+{
+    $events = Mockery::mock('Illuminate\Contracts\Events\Dispatcher');
+    $events->shouldReceive('dispatch');
+
+    return new Cart(
+        new SessionDriver(new SessionMock, 'TESTKEY'),
+        $events,
+        'cart',
+        require __DIR__ . '/../../Helpers/ConfigMock.php',
+    );
+}
+
+beforeEach(function () {
+    $this->cart = makeFakeableCart();
+});
+
+afterEach(fn() => Mockery::close());
+
+describe('Cart::fake()', function () {
+    test('returns the CartFactory instance', function () {
+        expect($this->cart->fake())->toBeInstanceOf(\Wearepixel\Cart\Testing\CartFactory::class);
+    });
+
+    test('swaps driver to NullDriver', function () {
+        $this->cart->fake();
+        expect($this->cart->getDriver()->getDriverName())->toBe('null');
+    });
+
+    test('Cart operations work after fake()', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 50.00, 2);
+
+        expect($this->cart->isEmpty())->toBeFalse();
+        expect($this->cart->getContent()->count())->toBe(1);
+    });
+
+    test('assertContains passes when item is in cart', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 50.00, 1);
+
+        expect(fn() => $this->cart->assertContains(1))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertContains fails when item is not in cart', function () {
+        $this->cart->fake();
+
+        expect(fn() => $this->cart->assertContains(99))->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertCount passes with correct count', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'A', 10.00, 1);
+        $this->cart->add(2, 'B', 20.00, 1);
+
+        expect(fn() => $this->cart->assertCount(2))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertTotalQuantity passes with correct quantity', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'A', 10.00, 3);
+
+        expect(fn() => $this->cart->assertTotalQuantity(3))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertSubTotal passes with correct subtotal', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 50.00, 2);
+
+        expect(fn() => $this->cart->assertSubTotal(100.00))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertTotal passes with correct total', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 50.00, 1);
+
+        expect(fn() => $this->cart->assertTotal(50.00))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertConditionApplied passes when condition exists', function () {
+        $this->cart->fake();
+        $this->cart->condition(new CartCondition(['name' => 'GST', 'type' => 'tax', 'value' => '10%', 'target' => 'subtotal']));
+
+        expect(fn() => $this->cart->assertConditionApplied('GST'))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertConditionApplied fails when condition does not exist', function () {
+        $this->cart->fake();
+
+        expect(fn() => $this->cart->assertConditionApplied('GST'))->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertEmpty passes when cart is empty', function () {
+        $this->cart->fake();
+
+        expect(fn() => $this->cart->assertEmpty())->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertNotEmpty passes when cart has items', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 10.00, 1);
+
+        expect(fn() => $this->cart->assertNotEmpty())->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+});