public class Calculator
{
  public double Sum(double first, double second)
  {
    return first + second;
  }
}

[Fact]
public void Purchase_succeeds_when_enough_inventory()
{
  // Arrange
  var store = new Store();
  store.AddInventory(Product.Shampoo, 10);
  var customer = new Customer();
  // Act
  bool success = customer.Purchase(store, Product.Shampoo, 5);
  // Assert
  Assert.True(success);
  Assert.Equal(5, store.GetInventory(Product.Shampoo));
}

[Fact]
public void Purchase_succeeds_when_enough_inventory()
{
  // Arrange
  var store = new Store();
  store.AddInventory(Product.Shampoo, 10);
  var customer = new Customer();
  // Act
  bool success = customer.Purchase(store, Product.Shampoo, 5);
  store.RemoveInventory(success, Product.Shampoo, 5);
  // Assert
  Assert.True(success);
  Assert.Equal(5, store.GetInventory(Product.Shampoo));
}

public class CustomerTests
{
  [Fact]
  public void Purchase_succeeds_when_enough_inventory()
  {
    Store store = CreateStoreWithInventory(Product.Shampoo, 10);
    Customer sut = CreateCustomer();
    bool success = sut.Purchase(store, Product.Shampoo, 5);
    Assert.True(success);
    Assert.Equal(5, store.GetInventory(Product.Shampoo));
  }
  [Fact]
  public void Purchase_fails_when_not_enough_inventory()
  {
    Store store = CreateStoreWithInventory(Product.Shampoo, 10);
    Customer sut = CreateCustomer();
    bool success = sut.Purchase(store, Product.Shampoo, 15);
    Assert.False(success);
    Assert.Equal(10, store.GetInventory(Product.Shampoo));
  }
  private Store CreateStoreWithInventory(Product product, int quantity)
  {
    Store store = new Store();
    store.AddInventory(product, quantity);
    return store;
  }
  private static Customer CreateCustomer()
  {
    return new Customer();
  }
}

public class CalculatorTests
{
  [Fact]
  public void Sum_of_two_numbers()
  {
    double first = 10;
    double second = 20;
    var sut = new Calculator();
    double result = sut.Sum(first, second);
    Assert.Equal(30, result);
  }
}