public void Buy_First_To_Fourth_Each_One_Book()
        {
            // arrange
            var target        = new HarryPotterDiscount();
            var shoppingItems = new SaleBook[] {
                new SaleBook {
                    ISBN = "9573317249", Name = "哈利波特(1):神秘的魔法石", Price = 100, Count = 1
                },
                new SaleBook {
                    ISBN = "9573317583", Name = "哈利波特(2):消失的密室", Price = 100, Count = 1
                },
                new SaleBook {
                    ISBN = "9573318008", Name = "哈利波特(3):阿茲卡班的逃犯", Price = 100, Count = 1
                },
                new SaleBook {
                    ISBN = "9573318318", Name = "哈利波特(4):火盃的考驗", Price = 100, Count = 1
                }
            };

            var expected = 320m;

            // act
            var actual = target.CaculateDiscount(shoppingItems);

            // assert
            Assert.AreEqual(expected, actual);
        }
        public void Buy_One_Book()
        {
            // arrange
            var target        = new HarryPotterDiscount();
            var shoppingItems = new SaleBook[] {
                new SaleBook {
                    ISBN = "9573317249", Name = "哈利波特(1):神秘的魔法石", Price = 100, Count = 1
                }
            };

            var expected = 100m;

            // act
            var actual = target.CaculateDiscount(shoppingItems);

            // assert
            Assert.AreEqual(expected, actual);
        }