Kata09: Back to the Checkout

Kata09.cs

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{

    static void Main()
    {
        var product = new Product(1) { Price = 10 };
        var product2 = new Product(2) { Price = 1 };

        var orderItem = new OrderedItem { Product = product, Count = 10 };
        var orderItem2 = new OrderedItem { Product = product2, Count = 2 };

        var discountRule1 = new DiscountRule { Product = product, NumberOfProductsToApplyTheRule = 3, SpecialPrice = 25 };
        var discountRule2 = new DiscountRule { Product = product, NumberOfProductsToApplyTheRule = 5, SpecialPrice = 40 };

        var total = new DiscountEngine().Calculate(new[] { orderItem, orderItem2 },
            new IPricingRule[] { discountRule1, discountRule2 });

        Console.WriteLine(total);
    }


    public class Product : IEquatable<Product>
    {
        public Product(int id)
        {
            Id = id;
        }

        public int Id { get; }

        public string Name { get; set; }

        public decimal Price { get; set; }

        public bool Equals(Product other)
        {
            if (other == null)
            {
                return false;
            }

            return NonNullEquals(other);
        }

        public override bool Equals(object obj)
        {
            if (obj == null || obj.GetType() != GetType())
            {
                return false;
            }

            return NonNullEquals((Product)obj);
        }

        public override int GetHashCode()
        {
            return Id.GetHashCode();
        }

        private bool NonNullEquals(Product product)
        {
            return Id == product.Id;
        }
    }

    public class OrderedItem
    {
        public Product Product { get; set; }

        public int Count { get; set; }
    }

    public interface IPricingRule
    {
        Product Product { get; set; }

        int NumberOfProductsToApplyTheRule { get; set; }

        IPricingRuleResult Calculate(int numberOfItems);
    }

    public interface IPricingRuleResult
    {
        decimal TotalCost { get; }

        int NumberOfProducts { get; }
    }

    private class PricingRuleResult : IPricingRuleResult
    {
        public decimal TotalCost { get; set; }

        public int NumberOfProducts { get; set; }
    }

    public class DiscountRule : IPricingRule
    {
        public Product Product { get; set; }

        public int NumberOfProductsToApplyTheRule { get; set; }

        public decimal SpecialPrice { get; set; }

        public IPricingRuleResult Calculate(int numberOfItems)
        {
            if (numberOfItems < NumberOfProductsToApplyTheRule)
            {
                return new PricingRuleResult { NumberOfProducts = 0, TotalCost = 0 };
            }

            var numberOfGroups = numberOfItems / NumberOfProductsToApplyTheRule;

            return new PricingRuleResult
            {
                NumberOfProducts = numberOfGroups * NumberOfProductsToApplyTheRule,
                TotalCost = numberOfGroups * SpecialPrice
            };
        }
    }

    public class DiscountEngine
    {
        public decimal Calculate(IEnumerable<OrderedItem> products, IEnumerable<IPricingRule> rules)
        {
            var result = products.GroupJoin(rules, p => p.Product, r => r.Product, (p, r) =>
                new
                {
                    OrderItem = p,
                    Rules = r.OrderByDescending(rule => rule.NumberOfProductsToApplyTheRule)
                }
            );

            decimal total = 0;

            foreach (var orderItem in result)
            {
                var count = orderItem.OrderItem.Count;

                foreach (var pricingRule in orderItem.Rules)
                {
                    if (count == 0)
                    {
                        break;
                    }

                    var calc = pricingRule.Calculate(count);

                    total += calc.TotalCost;
                    count -= calc.NumberOfProducts;

                }

                if (count != 0)
                {
                    // applying default price
                    total += count * orderItem.OrderItem.Product.Price;
                }
            }

            return total;
        }
    }
}